active selection of items to be remembered: the role of timing

COGNITIVE PSYCHOLOGY 6, 61-83 (1974)

Active Selection of items to be Remembered:

The Role of Timing1t2

PETER HAMILTON

University of Stirling, Scotland

AND

ROBERT HOCKEY

University of Durham, England

A series of experiments was carried out in which Ss were required to extract critical stimuli from a stream of nine spoken inputs, presented at various rates, and report on these after the presentation of each list. The critical items were normally digits at positions 2, 4, 6, and 8 in the input sequence. Subjects were required to employ either an “active” extraction strategy, aimed at achieving temporary storage only of items to be remembered, or a “passive” strategy, involving storage of all inputs with subsequent extraction of critical items. The initial experiment showed that the active strategy markedly improved performance efficiency as the presentation rate decreased; passive performance remained relatively stable. Experiments 2 and 3 indicated that the level of active performance was higher when critical items were categorically different from unwanted items. There were indications that this effect was independent of the effect of changes in the presentation rate.

The final experiments in the series showed that when Ss were denied the opportunity of predicting the time of arrival of critical items active performance hardly benefitted from a reduction in rate.

A “controlled activation” process is proposed as a basis for S’s ability to modulate his state of alertness, so as to maximize receptivity for critical stimuli arriving at well-defined points in time.

Research on the problem of how particular stimuli are selected by the organism for processing has emphasized man’s ability to economize on stimulus loading by selecting particular types of input at the expense

’ Requests for reprints should be sent to Peter Hamilton, Department of Psy- chology, University of Stirling, Stirling, Scotland.

‘The initial experiments in this series were conducted while both authors were at the M.R.C. Applied Psychology Unit, Cambridge, England. The authors wish to thank Ulric Neisser for his helpful and constructive comments on an earlier version of this paper.

61 Copyright @ 1974 by Academic Press, Inc. All rights of reproduction in any form reserved.

62 HAMILTON AND HOCKEY

of others. The experimental paradigms used have normally involved presenting S with simultaneous competing inputs and requiring him to monitor one or all of them. Two principal forms of selective operation are clear from this work. The first is s’s attempt to isolate a class of information designated as critical. A technique common to this area 1s shadowing (Cherry, 1953; Treisman, 1960), where input on one ear is monitored and that on the other ignored. The second is S’s ability to “switch” attention in an articulate manner between competing inputs. A summary of the original work in this area is given by Broadbent (1958) and the same author’s recent monograph (Broadbent, 1971) presents an up-to-date synthesis of the field as a whole.

The purpose of this paper is to evaluate man’s ability to extract critical information from a stream of stimulus events occurring se- quentially in time on the same sensory channel. There is no particular precedent for this line of enquiry, though there are general indications that such a selective operation is possible. Cherry ( 1953), in some of his experiments on speech recognition, found that Ss were, under certain conditions, able to separate mixed messages presented over the same sensory channels. Similarly, Broadbent (1952) showed that Ss were able to extract one of two synchronous but interleaved messages, provided that prior instructions were provided about which of the messages were required. In addition, there is the intuitive idea that economies in information processing may be achieved not only by filtering out information on some channels but by allotting full processing capacity to only some of the stimuli arriving on the attended channel.

One clear example of such a process is the preparation to receive the stimulus, assumed to occur during the foreperiod of a reaction time trial (Posner & Boies, 1971), and most clearly indicated by the pronounced negative shift in EEG baseline known as contingent negative variation, or CNV (Walter, 1964). Furthermore, there is evidence (Tecce, 1972) that Ss are able to exercise considerable control over the timing and amplitude of the CNV to meet different attentional requirements. Our research is concerned with the possibility that Ss are able to make use of such timed preparations over an extended sequence, as in listening to speech, so that only some inputs on the channel “receiving attention” are effectively processed. The possible role attributed to timing and rhythm in this view has a clear affinity with Neisser’s (1967) argument that efficient verbal memory span performance depends on the use of underlying rhythmic structures (an idea originally suggested by Lashley, 1951). More recently, a comprehensive elaboration of Lashley’s views on rhythmic action has been put forward by Martin ( 1972), in a paper which clearly demonstrates the central importance of rhythm (temporal

TIMING OF ITEM SELECTION 63

patterning) in the production of serial behavior. The present research is an attempt to examine one aspect of timing in attention and memory, even though it is not linked to the above viewpoints in any systematic way. Basically, we are interested in the role of cyclical timing operations in the extraction of critical items from a verbal sequence.

Our general approach is to compare the effectiveness of two attentional strategies. The first, which we call an “active” strategy, requires S to prepare specifically for the reception of critical items, normally occurring at regular intervals in a nine-item auditory sequence. The second, “passive” strategy, requires him to extract the same critical items from memory having allotted equal attention to all nine inputs. The primary interest is in the possibility of advantage accruing from use of the active strategy: Can Ss modulate their state of preparation over an extended sequence? Experiment 1 will show that performance may, indeed, im- prove when an active strategy is used, as indicated by the eiIiciency of recall of wanted items, but also that this improvement is critically dependent on the rate at which items are presented. In subsequent experiments an attempt is made to examine the processes involved in the operation of an active strategy, and to understand why this mode of operation is so markedly rate-dependent. In the final analysis the data presented here offer support to an explanation in terms of a fluctuating level of readiness to accept information, the rate limitation being set by the time required to attain maximal readiness at the various critical points in the sequence.

EXPERIMENT 1

This initial experiment was carried out by simply instructing Ss in the rationale of active and passive strategies and requesting them to use one or the other as indicated. Subjects heard a list of nine digits delivered at different rates and were asked to report the second, fourth, sixth, and eighth digit in the sequence after delivery.

Method

Instructions

The task was explained to a group of Ss who were then given a “lecture” on the active and passive techniques. Instruction was not rigid, being aimed at producing complete understanding in each S, but key concepts were made explicit.

If you’re told to operate actively, try to “grab” the numbers you need as they arrive. Try to concentrate as much as you can when a number you need comes along. If you do this well you’ll see that you finish up with


only 4 numbers in your memory, and you report those on your sheet. If you can’t make a reasonable guess at a location, leave a blank.

If you’re told to operate passively, try to forget about the job in hand as the numbers come in. Listen to all of the numbers equally hard, and don’t try to practice them or organise them in groups. When the numbers end, go into your memory and pick out the numbers at positions 2, 4, 6, and 8. Report them on your sheet. If you can’t make a reasonable guess at a location, leave a blank.

Instruction continued with sample sequences until all subjects reported understanding. Samples of all the delivery rates used in the experiment were given.

Design and Procedure

The Ss were tested as a group. Twenty-four sequences of stimuli consisting of random arrangements of the digits l-9 drawn without replacement were broadcast by a tape-recorder. There was an interval of 1 min between each sequence for recall and reinstruction if required. The sequences were organized in 12 blocks of two, six requiring active strategy and six passive. Intervals between onset of items were 0.375, 0.5, or 0.75 set, each rate being allocated to two active and two passive blocks. The order of presentation of the 12 blocks was balanced over time with respect to the rate and strategy variables. During the recall interval between sequences, Ss were always reminded of the strategy required for the next trial. Delivery of the digits was immediately preceded by 6 set of “lead-in” by a metronome ticking at the appropriate rate to allow S to establish the pace. Answers were written down on the response sheet in groups of four with blanks where required. Subjects could write the responses in any order they wished, so long as they placed them in their correct position in sequence on the sheet.

Subjects

The Ss in this experiment were 17 Cambridge housewives, who were paid for their services. Their ages ranged between 22 and 55, with a median age of 33.

Results

The data reported in this and all other experiments in this series result from scoring on a strict position criterion. That is, an item must be located correctly in the reported sequence to score. All of the experiments were also scored on a lax “item only” criterion, with no sub- stantial effect save a general increase in score.

The mean per cent error in report under both strategies at each rate is shown in Fig. 1, with log set/item as the abscissa. There is marked


- ACTIVE STRATEGY

- - - - PASSIVE STRATEGY

SECONDS PER ITEM (LOG. SCALE)

FIG. 1. Error rates under active and passive processing strategies as a function of stimulus input rate expressed as seconds per item.

effect of rate on the efficiency of the active strategy, but little effect for the passive. Effects of rate in the more usual “whole report” immediate recall task are ambiguous and tend to be quite small (Broadbent, 1971, p. 356 ff.). It is, therefore, perhaps not surprising that the passive condition, which, like these, demands full storage of the list, does not show a clear rate effect. The effectiveness of the retrieval process is about the same at all rates because it occurs after storage.

At the slow rate, an active strategy is considerably better than a passive one, but active performance is adversely affected by the rate increase, so that at 0.375 set/item, it differs little in achieved efficiency from passive. Sixteen of the 17 subjects show decreased efficiency at the fastest rate relative to the slowest when an active strategy is used (sign test, p < .OOl). The ability of Ss to benefit from active extraction of stimuli seems, thus, to be limited by event rate. It should be noted that the passive data are presented primarily as an indication of S’s performance under a reasonable alternative strategy. The more interesting rate limitation on active performance is indicated by the slope of the active function relative to the horizontal, rather than the angle at which it intercepts the passive function. The lack of a rate effect in passive


operations does, however, suggest that improvement in performance under the active strategy is not achieved at the point of retrieval and, consequently, that critical inputs are selectively treated at the point of intake.

Discussion

The “extraction” process appears to result in a relatively high status in storage for critical stimuli. At the point of intake this implies some form of selection procedure which allows only these inputs to be ef& ciently processed. On one level it is possible to regard this “selection- in-time” as a special case of the selective listening situation. Unwanted stimuli may then be seen as analogous to inputs to the unattended ear. Given this assumption the variables likely to determine performance are indicated by previous work. These follow.

(a) The integrity of the sets of wanted and unwanted items. From the extreme case illustrated in Experiment 1, where all inputs are drawn from the same population the situation may be varied so that wanted and unwanted item sets are drawn from different parent populations (cate- gories), or so that members of each set are linked by sequential redundancy. In dichotic listening, performance improves as the distinctiveness of the two channels is increased in these respects. For selection- in-time we should anticipate that variations in the nature of unwanted items will affect performance in a similar way. Such a mechanism is, of course, the basis of Treisman’s (1960) “dictionary unit” concept, where the system sets up a bias towards the acceptance of certain classes of verbal material. A more recent statement of the same kind of process in Broadbent’s (1971) “pigeon-holing” concept.

(b) The ‘acquisition strength” of wanted item. This terminology is adapted from the work of D. A. Norman and W. A. Wickelgren (see Wickelgren ( 1970) for a review). The “acquisition strength” of an item is an index of its impact on the system at input and, consequently, of the probability of its being recalled. In the theory of Broadbent (1958) an attentional “filter” could be biased to one of a set of competing input channels to ensure maximum acquisition strength for items arriving on that channel. This process was seen (Broadbent & Gregory, 1964) as independent from that affected by the integrity of wanted and unwanted channels. Furthermore, the filter was limited in the rate with which it could move from maximum bias on one channel (presumably through some zero or neutral point) to maximum bias on another.

The filter concept, however, implies a “negative” basis for selective attention. Inputs of a given type are well processed because competing stimuli are gated and, thus, not analyzed by the system (in Treisman’s


( 1960) modification unwanted inputs are “attenuated,” but, nevertheless, partially blocked in some way). In the present context we should not wish to preclude what might be called a “positive” interpretation, that critical stimuli are selected by means of their receiving some additional treatment. Neisser (1967) suggested that a particular stream of information gains access to attention by means of a constructive “analysis by synthesis” carried out on the input. In considering the results of the present experiment it is possible that the natural rhythm of the extraction operation may serve as a carrier for this construction process, or that Ss are able to provide a timed “amplification” to ensure enhanced reception of wanted items. Neither of these possibilities involves unwanted items being gated or blocked in any way. Operationally, these positive and negative interpretations are nearly indistinguishable, since a partially-effective filter will have all of the characteristics of a critical stimulus amplifier. For the present, we would conclude from Experiment 1 that selection-in-time involves a modulation of the acquisition strengths of successive inputs such that critical items are relatively better stored.

On the adopted working hypothesis both the effectiveness of selection within the channel and the distinctiveness of the sets of dictionary units for wanted and unwanted items should set limits to performance. But the nature of their interaction is not known, and the next two experiments were made to explore this area. Both of these provide for the joint manipulation of input rate and the nature of unwanted stimuli.

EXPERIMENT 2

In this experiment the nature of the unwanted items is the main variable of interest. In particular, the effects of a change in the category of unwanted items and of variability within a category are examined. All Ss were instructed generally as in Experiment 1, but requested to employ an active extraction strategy at all times.

Method

Materials

Nine-item input sequences of four different types were recorded on tape. All had randomly-selected digits at positions 2, 4, 6, and 8 of each sequence. The five interleaved (unwanted) stimuli were either (a) random digits ( RD) drawn without replacement from the five remaining digits, (b) random letters (RL) drawn without replacement, (c) a single digit ( SD) repeated five times, (e.g., 5 8 5 4 5 7 5 2 5)) a different digit being repeated in each trial, or (d) a single letter (SL) repeated five times (e.g., J 8 J 4 J 7 J 1 J). Sixteen lists of each type were used,


eight being recorded at a rate of 0.75 set/item and eight at 0.375 set/ item. The order of delivery of the resulting 64 sequences was randomized. Each sequence was preceded by 5 set of lead-in to indicate delivery rate. A period of 10 set was allowed for writing down each answer.


After instruction and prior to the experiment Ss went through a train- ing sequence which exposed them to each of the eight condition/rate combinations on two successive trials. During this period the principles of active selection were continually emphasized. There followed the un- interrupted presentation of 64 randomized sequences, with timing an- nounced by the metronome lead-in.

Subjects

The Ss were 15 Cambridge housewives, who were paid for their services. Ages ranged from 20 to 58 (median 30) years.

Results and Discussion

The results of this study are presented in Fig. 2 with percent error plotted against set/item.

At both input rates there appears to be an effect of variation in the

501

40 -

8

& z 30-

Y

f 20 -

Z

2

10 -

04 0475 0.750

SECONDS PEP ITEM

FIG. 2. Error rates at 0.375 and 0.75 set/item as a function of type of inter- polated material (random digits; random letters; single digits; single letters). Active processing strategy is employed throughout.


type of unwanted material. Furthermore, such variation results in an arrangement of conditions which is intuitively plausible. Single unwanted inputs from the letter set produce better performance than single digits. Similarly, random unwanted items from the letter set result in less errors than random digits. A Friedman two-way analysis of variance shows that the error rates for the four conditions are significantly different for both the slow rate ( xv2 = 25.7, df = 3, p < .OOl) and the fast rate (xr 2 = 34.4 df = 3, p < ,001). In addition, taking slope indices, there is a sign&ant difference between the effects of rate on the four conditions (xv 2 = 15.72, df = 3, p < .Ol). This seems to imply an interaction between type of unwanted material and the effects of rate, but there is clearly a ceiling effect in operation. One way of checking the reality of the interaction is to make the task more difficult (say, by increasing the list length to five wanted and six unwanted items), although this approach may run into problems of limitations due to memory span. A more useful strategy would seem to be to examine the effect of expanding the range of rates used. The appearance of Fig. 2 may simply reflect the fact that we have been looking at the efficiency of the extraction process in the rather limited range of 0.75 to 0.375 set/item.

EXPERIMENT 3

In this experiment we examine the efficiency of active extraction over a much wider range of input rates (from 2 set/item to 0.2 set/item). The main object of this is to give us a more complete picture of the form of the rate limitation, in relation to the nature of unwanted items.

Method


Procedural details are the same as for previous experiments. Subjects were asked to employ an active strategy only. Forty-eight lists were recorded on tape. These were nine-item lists, with alternating letters and digits, digits being the wanted items. Both single (SL) and random (RL) letter combinations were used, 24 lists of each type, in order to examine the effect of the nature of the unwanted items (e.g., (SL) Y 5 Y 4 Y 9 Y 2 Y; (RL) M 6 R 2 E 8 J 7 L). The lists were recorded at six different rates, 2.0, 1.0, 0.5, 0.33, 0.25, and 0.2 set/item, in randomized order of both rates and list types. Each list was introduced by a 5-set period of metronome lead-in. As before, a post-list period of 10 set was allowed for writing down the wanted digits.


Subjects

Twenty-two students from Durham University acted as Ss in the experiment, as part of a practical course in psychology. They were tested as a group.

Results and Discussion

The performance data is illustrated in Fig. 3, where percent error is plotted against log set/item. There is a very clear effect of rate on both types of list, performance falling off as the time allowed decreases in both cases. With this scaling rate variation produces a function which tracks at a negatively accelerated rate from the fastest rates where performance is relatively poor to a point where a ceiling of perfect performance is reached. It is clear that if the effect of unwanted material is assessed between any two rate points within this range an apparent interaction may or may not be observed, depending on the exact rates employed in the experiment. The curves appear to be similar in type, having inherently different “efficiency” parameters. If this is so, and an appropriate monotonic transformation of the ordinate can be found, two parallel lines would result, signifying a zero interaction. (There is an obvious analogy here with the use of ROC curves in signal detection theory.) One transformation which appears to create such conditions is

SECONDS PER ITEM (LOG SCALE)

FIG. 3. Error rates in the range 0.2-2.0 set/item as a function of type of inter- polated material (RL random letters; SL single letters). Active processing strategy is employed throughout.


the normalizing of the ordinate. We have not pursued this line since there is clearly insufficient resolution about each data point in such an experiment to enable firm conclusions to be drawn.

The problem is clearly one which merits further consideration, but we have decided to proceed on the assumption that the rate limitation effect occurs independently of changes in the nature of unwanted material. It is probably fair to say that this difficulty is not critical to the arguments presented in the remainder of the paper, although ulti- mately the exact nature of the limitation will depend on a more thorough investigation of these variables. From our original argument following Experiment 1 we now pursue the idea that the rate limitation is im- posed by the required oscillation frequency of either a gating or an amplifying process and its effect on the acquisition strengths of critical items. It is typical of such mechanisms that their output is a negative function of required oscillation frequency, and in that case, the achieved differential between the acquisition strengths of wanted and unwanted items would diminish with increased input rate.

The idea of selection-in-time is of limited value unless the locus of the process can be specified with respect to the various stages of information throughout. At a trivial level of description, one may state that “the subject selects and rehearses only certain critical inputs.” The primary concern here is to indicate how S is able to time the switching between a state of “monitoring the environment” and a state of “storing inputs” so as to accord preferential treatment to those inputs.

We have isolated three possible processes which appear worthy of consideration. These are termed retrospective processing, item processing, and timed processing. All three are illustrated in Fig. 4.

In retrospective processing all inputs are processed to the point where they are named, “tagged” for position-in-sequence, and categorized as wanted or unwanted. Wanted items are then subjected to a secondary process (such as rehearsal) which improves their status in store (“store input”).

In item processing inputs are simply “tagged” for position-in-sequence without further processing; only those items with “critical” tags proceed to the naming and storage level.

In timed processing the subject is assumed able to prepare to handle a sensory input, so that those items arriving when he is in a state of maximum preparedness are optimally processed and stored. The al- gorithm shows that, whereas in retrospective and item processing each input is allotted attention, in timed processing the subject is passive with respect to intake of stimuli until a critical point in time. The “yes” route on the figure can, thus, be taken as indicating a switch to some pre-


r IDENTIFY SENSORY.

STORE

POSITION IN

SEQUENCE

lNP”T

LISTEN

,DENTlFY

INPUT AND

STORE

WILL CRITICAL

I REMAIN PASSIVE

OR REHEARSE

STORE

FIG. 4. Three systems for extracting stimuli from an input stream. Top: retrospective processing in which all items are fully categorized; Middle: item processing in which items are counted, then critical items submitted to further processing; Bottom: timed processing in which any input in a given time zone is processed into storage.

paratory state which maximizes the effectiveness of storage. This behavior is envisaged as being similar to that found during the foreperiod of a reaction time trial. On current evidence the time required to move from zero to maximum preparedness is of the order of 590 msec (Posner & Boies, 1971). The complication introduced here is that S is assumed able to articulate his “preparations” in a rhythmic manner through time.

The three techniques described above should simply be taken as repre- sentative of large classes of possible strategies dependent on processes of different levels of complexity. The most complex process would presumably be the post-categorical operations envisaged in retrospective processing, the simplest the “preparations” involved in timed processing which would apparently require no preprocessing of input (e.g., for identification) as a preliminary to storage. As means of extracting items from an input sequence all three techniques could be rate-limited, since all involve operational cycles which when they are made to increase in frequency would make less time available for processing of each critical input, thus reducing its acquisition strength.

TIMING OF ITEM SELECX-ION 73

Experiments 5 and 6 in this series were made to determine the locus

of the rate effect with respect to the three modes of function proposed above. The rationale of the experiments is fairly simple. Situations may be found in which, say, retrospective processing is the only viable mode of operation. If the data found under these circumstances differ from those in situations where all three types of processing strategy are intuitively possible it is unlikely that subjects are using retrospective processing in these latter situations.

As a preface to these experiments a study was carried out to determine whether the effects under discussion are found under more formally controlled circumstances than those of “persuasive instruction.” The paradigm is based on that employed by Brown (1960) in which pre or post list instructions are given as to which stimulus category from a two-category list is to be reported. With pre list instructions (say “letters” or “digits”) some form of active selection may be possible, whereas it is precluded by delaying instruction until after delivery. This study is reported as Experiment 4.

EXPERIMENT 4

Method


Forty-eight lists were recorded on tape, all lists consisting of alternating letters and digits (e.g., L 4 B 2 F 9 J 3 N), 16 at each of the presentation rates 1, 0.5, and 0.33 set/item. Letters and digits were designated the wanted items equally often. The instructions “letters” or “digits” were given either before presentation (pre instructions) or afterwards (post instructions) on 24 trials each. The interval between instructions and the beginning and end of the Iist was approximately 1 sec. Following the technique of Brown ( 1960), a further instruction was incorporated in order to control for the inevitable delay in recall for the post condition. Subjects were told “left” or “right” indicating the side of their response sheet on which to write. Thus, for the pre instruction, the sequence might be: “letters’‘-list presentation-“left,” and for the post condition: “right’‘-list presentation-“letters.” Subjects were tested in blocks of six pre or post trials, with rate and letters/digits combinations tested once each during each block. There were four blocks of each condition, all Ss carrying out the eight blocks in the order pre-post-post- pre/post-pre-pre-post. A break of 30 set was given after each block and a break of 1 min after the fourth block. During these breaks


Ss were reminded of their task and told about the form of the instructions in the next block. They were told that they could make use of pre instructions by using the active listening strategy, while with the post instructions they should not attempt to guess which category of items would be required, but to pick them out afterwards, using the passive listening strategy.

Subjects

Twenty students at Durham University took part in the experiment as part of a practical course in psychology. They were tested in a group.

Results

The percentage of errors made under each instruction condition and rate is shown in Fig. 5. The results are very clear, performance falling off with rate for pre and improving slightly with rate for post instructions.

For the pre condition, combining letters and digits error totals, 14 out of 17 Ss perform better at the slower than at the faster rate; this is a highly significant difference (sign test, p = 0.006). The slope is not significant, however, for the post condition, 12 Ss out of 20 performing better at the fast rate.

_--- ,--A

-,----- ;C--

*-------- -,-* ----- ---- -----*

c _---

A DIGIT RECALL

0 LETTER RECALL

__- POST INSTRUCTION

\- PRE lNSTRUC>

O-l 0.33 0.55 I.00

SECONDS PER ITEM (LOG SCALE)

FIG. 5. Error rates for digit (A ) and letter ( l ) recall under pre (-) and post ( - - - -) instruction conditions at three stimulus input rates.


This result is in general agreement with that obtained in the first experiment; active extraction drops off as presentation rate is increased, with passive performance showing very little effect of rate (or, if any- thing, the opposite trend). In terms of average performance level, it may be observed that the recall of digits in the present experiment (Fig. 5) is considerably better than that in the equivalent condition of Experi- ment 2 (RL in Fig. 2), particularly at the fastest rate. There are several differences between the two situations which might account for this. For example, Ss in Experiment 4 may have found the strategies easier to use when given a more formal rule to follow, or they may simply have been a better group. In addition, from Fig. 3 it can be seen that the operating function for this condition is particularly steep at the fast rate (0.33 sect item), so that the obtained estimate of efficiency in any experiment may vary considerably with small variations in procedure. These questions are secondary to our main line of investigation, however, and are not pursued at this point. The results of Experiment 5 demonstrate that active extraction is subject to the same kind of rate limitation, whichever method of inducing the strategy is used.

In effect, then, the pre/post situation and the original active/passive situation are equivalent, as far as the operation of the extracting strategy is concerned. In Experiment 5 the pre/post paradigm is maintained in order to examine the effect of rate in a situation where only retrospective processing is possible. This is done by presenting the four digits and five letters in a nonregular sequence. If the earlier effects of rate on active performance are due to limitations in the efficiency of retrospective processing then this condition would show the same order of rate slope as one in which letters and digits alternate in the sequence. How important for the rate limitation effect is it that the position of wanted items in the sequence is known in advance?

EXPERIMENT 5

Method


The general procedure and design for this study is similar to that of Experiment 4. A total of 64 lists were presented on tape to Ss in a pre- post paradigm. Each list contained five letters and four digits, and the recall cue asked subjects to report either one or the other category of items. In addition to the alternating sequence used in the previous experiment, a further condition was used in which the letters and digits


were presented in a nonpredictable sequence (e.g., G 3 5 F K 6 L B 1). These two conditions were designated “fixed” and “varied,” respectively. In the varied condition the lists were constructed with the restriction that no more than two letters or digits could occur in succession within a list. Two rates of presentation (1 and 0.33 set/item) were used in the experiment. The two list types were presented in blocks, as were the pre and post instructions, while the rates and recall instructions were balanced within each block. All Ss carried out the task in the order pre/fixed-post/varied-post/fixed-pre/varied-post/varied- prelfixed-prelvaried-post/fixed. Each block contained two lists of each recall instruction/rate combination, eight lists in all.

Subjects

Sixteen students at Durham University took part in the experiments as part of a practical course in psychology. They were tested in a group.

Results

The percentage of errors made for letters and digits at each rate is shown in Fig. 6, for pre and post instructions and for each list type. For pre instructions the fixed condition gives rise to a significant advantage of the slower rate over the faster rate (combining letters and digits

t

. FIXED ITEM ORDER

50 A VARIED ITEM ORDER

--- POST INSTRUCTION

E I 40 -

5 &I4 ”

g 30-

f 2

20 -

1oQ1 oi3 PO0

SECONDS PER ITEM

F’IG. 6. Error scores at two stimulus input rates under pre (-) and post (- - - - ) instruction when critical item order is fixed ( 0 ) and varied ( A ) .


recall errors, 14 out of 16 Ss make less errors at the slow rate, p = .002); whereas, no rate effect is evident for the varied list (eight Ss out of 14 that show a difference make less errors at the slower rate, p > -05). For the post instructions, no significant effects of rate were obtained.

Discussion

These data show that when all inputs are categorized at a relatively sophisticated level the rate effect is eliminated. Subjects derive no advantage from a reduction in input rate over the range used when they are not able to predict the arrival of critical items in the input sequence. Although it is possible that retrospective processing might yield advan- tages at even slower rates than those employed, we conclude that selection at input can occur without categorization of each item, provided that the locations of critical stimuli are known in advance. The dramatic rate limitation effect thus seems to be due to either item processing or timed processing.

In Experiment 6 an attempt is made to discriminate between stimulus selection which is based on an item count (item processing) and that which is mediated by a phased change in state aimed at maximizing acquisition strength over a given time zone (timed processing). In essence it is argued that an item count can proceed independently of the time of arrival of inputs. Therefore, if critical items are selected on the basis of their position in the count, and if the time of arrival of items is randomized, performance should not suffer. On the other hand, if the cycle of operations (receive-store-receive-store, etc.) is rhythmic and time- dependent randomization of input timing should cause disruption. In the experiment which follows regular input timing is compared with irregular timing at two presentation rates. For this study we revert to the original instruction method of inducing active and passive strategies, and all-digit lists.

EXPERIMENT 6

Method

Materials and Procedure

Two conditions of input timing were compared in this experiment, (a) regular timing, as used in all previous studies, and (b) irregular timing, where the interval between successive items in the sequence varies throughout the list. Input rates of 1 and 0.33 seclitem were used for the regular lists, and average rates approximating these for the irreg-


ular lists. These were recorded by the. speaker attempting to produce an irregular sequence using simple visual aids (e.g., 64 9 28 51 7 3) ; all that was aimed at was a series of lists that were not regular, but in which each item was clearly pronounced (considerable practice was required to carry out this task successfully). The average rates for the irregular lists were estimated as 0.83 and 0.34 set/item. Nine-digit lists were used, the second, fourth, sixth, and eighth items again being designated wanted items. Two sets of 20 lists (one for each condition) were recorded on tape, and presented to separate groups of Ss. Each set contained ten lists of each rate, in a randomized order, with a S-set period of (regular) metronome lead-in to each list. Within each condition Ss were allocated to active or passive strategies, as described in Experi- ment 1.

Subjects and Design

Twenty four Ss were tested in the regular timing condition, 12 each under active and passive instructions, while 23 subjects heard the lists under the irregular condition ( 11 active and 12 passive). All were students at the University of Durham. They were tested in two groups, corresponding to the regular and irregular timing conditions.

Results

Figure 7 shows the percentage of errors made under each timing condition, as a function of rate and strategy. For the regular timing, the active strategy shows a marked rate effect, as in previous experiments (all 12 Ss recalling more at the slow rate, p < .Ol), while the passive strategy shows a small, though insignificant effect in the opposite direc- tion (nine Ss out of 12 are better at the fast rate, p = .073). When items are presented in an irregular temporal sequence, however, the slope of the rate effect for active is much reduced, and no longer significant (seven Ss out of 11 recalled more at the slow rate, with one S showing no difference, p > .05). Again, no rate effect is obtained for the passive strategy, seven out of 12 Ss performing better at the fast rate. It is perhaps worth noting that active performance at the fast rate is seemingly more efficient for the irregular condition, though the difference is not significant on a median test (x2 = 1.08, df = 1, p > .05). Figure 7 also indicates a (nonsignificant) tendency for passive performance to be generally higher in the irregular condition, so that there is evidently no overall impairment due to irregularity of timing (such an effect might be expected, for example, if the spoken inputs were not as well artic- ulated in the irregular condition, or if temporal irregularity affected some general secondary processing characteristic such as rehearsal). The ef-


SECONDS PER ,TEM

FIG. 7. Error scores at two stimulus input rates under active (-) and passive (- - - -) strategies when critical items arrive regularly (0) and irregularly (A) in time.

feet, however, seems to be quite specific and concerned with the difficulty of adequately preparing for the reception of critical items. It is not sufficient to know the position of wanted items in the input sequence: their time of arrival must also be predictable for maximum benefit to be obtained from a slow presentation rate. On this hypothesis, regular timing should be better than irregular timing even when the minimum onset-to-onset time in the latter is equal to the constant 1.0 set of the regular condition. This prediction has not yet been tested.

It is important to be able to demonstrate that the observed difference between the timing conditions in the efficiency of the active strategy is due to differential reception of items, rather than to ease of tagging for order in sequence. One could, perhaps, expect more difficulty in order preservation in the case of irregular lists, since some items are received close together in time; this could lead to impairment in recall and a reduction in the normal advantage of the slow rate during active extraction. In order to check on this possibility an error analysis was carried out, classifying each response not correct as either an item error or order error. Although this is, to some extent, arbitrary, item errors were defined as digits present in the input but missing from the response, and


TABLE 1 Analysis of Errors for Active Strategies in the

Two Timing Conditions of Experiment 6

Order errors Item errors Total errors

Regular (N = 11)

Slow Fast

27 (31%) 102 (23%) 59 336 86 438

Irregular (N = 12)

Slow Fast

78 (29%) 60 (20%) 187 238 265 298

order errors as items recalled correctly but in the wrong position in the sequence. Table 1 gives the breakdown of errors for the two active conditions, in terms of the total number of item and order errors: order errors are also expressed as a percentage of all errors. It is clear that there are no real differences in the proportion of order errors between the regular and irregular conditions, at either rate of presentation, though order errors are more prevalent at slow rates for both conditions. In terms of absolute numbers of errors, the greatest advantage of regular timing over irregular timing seems to be the reduction of item errors at the slow presentation rate. These data offer further support to the interpretation of the results of the experiment in terms of impired reception of items presented irregularly in time, rather than increased diffi- culties in retrieval or order tagging.

Discussion

When inputs arrive randomly in time the increment in performance accruing from a reduction in presentation rate is relatively small. This result indicates that selection of items from a sequence is mediated by a time-dependent process, rather than a simple count of items. On this evidence, and considering that a “retrospective processing” hypothesis was rejected on the basis of earlier experiments, we would suggest that the organism is able to modulate his state of receptivity in a rhythmic manner with respect to items arriving in a well-defined temporal pattern. Such a process results in heightened receptivity (and increased acquisition strength) for items to be remembered.

As suggested earlier it is economical to equate the kind of operation evident in the present “acquisition timing” situation with that seen in more detail during the foreperiod of a reaction time task (Posner & Boies, 1971). It is clear from such studies, as well as from those showing the concommitant development of CNV during the preparation interval ‘Tecce, 1972), that the individual is capable of exercising a fair degree


of control over the timing of his preparations. In the acquisition timing situation S may be regarded as being required to produce a series of such timed preparations during the input of any one sequence. Looked at in this way, the experiments reported here would seem best interpreted as demonstrating that the individual has the facility of fast control of a process which we shall refer to as “controlled activation,” and that he is able to use this facility to aid the structuring of his behavior through time.

Although it was necessary in this series of experiments to pre-define critical points in the input sequence, thus demanding periodic control of activation, a more interesting general implication of these findings is that the organism may be able to dispose these bursts of heightened receptivity through time so as to extract maximal critical information from any temporally-structured input stream. An obvious example is listening to a stream of language. Here, both syntactic and semantic constraints may dictate regions of high priority in advance, so that timed preparation would, in principle, be possible. The same suggestion is made by Martin ( 1972), though in terms of the intrinsic temporal structure of speech. He argues that, since speech is rhythmically patterned in time, the listener is able to anticipate the development of the pattern, and so make use of “attentional cycling” strategies (attending only at critical points in the sequence, such as accented syllables, while processing previous inputs during low-information phases of the speech). A rhythmic basis for such a mechanism is, in fact, quite likely, since these cues may be monitored at a fairly low level of analysis, even though Ss may also be able to make some use of the additional linguistic constraints men- tioned above. It has, to date, proved difficult to design experiments which adequately test this idea. It is known from the use of probe latency techniques (Kennedy & Wilkes, 1963) that the latency for a word in a sentence is related to its status in the deep structure of the sentence. What is not clear is whether variation in listening behavior plays any part in the process, in addition to a restructuring in memory (i.e., the kind of activity which might be expected to occur following passive processing in the present study). We would agree with Martin ( 1972), that such a mechanism is probably needed to account for the speed at which speech processing occurs.

Controlled activation, as we have used the term to account for the findings arising from the present group of studies, is manifestly concerned with the selective intake of information. On the other hand, most current theories of attention (Broadbent, 1971; Deutsch & Deutsch, 1963; Treis- man, 1969) relate not only to intake but to all types of activity which can dominate the organism’s behavior. Thus, attention can be devoted


to the guiding of a particular motor sequence or to the retrieval of a set of items from memory. It may be the case that controlled activation, considered as a general property of the attention system, is involved in the selectivity of behavior at all levels. Since its effect in information intake is apparently to increase the acquisition strength of critical items, an analogous function operating in memory retrieval or in the selection of a motor response would be to increase the strength of a required “pattern of activity” relative to the strengths of all competing patterns. The process may thus be a much more general one than is evident in the rather specific information-processing situation we have focussed on in this paper. Its role may be one which relates to the general problems involved in the control of behavior on a serial basis-the continued domi- nance of activity currently relevant to the organism, and the sequential organization of different activities in any ongoing behavior pattern.

It is perhaps too early to speculate on the nature of such a process, although the idea of a general facility of temporal control over the production of all patterns of behavior is not a new one. It was suggested by James (1899) as the basis of mental concentration, and was, of course, a central theme in Lashley’s (1951) important theoretical outline of the mechanisms underlying serial action. In recent years, both Martin ( 1972) and Neisser (1967) have discussed aspects of this question, while in a different area, Vanderwolf (1971) has provided tangible evidence for the operation of a nonspecific “triggering” mechanism underlying the initiation of voluntary movement patterns in neurophysiological studies of animal behavior.

As yet, there has been little direct study of these ideas or their im- plications, and this may be partly to do with the lack of any clear operational statement of their empirical consequences. Yet there are grounds for believing that such a process might be essential to the production of any coherent behavior sequence. We are perhaps now in a position to begin to formulate some of these ideas more precisely.

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(Accepted August 22, 1973)

active selection of items to be remembered: the role of timing

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