Acta Psychologica 69 (1988) l-17

North-Holland

VISUAL LETTER-MATCHING AND THE TIME COURSE OF VISUAL AND ACOUSTIC CODES

Marisa CARRASCO * and Ronald A. KINCHLA

Prrnceton Unwersity, Princeton, USA

Jesus G. FIGUEROA

Metropolitan Unruersrt_v of Mex~o. Mextco

Accepted January 1988

Two experiments on visual letter-matching have been widely cited in the literature as Indicating a

role of acoustic codes in such tasks. Posner and Mitchell (1967) concluded that a visual code is

sufficient to compare two identical letters (e.g.. AA), whereas a slower forming acoustic code is

required to compare nonidentical letters, whether with the same name (e.g.. Aa) or with different

names (e.g., AB or aB). Thorson. Hochhaus. and Stanners (1976) asked subjects to compare

sequentially presented letter pairs and found that visual confusability increased latencies at short

delays (ISIS). whereas acoustic confusability increased latencies at longer delays. Our original

objective was to use these paradigms to reveal differences in processing by English and Spanish

speakers (Spanish has a much more direct grapheme-phoneme correspondence). Extensive testing

of both native English and Spanish speakers reveals a highly consistent pattern of results that

clearly differs from those reported in the aforementioned papers. The new data fail to indicate any

role of acoustic codes in such tasks.

A commonly held view of visual letter perception is that first an internal visual representation (code) is formed, followed by a more slowly formed acoustic or phonetic representation. This dual-code model is based primarily on two widely cited letter-matching studies by Posner and Mitchell (1967) and Thorson et al. (1976). The sequence of visual, then acoustic, representation was initially presented as strictly serial (Posner and Mitchell 1967), although Posner (1978) now acknowledges that the two codes may be evoked in parallel, but with

* We thank Sam Clucksberg, Michael I. Posner and Allan Singh for helpful comments on this

article. and Miguel Zavaleta, Olga Mora, and Ana Ma. Riquelme for their assistance in writing the

program of experiment 2 and in testing Spanish speakers.

Requests for reprints should be sent to M. Carrasco, Dept. of Psychology. Wesleyan Univer-

sity. Middletown, CT 06457, USA.

OOOL6918/88/$3.50 0 1988. Elsevier Science Publishers B.V. (North-Holland)

one forming faster. Whether in serial or in parallel, the visual code is

generally assumed to form before the acoustic one in these letter-

matching tasks.

Posner and Mitchell (1967) employed two forms of a visual letter-

matching task. Subjects were asked to view a pair of simultaneously

presented letters and then quickly press one of two keys depending

upon the relationship between the letters. In one form of the task.

‘ph_vsicuI matching ‘. they were to press one key if the letters were

physically identical (e.g., AA, bb) and another key if they were physi-

cally different (e.g., Aa, AB, or aB). In a second form of the task, ‘~unze

matching’. they were to press one key if the names of the two letters

were the same (e.g., AA, Aa), and the other key if they were different

(e.g., AB, aB).

Posner and Mitchell primarily pointed to four features of their

results as supporting a dual-code model. It will be useful to first list

these empirical features (designated 1, 2, 3, and 4) and then indicate

how they can be interpreted in terms of the dual-code model (desig-

nated l’, 2’, 3’, and 4’, respectively):

(1) On name matching tasks ‘same’ responses to physically identical

letters (e.g., AA) were faster than to letters simply having the same

name (e.g., Aa).

(1’) Physically identical letters such as AA must have the same name,

thus a ‘same-name’ response to them can be based solely upon the

initial visual codes rather than the slower forming acoustic codes

required for such pairs as Aa.

(2) ‘Different’ responses were faster on physical matching tasks than

on name matching tasks.

(2’) On name matching tasks a physical disparity is insufficient to

justify a ‘different’ response, since the letters may still have the

same name. Thus on name matching tasks, unlike physical match-

ing tasks, the subject must wait for the slower acoustic code to

form before making a ‘different’ response.

(3) On physical matching tasks the responses seemed independent of

the letters’ names. (For example, it took no longer to respond

‘different’ to Au than to AC.)

(3’) A response based solely on a visual code may occur before the

slower forming acoustic codes can produce any interference: e.g..

Acr may evoke a ‘different’ response before the name match can

produce response conflict.

hf. Carrasco et al. / Vmd und acousttc codes 3

(4) On name matching tasks a ‘same’ response occurred more rapidly to Cc than to any other physically different, but same name, pair (e.g., Aa or eE).

(4’) The physical similarity of Cc seemed to facilitate a ‘same-name’ match, as if the similarity of the initial visual codes facilitated the subsequent name match. ’

(Note that points 3 and 4 taken together suggest that visual codes can influence name codes but name codes may occur too slowly to affect responses based on visual codes; i.e. the acoustic code is formed later than the visual code.)

These findings were taken by Posner and Mitchell (1967) as evidence for two stages (nodes) of processing: physical matching tasks required only comparison of the more rapidly forming visual code (stage or node l), whereas, except for ‘same’ responses to identical letters, the name matching task required comparison of the slower forming name codes (stage 2 or node 2).

Subsequent research on the dual-code model has examined the effects of visual and auditory confusability on reaction time in sequen- tial letter matching tasks (e.g., Dainoff and Haber 1970: Thorson et al. 1976). According to Posner (1978), the results of these studies are best exemplified by the Thorson et al. experiment, in which subjects were asked to compare two sequentially presented letters, pressing one key if they were identical and another if they were different. The two inde- pendent variables were the delay between presentation of the two letters (0 to 2 seconds), and the type ofconfusuhility between particular letters (primarily acoustic such as ED or XS, or primarily visual such as EF or XK). The dual-code model predicts that visual confusability should be evident at the short delays, but that at longer delays acoustic confusability should occur. Thorson et al. reported that these two variables had an interactive effect on the latency of the ‘different’ responses.

As shown in fig. 1, ‘different’ responses to visually confusable pairs were slowest with a 0-msec delay and progressively faster with longer

’ Posner (1978) has proposed a somewhat different interpretation. suggesting that a modified form of physical identity existed between C and c: they differed only in size. Thus pairs of this

sort might be matched based on a type of physical identity earlier than they could using the slower forming acoustic (name) codes.

4 M. Carrasco et 01. / Visual and ucou.rtic code.7

720 - ACOUSTIC CONFUSABILITY

VISUAL CONFUSABILITY 600 -

.

560 I I I I I

0.0 0.5 1 .o 1.5 2.0

INTERVAL (set)

Fig. 1. RT for visually confusable (VC) and acoustically confusable (AC) pairs at each of the four

delays utilized by Thorson et al. (1976).

delays. In contrast, the corresponding latencies to acoustically confusa- ble pairs were fastest with a 0-msec delay and progressively slower with longer delays. These results were seen as further support for the dual-code model, implying that a visual code mediated comparisons at delays up to about 1 set, whereas an acoustic code mediated compari- sons at longer delays.

Our original objective was to investigate possible differences between native Spanish and native English speakers on letter comparison tasks. We were looking for differences since Spanish, unlike English, has a virtually invariant grapheme-phoneme correspondence; i.e., if you know how a word is spelled in Spanish you know how to pronounce it, and vice versa. Previous studies have shown that Spanish speakers show a different pattern of performance in studies involving acoustic coding of information. For example, following Conrad’s (1964) procedure, an acoustic confusion matrix of native Spanish speakers was obtained (Riquelme 1985). The number of acoustic confusions of Spanish speakers was significantly lower than that of English speakers. Evi- dence from certain short-term memory studies also indicates that acoustical interference does not impair the performance of Spanish speakers as much as it impairs the performance of English speakers (Bravo 1985; Duprat 1982). In the present study we employed the letter matching tasks used by Posner and Mitchell (1967) and Thorson et al.

(1976) since these two papers have been widely accepted as fundamen- tal studies of simultaneous and successive letter-matching tasks, respec- tively, and we were interested in studying the temporal aspects of both the visual and acoustic codes of Spanish speakers. In fact, current theoretical arguments concerning various models of letter coding take as fundamental the data reported in these papers (e.g., Hardzinski and Pachella 1980; Krueger and Shapiro 1982; Posner 1978; Proctor 1981; Proctor and Rao 1983a, b). These data seemed to clearly reveal a role for both visual and acoustic codes in visual letter-matching by English speakers. We felt a similar experimental approach might reveal a different role for such codes in Spanish speakers: specifically, we expected to find some differences in the temporal formation of these two codes. Finally, subsequent to conducting the experiments pre- sented here we learned of a reported failure to replicate Thorson et al.‘s results (Boles and Eveland 1983). This paper will be considered in some length in our discussion section.

As will be seen later, the comparison between Spanish and English speakers turned out to be less interesting than that between our results and those reported previously. The studies presented here represent a far more substantial evaluation of performance on the aforementioned letter-matching tasks. For instance, 42 subjects participated in our first experiment, compared to 4 in the Posner and Mitchell (1967) study, and 60 subjects participated in our second experiment compared to 16 in the Thorson et al. (1976) study.

Experiment 1

Experiment 1 essentially replicates Posner and Mitchell’s (1967) experiment II. ’

Each subject performed the two variants of the simultaneous letter-matching task

described earlier. The experiment was conducted in two phases, each involving the

same stimuli, apparatus, and procedure. Phase 1 was conducted in Mexico City with 21

native Spanish speakers and phase 2 was conducted at Princeton University with 21

native English speakers. All subjects could be best described as monolingual.

* The differences between our procedure and that of Posner and Mitchell (1967) wrre that our stimuli were presented on a video display (where the letters had an onset of less than 1.25 msec)

rather than on a conventional tachistoscope, and our letter display subtended about 3O of visual

angle rather than 2.5 “. Neither of these differences would seem lo be significant given the way in

which the Power and Mitchell results have usually been interpreted.

6

PHASE 1

Sutljec~t.~. Twenty one students from the Metropolitan University of Mexico par-

ticipated voluntarily. All subjects were native Spanish speakers and had normal or

corrected-to-normal vision. They had not participated in related experiments and were

naive as to the purpose of the experiment.

Stimuli. The same four letters utilized by Posner and Mitchell (1967) were used here:

A. B. C, and E in both upper-case and lower-case. All possible pairs of these eight

stimuli resulted in 64 letter pairs. Posner and Mitchell (1967) classified them as:

‘physical!): idmtlcul’ (e.g., AA), ‘nomrnal!v identicul’ (e.g., Bb). and ‘di//errnt’ (e.g.. CE

or Ce) types. In our analysis of this experiment, ‘different’ will be further subdivided

into same-cosr (e.g., AB) and mixed-case (e.g., Ab) pairs.

Each subject sat in a dark room 50 cm in front of an Apple II-plus monitor. Letters

appeared side by side. centered on the conventional Apple green screen which sub-

tended a 15.15“ x 18.30” visual angle. The upper-case letters were separated by 0.57 o

and subtended 1.3O horizontally x I .4 o vertically. and the lower-case letters subtended

0.80” x 0.910, respectively. Both letters of a pair were presented simultaneously. They

were defined by 15 successively generated raster lines. such that the letters were fully

formed in less than 1.25 msec.

Ap~urutus clnd procrdur~. Control of the experiment and data collection were per-

formed by an Apple II-plus microcomputer, using the Levy et al. (1979) ‘Pattern

Interpretation’ program. The program includes 64 trials under each of two matching

tasks. On each trial one of the 64 possible letter pairs was presented. There were two

distinct tasks: for the ‘ph_psicu/-matching /usk’ subjects were to respond ‘same’ if. and

only if. both letters were physically identical (e.g., AA, bb) and ‘different’ otherwise

(e.g., Aa, AB. aB). For the ‘nomr-mutc,hing rusk subjects were to respond ‘same’ if the

names of the two letters were the same (e.g., AA. Aa) and ‘different’ otherwise (e.g..

AB. aB). Subjects were always to respond as ‘quickly and as accurately as possible’.

Each subject performed both types of tasks using the same set of letter pairs. ’

Eleven subjects responded ‘same’ by pressing the ‘S’ key with their left index finger

and ‘different’ by pressing the ‘L’ key with their right index finger: this response

mapping was reversed for the other ten subjects. The order in which the two types of

matching tasks were performed was also counterbalanced across subjects.

’ The proportion of ‘same’ responses in the name-matching task was larger than in the physcal-

matching task. since nominally identical pairs require a ‘same’ response in the former and a

‘different’ response in the latter. Posner and Mlrchell’s (1967) experiment II :dso had unbalanced

‘same’-‘different’ frequencies. They reported that a control study in which the ‘samc’~‘different’

frequencies were balanced confirmed their major finding>. Thus. the same pattern of results was

obtained regardless of response probabilities.

PHASE 2

The stimuli, apparatus, procedure, and design were exactly the same as those in phase 1. However, in this phase the subjects were native English speakers. Twenty-one Princeton University students with normal or corrected-to-normal vision were paid for their participation. None was aware of the purpose of the experiment.

RESULTS AND DISCUSSION

The general pattern of results will be reported first. and then specific comparisons will be made with the results of Posner and Mitchell (1967).

General results

RTs were subjected to a three-way ANOVA: Group (Spanish versus English speakers) x Matching Task (Physical versus Name) x Type of Pair (physically identi- cal, nominally identical, same-case ‘different’, or mixed-case ‘different’ pairs). The second and third factors were within-subjects.

Matching task and type of pair both had a significant main effect: F(3, 120) = 6.21, p < 0.01, and F(3, 120) = 39.0, p < 0.001, respectively. In order to compare mean RTs to the four types of letter pairs on each type of task, we have plotted the RT on the name task against the corresponding RT for the physical task for Spanish speakers (fig. 2, upper panel) and English speakers (fig. 2, lower panel). Note that any point which falls above the positive diagonal on each graph indicates subjects responded faster to that letter pair on the physical matching task. This was the case for all the letter pairs except for the ‘different’ mixed-case pair for English speakers (DM, fig. 2. lower panel) where they were slightly faster on name matching. The most striking feature of these results however is the very similar pattern of results for both Spanish and English speakers. The relative magnitude of RTs was the same for both groups of subjects on the name-matching (PI, DS. DM, and Nl from shortest to longest respectively), and on physical matching (DM, DS, PI, and NI). There was also a significant interaction between matching task and type of pair: F(3, 120) = 11.63, p i 0.001. Specifically, matching task had a significant effect on all pair types, except the mixed-case ‘different’ one: F(1, 40) = 37.55, p < 0.001 for physically identical pairs, F(1, 40) = 5.70, p c 0.05 for same-name pairs, F(1, 40) = 3.89. p < 0.05 for same-case ‘different’ pairs, and F(1, 40) = 0.64 for mixed-case ‘different’ pairs. None of the other interac- tions was significant: matching task X group: F(3, 120) = 0.51, type of pair X group: F(3, 120) = 0.78, and group x matching task x type of pair: F(3. 120) = 1.99, p B 0.1. Error rates were less than 5% for each of the eight subconditions in each language group, and a three-way ANOVA showed no significant effects.

An important prediction of the dual-code model is that in intermixed blocks, RTs for same-case and mixed-case ‘different’ pairs should be similar. In the present experiment we found a same-case over mixed-case advantage for ‘different’ responses on name matching tasks for both Spanish (793 versus X29 msec) and English speakers (570 versus 605 msec): F(1, 46) = 5.67, p < 0.05.

SPANISH SPEAKERS

720

720 770 820 870 920

Name Matching

ENGLISH SPEAKERS

720

.r 670 _

520 1 520 570 620 670 720

Name Matching

Fig. 2. RT for the four types of letter pairs (physically identical - PI; nominally identical NI; different same-case - DS: different mixed-case - DM) under physical- and name-matching taska

for Spanish (upper panel) and English (lower panel) speakers.

There were significant correlations between the KTs of both groups of subjects for

each matching task: r = 0.37. p < 0.05 for physical matches, and r = 0.52. p < 0.001

for name matches. However, within each language group the correlations between

matching tasks were not significant: r = -0.09. p > 0.2 and r = 0.18, p > 0.2 for

Spanish and English speakers, respectively. These results suggest that both groups of

subjects were consistently modifying their strategies according to the type of matching

task, and that there could be a common underlying process mediating the performance

of both language groups.

M. Carrasco et al. / V&d and momtIc codes 9

Comparison with the results of Posner and Mitchell (I 967) Specific comparisons can be made between the results obtained in this experiment

and the four data ‘features’ that Posner and Mitchell advanced in support of the

dual-code model; these were designated features 1, 2, 3, and 4 in our introduction.

Comparisons of each feature with the corresponding feature of our data will be

designated 1, 2. 3, and 4 respectively:

(1) On name matching tasks ‘same’ responses to physically identical pairs were faster

than to same-name pairs. The same result has been reported by Posner and

Mitchell.

(2) In contrast to Posner and Mitchell’s finding, ‘different’ responses were slower

(rather than faster) on physical matching tasks than on name-matching tasks.

(3) On physical matching tasks ‘different’ responses to physically different pairs were

slower when both letters had the same name (e.g., Aa) than when they did not (e.g.,

AC). Thus in contrast to Posner and Mitchell’s study, responses here were not

independent of the letter names.

(4) Again in contrast to Posner and Mitchell, ‘same’ responses on name-matching tasks

for the Cc pair were not significantly different from the responses to any other

physically different, but same-name, pair. (The mean RT for Cc and cC pairs was

853 msec for Spanish and 617 for English speakers, whereas the mean RT for Au,

aA, Bh, hB, Ee, and eE was 863 msec for Spanish and 635 for English speakers.)

In summary: only the first feature cited in Posner and Mitchell (1967) was

replicated here, leaving little support in our study for their theoretical model. The fact

that the first feature was replicated implies no more than that it is easier to respond

‘same’ to two physically identical stimuli, than to two non-identical stimuli having a

common feature, e.g.. ‘same name’. Our data provide no reason for making more

complex theoretical assumptions regarding visual and acoustic codes. In particular,

there is clearly no support for the idea of a ‘slower forming’ acoustic code. In fact. in

our study name identity slowed decision to physically different pairs (feature 3)

whereas physical similarity, e.g., Cc, did not facilitate decision to nominally identical pairs (feature 4). By Posner and Mitchell’s logic, this would imply that the visual code

was formed after the acoustic one. However, in the absence of any additional evidence

such speculation seems unwarranted.

Experiment 2

This experiment was designed to contrast performance of English and Spanish

speakers on delayed comparisons of letters, rather than the simultaneous comparisons

of experiment 1. We used the successive letter-matching task employed by Thorson et

al. (1976), since it seemed to reveal the temporal course of visual and acoustic codes. As

in experiment 1 there were two phases. Phase 1 was conducted with native Spanish

speakers and phase 2 with native English speakers. The apparatus and procedure were

held constant throughout the two phases. A total of 60 subjects participated in this experiment.

PHASE 1

Sut$Tt.s. Thirty students from the Metropolitan University of Mexico participated in

one 45-min session to fulfill a class requirement. Some of them had participated in

experiment 1 but were naive as to the purpose of the experiment. All subjects were

native Spanish speakers and had normal. or corrected-to-normal, vision.

Sll??dl. The same types of letter pairs utilized by Thorson et al. (1976) were used.

Specifically, these were: plz~r~.sical& identrcal pairs (e.g.. T-T); ~~i.suuI!~~ ~on~u.sahle pairs

(P-R, E-F. X-Y, M-W. T-I, Y-V, X-V. K-X) which share a high number of visual

features according to the feature list of Gibson (1967) and have low acoustic confusion

values; and rlisuall,:-plus-cr~ou.~tl~ul!~ ~onfu.suh/e pairs (M-N, K-A. E-B. T-P, B-D. T-E.

B-T, B-P). The only exception to the Thorson et al. stimuli stems from our use of

Spanish speakers. The pairs G-C, S-F, D-E, C-T, B-V, C-P, S-E. G-P were used as

uc.ou.stic.u/(~ con/usuh/e pairs, since these have high confusion values in an acoustic

confusion matrix of native Spanish speakers obtained by Riquelme (19X5) following

Conrad’s (1964) procedure, as well as a low number of shared visual features. Since the

visually-plus-acoustically confusable pairs used by Thorson et al. have intermediate

acoustic confusion values for Spanish (as well as for English speakers), the same letter

pairs were used. 4

Procwiure und dmign. Each subject sat in a dark room, 70 cm in front of an Epson

monitor. The stimuli were upper-case letters subtending approximately 0.6 o vertically

and 0.3O horizontally, and appeared on the conventional green Epson screen.

Following Thorson et al’s procedure, a letter pair was preceded by two horizontal

rows of block dots, subtending 2.3O horizontally and 1.3” vertically. designating the

fixation area in which the letters would appear. Each letter appeared in the same

position, centered in the fixation field. The first letter was presented for 500 msec.

Then after a delay (ISI) of 17, 250, 500, 1000. or 2000 msec, the second letter was

displayed. Subjects were instructed to press one of two keys indicating ‘as quickly and

as accurately as possible’ whether the two letters were the ‘same’ or ‘different’. Fifteen

subjects responded ‘same’ by pressing the computer’s ‘.I’ key with their right index

finger and ‘different’ by pressing its ‘F’ key with their left index finger. This response

mapping was reversed for the other 15 subjects. Both RTs. measured from the onset of

the second letter, and correctness of response were recorded.

Both of the independent variables in the experiment, type of confusion and ISI.

were manipulated within subjects. Each of the eight letter pairs in the three conditions

was presented twice at each of the five delays. resulting in 240 ‘different’ trials. In

addition, 120 ‘same’ trials were presented. producing a total of 360 teat trials. which

4 For all types of letter pairs a large number of common letters is repreaentrd in each set,

therefore different results can be attributed to the pairings of the letters and not to the particular

set of letters chosen for each condition (Boles and Eveland 1983).

M. Carrasco et al. / Visual and ucoust~c codes 11

were randomly ordered for each subject. The computer initiated each trial 5 set after the subject had responded. The experimental session was preceded by 30 practice trials. ’

PHASE 2

Method

Subjects. Thirty Princeton University students, native English speakers. with normal or corrected-to-normal vision, were paid for their participation in a 45-min session. Although some of them had participated in the first experiment. none was aware of the purpose of the experiments.

Stimuli. The same letter pairs for each type of pair utilized by Thorson et al. were used. The acoustically confusable pairs were: A-O, E-D, E-P, F-S, F-X, N-A, P-Q, X-S, which have high acoustic confusion values in Conrad’s (1964) matrix and low visual confusion values in Gibson’s (1967) feature list. In phase 1, the corresponding acousti- cally confusable pairs were consistent with the acoustic confusability matrix for Spanish speakers.

Procedure and design. The apparatus, procedure, and design utilized in this phase were exactly the same as those in phase 1.

RESULTS AND DISCUSSION

The general pattern of results will be reported first, then specific comparisons will be made with the results of Thorson et al. (1976).

Generul results

Table 1 presents the geometric mean of the 16 RTs in each of the 15 subconditions for each language group.

’ This experiment differed from the original Thorson et al. (1976) experiment in four minor ways:

(a) They presented the stimuli in a Scientific Prototype tachistoscope. Carrasco et al. (1985) have

already conducted Thorson et al’s (1976) experiment with 30 Spanish speakers presenting the

stimuli in a Scientific Prototype tachistoscope. The obtained results did not agree with those

reported by Thorson et al., but do agree with the results to be reported in this paper with both

Spanish and English speakers. (b) They used subject-initiated trials. (c) They only had four delays

(ISIS of 0, 500, 1000, and 200 msec.) In the present experiment the O-IS1 was substituted by an IS1

of 17 msec to make the offset of the first letter and the onset of the second letter discernible, and an IS1 of 250 msec was incorporated in order to have a more detailed mapping of the temporal

course of the codification process. (d) They presented the trials over two l-hour sessions. None of

these differences would seem to be significant given the way in which the Thorson et al. results have been interpreted.

Table I

Mean reaction time (in msec) for acoustically confusable (AC). visually confusable (VC), and

visually-plus-acoustically confusable (VC + AC) pars.

IS1

(in msec)

Spanish speakers

AC VC VC+AC

English speakers

.4C V(‘ VC+AC

17 x94 962 972

250 x90 916 905

500 900 1027 951

1000 923 1036 964

2000 971 1068 1058

x09 x7x

197 857

816 X91

799 891

830 934

x47

X18

x44

X46

x59

The results for the visual!~ confu.sahlr and the ucouxticul!~ confusuhle pairs are

shown graphically in fig. 3. Note that. as in experiment 1, the results for both language

groups were highly consistent with each other. Although the scale values were some-

what different, the basic pattern of the results is virtually identical. The RTs were

consistently longer for the visually confusable pairs. and greater delays led to longer

RTs for both types of letter pairs.

‘Different’ RTs were subjected to a three-way ANOVA: Group x Confusion Type

X ISI. The second and third factor were within-subjects. Confusion type had a

significant main effect: F(2, 116) = 58, p < 0.001. and was significant at all five ISIS at

the 0.001 level. The RTs were fastest for the acoustically confusable letter pairs and

slowest for the visually confusable letter pairs. IS1 also had a significant main effect:

F(4, 232) = 15.39, p < 0.001, and was significant for each confusion type at the 0.001

level. Error rates were less than 5% for each of the subconditions. and a three-way

ANOVA showed no significant effects.

An analysis of the data shown in fig. 3 yielded no statistically significant interaction

between type of confusion (acoustic or visual) and delay. Specifically, neither the

three-way interaction nor any of the two-way interactions was significant: Group x

Confusion Type X ISI: F(4, 232) = 0.97; Confusion Type X ISI: F(4. 232) = 1.98. p <

0.1: Group x ISI: F(4, 232) = 2.23. p < 0.1: and Group x Confusion Type: F( 1, 58) =

1.07. p > 0.2. If one considered the third type of confusable pairs (AC + VC), there

was a weak but statistically significant interaction between delay and confusion type:

F(8. 464) = 2.14. p < 0.05. This interaction may be due to the first data point of the

acoustically-plus-visually confusable pairs (see table 1).

Comparrson uxtth the results of Thorson et ui. (I 976) The results obtained in both phases of this experiment clearly differ from those of

Thorson et al. (1976). The delay between presentation of the two letters and the

confusion type of letter pairs did not have an interactive effect on the latency of the

‘different’ responses (see fig. 1). RTs neither increased at the 2000.msec IS1 in the

acoustically confusable condition, nor decreased in the visually confusable condition.

even though these were the critical points in the Thorson et al. study (see fig. 3). In our

hf. Carrasco et al. / Visual and acoustic codes 13

Spanish Speakers

900

850 ;I

0 250 500 750 1000 1250 1500 1750 2000

IS1 (in msec)

English Speakers 1000

950

2 900 E c

.Y?/

2 850- AC

7501

0 250 500 750 1000 1250 1500 1750 2000

ISI (in msec)

Fig. 3. RT for visually confusable (VC) and acoustically confusable (AC) pairs at each of the five

delays utilized with Spanish (upper panel) and English (lower panel) speakers. (Note the high

similarity between these two graphs and compare them with the graph in fig. 1.)

experiment visual confusability has a greater effect than either of the other confusion

types at all ISIS, providing no evidence that a phonetic code is used in successive

letter-matching, even at the longest delay of 2 sec.

General discussion

Note that in spite of language differences the results of Spanish and

English speakers were very similar in both experiments. Thus, our

original interest in finding performance differences between native

Spanish and English speakers led instead to virtually identical patterns

of results. In fact the highly similar performances of both language

groups indicates the reliability of our results, since the two phases in

each experiment can be taken as replications. What emerges as most

important is the degree to which our results differ from those of Posner

and Mitchell (1967) and Thorson et al. (1976).

The one feature of Posner and Mitchell’s data that was replicated in

our experiment 1 does nor suggest a ‘dual-code’ model, simply a visual

code. The surprising pattern of our data led us to search the literature

on letter-matching for any other reasons to question the generality of

the Posner and Mitchell and/or Thorson et al. results. We found that

Anderson (1975) reported, among other things, that on physical match-

ing tasks ‘different’ responses to physically different pairs were slower

when both letters had the same name (e.g., Aa) than when they did not

(e.g., AC). This is consistent with our results but not with the Posner

and Mitchell (1967) data.

We also noted that Crist (1981) pointed out that for specific pairs of

letters, name matches may be either faster or slower than physically

identical matches, even though they are slower on average. Thus. he

concluded, Posner’s (1978) inference of ‘isolable’ systems, is not sup-

ported by an analysis of responses to individual stimulus pairs.

Finally, in our experiment 1 ‘different’ responses were faster for

same-case than for mixed-case on name-matching tasks. Although

Posner and Mitchell (1967) did not perform such an analysis, their

model is clearly contradicted by these results. This specific aspect of

our data is consistent with findings by Hellige and Webster (1981) and

Boles and Hellige (1984). Thus Posner and Mitchell’s assumption that

an acoustic code mediates same-case and mixed-case comparisons

seems doubtful.

As we mentioned in the Introduction, Boles and Eveland (1983) also

failed to replicate Thorson et al.‘s (1976) results. 6 Both our results and

’ Power himself (personal communication, March 1987) acknowledges that the replicability of

Thorson et al.‘s study is questionable, and he now recognizes some problems with the original theoretical view he Lad of letter-matching.

M. Carrasco et al. / Visual and acoustic codes 15

those of Boles and Eveland indicate that visual confusion exerts a greater effect than either of the other types. Thus again, there is no evidence that an acoustic code is used in successive letter-matching even at the longest delay of 2 sec. Boles and Eveland not only failed to replicate the study of Thorson et al. (1976) but also that of Dainoff and Haber (1970) which had been considered as evidence for name codes in same-different matches. Rather than name codes, they found evidence for visual or physical codes in same-different matches, even when the stimuli to-be-matched were presented successively with rela- tively long delays (2 set).

In summary, the present experiments raise serious questions regard- ing the widely accepted dual-code model of visual letter-matching, particularly the role of more than visual codes in such tasks. Note that in this study we did not find evidence of an acoustic code in either simultaneous or successive letter-matching, for neither Spanish speakers nor English speakers. The very similar performance of both language groups suggests that acoustic coding is not involved in visual letter matching for either language, even though Spanish has a more direct grapheme-phoneme correspondence than English. Many of those who have proposed models for visual letter-matching have accepted the results of Posner and Mitchell (1967) and Thorson et al. (1976) as fundamental data to be explained by their theory (e.g., Hardzinski and Pachella 1980; Posner 1978; Proctor 1981; Proctor and Rao 1983a,b). Thus it would seem that these theorists should now question the replicability of the results of Posner and Mitchell and of Thorson et al. It should be emphasized that we are not questioning the role of acoustic codes in some other tasks. For example, Sperling (1960) found acoustic confusions in his classic study of iconic memory, and Conrad (1964) demonstrated a highly significant association between errors in immediate recall of letter sequences and listening errors. The data from these tasks clearly imply some form of acoustic coding. What we are questioning is the dual-code model of visual letter-matching, specifi- cally the role of an acoustic code in such tasks.

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