A response-type reaction time effect found in the S. Sternberg high speed memory scanning paradigm

Acta Psychologica North-Holland

75 (1990) 279-292

279

A RESPONSE-TYPE REACTION TIME EFFECT FOUND IN THE S. STERNBERG HIGH-SPEED MEMORY SCANNING PARADIGM * S.M. WILLIAMS Institute for Health Studies, Colchester,

C. COOPER

UK

and J.A. HUNTER

University

of Ulsterat Coleraine, Northern Ireland

Accepted

February

1990

A response-type (‘present’ responses faster than ‘absent’ responses) reaction-time effect was found in the S. Stemberg high-speed memory scanning paradigm by means of presenting the target/probe stimulus briefly in peripheral vision. The probe was presented 3 degrees left or right of fixation. The large (113 msec) response effect did not interact with the marked reaction-time effect of two types of stimulus differing in ease of nameability, nor with two laterality variables. An explanation of the effect consistent with Stemberg’s (1975) multiple-stage processing model is presented: the effect appears to arise at the stage of binary decision as to whether the probe was present in the memory set or not. The mechanism of the effect depends on the possibility of the memory stimulus priming the probe stimulus in the ‘present’ condition. Such priming must be much more effective where probe stimulus and the trace of the memory stimulus do not, because of their separation within the visual field, mask each other either backwards or forwards. The experiment found the orthodox linear effect of memory set size, not interacting with type of response, consistent with the common attribution of an exhaustive serial comparison process. Some relevance of the paradigm to laterality was also exhibited. A confirmatory experiment replicated the response-type effect with the probe presented above as well as to left and right of fixation, and showed no effect in a control condition of central presentation.

In recent years an influential example of cognitive chronometry was work of S. Stemberg (1966, 1970) with a ‘high-speed memory-scanning’ task. He gave subjects sets (of varying number) of positive stimuli to memorize, then flashed a ‘probe’ tachistoscopically and measured the time taken by the subject to decide whether the probe was a member of * Requests for reprints should be sent to S.M. Williams, Road, Colchester, Essex, CO4 5HG, UK.

Institute

OOOl-6918/90/$03.50

B.V. (North-Holland)

0 1990 - Elsevier Science Publishers

for Health

Studies,

Boxten

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the positive set of possible stimuli. The fact that the time increased linearly with the size of the set was clear evidence of serial internal processing. Klatzky and Atkinson (1971) flashed the probe in this paradigm either to the left or to the right visual field which should give an indication of whether the right and left hemispheres behave serially. Presentation was unilateral in counterbalanced blocks of trials. This as well as other features of their procedure (400 msec exposure of the probe; two different fixation points with the probe always presented in the same spatial location which was left of one point and right of the other) suggested that control of fixation to ensure the probe was initially processed by the intended hemisphere was poor, in spite of the often stressed (e.g. McKeever et al. 1972) importance of this. For this reason the probe should be presented randomly to the left or right of fixation. Different conditions of Klatzky and Atkinson’s experiment used as probes verbal and nonverbal stimuli, to determine whether this affects the nature of the processing. Ironically, their verbal probes in fact permitted spatial (putatively right-hemispheric) processing while their nonverbal probes were pictures of objects whose initial letter had to be determined (verbal, putatively left-hemispheric processing) and sought for in the positive memory set. They found for both types of probes and both sides of lateral presentation a linearly increasing function diagnostic of serial processing. Klatzky and Atkinson also found equivalent slopes for ‘present’ (the probe is a member of the positive stimulus set) and ‘absent’ (the probe is not a member of the positive stimulus set) response times at different sizes of the memory set. This is what Sternberg regards as evidence for the scanning through the memory set being exhaustive for each trial rather than selfterminating (i.e. where scanning is terminated when a match is found and the response initiated). When the probe is absent from the memory set the scan must be exhaustive anyway; when it is present a non-exhaustive scan will be terminated on average halfway through the memory set and so the slope of response time against memory set size will be approximately half that for ‘absent’ responses. However, Klatzky and Atkinson found an effect of serial position within the memory set on latencies for ‘present’ responses. This seems to argue against exhaustive scanning. However it is noteworthy that they presented the positive memory set at the start of each trial as a single left-to-right visual display. This procedure may encourage serial position effects because of left-to-right scanning induced by reading habits.

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The most intriguing aspect of Klatzky and Atkinson’s data, however, is something only discernible from their table of reaction times and neither mentioned in their text nor tested for significance. This is that there appears to be a substantial difference (110 msec) between the time taken for ‘present’ (which they call ‘positive’) and that taken for ‘absent’ (‘negative’) responses. Sternberg (1975) acknowledged that in one stage of the internal processing done during a trial of his paradigm a binary decision between responses had to be made which might take longer for ‘absent’ responses. Wickens et al. (1981) asserted the difference is but a trend and typically the two sorts of reaction time are ‘very nearly coincidental’. Wickens et al. (1981), in a version of the Sternberg paradigm which manipulated proactive interference between trials and interference interpolated between presentation of the memory set and the probe in a particular trial, discussed at length the implications for the understanding of retrieval of present/absent differences as small as 15 msec and 40 msec. Thus if the Klatzky and Atkinson difference of 110 msec is reliable it seems that lateralized presentation in the Sternberg paradigm creates a large and interesting effect worth replicating. Finally, Klatzky and Atkinson found a significant interaction between the side of visual presentation and the stimulus type (encouraging verbal or nonverbal processing) i.e. faster nonverbal processing in the left-visual-field/ right-hemisphere (LVF-RH) and faster verbal processing in the right-visual-field/left-hemisphere (RVF-LH) along the lines theories of hemispheric specialization would predict. (The equation of visual field with hemisphere is adopted as a convention promoting clarity of communication with no intention of dismissing interpretations of laterality experiments in terms of postexposural e.g. iconic scanning.)

Experiment 1 Method Experimental design A repeated measures design was used, with one between- and four within-subjects factors. Each subject sat two blocks of trials, one using the digits O-9 and the other using the symbols $, -, /, <, ?, @, %, *, ! and f: (after Cohen 1973) as stimuli for the memory-scanning task. These latter symbols were chosen as being easily discriminable but difficult to name. In contrast to Cohen (1973) some subjects spontaneously

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reported that they were able to name these symbols, though naturally the names were longer than for digits. A pilot study showed that with random - and therefore clearly unnameable - shapes, Sternberg-style scanning was impossible. The order of presentation of the blocks was counterbalanced across subjects. The four within-subjects factors were stimulus type (digits or symbols), the number of characters in each list (2, 4 or 6) probe position (to the left or right of the fixation point), and probe presence (probe present or absent from list). Four trials were given in each condition, the average reaction time from these four trials being used in all subsequent calculations. Thus there were 48 trials per block. The procedure used to generate the lists of characters ensured that no character appeared more than once in any given list. The order of the 48 trials within each block was independently randomized for each subject. If a subject made an incorrect response on any trial or took more than 3 set to respond an equivalent trial was inserted at the end of the block. Thus the experiment continued until each subject answered 48 items correctly in each block. This procedure can tend to pile up difficult trials at the end of the block, impairing the randomization, but inspection of the data revealed this was not a severe problem here. The between-subjects factor was the hand used to indicate whether or not the probe had appeared in the list. For half the subjects probe presence was indicated by pressing the key in the preferred (right) hand, and probe absence by pressing the key in the left hand, this being reversed for the remaining subjects. Apparutus

The presentation of instructions, stimulus generation, stimulus presentation and reaction time recording were controlled by an Acorn Master microcomputer. Two hand-held response keys allowed the subjects to indicate whether or not they thought that each probe character had appeared in the previous list. Reaction times were recorded with millisecond accuracy using an interrupt-driven clock (after Cooper et al. 1985). and the onset of each probe was synchronized with the raster scan. Stimuli were presented (in yellow) on a Microvitec model 1451 colour monitor. chosen because of its relatively low-persistence phosphor. Subjects’ eyes were positioned 0.86 m from the surface of the screen by a nose-restraint as described by Cooper (1982). This ensured that their noses were perpendicular to the horizontal midpoint of the screen and that the probes were presented three degrees to the left or right of the central fixation point. Subjects used binocular vision throughout. Subjects

Twenty subjects (17 male and 3 female) volunteered to take part in this experiment. Most were students or technical staff at the University of Ulster, with ages ranging from 20 to 60 years. All were given a 12-item handedness questionnaire (Annett 1985: 187, modified). Only four gave any ‘left’ responses (individually 1, 2, 2 and 7 such responses out of the 12). The subject giving 7 ‘left’ responses was not excluded, because this would bias subjects having a generalized unilateral awareness to show a spurious hemispheric difference. Procedure

Subjects were seated in front of the monitor as described above. After ascertaining which response button was held in which hand, they were familiarized with the range of

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digits or symbols which would form the stimuli for the first block of trials. They were then told that between two and six of these characters would appear, one after the other, at the centre of the screen, followed after a pause by an ‘X’ in the same position. Subjects were told that they should fixate the ‘X’ all the time that it was present on the screen and that the probe character would be presented to either the left or the right of this point but that they should not try to anticipate which. They were asked to indicate whether or not the probe character had been in the list by pressing the appropriate button ‘as quickly as possible whilst trying not to make any errors’. There followed ten practice trials in which full feedback was given, the list and probe being shown after each response. All trials were self-paced, the subject pressing either key to initiate the next trial. The experimenter also told subjects that they could pause for a rest whenever they wished. After the practice trials came the 48 trials of the first block (in fact, due to the need to repeat trials where the reponse had been an error, 48-plus trials). Similar instructions and examples were then given for the second block (symbols or digits), followed by the 48-plus trials of that block. Subjects heard two tones during all trials. A gong-like warning tone accompanied the onset of each ‘x’, and then after the response a high pitched gong signified a correct response with a harsh ‘bleep’ indicating an incorrect response. This provided both the subject and experimenter with feedback about the progress of the session. For every trial each member of the stimulus list was presented at the centre of the screen for 1.2 set, with an 0.5 set inter-stimulus interval. The screen remained blank for 1.5 set after the offset of the last member of the list. Then the ‘X’ was presented at the centre of the screen for 1.5 set and the warning tone was sounded. Then the probe character was shown for 80 msec at a position three degrees to the right or left of the ‘X’. Response times were measured from the offset of this character for technical reasons. The complete session took approximately 40 minutes to complete, and it was felt that further trials would lessen the quality of the data due to fatigue and weakening concentration. Results

A complete ANOVA testing all interactions of any level as well as main effects was performed on the reaction times with the four within-subjects independent variables Response Type (present/absent), Memory Set Size (2, 4 or 6 items), Stimulus Type (digits or difficult-to-name symbols), Side of Probe (left or right) and one between-subjects variable Hand for Response ‘Present’ (preferred or nonpreferred). The significant terms were Response Type (F&18) = 22.8, p < O.OOl), Memory Set Size (F(2,36) = 19.9, p < O.OOl), Stimulus Type (F(lJ8) = 19.5, p < 0.001) and Side of Probe x Hand for Response ‘Present’ (F(1,18) = 9.3, p < 0.01). Fig. 1 shows (i) that with all four stimulus X response types there is strong indication of serial processing of the positive memory set, though this is clearer for digits than for symbols; (ii) that for both types of character and all memory set sizes, ‘absent’ responses take longer than ‘present’ responses (113 msec longer on average); (iii) that symbols always take longer to process than do digits; (iv) that the slopes of the functions for comparable ‘present’ and ‘absent’ responses are approximately equal (as is also indicated by the absence of a significant Response

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S. M. Williams et al. / Response-type RT effect in the Sternbergparadigm 1300 0 S

Digits Diglts

‘present”

0

Characters

“present”

“absent”

.

Characters

“absent”

b

l

1200.

1100

:: 6 0 2

1000

E c 0 E ._ c 0 ;;-

900

2 cc

600

600

1

I

0

4

2 s,ze

of memory

6

set

Fig. 1. Reaction time as a function of memory set size for digits and for symbols present or absent from the set.

Type x Memory Set Size interaction), suggesting exhaustive memory search even for ‘present’ responses; further analysis also suggested that the serial position of a character within a positive memory set did not influence reaction time to it, again indicating exhaustive memory search; (v) that the slope of the reaction time (RT) functions suggests a comparison time per item of 35 msec for digits. For symbols it is not unequivocally appropriate to draw linear functions. However, trend analysis showed a significant linear trend for symbols (t = 3.05, p c 0.01) and no trace of a quadratic trend (t = 0.25, p = 0.81).

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Williams et al. / Response-type RT effect in the Sternbergparadigm

Table 1 Mean RTs (in msec) for each side of presentation ‘present’.

of probe

Side of presention

Hand used for response Preferred (right) Nonpreferred (left)

and each hand

285

used for response

of probe

Left

Right

972 1049

933 1102

‘present’

The significant interaction is described by the means in table 1. It seems plainly enough the effect of spatial compatibility between irrelevant stimulus location and hand for ‘present’ responses known as the ‘Simon effect’, underlining the importance with manual reaction time measures comparing the two visual hemi-fields of controlling and analysing response hands. Trend analysis on point (iv) established that the slopes of reaction time to digits for ‘present’ and ‘absent’ responses were not significantly different (F&19) = 0.08). The slope for ‘present’ responses was 30 msec per digit, the slope for ‘absent’ responses was 37 msec per digit. As described in the Method, trials eliciting an error were discarded and replaced by analogous trials until an error rate of zero in each condition was achieved. The numbers of discarded trials for each side of presentation of the probe and each type of stimulus were, however, totalled for each subject. Descriptive statistics for the two variables showed cell variances to be approximately a function of cell means, so the scores were transformed according to the formula Y = sqrt (X + l/2). An ANOVA on these transformed errors testing both variables and their interaction found, unsurprisingly in view of the reaction time evidence, a significant effect of stimulus type (F(lJ9) = 35.0, p -C 0.001) with symbols yielding more errors than digits, but no effect of side of presentation of the probe (F< 1). However, there was a significant interaction between the two variables (F&19) = 5.49, p < 0.05). The means are shown in table 2. There does seem here to be some sign of an interaction between visual hemi-field and verbal/nonverbal nature of the stimulus, with the verbal stimulus showing no visual-field advantage and the nonverbal stimulus a left-visual-field ad-

Table 2 Mean transformed

errors

for each side of presentation Side of presentation

Type of character Digit Symbol

of probe and each type of character. of probe

Left

Right

0.79 0.84

0.71 1.03

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Table 3 Mean reaction times (in msec) for each level of each within-subjects variable in experiment 1 Digits Visual field:

LVF

R type:

Present

Absent

Present

Absent

Number 2 4 6

712 845 922

871 992 1002

775 855 953

889 949 1070

RVF

Characters Visual field:

LVF

R type:

Present

Absent

Present

Absent

Number 2 4 6

1002 1086 1134

1111 1215 1236

1113 1027 1066

1063 1231 1222

RVF

vantage, of a type compatible with current theories of hemispheric specialization contralateral innervation. However, there was no significant simple main effect symbols. Since there was no main effect of the between-subjects variable Hand Response ‘Present’ the data were pooled across this variable and table 3 gives simple means of each of the experimental (within-subjects) variables.

and for for the

Experiment 2 The main finding of the principal experiment seemed large and the general pattern of data satisfactory but three issues seemed to call for further resolution. Firstly, could the assumption that the present-absent difference was created by means of lateralized presentation be supported by a control condition within the experiment using central presentation for the probe as well as for the memory set? Secondly, in view of the thinness of the evidence for hemispheric effects was the finding due to eccentric rather than specifically lateralized presentation of the probe? Thirdly, could the additional finding of the common Simon effect be used to help specify the internal locus of the response-type effect? A corollary partial replication and extension of the experiment was run to investigate these issues. The design of experiment 2 was identical to that of its predecessor in all respects except those to be described. The main difference was that the probe was presented not only to left and right of fixation but also centrally and three degrees either above or below fixation, so that the variable Side of Probe became Probe Location with five levels arranged in a random order of presentation through the session. The length of the session was not greatly different because the block of trials with symbols which had

SM.

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given less clear-cut data in the first experiment was dropped from the second. To prevent masking between the fixation cross and the central probe a square box with side of 1 degree was used with fixation directed towards the empty space within it. There were twenty-three right-handed subjects (two fully left-handed subjects were dropped from the data analyses to be reported in order to preserve comparability with experiment 1). The numbers of left-handed responses on the Annett questionnaire of subjects showing any were 1 (seven subjects), 2, 2, 3, 4, 5. Histograms for each subject for each probe location confirmed the assumption of approximate normality for an ANOVA. In the ANOVA the significant main effects were Response Type (F(1,21) = 10.6, p -CO.Ol), Memory Set Size (F(2,42) = 48.0, p < 0.001) and also Probe Location (F(4,84) - 3.21, p c 0.05). This last new effect was hue to probes above or below fixation giving longer and central probes slightly faster

Fig. 2. Reaction time as a function of memory set size for digits with probes present or absent from the set shown at any of five possible locations relative to fixation.

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Table 4 Mean RTs (in msec) for each side of presentation of probe and each hand used for response ‘present’, for each type of response.

(a) ‘Present’ responses

Side of presentation of probe

Hand used for response ‘present’ Left Right

Left

Right

692 199

698 768

(b) ‘Absent’ responses Side of presentation of probe

Hand used for response ‘present’ Left Right

Left

Right

754 853

817 820

times. The effect of Response Type interacted significantly with Probe Location (F(4,84) = 2.6, p < 0.05) and it can be seen from fig. 2 that this was because there was no real effect of Response Type for central or below-fixation probe presentation. The other significant interactions were Probe Location X Hand for Response ‘Present’ (F(4,84) = 3.02, p < 0.05) reflecting a replication of the Simon effect and Probe Location x Hand for Response ‘Present’ X Response Type reflecting a differential Simon effect for the two types of response. As can be seen from table 4 the Simon effect is greater for ‘absent’ responses. Interesting interactions where F was less than 1 were: Memory Set Size x Probe Location, suggesting serial memory search operated for all probe locations; Memory Set Size x Response Type, suggesting exhaustive memory search (as do the parallel functions in fig. 2); and Memory Set Size x Response Type X Probe Location, suggesting exhaustive memory search operated for all probe locations. reaction

Discussion The main finding was that lateralizing the probe in a Sternberg high-speed memory-scanning task gave rise to a large (113 msec on average) increment of duration on ‘absent’ responses as compared with ‘present’ responses. The attribution of the increment to lateralization is

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inferential but immensely probable in view of the large corpus of studies showing no increment with central presentation of the probe. The same finding is apparent on inspection of data provided by Klatzky and Atkinson (1971) though it is neither discussed nor analysed there. What is particularly noteworthy is that the Response Type effect did not interact with any of the other variables analyzed, namely Memory Set Size, Stimulus Type, Side of Probe and Response Hand. The failure of these latter two to interact significantly with the Response Type effect argues strongly against any conceptualization of the increment as due to transcallosal transmission time. Nevertheless the fact that the effect was found with presentation of the probe unpredictably to the left or right of fixation might have been consonant with an interpretation in terms of the independent ‘activation’ (e.g. Kinsbourne, 1975) or ‘warming up’ of the two cerebral hemispheres. This in turn might tend to throw the brain into a dual-processing rather than a single-processing mode. Such a line of speculation relates the Sternberg paradigm to the more straightforward task of visual perceptual matching where an analogous increment of ‘different’ over ‘same’ RTs and its possible interpretation in terms of a parsimonious single process rather than dual processes more adaptable to the whole range of evidence has already been explored and debated (Bagnara et al. 1983; Sergent 1984; Boles et al. 1984). However, this line of explanation is rendered insufficient by the discovery in experiment 2 of the same effect with presentation of the probe above rather than to left or right of fixation. Experiment 2 also confirmed as a control condition what can be surmised from the rest of the Sternberg literature: that the response-type effect is not found with the normal central presentation of the probe. On the basis of a great deal of empirical evidence with his memoryscanning paradigm Sternberg (1975) offered an information-processing model of four stages intervening between presentation of the probe and the timed response. He called these Stimulus encoding, Serial comparison, Binary decision and Response translation & organization. He also felt that the experimental factor of Response type had a selective effect on the duration of the Binary decision stage. Certainly in this experiment the failure of the Response Type effect to interact with Memory Set Size in spite of the latter’s large main effect argues not only for exhaustive scanning even with ‘present’ responses but also against any implication of the effect in the stage two Serial comparison. Moreover, the large effect of Stimulus Type in experiment 1 shifts attention away

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from the Response translation & organization stage four. It seems most likely that probe digits can be encoded more rapidly than difficult-toname symbols. Moreover, since a ‘present’ probe will always have been preceded by another token of the identical character during presentation of the positive memory set, it seems quite plausible that this identical character should prime or in other words speed the Stimulus encoding of the probe. However, if both Stimulus Type and Response Type have their effect on reaction time in the Stimulus encoding stage it is difficult to explain the observed absence of any interaction of Response Type x Stimulus Type. If Response Type has its influence on the fourth Response translation & organization stage why is its magnitude so dramatically altered by the stimulus presentational variable of lateralization? By a process of elimination it does seem after all it is most likely that Response Type has its influence on the Binary decision between the two possible responses. This argument by elimination is greatly strengthened by positive evidence in experiment 2. Both experiments showed a ‘Simon effect’ whereby probes ipsilateral to the hand responding ‘present’ were responded to faster. Experiment 2 showed this effect to be substantially larger for ‘absent’ than for ‘present’ responses. Since the Simon effect is thought to have a locus at the Binary decision stage (Simon et al. 1981), and it interacts with Response Type, it follows from the additive factor logic that the main effect of Response Type too has its locus at this stage. It remains to be explained why the decision between the two responses should show a particular lengthening for the ‘absent’ response with eccentric presentation. With simultaneous matching of vertically arranged pairs of letters it is known that the ‘same’/ ‘different’ decision takes longer the further the stimuli are from fixation (Lefton and Haber 1974). It should be noted that in the present experiment with the positive memory set presented successively at the point of fixation and unpredictable unilateral presentation of the probe, fixation control is probably especially good. If with three degrees eccentric presentation of the probe the whole Binary decision stage takes longer it would be natural to expect a small Response Type difference with central presentation to be proportionally greater with eccentric presentation. Lefton and Haber (1974) also find that with increasing eccentricity from fixation the relative durations of ‘same’ and ‘different’ responses likewise alter. Many find the ascription of exhaustivity to the

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Serial comparison stage implausible, but within the Sternberg model as presented it seems very likely that an exhaustive series of high-speed comparisons followed by a decision based on them might actually be faster than a partial’ series of comparisons broken off by a decision (to self-terminate and desist from further comparisons) followed by yet another decision (as between response types). The speed of individual comparisons in this experiment (35 msec for digits) seemed comparable to the typical Sternberg experiment - certainly no slower. The best explanation of the Response Type effect derives from the fact that the Stemberg paradigm is a form of multiple successivematching task. In such tasks the possible role of priming of some sort, perhaps between the memory trace and a match being inspected at the Binary decision stage, can not be ruled out (Proctor 1981; a related form of priming was reported recently by Simon (1988)). And it is intuitively clear that such priming is likely to be more effective with distinct spatial separation reducing masking phenomena between the primed and priming items. The other findings of the experiment need little discussion. The significant and reliable Side of Presentation X Response Hand interaction demonstrated the importance for those interested in using manual reaction-time paradigms to explore visual lateral hemi-field differences as a possible index of hemispheric asymmetries of controlling and analysing Response Hand as recommended by Sergent (1984). This experiment concurred with Klatzky and Atkinson (1971) in finding the Sternberg paradigm effects too robust to be influenced by visual hen&field of presentation. Nonetheless, an analysis of errors did show that even in this experimental paradigm the sort of Side of Presentation X Stimulus Type (digit or difficult-to-name symbol) interaction that current ideas about hemispheric specialization and contralateral visual innervation would predict does indeed appear.

References Ann&t, M., 1985. Right, left, hand and brain: The right shift theory. London: Erlbaum. Bagnara, S., D.B. Boles, F. Simion and C. UmiltB, 1983. Symmetry and similarity effects in the comparison of visual patterns. Perception and Psychophysics 34, 578-584. Boles, D.B., S. Bagnara, F. Simion and C. Umilti, 1984. Hemispheric mediation of same-different judgments: A reply. Perception and Psychophysics 35, 596-600. Cohen, G., 1973. Hemispheric differences in serial vs parallel processing. Journal of Experimental Psychology 91, 349-356.

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Cooper, C., 1982. An experimental investigation of Freudian defences. Unpublished doctoral dissertation, University of Exeter, England. Cooper, C., R. Kirby and J. May. 1985. A millisecond timer for the BBC microcomputer. Farnborough, Army Personnel Research Establishment: Working Paper WP14/87. Kinsbourne, M., 1975. ‘The mechanism of hemispheric control of the lateral gradient of attention’. In: P.M.A. Rabbit and S. Domic (eds.), Attention and performance V. London: Academic Press. pp. 81-97. Klatzky, R.L. and R.C. Atkinson, 1971. Specialization of the cerebral hemispheres in scanning for information in short-term memory. Perception and Psychophysics 10. 335-338. Lefton, L.A. and R.N. Haber, 1974. Information extraction from different retinal locations. Journal of Experimental Psychology 102, 975-980. McKeever, W.F., M. Suberi and A.D. van Deventer, 1972. Fixation control in tachistoscopic studies of laterality effects: Comments and data relevant to Hines’s experiment. Cortex 8, 4733480. Proctor, R.W., 1981. A unified theory for matching-task phenomena. Psychological Review 88, 291-326. Sergent, J., 1984. Hemispheric mediation of same-different judgments. Perception and Psychophysics 35. 592-595. Simon. J.R., 1988. A ‘priming’ effect in a choice reaction time task. Acta Psychologica 69, 45-60. Simon. J.R.. P.E. Sly and S. Vilapakkam, 1981. Effect of compatibility of S-R mapping on reactions toward the stimulus source. Acta Psychologica 47, 63-81. Sternberg, S., 1966. High-speed scanning in human memory. Science 153. 652-654. Sternberg, S., 1970. ‘Memory-scanning: Mental processes revealed by reaction-time experiments’. In: J.S. Antrobus (ed.), Cognition and affect. Boston; MA: Little. Brown. Stemberg, S., 1975. Memory scanning: New findings and current controversies. Quarterly Journal of Experimental Psychology 27. l-32. Wickens, D.D., M.J. Moody and R. Dow, 1981. The nature and timing of the retrieval process and of interference effects. Journal of Experimental Psychology: Genera1 110, l-20.