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In Cognitive Neuropsychology,18(8),673-696 Letter Position Dyslexia 1

Letter Position Dyslexia

Naama Friedmann

Tel Aviv University, Israel

Aviah Gvion

Loewenstein Rehabilitation Center, Israel

Many word-reading models assume that the early stages of reading involve a separate process of

letter position encoding. However, neuropsychological evidence for the existence and selectivity

of this function has been rather indirect, coming mainly from position preservation in migrations

between words in attentional dyslexia, and from non-selective reading deficits. No pure

demonstration of selective impairment of letter position function has yet been made. In this paper

two Hebrew-speaking acquired dyslexic patients with occipito-parietal lesions are presented, who

suffer from a highly selective deficit to letter position encoding. As a result of this deficit, they

predominantly make errors of letter migration within words (such as reading �broad� for

�board�) in a wide variety of tasks: oral reading, lexical decision, same-different decision and

letter location. The deficit is specific to orthographic material, and is manifested mainly in medial

letter positions. The implications of the findings to models of reading and attention are discussed.

In recent years several distinct types of acquired dyslexia have been identified. These discoveries owe

much to a fruitful interaction between the study of reading disorders and information processing models.

The models of normal single word reading have been constructed and shaped by the identification of

selective deficits, each indicative of failures in different parts and stages of the reading process (e.g.,

Coltheart, 1981; Patterson, 1981). On the other hand, new dyslexic patterns have been identified

following predictions derived from information processing models and other, already known, types of

dyslexia have become better understood through their use. Most of the components of these word-reading

models have been found to have correlates in selective reading deficits. The later stages of reading, such

as the orthographic lexicon, the grapheme to phoneme converter, and the connections between them,

have been found to be selectively impaired in various central dyslexias (Shallice & Warrington, 1980).

We thank Uri Hadar, David Swinney, and Karalyn Patterson for their helpful comments and discussions. Address correspondence to Na�ama Friedmann, School of Education, Tel Aviv University, Tel Aviv 69978, Israel. e-mail [email protected].

LETTER POSITION DYSLEXIA 2

However, for some of the theoretically postulated functions of the earlier stages of visual analysis,

dyslexic correlates have not yet been identified.

In this paper we present a new type of peripheral dyslexia, which, though predicted by the model, has not

previously been demonstrated clinically. This dyslexia is a result of a highly selective deficit to the visual

analysis system.

According to Ellis and Young (1988), the visual analyzer has three distinct functions:

1. Letter identification.

2. Letter to word binding: allocation of letters to the word they belong to (or an attenuation filter that

reduces input from words outside the appropriate window. Shallice, 1988).

3. Encoding of letter position within a word (Ellis, 1993; Ellis, Flude, & Young, 1987), or its position

relative to exterior letters (Humphreys, Evett, & Quinlan, 1990; Peressotti & Grainger, 1995).

Each of these three functions is predicted to be susceptible to a selective deficit, causing completely

different patterns of errors.

When the first function is impaired, the result should be a failure to identify letters correctly. This has

been shown to be the case in letter agnosia which is a deficit located in the letter identification function

of the visual analyzer, in which patients fail to recognize letters, even in isolation. Another type of letter

identification error occurs when patients make visual paralexias - reading n instead of m, or b instead of d

(termed �visual dyslexia� by Marshall & Newcombe, 1973; Newcombe & Marshall, 1981; see also

Lambon Ralph & Ellis, 1997). A different type of deficit in processing letters or part-of-the-word appears

in neglect dyslexia and its close relative, positional dyslexia. In these dyslexias, patients fail to read and

report letters in a specific side of the word - either left or right, or in a specific position in the word

(Arguin & Bub, 1997; Caramazza & Hillis, 1990; Ellis, et al., 1987; Ellis, Young, & Flude, 1993; Katz &

Sevush, 1989; Kinsbourne & Warrington, 1962; Riddoch, Humphreys, Cleton, & Fery, 1990; Young,

Newcombe, & Ellis, 1991; Warrington, 1991). Interestingly, these patients successfully encode the

position of letters, as indicated both by the consistent omission of letters in a specific spatial position,

and by letter substitutions at the impaired side, which usually preserve letter position and word length.

(Caramazza & Hillis, 1990; Ellis et al., 1987; Young et al., 1991).

When the second function of the visual analyzer fails, the reader should fail to allocate letters to the

words they belong to. In normal skilled readers, this causes occasional letter migrations between words

when words are presented briefly (Allport, 1977; McClelland & Mozer, 1986; Mozer, 1983; Shallice &

McGill, 1978; Treisman & Souther, 1986; Van der Velde, 1992). A more permanent failure of the letter-


to-word binding function causes attentional dyslexia, which is a similar phenomenon, but with a much

higher error rate: patients fail to relate letters to the words they belong to,1 and the result is letters that

�migrate� from one word to the other even in the absence of time limit (Shallice & Warrington, 1977;

Warrington, Cipolotti, & McNeil, 1993).

A selective impairment to the third function - identifying the position of letters within a word - has never

been demonstrated. The only evidence from neuropsychology in favor of the within-word-position

function of the visual analyzer has been indirectly deduced from the pattern of letter migration in

attentional dyslexia, where letters migrate primarily to corresponding positions in the other word, and

preserve their within-word positions (Shallice & Warrington, 1977; Saffran & Coslett, 1996).

The current study offers direct evidence for this function of the visual analyzer from two Hebrew-

speaking individuals who present a selective deficit to positioning of letters within a word, with intact

letter identification and intact binding of letters to words (henceforth: Letter Position Dyslexia, LPD).

The special nature of Hebrew orthography offers an interesting test case for the study of reading. Hebrew

is a Semitic language which is written from right to left in Hebrew letters. Vowels are usually not

represented in Hebrew orthography, and many words comprise only consonant letters. For example, the

vowels /a/ and /ε/ are almost never represented (except for at the end of words), and therefore words that

sound completely different are written exactly the same. For example, /kerex/ (volume), /karax/ (bound),

and /krax/ (metropolis) are all written the same: כרך (KRK). The vowels /i/, /o/ and /u/ are represented

only in some of the words. Furthermore, even when a vowel is represented orthographically by a letter,

this letter is usually ambiguous between several vowels and consonants (the letter for example can be "ו "

read as /o/; /u/; /w/ or /v/). (And some consonants are ambiguous too.) This leads to numerous reading

possibilities for almost every Hebrew letter sequence, some of these readings being existing words (a 4-

letter word, when represented by consonants only, can actually be read in more than 63 different ways!).

As a result, when a letter erroneously changes position in a sequence, there are many possible ways to

read the new sequence. This increases the probability that at least one of them will be an existing word,

and therefore lexical knowledge cannot always compensate for letters that are perceived in a wrong

position. These properties of the Hebrew orthography (together with the Semitic morphological system,

see Discussion) make a selective deficit of letter migration within words easier to detect in Hebrew.

1 Or fail to set the attentional window (Humphreys & Riddoch, 1992) or to attenuate irrelevant neighboring words

(Shallice, 1988).


Two Hebrew-speaking dyslexic patients with Letter Position Dyslexia are presented: they identify letters

correctly, but are impaired in assigning letters to their proper position within a word. Thus, they

predominantly make letter-order errors such as reading בשלנית for BSLNIT=/bashlanit/= a) בלשנית

woman who likes cooking for BLSNIT=/balshanit/=female linguist). This pattern suggests that letter

position encoding is a separate function of the visual analysis system.

SUBJECTS

Two right-handed Hebrew-speaking patients, BS and PY, participated in the study. Both were referred to

the clinic with reading difficulties following a left hemisphere lesion. Both had no prior reading disorder.

BS was a 75 year-old right-handed man, who worked prior to his impairment as a graphic editor and a

calligraphy artist and had a bachelor degree from the academy of arts. He was admitted to the aphasia

clinic for language therapy. Three months prior to his admission BS underwent left parieto-occipital

tumor removal craniotomy, which was complicated 5 days later with hematoma at the bed of the tumor

which resulted in a mild right hemiparesis, right field hemianopsia, and language disorders. Upon

admission, neurological analysis of the CT scan reported left parieto-occipital lesion; BS was oriented in

time and place and was attentive and cooperative. His main complaint was difficulties in reading.

Language assessment using the Hebrew version of the WAB (Kertesz, 1982, Hebrew version by Soroker,

1997) revealed fluent speech with very mild nominal difficulties in spontaneous speech (WAB

Spontaneous speech=18/20) as well as mild nominal difficulties in confrontation naming (WAB Object

naming=50/60) with no phonological or verbal paraphasias (only circumlocutions and �Don�t know�

responses), good repetition (WAB Repetition = 93/100) and fairly good auditory comprehension (WAB

Auditory Word Recognition = 58/60; WAB Sequential Commands=68/80). His writing to dictation was

much better than his reading, with only nine letter position errors and two homophone substitutions in

writing 155 words.

PY was a 70 year-old man, a right-handed army veteran with high school education. He was tested 4

months post onset of a left ischemic parieto-occipital infarct, which resulted in a right hemiparesis, which

was already resolved at the time of the language assessment, and with no visual field deficits. Upon

admission, neurological analysis of the CT scan reported parieto-occipital infarct in the left hemisphere.

Language assessment revealed nearly intact performance in all language modalities except reading:

spontaneous speech (WAB Spontaneous Speech=20/20), sentence repetition (WAB Repetition = 93/100),

naming (WAB Object Naming=57/60, Fluency=14/20), and auditory comprehension (WAB Auditory


Word Recognition=60/60, WAB Sequential Commands=80/80). No phonological paraphasias were

present in his spontaneous speech and naming.

No visuo-spatial neglect was found for either BS or PY when tested with the Behavioral Inattention Test

neglect battery (BIT- Wilson, Cockburn, & Halligan, 1987) all subtests including picture copying and

human figure drawing were intact and included no sign of spatial or configural deficit. Simultanagnosia

was also ruled out by the patients� performance on complex picture description tasks which was

completely fluent and normal (the cookie theft from the BDAE, Goodglass & Kaplan, 1983, and a

sequence of four detailed pictures). In addition, no deficit in object perception and identification was

observed in context of several objects appearing simultaneously in the visual field.

Reading of single letters was good for both patients, even in short presentation: In single letter naming

tasks, BS correctly read 52 of 54 letters (10 items with no time limit, 22 presented on computer screen for

1 second, and 22 for 0.7 seconds); BS could not perform the task in shorter presentation times. PY read

44/44 letters correctly in 0.1 sec computerized presentation.

Error Types and Effects on Reading

The reading pattern presented by the patients was unique in both in the error types and in the effects on

reading (namely, which word stimuli induced more errors). The predominant error was letter migrations

within words. Unlike in the central dyslexias, no regularization, semantic paralexias or lexicalizations

(except for �migratable nonwords�, see Experiment 2, p. 12 ) were observed. In addition, there were no

visual paralexias, no errors typical to neglect dyslexia, no letter-by-letter reading, and only a few letters

migrated between words.

In order to assess the effects on reading performance such as regularity, imagibility and grammatical

category, the relevant subtests of the Hebrew PALPA (Kay, Lesser, & Coltheart, 1992, Hebrew version

by Gil & Edelstein, 1999) were devised.

The patients did not show effects of regularity on PALPA 35 (Table 1, No effect for each patient

separately using χ2, P>0.1, and no effect for both subjects combined, using Mantel Haenszel test for

collection of 2X2 tables, χ2=0.071, P>0.5).

The patients also showed no effect of imagibility on PALPA 31 (TABLE 2, no effect for each patient

separately using χ2, P>0.1, and no effect for both subjects together using Mantel Haenszel test, χ2=0.65,

P>0.1). The relatively small number of items in the PALPA imagibility subtest is not responsible for the

lack of imagibility effect. When we added to the PALPA results a reanalysis by imagibility of all the


words read in Experiment 1.1 (400 items for BS, 356 items for PY), there still was no significant effect

of imagibility (BS performed 80% in low imagibility words and 82% in high imagibility words, using χ2,

P>0.05; PY performed 88% correct in low imagibility words and 92% correct in high imagibility words,

again, using χ2, P>0.05).

Word length was not a critical factor for reading performance either � increasing length did not cause

increase in error rate (Table 3).2 No linear trend was found using logistic regression for PY or for BS

(without the 3-item cell), P>0.1. Long words of 5 and 6 letters that do not have lexical anagrams (and, as

will be shown soon, are therefore less prone to letter migration errors) were read better than short 4-letter

words that do have lexical anagrams (93% correct in 5-6-letter no-anagram words vs. 66% in 4-letter

words with anagrams).

The grammatical category effect that was found using Hebrew PALPA 32 was very different from the

�classical� grammatical class effect (as witnessed for example in the reading of many patients with deep

dyslexia, see Coltheart, 1980; Marshall & Newcombe, 1980; Morton & Patterson, 1980; Patterson,

1979). For example, function words, which are usually the hardest category, turned out to be the best

category for our patients. Function words were significantly better than verbs for PY and for both

subjects combined (using Mantel Haenszel test χ2=6.87, P=0.009); all other comparisons of function

words with other categories for each patient and for the sum were non-significant in the direction of

better reading of function words, using χ2 and Bonferroni adjustment (Table 4). Even after adding the

results of a reanalysis by grammatical category of all the stimuli in Experiment 1.1 to the PALPA results

(400 items for BS, 356 items for PY), grammatical category effect was still in the same direction, with

function words being not significantly different from nouns, and significantly better than verbs and

adjectives.

2 In order to allow a clear estimate of length effect we looked only at non-migratable words, because in

migratable words the migration potential depends on number of letters, and middle migration is only possible in words of four letters and longer.


TABLE 1

Reading of Regular � Irregular Words

Subject Regular Irregular

BS# 75% (24/32) 75% (24/32)

PY# 100% (32/32) 94% (30/32)

Total# 88% (56/64) 84% (54/64)

% correct (number correct/total). # n.s., p>0.1.

TABLE 2

Reading of Words of High and Low Imagibility

Subject Low imagibility High imagibility

BS# 87% (26/30) 77% (23/30)

PY# 93% (28/30) 90% (27/30)

Total# 90% (54/60) 83% (50/60)

% correct (number correct/total) # n.s., p>0.1.

TABLE 3

Reading of Words as a Function of Number of Letters

2 3 4 5 6 >6

BS 100% (11/11) 97% (28/29) 83% (20/24) 90% (38/42) 85% (11/13) 33% (1/3)

PY 100% (11/11) 92% (35/38) 93% (14/15) 98% (41/42) 93% (13/14) 100% (8/8)

% correct (non-migratable words)

TABLE 4

Reading Words of Different Grammatical Categories

Subject Verb Noun Adjective Function word

BS 70% (14/20) 75% (15/20) 80% (16/20) 90% (18/20)

PY 70% (14/20) 80% (16/20) 90% (18/20) 100% (19/19)

Total 70% (28/40) 78% (31/40) 85% (34/40) 95% (37/39)

% correct (number correct/total)


This unusual pattern of grammatical category effect, with best reading performance on function words, is

probably only a by-product of the differential liability of these categories to letter migrations: function

words were the least susceptible to migration errors due to their small number of letters, and verbs and

adjectives were the most vulnerable due to the nature of Semitic word templates, which makes verbs and

adverbs interchangeable by changing the location of a single letter (see Discussion for further detail on

the effects of word length and Semitic templates on liability to migration errors). These two factors were

probably responsible for the unusual effect of grammatical category on reading,

It seems, then, on analysis of both error types and various effects on reading, that this deficit is not

consistent with any of the known dyslexias. We therefore proceeded to examine the nature of the

positional deficit in reading in greater detail.

CONTROL SUBJECTS To obtain data on normal performance in the following experiments, all tests were also administered to

10 control subjects without reading deficits aged 32-75, 5 men and 5 women. All subjects were native

speakers of Hebrew, without language or reading disorders. Two of the control subjects were matched in

age, gender, and education level to the two dyslexic patients, and their data were analyzed separately.

The performance of all control subjects on all tasks was above 95% correct; the exact performance rate

for each test is given in Appendix A.

EXPERIMENTAL INVESTIGATIONS

Experiment 1: Reading �migratable� words

The first experiment examined reading of migratable words, namely, words that consist of letters which,

in a different order, could assemble at least one additional word (A relevant example in English is the

letter set [b,r,e,a,d] which could be read as beard, bared, bread and debar).

Our assumption was that if letter position information is not available to the patients, they might rely on

lexical knowledge, either as a part of the automatic reading process or as a compensatory strategy to limit

themselves to existing words. For many words, these lexical constraints will lead to a single possible

reading: the only word that could be constructed from the letters identified. For example, for the English

letter set [b,u,t,t,e,r], only one reading possibility is allowed by the lexicon � the word �butter�. In these

words then, a deficit in letter location will mostly be compensated. The deficit will become more

apparent in letter sets which have more than one lexical reading. In these cases, the lexicon�s help is not


enough, and the patients are bound to make more errors of letter migration within a word (reading

�bread� as �beard�, for example).

We therefore compared words for which the letters could be ordered to form a different word (migratable

words) with words that are built from letter sets that have a unique reading (non-migratable words). We

used oral reading and same-different decision tasks.

1.1 Oral reading of migratable words

Method

Migratable and non-migratable Hebrew words were presented in large print (18 pt. font) without time

limitation (54 of the non-migratable words presented to PY were presented on a computer screen for 0.1

seconds. His performance on the time limited task was similar to that of the untimed task, and the results

were collapsed together).

The migratable words included words with middle letter migration potential (to form another existing

word, see example 1), and words with a potential of exterior letter migration (see example 2). The

patients were asked to read the words aloud, and no response-contingent feedback was given during the

test.

(1) Migratable word pair, middle migration: הספיק-הפסיק

HSPIK-HPSIK (/hispik/=managed - /hifsik/=stopped)

(2) Migratable word pair, exterior migration: משיכה-שמיכה

SMIXH-MSIXH (/smixa/=blanket - /meshixa/=attraction, withdrawal, pull)

Migratable and non-migratable stimuli were 2-8 letters long (average length for migratable words was

4.1, average length for non-migratable words was 4.2 letters) and were balanced for lexical category

(6:2:1 ratio of nouns, verbs, and adjectives respectively for both the migratable and non-migratable

words).3

3 In tests in which some items are missing (when the patients did not complete the whole test), the relevant

Method sections include length and grammatical category data for the items that were actually read by the patients and were included in the results, and not for all the pre-planned items.


Results TABLE 5

Reading of Migratable Vs. Non-Migratable Words

Subject Migratable words Non-migratable words

BS* 74% (207/278) 89% (109/122)

PY* 84% (192/228) 95% (122/128)

Total* 79% (399/506) 92% (231/250)


* p<0.005

The results presented in Table 5 show that BS is more impaired in reading than PY. However, they share

the same error pattern. For both patients, migratable word reading was more impaired than reading of

non-migratable words (the difference was significant for each individual patient using χ2, P<0.005, and

for the sum using Mantel Haenszel test χ2=20.82, P<0.0001). Within-word migrations were by far the

most frequent error in the reading of migratable words, accounting for 87% (93/107) of the errors in

migratable words. In non-migratable words, errors were mainly mixed errors of letter addition and letter

migration in middle positions, all producing other existing words. Lexical knowledge was probably

responsible for the more preserved reading of non-migratable words.

Frequency was not responsible for the difference in performance between migratable and non-migratable

words. In a post-hoc assessment of the frequency of words in this test, we obtained frequency estimation

ratings from 50 native speakers of Hebrew, who rated the words in this test on a scale of 1-7, 7 being

�very frequent�. In addition, we used words from the frequency estimation database for Hebrew words by

Ram Frost. The average frequency estimations of migratable and non-migratable words did not differ

significantly (t<1). Average frequency of non-migratable words was 4.45 (SD=1.38), average frequency

for migratable words was 4.43 (SD=1.39). Words with exterior migration potential had average

frequency of 4.43 (SD=1.37), and words with middle letter migration 4.44 (SD=1.41).

Most of the migration errors involved middle letter migration to another middle position. Only three

migrations from exterior positions occurred for both patients together. For this reason, migratable words

in the following tests were based only upon medial letter migration (see Table 14 for a full analysis of

exterior vs. medial position pure migration errors in this test.) Note that the good reading of migratable

words with only exterior letter migration potential means that Table 5 actually presents an overestimation

of the patients� ability to read migratable words with middle letter migration potential.


1.2 Same-different decision

Method

120 Hebrew word pairs were presented in random order. The test included 40 pairs of words differing in

the relative order of middle letters ( תריס-תירס TIRS-TRIS =corn-shutter), 40 pairs of words differing in

the identity of a single middle letter ( אסור-אפור AFOR-ASOR =gray-prohibited), and 40 pairs of identical

words ( מחשב-מחשב MXSV-MXSV=computer). Word stimuli were balanced for length and grammatical

category between conditions. Each condition included 48 nouns, 24 verbs, and 8 adjectives. Words were

4-7 letters long, with an average length of 5.0 for each condition.

The word pairs were presented visually without time limitation, and the patients were asked to determine

whether the words in the pair were identical or not, without reading them aloud. The test was

administered twice to PY: once without time limit, and once with 1 second exposure for each pair. The

timed test was administered one month after the untimed test. PY�s performance on the two tasks was

identical (except for the �same� pairs, in which the unspeeded presentation yielded 35 items correct, and

the speeded yielded 32 correct items) and therefore the results were lumped together in Table 6.

Results

The patients� performance is presented in Table 6. The results were striking: a chance performance was

evinced in same-different decision of words that differed in letter order (not significantly different from

chance for each patient and for the sum, P>0.1, binomial test). In marked contrast, the performance on

same-different decision for words that differed in letter identity was relatively intact and differed

significantly from the different-order pairs (for each individual patient P<<0.0001, and for the sum using

Mantel Haenszel test, χ2=50.09, P<<0.0001), and from chance (for each patient and for the sum,

P<0.0001, using the binomial test), and did not differ from the performance in the �same� condition (for

each subject P>0.1, and for the sum, Mantel Haenszel χ2=2.27, P>0.1). Thus, in this task too, the

patients� errors were mainly errors of letter migration within words.


TABLE 6

Same-Different Decision: Different Order and Different Letter Pairs

Subject Different order Different letter Same

BS 60% (24/40) 95% (38/40) 93% (37/40)

PY 48% (38/80) 93% (74/80) 84% (67/80)

Total 52% (62/120) 93% (112/120) 87% (104/120)


Summary

From both experiments it is evident that letter migration within words is the predominant error type in the

reading of these two patients. The scarcity of letter substitutions in the various reading tasks, and the

good performance in same-different decision of words that differ in one letter, suggest that the deficit is

selective to letter location, and that letter identification is relatively unimpaired.

Experiment 2: Nonword reading

Reliance on lexical knowledge in the absence of letter position information is useful when reading non-

migratable words. Nevertheless, exclusive reliance on the lexicon could become an obstacle in reading

nonwords that are derived from existing words by changing the letter order: it could lead to misreading of

nonwords as words. For example, in the absence of letter position information, the nonword �pincel� is

prone to be read as �pencil�. It is therefore interesting to examine the nonword reading performance of

patients who have impaired position encoding, and rely on lexical information. Nonword reading was

examined by means of two types of tests: reading aloud and lexical decision (both without time

limitation).

2.1 Oral reading of non words

Method

We compared nonwords that can be rearranged as Hebrew words following middle letters rearrangement

(migratable nonwords) with nonwords that cannot (non-migratable nonwords). Nonwords were

constructed by changing a single letter in existing words. All nonwords were 4-6 letters long, with an

average of 5.0 letters for each condition.


Results

The patients� performance in this task is presented in Table 7. Both patients showed tendency toward

reading migratable nonwords worse than non-migratable nonwords. For PY, it was significant (χ2=10.80,

P=0.001); BS showed only non-significant tendency (χ2=0.74, P>0.1), and both of them together showed

significant difference using Mantel Haenszel test (χ2=6.81, P<0.01). Unlike the non-migratable nonword

reading, only 4% of the errors in the migratable nonwords formed another nonword: 23 of the 24 errors

in the migratable nonwords were migration errors that formed an existing word, and only one was

addition error that formed another nonword. In the non-migratable nonword reading, errors were

migrations that formed another nonword, and letter substitutions that formed a word.

TABLE 7

Reading Aloud � Migratable and Non-Migratable Nonwords

Subject Migratable nonwords Non-migratable nonwords

BS# 52% (15/29) 65% (11/17)

PY* 66% (19/29) 97% (33/34)

Total* 59% (34/58) 86% (44/51)

% correct (number correct/total) # n.s. ; *p=0.01

2.2 Lexical decision

Method

Twenty eight migratable nonwords and 28 Hebrew words were presented individually to the patients,

printed on paper (18 pt. font). Sequences were 5-6 letters long, with an average of 5.3 in both conditions.

The patients were asked to determine whether the sequence was an existing word or not.

Results

As shown in Table 8, migratable nonwords were judged as words almost half of the time (using the

binomial test, not significantly different from chance, for each patient and for the sum, P>0.05). Words

were judged as nonwords only 2% of the time, and were significantly better than nonwords (for each

patient P<0.0005, and for the sum using Mantel Haenszel test, χ2=26.21, P<0.0001). The results indicate

that the patients rely on lexical knowledge, and this tendency leads them to read migratable nonwords as

words with the same letters, but different letter order.


TABLE 8

Lexical Decision Task

Subject Migratable nonwords

judged as words

Words

judged as nonwords

BS* 50% (14/28) 4% (1/28)

PY * 39% (11/28) 0% (0/28)

Total* 45% (25/56) 2% (1/56)

* p <0.0005

The pattern that emerges from the experiments presented so far regarding reliance on the lexicon is that

lexical knowledge helps to confine reading to the right word in non-migratable words, is less helpful in

migratable words, induces errors in migratable nonwords, and does not make a difference in (non-

migratable) nonword reading.

Experiment 3: letter location tasks

In order to focus on the letter location deficit we explored the locational ability directly, by asking the

subjects to name a letter according to its serial position in the word, and to determine the position of a

letter in a presented sequence.

3.1 Naming letters according to position in a word

3.1.1 Visual presentation

Method

115 non-migratable Hebrew words of 3-6 letters (average length 4.3) were visually presented to one of

the patients (BS). In each word, the patient was asked to name a letter according to its serial position

provided by the experimenter, without time limitation (�What is the second letter in this word?�).

Results

The results again indicate a deficit in letter location, which is most evident in middle positions (χ2=5.59,

P<0.05) (see Table 9). Errors were naming letters that appeared in a different serial position in the word.


TABLE 9

Naming Letters According to Position � Visual Presentation

Subject Middle position First / Last position

BS* 74% (56/76) 92% (36/39)


* significant difference between middle and exterior position using χ2, P<0.05

A demonstration of the difficulty BS experienced in these letter location tasks could be seen in his

response when trying to name the middle letter in the 5-letter word MTRIA (/mitria/ -umbrella):

�tee. no... there isn�t really a middle letter...ar?�

Interestingly, in all 20 cases in which the patient failed to name middle letters according to their position,

he succeeded in oral spelling of the whole word (letter by letter).

3.1.2 Auditory presentation

Method

Letter naming by position was tested in auditory presentation as well. In this task, the experimenter said a

Hebrew word, and the patient had to use his mental imagery of the orthographic representation of the

word in order to name a letter according to its position (�What is the second letter in the word broad?�).

Words were 3-6 letter long with an average length of 4.3 letters. All words had a potential of middle

letter migration. Words in the first/last letter naming condition had exterior letter migration potential as

well (such as the word בוחר [=voter, chooses], which has a middle letter migration potential for בחור

[=guy], but also exterior migration potential for בורח, חרוב, רחוב [=runs-away, carob, street]).

Results

The results of the auditory presentation were similar to those observed in the visual presentation: naming

middle letters was poor, while first and last letter naming was intact (Table 10; significant difference

between middle and exterior position for each subject, P<0.05, and for the sum using Mantel Haenszel

test, χ2=9.6, P<0.005). Although in Hebrew there are several homophone letters, and the phoneme to

grapheme relation is highly irregular, the patients did not make letter selection errors (homophone letter

selection), only migration errors. This indicates that they did not lean on phonological analysis for this

task, but rather retrieved the items from the orthographic lexicon.


TABLE 10

Naming Letters According to Position - Auditory Presentation

Subject Middle position First / Last position

BS* 67% (51/76) 100% (9/9)

PY* 69% (36/52) 100% (18/18)

Total* 68% (87/128) 100% (27/27)


* p<0.05

3.2 Determining letter position within a symbol sequence

Method

Sequences were presented on a computer screen. Each sequence consisted of four pound symbols, and a

Hebrew letter in one of the five possible positions within the sequence מ# # ( # #). Each sequence was

presented for 0.1 second to PY and for 1 second (30 sequences) or 0.7 second (10 sequences) to BS. The

patients were asked to locate the position of the letter (say the serial position from right to left).

Results

In this task, again, both patients showed impairment in letter location, primarily in medial positions, as

shown in Table 11. The difference between middle and first/last positions was significant only for BS,

χ2=6.67, P=0.01, and not significant for PY, χ2=1.69, P>0.1, significant for both using Mantel Haenszel

test, χ2=5.89, P<0.05.

TABLE 11

Determining Letter Position in a Sequence

Letter position Subject

Middle First/last

BS* 67% (16/24) 100% (16/16)

PY# 61% (11/18) 83% (10/12)

Total* 64% (27/42) 93% (26/28)


* p<0.05. # n. s.

Summing up, direct assessment of the patients� ability to locate letters within words and sequences

indicates a deficit in this function. The vulnerability of the middle letters, observed in the previous

reading tasks, was also detected in the letter location tasks.


Experiment 4: �Classical� attentional dyslexia

The results indicate that the patients suffer a problem with encoding the position of each letter within the

word, which causes migrations within words. Do they also evince letter migration between words, like

the patients reported in Shallice and Warrington�s (1977) classical study?

The patients described by Shallice and Warrington (and the Alzheimer�s patient in Saffran & Coslett,

1996) showed migration of letters between words which in most cases preserved their within-word

position (the first letter in one word migrated to the first position in the other word).

A patient with letter migrations within words will not necessarily suffer from migrations between words

as well (see for example Humphreys, Riddoch, & Muller, (in Riddoch et al. 1990), and Saffran & Coslett,

1996, for a related discussion of attentional deficit between and within words; and Duncan, 1987, for a

comparison of between- and within- word migration in normal reading). However, in case letters do

migrate between words, we surmised that the pattern of migration between words would be different for

LPD patients, since the patients� ability to encode letter position is impaired. For these patients, letters

are expected to migrate also to non-corresponding positions, without preserving the original position

within the word.

In order to check whether migrations between words occur in these patients� reading and to examine

position preservation, semantically unrelated Hebrew word pairs were presented for oral reading, without

time limitation. Words were 4-6 letters long, and shared 2-3 letters in the beginning, the end or the

middle. They were constructed in such a way that two letters, separately or combined, could migrate to

the second word and create two other words (see examples (3), (4) below). The distance between words

was 0.2 cm - the regular spacing between 20 point letters in font David.

אלים כדים (3)

ALIM KDIM /elim/-/kadim/ =gods,violent � jars

possible migrations: כלים KLIM (/kelim/ =tools); אדים ADIM (/edim/ =vapor)

חולבת חושפת (4)

XOLBT XOSFT /xolevet/-/xosefet/ =milks-exposes possible migrations: חולפת XOLFT (/xolefet/ =passes); חושבת XOSBT (/xoshevet/ =thinks)

Results

Few migration errors between words were found for both patients. The errors were not consistent with

the position preservation reported for �classical� attentional dyslexia: 55% of the migrations between


words did not preserve the original position (no significant difference was found between position

preserving and non-preserving errors for each subject, P>0.1, and for both of them together using Mantel

Haenszel test, χ2=0.25, P>0.1). Furthermore, letter position preservation in the remaining 45% of the

migration errors does not necessarily indicate preserved knowledge of letter position. It may be the case

that it is merely the lexical constraints that prevented the letters from migrating to a different position: in

all the position-preserving cases, there was no other position to insert the migrating letter and still keep

the sequence a lexical item.

TABLE 12

Letter Migrations Between Words

Migrations between words

/ word pairs

Error type

Preserving position Not preserving position

BS 16% (16/100) 38% (6/16) 63% (10/16)

PY 12% (15/124) 53% (8/15) 47% (7/15)

Total 14% (31/224) 45% (14/31) 55% (17/31)

Experiment 5: Reading nonlinguistic material - numbers and icons

In order to assess whether the reading deficit was limited to linguistic-orthographic material, reading of

numbers and icons was examined.

5.1 Oral reading of numbers

The patients were asked to read aloud 28 numbers. The numbers were 3-6 digits long.

Results

BS�s number reading was 71% correct (20/28); PY�s reading was 86% correct (24/28). In number

reading there were no errors of migration of the type evinced in word reading (e.g., reading 1423 instead

of 1243). The 12 errors in the oral reading task were digit substitutions only, nine of them were doubling

of digits: replacing a digit with a different existing digit in the same number (reading 1223 instead of

1243)).4 It is hard to determine whether the doubling errors were a result of random digit substitution or

migration of a digit without deletion of the digit from the original position.

4 In addition, BS had six �decimal � errors (such as naming 200 as 2000): three pure decimal, and three mixed

with substitutions, that were also included in TABLE 13.


5.2 Same-different decision - numbers

The patients were asked to make same-different decisions on 3-6 digit number pairs. Half of the pairs

were identical, and half differed in digit order, but consisted of the same digits. The same and different

pairs were randomly ordered.

Results TABLE 13

Same-Different Decision - Numbers

Same-different decision

BS 91% (29/32)

PY 95% (35/37)

Total 93% (64/69)


As seen in Table 13, the performance of both patients in the same-different task was good. The errors in

same-different decision consisted of four false alarms (calling a same pair �different�), and one

misdetection (calling a different pair �same�). The results of this test were significantly different from

the equivalent task in word reading: unlike in words, where the patients could not detect a difference in

letter position and performed at chance in same-different decision task, in the number task they

performed significantly better than chance (for each subject, and for the sum using binomial test,

P<<0.0001), and significantly different from the same-different order decision in the word task (for each

patient, and for the sum, χ2=33.28, P<<0.0001).

5.3 Digit position within a number

Thirty-five numbers of 4-6 digits were presented on paper (in 20 pt. font). The patient (BS) was asked to

name a digit according to its position in the number. (The experimenter asked �What is the second digit

in this number?� pointing to the number). Only digits in medial positions were included.

Results

BS scored 89% correct (31/35) on this task. All four errors involved naming of a digit from a different

position.

5.4 Icon position

Twelve sequences of 5 icons were presented separately to BS. The size of the icons was the same as the

size of letters in the other tests. The patient was asked to name an icon according to its position within a

sequence (�What is the second icon in ☺ " # $ %?�).


Since Hebrew is read from right to left, the patient was instructed to refer to the rightmost icon as the

first. Only medial positions (2nd, 3rd, and 4th icons) were tested. A short training session preceded this test

in which the patient was asked to name each icon separately.

Results

BS performed perfectly on this test: he detected 12/12 correct icon positions.5

Although more data are needed, especially regarding icon reading, it seems that the patients� deficit

manifests mainly in words. In icon-sequence naming the deficit was not observed at all, and in number

reading the error pattern was different, and smaller in extent. This adds to the data that indicate that the

locational deficit does not extend to objects and pictures from the patients� good performance on

complex pictures, which they flawlessly drew, copied and described (see Subject description, p.7 ).

6. Further analyses: toward a characterization of LPD

6.1 Medial vs. first and last letter migration – Exterior letter advantage

A comparison of migration errors in different positions within words revealed that not all letter positions

were equally impaired. A reanalysis of the reading of migratable words in Experiment 1.1 by position of

potential migration, summarized in Table 14 (only pure migration errors, words with both middle and

exterior migration potential counted twice), showed that medial letters were 10 to 38 times more

vulnerable to migration than letters in first and last positions (for each individual patient, P<0.0001, and

for the sum using Mantel Haenszel test, χ2=55.10, P<<0.0001). Migration errors occurred mainly in

medial positions, while first and last letters were relatively migration-proof.6 (Average frequency

estimations for words with middle and for words with exterior migration potential did not differ

significantly [t<1]. Frequency estimations for words with only exterior migration potential was 4.43

[SD=1.37], and for words with middle letter migration 4.44 [SD=1.41]).

5 It might be that this task was easier due to the lower similarity between these items compared to letters. Future

studies might use more similar non-orthographic stimuli to avoid this problem. 6 A tendency toward exterior position advantage was also found in number reading: In the oral number reading, 3

out of 4 migration errors PY made were in medial position, and 7 out of 11 errors (including doubling) of BS were in medial position. This parallelism between letters and numbers with regard to exterior position advantage in normals was reported by Hammond and Green (1982) and Mason (1982).


TABLE 14

Word Reading: Medial vs. Exterior Letter Migration

Subject Medial Exterior

BS* 20% (38/194) 1% (1/195)

PY* 12% (21/174) 1% (2/171)

Total* 16% (59/368) 1% (3/366)

number of errors of a certain type / total words with potential migration of this type

* p<0.0001.

The data about word reading (Table 14) as well as about letter position tasks (Tables 9, 10, and 11)

indicate that migration errors occur mainly in medial positions, and almost never in first and last

positions. The privileged status of end letters is in accordance with numerous previous findings on

normal reading and impaired reading, obtained in different experimental tasks. Bradshaw, Bradley, Gates

and Patterson (1977); Bradshaw and Mapp (1982); Grainger and Segui (1990); Humphreys, Evett and

Quinlan (1990); Humphreys, Evett, Quinlan, and Besner (1987); Mason (1982); Merikle and Coltheart

(1972); Perea (1998) and others have shown that in normal reading, letters at the beginning and end of

words are processed differently from and faster than letters within.

The aforementioned studies dealt mainly with letter identification. Humphreys, Evett and Quinlan (1990)

have studied letter position processing as well, and have found lack of position specificity for medial

letters: End letters were found to be more accurately tied to their relative position than were internal

letters in normal word-reading. Estes (1975) directly examined transpositions in normal reading, and

found that twice as many occur in middle than in end positions in words, and in a 4:1 ratio in nonwords.

Why are exterior letters identified and located better than middle letters? Some researchers have

suggested that exterior letters are the ones that are used to access the subset of word candidates, and that

these are the positions that provide more activation to the lexicon (Forster, 1976; Grainger & Segui,

1990).7 This importance of exterior letters might be the reason that in normal reading, word processing

begins at the ends, namely that exterior letters are accessed faster than interior ones (Bradshaw, et al.,

1977; Bradshaw & Mapp, 1982; Mason, 1982; Merikle & Coltheart, 1972). If indeed initial and final

letters have priority in word processing, it might be the case that our patients, with limited attentional

capacity (for reading), allocate attention to these positions, while interior letters are not attended. As a

7 If the access code to the input lexicon is the initial letter and the word�s length, then it is evident why last letter

position in essential for lexical access: a word�s length is determined by the serial position of the terminal letter. (See Mason, 1982 for a similar account regarding last item in words and numbers.)


result, illusory conjunctions between letters and positions occur in medial positions. In this respect, LPD

readers are somewhat like participants in an attention task of the kind used by Treisman and Schmidt

(1982) in which subjects were required to report the two digits that appear before and after a letter string.

There too, participants experience illusory conjunctions in the unattended letter string appearing between

the two attended exterior digits.

Another contribution to the better preservation of position in exterior letters might be that they have less

neighbors, and therefore smaller number of transposition opportunities (or competing positions)

compared to middle letters. However, we need to account somehow for the magnitude of difference

between errors on middle and exterior letters. As Estes (1975) noted, an error ratio of 2:1 of middle

compared to end letters would be expected, but our findings show a much higher ratio of 38:1 for one

patient, and 10:1 for the other. Mozer (personal communication), suggests that the observed ratio can be

explained by some sort of non-linear lateral interference, in which having two neighbors causes

uncertainty far greater than twice the uncertainty caused by having a single neighbor.

Note, that the witnessed pattern of letter location errors can result either from complete loss of location

information or from an enhanced uncertainty with respect to letter location, with letters being encoded in

a probabilistic way, high probability on the letter�s actual position and a distribution of lower probability

around this position. More empirical data regarding the rate of transpositions between neighbouring

positions compared to non-neighbouring positions might help determine which of the possibilities

describes the deficit better.

6.2 Word frequency

An analysis of the migration errors was made, which examined the directionality of errors with respect to

word frequency. We examined whether errors were more likely to occur from less frequent to more

frequent word, or whether words were read and errors were made regardless of their relative frequency.

Since Hebrew does not have an updated frequency list for all lexical items, relative frequency of

migratable words that the patients read with migration errors was determined by 25 normal readers,

native speakers of Hebrew. Judges were asked to decide, for each written word pair, which word was

more frequent. The 30 word pairs that were agreed upon by at least 80% of the judges were included in

the analysis.


TABLE 15

The Effect of Target Word Frequency on Letter Migration

Non-frequent to frequent Frequent to non-frequent

BS* 17/30 8/30

PY** 11/30 1/30

Total** 47% (28/60) 15% (9/60)

significant difference between columns using χ2 *p<0.05, **p<=0.001

Analysis of the reading of these 60 words shows a directional pattern of migration errors, with a

significant difference between frequent and non-frequent words. The patients tended to read non-frequent

words as their frequent counterparts, and not vice versa (Table 15). For instance, the word "טפלון"

(Teflon) was read by both patients as � טלפון � (telephone), while the other direction was not witnessed -

�telephone� was not read as �Teflon�. (100% of the judges rated the word �telephone� as the more

frequent word.) This difference was significant for each patient, P<0.05; and for the sum using Mantel

Haenszel (χ2=13.17, P<0.0005). In a further test that required reading of 24 word pairs of frequent and

non-frequent words (each word was separately presented, in random order), the same tendency toward

the frequent reading was observed, although in a relatively small number of errors. BS had 4 out of 5

migration errors toward the frequent reading; PY had 2 out of 3.

These results show preference for the more familiar or frequent word, and a tendency to read the more

frequent anagram in cases of failure to encode letter position, thus supporting the contribution of lexical

knowledge to word reading in Letter Position Dyslexia.8

6.3 Is this an orthographic input deficit or a phonological output deficit?

Until now we have ascribed letter migration in our patients� reading to orthographic-visual input deficit.

Theoretically, however, an input deficit is not the only possible explanation, as incorrect reading aloud

can be also due to phonological output deficit. The data show, however, that a phonological deficit is not

the source of our patients� letter migrations. If there were an output deficit, we would expect the deficit to

be manifested also in speech contexts that do not involve reading, such as spontaneous speech,

confrontation naming, and word repetition. In addition, we would not expect to find the deficit in reading

tasks that do not require reading aloud. The data were just the opposite. Neither patient showed any sign

of phonological deficit in speech contexts: They repeated words normally, and there were no

8 This finding is more consistent with position uncertainty at the first perceptual stages of position encoding,

which is later assisted by the lexical level, either by feedback from the lexical level to the visual analysis stage or by some type of guessing procedure, than with wrong position perception at the very first perceptual stages that are not by themselves sensitive to word frequency.


phonological paraphasias in their spontaneous speech or in oral confrontation naming. In addition, their

migration errors affected word comprehension: Words that were read with a migration error were

assigned the meaning of the response word and not the target word. Had the problem been a phonological

output deficit, they would have read the word aloud incorrectly, but understood it as the target word.

Furthermore, the results of the same-different decision, the lexical decision and the letter location tasks

indicate that the same errors occurred when the patients were not required (and even asked not) to read

aloud.9

The error pattern also favors an orthographic-input over a phonological output explanation: Errors

preserved the letters but not the sounds of the target word. Namely, the migrated units were graphemes

rather than phonemes. For example, the 2-syllable mishpat became mefashet ( שטפמ ! פטשמ( . These

words share the same letters, but they do not have any syllable in common. Furthermore, the dissociation

found between orthographically migratable and non-migratable words cannot be explained by a

phonological-output deficit.

In light of all these facts, we can confidently conclude that the deficit of our patients lies in the early

stages of input analysis rather than in the late phonological output processes.

DISCUSSION

We have reported a series of experiments examining the word reading of two individuals with acquired

dyslexia. The main characteristic of their reading deficit was within-word migrations of middle letters

that accounted for approximately 90% of the errors. The main effects on reading accuracy were the

factors that determined the string�s liability to migration: the lexical status and frequency of the possible

migration outcomes compared to the lexical status and frequency of the target. Reading was the poorest

for nonwords that had existing word anagrams, and for words that had high frequency anagrams. Target

words with no lexical anagram were read with fewer errors. Regularity, semantic content, length, and

grammatical class effects were not exhibited by our patients.

The deficit proved highly selective. Letter identification was intact, and letter-to-word binding was

relatively spared. Very few �classical attentional dyslexia� errors in which letters migrated between

words occurred, and when they did, letters did not keep their within-word position

9 Still the possibility of �inner speech� exists. A possible way to prevent inner speech in future studies would be

to include �articulatory suppression� (Baddeley, 1990) in same-different and lexical decision tasks.


This pattern of results indicates a novel type of peripheral dyslexia which stems from a deficit in the

visual analysis stage. Only one function of the visual analyzer is impaired here: the ability to locate

letters within a word. Letter identification and letter-to-word binding are relatively spared. Such a

dissociation immediately reflects on the different separate functions of the normal visual analyzer.

Specifically, it offers direct evidence in favor of the single word reading model suggested by Ellis and

Young (1988 and Ellis et al., 1987) which postulates letter location as one of the functions of the visual

analyzer which can be separately impaired. Until now, the existence of a separate function of letter

location has been inferred from indirect evidence from patients with attentional dyslexia, who present

letter migrations between words, but preserve letter position within words (Shallice & Warrington,

1977), and from patients with neglect dyslexia who tend to keep letter position and word length when

substituting letters in one part of the word, thus showing successful assignment of letters to word

positions (Caramazza & Hillis, 1990; Ellis et al. 1987; Young et al., 1991). However, although predicted

from the model, a selective impairment to the letter location function has not been reported until now,

and it is presented here for the first time.

Why was it easier to detect LPD in Hebrew?

This selective deficit of letter migration within words has not been identified until now probably due to

the nature of the languages in which dyslexia was studied. In Hebrew it is much easier to detect such a

deficit because, due to the nature of Hebrew�s �deep orthography� (Frost, 1992; Frost, Katz, & Bentin,

1987) and Semitic morphology, more letter migrations form legal sequences, and these sequences have a

higher probability to be read as another existing word. Thus, migration errors cannot be avoided by

reliance on lexical information, and a patient who suffers from letter position deficit is liable to produce

more migration errors in reading.

In Hebrew, vowels are usually not represented in the orthography.10 Consequently, the Hebrew reader has

to tackle a complicated reading task, equipped only with partial knowledge about the phonological

structure of the word. Without vowels, the reader has six possible readings for every consonant (a

reading for every vowel). For example, the consonant ,/can be read in the context of a word as /pa/, /pi , פ

/po/, /pu/, /pe/, and p with schwa (as well as /f/ with all these vowels). Some of these readings within a

letter sequence form existing words.11 For instance, the sequence ספר (SPR) can be read as sefer (book),

10 Hebrew has two orthography systems: pointed and unpointed. In the pointed system, diacritics are added to the

consonant letters and convey vowel information. The unpointed system does not contain the diacritics, and vowels are usually not represented in it. The majority of adult reading uses the unpointed system, whereas pointed writing is used mainly in children�s books and in poetry.

11 In addition to the underspecification of vowels, the orthographic representation of some of the consonants in Hebrew is also ambiguous. For example, the letter פ can be read both as /p/ and as /f/.


safar (counted), siper (told), sapar (barber), sfar (frontier), saper (tell-imperative), supar (tell-passive) and

sper (colloquial for spare wheel). As if to make things more complicated, even when vowels are

represented, they are ambiguous. The Hebrew letter " ו" can be read both as the vowels /o/ or /u/ and as

the consonants /w/ and /v/; the letter "י" can be read as /i/ and /ei/ and sometimes even as /a/, or as the

consonant /y/. As a result, even when vowels are represented, the degree of freedom in word reading is

still very large.

Due to this underrepresentation of vowels, almost all letter sequences that are formed by migration are

legal sequences (possible written words). The lack of vowel representation and therefore lack of markers

for syllable nuclei, allows almost every reading and every syllable partition, and increases the number of

possible readings per sequence. Even a sequence of four or five consonants can be a word, as opposed to

English, for example, where consonant-vowel alternation is usually required. Furthermore, the very large

degree of freedom in reading a Hebrew letter sequence makes it more probable that at least one of the

possible readings of a sequence that was created by migration will be an existing word (whereas in

English, for example, a migration of one letter usually forms only one reading and therefore rarely forms

an existing word.12)

The second factor that contributes to the high probability that a letter position error will result in another

existing word is the Semitic derivational morphology. Most of the words in Hebrew consist of a 3-

consonant root and a template (Bat-El, 1989; McCarthy, 1979). The fact that many roots differ only in

consonant order, and that there are templates that differ in the position of one vowel only, increases the

probability that once the target word is a possible word in Hebrew that is built from a root and a template,

letter migration would also create a morphologically well-formed sequence, and possibly an existing

word. For example, active and passive participles differ only in the location of one vowel; adjectives and

adjective-derived-nouns are written exactly like active and passive participles and also differ in vowel

position only. A migration of this vowel, for many words in these templates, would thus result in another

existing word. If a consonant migrates, then if other roots exist with the same letters in different order,

we might again end up with an existing word because the new root will be inserted in a legal template.

For example:

חבוש-חובש-חושב XoSeV-XoVeS-XaVuS (thinks-bandages,paramedic-bandaged, quince)

לדוג -גודל-גדול- דגול DaGuL-GaDoL-GoDeL-ladDuG (distinguished-big-size-to fish).


Our data have shown that migrations occur mainly when the sequence formed by migration is an existing

word. Since the combination of orthography and derivational morphology in Hebrew causes more

migrations to result in existing words, more migration errors were produced, and this made it easier to

detect letter position deficit in Hebrew.

Were any other cases of letter position deficit reported in the literature?

Careful perusal of the literature reveals some scattered evidence for letter migration within strings in

English as well. The patients reported in Price and Humphreys (1993); Shallice and Warrington (1977),

and in Warrington, Cipolotti, and McNeil (1993) who suffered from attentional dyslexia between words,

and the patient in Katz and Sevush (1989) who suffered from positional dyslexia, had letter migration

errors within letter arrays. When asked to name a specific letter within a letter sequence, these patients

made migration errors � naming a letter from a different position within the same sequence. This

accounted for 25% of the errors of Price and Humphreys� patient, 34% of the errors of Warrington,

Cipolotti and McNeil�s patient, 36% and 77% of Shallice and Warrington�s patients� errors, and for

100% of Katz and Sevush�s patient.13 Thus, these patients too were able in some cases to identify the

letters in the sequence but failed to attach them to their positions. Note that these patients� attentional

deficit was noticed within arrays of letters, not within words.14 Due to the character of English, and to the

reliance on lexical knowledge, letter position deficit in words, unlike in letter-arrays, could go

undetected.

All these reported dyslexic patients had letter position location deficit on top of other reading or general

visual attentional deficits: Some of them had hemi-neglect, some had letter identification deficits, and

some evinced the attentional deficit also in non-verbal pictorial material.

12 The reader is invited to try and find pairs of English words that differ only in the position of letters in middle

positions (such as �could� and �cloud�). In our experience, the are very hard to find in English, whereas in Hebrew such pairs are abundant, especially in verbs.

13 Transposition errors in words were reported in writing, though not as the main error type (Caramazza & Miceli, 1990; Caramazza, Miceli, Villa, & Romani, 1987; Hillis & Caramazza, 1989; McCloskey, Badecker, Goodman-Schulman, & Aliminosa, 1994). In addition, LB (described in Caramazza, Capasso, & Miceli, 1996; Caramazza et al., 1987) showed transposition errors both in spelling and in nonword reading. Unlike our patients, transpositions were not their main error type: letter substitutions and insertions were more frequent than transpositions.

14 Price and Humphreys (1993) report that their patient read 5-letter words with accuracy of only 55-58%, but they do not analyze the types of errors she made in word reading.


The current study presented two individuals with a pure letter location deficit, and thus enabled a closer

look at the exact characteristics of the Letter Position Dyslexia and the letter position function.15

Is the location deficit of our patients specific to orthographic material?

It is hard to say at this point. First, given the patients� flawless drawing and copying, and their good

description of complex pictures, as well as their good performance in the BIT test, it seems that the

deficit does not extend to objects and pictures. The good performance on icon-sequence naming goes in

the same direction, although more items are needed as well as information from stimuli that are more

similar to each other. What about numbers? Here the picture is less clear. Data indicated that number

reading was less impaired than word reading, and, crucially, the error pattern was completely different �

while in words the predominant error was migration, in number reading there were only substitutions and

doublings. There are two possible ways to interpret the observed difference between words and numbers.

It might be that the position deficit is indeed specific to letters, and that orthographic material is

processed separately from digits. However, it might also be that the deficit affects both letters and digits,

but the different nature of words and numbers causes the deficit to be manifested in words but not in

numbers. The two main differences between words and numbers that come to mind are, first, that

whereas letters in words are processed in parallel as a group, digits in numbers are processed

independently. If the attentional deficit is evident whenever several letters have to be processed in

parallel, this predicts a deficit in real words but much better performance in items that do not require

parallel processing, such as numbers (and non-words). A second important difference is that lexical cues

are available for words but not for numbers (except for significant numbers like 1968, 2000 etc., see

Patterson & Wilson, 1990). These two factors might also explain the similar performance of non-

migratable non-words (Experiment 2.1) and numbers (Experiment 5.1). Another factor that might have

influenced the different performance on numbers and words, especially given that the deficit seems

attentional in nature, is that in Hebrew words are read from right to left whereas numbers are read from

15 One reviewer has suggested that our patients might have suffered from word-form dyslexia (Warrington &

Shallice, 1980, termed by others �letter by letter reading� or �pure alexia�). However, the reading pattern did not conform with word-form deficit. Word-form dyslexics frequently use letter by letter reading, have difficulties reading long whole words of any type, their performance deteriorates considerably in short presentation, and their reading performance is greatly affected by length. However, in our patients, no letter-by-letter reading was presented, and their reading of non-migratable words was good (see Tables 5 and 6), even with short presentation time of 0.1 second (for PY). No length effect was found for non-migratable words: long words were not more difficult to read than short words if they did not contain a potential of within-word letter migration (Table 3). Furthermore, migratability was a far more important factor for reading than length. Long non-migratable words of 5 and 6 letters were read better than short 4 letter migratable words (93% correct in 5-6 letter non-migratable words vs. 66% in 4-letter migratable words). In addition, some individuals with word-form deficit (pure alexia) show deficits in single letter identification in naming (Arguin & Bub, 1994; Lott & Friedman, 1999; Rapp & Caramazza, 1991). The patients in the current study had good single letter reading even in short presentation, with no deficit in letter form


left to right. (See Seron & Noël, 1992 for a review of contradictory results regarding the parallelism

between letters and digits.) 16

In what follows we discuss the implications of the reading pattern of the described dyslexic patients for

three issues: First, we consider the contribution of lexical knowledge to reading in the absence of letter

position information. Then, we discuss the theoretical implications of LPD: we show that most

connectionist models for reading are not consistent with our data, and point to the constraints that the

data pose on these models. We then suggest an attentional account for the findings, that will account both

for within word migrations and for the relative resistance to migration of first and last letters.

LPD and lexical knowledge

In LPD, very much like in other known dyslexias, there was �an attempt to make lexical sense� (Marcel,

1980). This was demonstrated in several ways in the reading performance of our patients. First, exactly

as was found for migration between words (McClelland & Mozer, 1986), and for transpositions within

words in normal reading (Estes, 1975), the lexical status of the target string compared to the lexical

status of the potential migration responses played a crucial role in string reading: when the string was an

existing word without a lexical anagram (non-migratable word), it was read relatively well. When the

string was a nonword with a lexical anagram, performance deteriorated to around 50% correct: When the

patients were asked to read a nonword with lexical anagram (like puprle) there was a strong tendency,

both in reading aloud and in lexical decision tasks, toward lexicalization (purple).

This reliance on lexical knowledge is manifested in the error pattern in an additional way: the few

substitution and addition errors that occurred beside letter migrations mostly created existing words. This

ended up in lexicalizations in the case of nonwords, and in word substitutions in the case of words.

The relative frequency of the target word and its anagrams affected reading accuracy as well: Patients

preferred the more frequent word, and as a result read frequent words better, and tended to read a more

frequent transposed word instead of a low-frequency target.

Thus, although the letter position deficit probably affects all words to the same degree, letter position

deficit, together with the reliance on lexical knowledge, give rise to a hierarchy of liability to migration

errors: The least liable to migration are non-migratable words, namely words whose letters do not

identification or naming, and no errors based on form similarity between letters. Thus, the patients presented in the current study were not word-form dyslexics.

16 Similar findings regarding material-specific attentional deficit were also reported for neglect. Costello and Warrington (1987) and Patterson and Wilson (1990) described patients who had neglect (or positional) dyslexia for orthographic material in the presence of general neglect to the opposite side or with no general attentional deficit at all.


combine to form any other existing word; migratable words are more prone to migrations especially those

with high frequency anagrams, and migratable nonwords (nonwords whose letters compose an existing

word) are the most susceptible to errors.

One additional finding that should be accounted for in this regard is that although in nonwords patients

did not have lexical knowledge on their side, one of the patients read (non-migratable) nonwords at the

same good level he read existing (non-migratable) words. This fact can be explained by reference to

Prinzmetal�s (1981) and Prinzmetal and Millis-Wright (1984) findings regarding illusory conjunctions

and perceptual groups. Prinzmetal and Millis-Wright (1984) found that subjects made significantly more

illusory conjunction errors (incorrectly integrating a letter form and color) within words than within

nonwords. They accounted for this by Prinzmetal�s (1981) principle which stated that illusory

conjunctions are more likely to occur within a perceptual group. Because words are parsed into

multiletter perceptual units, and are processed by perceptual units larger than individual letters, and

nonwords are processed by individual letter units, illusory conjunctions occur more often within words

than within nonwords. Our patient�s good performance in nonword reading can be explained in a similar

vein: while his attentional deficit made him fail when he had to correctly integrate several positions to

several letters within words, such a failure was prevented in nonwords, since in nonwords attention is

allocated separately for every letter, and each time only a single position has to be combined with a

single letter.

LPD and connectionist models of reading

McClelland and Rumelhart (1981; Rumelhart & McClelland, 1982) proposed an interactive-activation

model for word reading. According to this model, at the letter-nodes, letters are already encoded

separately in each position within the word. Namely, for each position within a word, there is a full

separate set of letter detectors. This model was utilized to account for several dyslexias. Katz and Sevush

(1989), for example, suggested that positional dyslexia is caused by selective damage to the activation of

specific letter position nodes, in their case the first letter node. Is it possible that the LPD as described

here is a deficit to middle-letter nodes? We believe not, for two reasons: First, the absolute position did

not play a role here, only the relative position: the third letter could be completely spared when it was the

last letter of a three letter word, but impaired when it was medial in a longer word. Second, unlike in

positional dyslexia, there were no letter identification errors, only wrong positioning of medial letters.

Actually, there is no way a model with position-specific letter detectors can account for correct

perception of letter in a wrong position. The deficit that causes letter migration within words cannot be

ascribed to any of the stages of this type of model, because the first stage of letter identification is already

position specific.


Similarly, McClelland and Rumelhart�s models do not appear to offer any way to accommodate the

finding that a word activates a similar word which is a result of medial letter migration more than a

similar word which is a result of medial letter substitution. For example, these models cannot account for

the fact that the target broad yields more board responses than blind responses, since according to these

models a letter in a certain position does not contribute to the activation of a word that contains the same

letter in different position. Such a model can only handle within-word migrations by introducing cross-

talk between neighboring letter positions (Peressotti & Grainger, 1995).

Some later connectionist models represent words and letter order using letter clusters, or trigrams

(BLIRNET, Mozer, 1987; Seidenberg & McClelland, 1989). The relative position within a word is

encoded within each trigram and in the trigram combinations. Transposition errors in these models are

explained in terms of �spurious activations which involve clusters whose letters are present in the display

but in a slightly rearranged order�. However, for a trigram representation model to account for single

transposition of adjacent letters, many spurious clusters are required: for example, as CALM and CLAM

share only 2 out of 6 trigrams, for �CLAM� to become �CALM�, four spurious trigrams have to be

activated: two different-order, and two different letter trigrams. This makes trigrams a less natural

representation compared to single letters to explain within-word migrations.17 Furthermore, one of the

interesting features of these models (Mozer 1987, 1991) is that they produce transposition errors within

non-word sequences, but not within existing words. Recall, that the LPD readers exhibited a quite

different pattern, with impaired position within words, and less impairment in non-words. Thus,

additional assumptions must be added to these models in order to account for the pattern manifested by

the current findings of transpositions within words.

A later proposal of Mozer is perhaps more suitable to account for transposition errors: according to the

spatial uncertainty hypothesis (Mozer, 1989), parallel processing encodes only letter identities, while

focal attention is required to register the position of these letters and bind letters to their location

attribute. When focal attention is prevented in normal readers (for example by short exposure duration),

letter position errors occur. It therefore might very well be that LPD reading disorder is attention

related.18

17 One exception is PABLO, a programmable blackboard model (McClelland, 1986), which was designed to

activate anagrams of the target word especially in medial positions, by encoding medial letters in pairs rather than in trigrams: letters are represented in PABLO as following and preceding another letter (xA, Ax).

18 Our findings regarding different liability to migrations of different letter positions (see Section 6.1) pose another type of problem to McClelland and Rumelhart�s models, and actually to every model that assumes equal activation to all letter positions in visual word recognition. These findings join the large body of results that suggest


An attentional account for LPD

An account for the deficit in LPD should be one that explains letter migration within words, taking into

account the difference between medial and exterior positions, lexicality effects and frequency effects. We

suggest that the impairment lies in the early stage of word processing � the visual analysis of

orthographic input, and more specifically in the letter location function of the visual analyzer, which is

responsible for encoding the relative position of letters within words. Although different views regarding

the role of attention in word reading have been suggested, spatial attention appears to play a role in letter

localization (see McCann, Folk, & Johnston, 1992 for a review). According to Treisman and Souther

(1986), the location of letters sometimes fails to be registered when attention is overloaded in normal

readers. This might also be the explanation for the deficit of letter location in our dyslexic patients. These

patients may suffer an attentional deficit (possibly specific to orthographic material) which prevents them

from locating letters within words, or from integrating letters with their relative position-within-word

features. As we have suggested in Section 6.1, since first and last letters are accessed separately from the

rest of the letters, they are correctly located. On the other hand, middle letters are processed together and

therefore they are not integrated correctly with their relative within-word position. In the absence of

attention to integrate middle letters with their correct positions, two theoretical possibilities exist for the

next stage of reading: illusory position perception or accessing lexical information with partial

information. The first possibility is that illusory conjunctions between letters and their positions occur, as

a result of which letters are actually perceived in a wrong position. This possibility is somewhat

problematic given the lexicality and frequency effects on these conjunctions. (Treisman & Souther, 1986,

who also found lexicality effects on migration between words, suggested that lexical status and top down

processes can have an effect on perception and sensory evidence). The second possibility is that the

patients are left with the partial information of free-floating middle letters. With this partial information,

they consult the lexicon, and come up with a lexical entry that is adequate for the information � namely a

word that starts and ends with the correct letters, and that includes the same middle letters.19 This can

easily account for both lexicality and frequency effects: The first appropriate item to be retrieved from

the lexicon will probably be a frequent, existing word.

that different letter positions have different weight in word processing (Humphreys, Evett, & Quinlan, 1990; Perea, 1998).

19 See Caramazza and Hillis, 1990; Patterson and Wilson, 1990; and Riddoch, 1990 for related discussions of compensation strategies and the contribution of top-down processing to successful identification of words in neglect dyslexia.


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Appendix A – control subjects’ performance

Experiment Matched subjects All control subjects

1.1 Reading migratable words F 97% 99% 1.2 Same-different decision 95% 99% 2.1 Nonword reading 100% 100% 2.2 Lexical decision 100% 100% 3.1.1 Visual letter location 100% 100% 3.1.2 Auditory letter location 100% 99% 3.2 Position within a sequence 100% 99% 4. Word pair reading 96% 98% Errors in Experiments 1.1, 1.2 were migration of medial letters in migratable words.

Errors in Experiments 3.1.2 and 3.2 were migrations of medial letters.

Errors in Experiment 4 were position preserving.

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