SPATIAL LOCALIZATION IN SEQUENTIAL LETTER DISPLAYS' PAUL J. HEARTY AND D.J.K. MEWHORT Queen's University ABSTRACT
Two experiments investigated the ability to locate letters presented successively along a horizontal row. The letters were displayed for 5 msec, and the inter-letter interval varied between 0 and 200 msec. In the first experiment, localization decreased as the inter-letter interval was increased to 50 msec. With further increments in inter-letter interval, performance improved. In the first experiment, however, there was a correlation between the positions of the letters in space and in time. The second experiment indicated that the recovery in spatial localization with inter-letter intervals greater than 50 msec is spurious, i.e., it does not occur if the correlation is minimized. The data were discussed in terms of a recent model for the sequential organization of
report. subjects are shown a number of letters or digits simultaneously and are required to report as many of the items as possible. Here, the subject is provided with a multi-element display in parallel but is required to construct a sequential report. When subjects in such a task are free to report in any order, they begin with the left item and proceed toward the right (Bryden, 1966; Heron, 1957; Mewhort, 1966), and accuracy of report is superior for items from the left side of the display (Heron, 1957). A number of attempts have been made to explain the accuracy and order of report data. Heron (1957) proposed a model in which a postexposural attentional mechanism is responsible simultaneously for both the left-to-right order of report and the left-side superiority. He suggested that the stimulus exposure activates neural traces which are subject to decay and that an attentional mechanism "scans" the traces from left-toright. Responses are produced in the order used by the scan. Thus, the model accounts for left-to-right reporting as a direct consequence of ordered processing; the left-side superiority emerges because right-side traces fade before responses can occur. When the order of report is constrained by the experimenter, the pattern of results is more complex. Suppose subjects are directed to report either from left to right or from right to left. When the cue specifying IN A SIMPLE TACHISTOSCOPIC TASK,
"This research was supported by a grant from the National Research Council to DJKM (Grant No. AP-318), and the paper is based on an unpublished MA thesis by PJH. Address for reprints: Paul J. Hearty, Department of Psychology, Queen's University, Kingston, Ontario K7L 3N6. 348 CANAD. J. PSYCHOL/REV. CANAD. PSYCHOL. 1975, 29 (4)
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direction of report is presented prior to, or immediately after, the stimulus exposure, accuracy is better for the side reported first. When the cue is delayed, however, a left-side superiority emerges regardless of the direction of report (Cornett, 1972; Scheerer, 1972). If the direction of the scan can be controlled by instructions, and if subjects use a left-to-right scan by default whenever the cue has been delayed too long, the data can be explained in terms of Heron's ideas (e.g., Scheerer, 1972). Here, the side superiorities with immediate instructions would reflect directed scanning, and the left-side superiority, which emerges with the delayed cue, would reflect the default condition. However, Mewhort and Cornett (1972) have shown that even with an immediate right-to-left cue, instructions do not control the direction of the scan. A satisfactory model must explain the effect of instructions without assuming that they alter the direction of the scan. Mewhort (1973; Mewhort & Cornett, 1972) proposed a two-stage model which distinguishes between the input scan and rehearsal or response organization processes within short-term memory. He argued that the scan operates only from left to right and that it orders spatially discrete, but simultaneous, items along a temporal dimension. Thus, the scan converts the spatial dimension to a temporal one in preparation for rehearsal in short-term memory. The rehearsal process involves an iterative refreshing of item representations and requires material which has been ordered on a temporal dimension. The model permits material to be sustained without rehearsal for only a brief time, and once rehearsal has been initiated, the organization for rehearsal is fixed. For a free-recall case, the model assumes that the scan loads short-term memory and that rehearsal follows the organization implied by the scan. Thus, left-to-right reporting and a left-side superiority are direct results of an interaction of iterative rehearsal with loss during report. For the ordered-recall case, particularly report from right to left, the situation is more complicated. Here, the scan loads memory by ordering items (from left to right) on a temporal dimension. If the cue is available, the subject can ignore the order implied by the scan, and rehearsal follows the order dictated by the cue. However, when the cue has been delayed, the subject must initiate rehearsal, and as a result, rehearsal follows the only organition available, that established by the scan. Consequently, the left-side superiority emerges. To test the model, Mewhort (1974) presented a row of letters sequentially starting either from the left or from the right. The time between successive letters was 0, 25, 50, or 100 msec. The materials consisted of either first-order or fourth-order approximations to English, and subjects were free to report in any order. For left-to-right presentations, the order
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of report was left to right at all inter-letter intervals. For right-to-left presentations, however, the order of report varied with the inter-letter interval. As the interval increased from 0 to 50 msec, order of report changed from a left-to-right direction to one matching the direction of presentation, i.e., from a spatial to a temporal organization. Mewhort argued that increasing the inter-letter interval disrupted the scan and forced the rehearsal mechanism to adopt the temporal organization implicit in the order of presentation. Accuracy data, particularly variation in the size of the familiarity effect, were consistent with die argument. The assumption that increasing the inter-letter interval disrupts the scan is crucial to Mewhort's (1974) test of the model. He recognized the importance of the assumption but was unable to support it empirically. Instead, he suggested that the manipulation deprives the scanning mechanism of necessary spatial information and thus, attempted to explain, on theoretical grounds, why it might disrupt the scan. The present experiments were designed to furnish empirical support for the latter position. Specifically, the experiments measure the usability of spatial information under conditions comparable to those of Mewhort (1974), and if his explanation is correct, increasing the inter-letter interval should reduce the subjects' ability to access spatial information. EXPERIMENT I
In the first experiment, 8 letters of 5 msec duration were presented sequentially along a horizontal row. The direction of presentation was either left to right or right to left. The inter-letter interval varied between 0 and 200 msec, and the subjects were required to locate a designated letter in the display. The presentation conditions are almost identical with those used by Mewhort (1974), and if his explanation of scan disruption is correct, increasing the inter-letter interval should result in lower spatial localization. Method Subjects. Eight volunteers from the undergraduate and graduate classes of Queen's University served as Ss. Materials and Apparatus. One hundred and ninety-two 8-letter sequences were generated. The sequences were random with the restriction that no letter was repeated within any sequence. The letters were displayed on a cathode-ray tube (CUT) supplied with fast-decay phosphor and slaved to a PDp/8e computer. A letter was presented by brightening the appropriate dots in a matrix of 7 rows and 5 colums on the CRT. A complete letter sequence, centered on the display, subtended a visual angle of approximately 4° by 28'.
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Procedure. On each trial, a fixation dot appeared on the centre of the display, and E called out a letter. The letter display followed, and the letter designated by E was always present in the display. The S was required to indicate the spatial position of the designated letter within the 8-Ietter display and responded with a digit 1 to 8. The response referred to the location of the designated letter counting from the left. The letters of the display were presented one at a time and in sequence starting from either the left or the right. Each letter was exposed for 5 msec, and the interletter interval was one of 0, 50, 100, or 200 msec. Before starting the experiment proper, 40 practice trials were administered. The latter were identical with the trials to follow. Instructions to Ss were illustrated with the practice trials and stressed that the letter designated by E was always present in the display but could take any of the 8 spatial positions. Design. The design was within-subjects. Each S received 192 trials, 3 replications for the factorial combination of three variables: direction of presentation, inter-letter interval, and spatial position of the critical letter. Each trial used a different letter sequence. Both the pairing of letter sequences to conditions and the order in which conditions occurred were determined randomly. RESULTS
An accuracy measure was taken by computing the number of responses which matched the position of the designated letter. In all cases, accuracy was slightly better for letters presented at the ends and near the fixation point, i.e., accuracy across the stimulus array took the symmetrical Wshape typical of partial-report studies. In Figure 1, the percentage of correct responses is shown for each combination of inter-letter interval and direction of presentation. As the inter-letter interval was increased from 0 to 50 msec, accuracy of localization decreased sharply. With further increases in the inter-letter interval, however, performance improved. An analysis of variance indicated that the effect of inter-letter interval was highly significant, F(3,21) = 16.88, p < .0001. In addition, at an interval of 0 msec, right-to-left presentations yielded slightly higher performance than those from left to right. However, at longer intervals, the relation reversed. Thus, although both directions of presentation showed essentially the same trend, there was a significant interaction of inter-letter interval with directions, F(3,21) = 4.95, p < .0096. Performance in the present task is necessarily limited by the subjects' ability to identify the items. If performance were affected seriously by the limitations, the task could not be used as a method for measuring spatial localization. In such a case, however, performance should tend to chance (12.52). It is clear from Figure 1 that performance is well above chance. Nevertheless, it is possible that subjects fail to identify the material on a small proportion of the trials. In particular, many of the errors may reflect a failure to identify the material rather than a failure to locate it correctly.
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100
75
O
I
50
25
LEFT-TO-RIGHT RIGHT-TO-LEFT 50
100
200
INTER-LETTER INTERVAL (MSEC)
FIGURE 1. Percentage correct for each combination of interletter interval and direction of presentation (Experiment i).
If this were so, incorrect responses should tend to be distributed evenly across the response alternatives. To consider the latter possibility, an error analysis was conducted. In the analysis, the number of errors was calculated as a function of the distance from the correct response. For example, suppose the correct position is 5 and that the response is either 3 or 7. In such a case, the distance is 2. Because the number of errors varies across conditions, the errors at each distance are expressed as a proportion of the total number of errors in the condition (see McCrary & Hunter, 1953). Table i shows the error distributions for each condition. In addition, the table shows the chance distribution for the condition and the deviation of the data from chance. The chance distributions were calculated on the assumption that all errors are equally likely, and because the total number of errors varies across the conditions, separate distributions were calculated for each. As is clear from the table, the bulk of the errors are concentrated close to the correct item. Thus, the errors indicate a failure of precise localization.
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TABLE I PROPORTION OF TOTAL ERRORS AS A FUNCTION OF DISTANCE (ROW 0 CONTAINS THE DATA OBTAINED, ROW C SHOWS THE CHANCE CALCULATION, AND ROW D SHOWS THE DIFFERENCE)
Distance Direction
ILI
LR
0
LR
50
LR
100
LR
200
RL
0
RL
50
RL
100
RL
200
0 C D 0 C D O C D O C D O C D O C D O C D O C D
1
2
.588 .273 .315 .646 .270 .377 .806 .267 .539 .833 .271 .562 .483 .262 .221 .566 .267 .299 .743 .272 .471 .922 .266 .655
.162 .212 -.050 .162 .227 -.065 .075 .226 -.151 .100 .243 -.143 .167 .205 -.038 .255 .229 .026 .135 .239 -.104 .039 .244 -.204
3
4
.059 .103 .143 .164 - . 0 6 1 —.084 .071 .071 .143 .188 -.117 -.072 .030 .045 .143 .200 -.171 -.098 .000 .033 .214 .143 -.214 -.110 .083 .133 .143 .162 -.029 -.060 .057 .066 .143 .185 -.119 -.086 .081 .027 .143 .201 -.174 -.062 .000 .039 .143 .207 -.168 -.143
5
6
7
.074 .122 - .048 .040 .098 - .058 .030 .085 - .055 .000 .071 - .071 .133 .124 .010 .038 .101 - .063 .014 .085 - .071 .000 .078 - .078
.015 .074 -.059 .010 .059 -.049 .015 .060 -.045 .033 .043 -.010 .000 .081 -.081 .019 .057 -.038 .000 .046 -.046 .000 .042 -.042
.000 .013 -.013 .000 .016 -.016 .000 .019 -.019 .000 .014 -.014 .000 .024 -.024 .000 .019 -.019 .000 .014 -.014 .000 .020 -.020
An additional point is of interest. During interviews after the experiment, Ss indicated that at shorter inter-letter intervals (especially 0 msec), they attempted to see all of the display and to estimate the spatial position of the designated letter. Thus, at the 0 msec condition, Ss attempted to follow the intent of the instructions. However, with longer inter-letter intervals (especially 100 and 200 msec), Ss reported a different strategy. Specifically, they noted the direction of presentation and counted the items until they arrived at the designated letter. If presentation was from left to right, Ss started at 1 for the first item; if presentation was from right to left, they started at 8 and counted backwards. In short, when dealing with the longer intervals, Ss used a temporal strategy. DISCUSSION
As the inter-letter interval was increased from 0 to 50 msec, accuracy of localization decreased sharply. Such a result is consistent with the loss
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of spatial information required by Mewhort's explanation of scan disruption. However, the data show that increasing the inter-letter interval beyond 50 msec resulted in a sharp recovery in performance. If the data at the longer temporal intervals reflect an increase in spatial resolution, they are inconsistent with the scan disruption idea. However, it is not clear that the data at these intervals do reflect spatial localization. The subjects' reports indicate that they adopted a temporal strategy at the longer intervals. If the subjects' reports represent an accurate description of their behaviour, performance at the longer inter-letter intervals does not reflect a recovery in spatial localization but rather reflects the use of a temporal strategy.
EXPERIMENT II
The second experiment was designed to determine if the recovery in performance with inter-letter intervals greater than 50 msec reflects a temporal strategy. The Ss in the first experiment reported a simple counting strategy. In effect, their strategy exploits the correlation between spatial position and temporal position inherent in the presentation conditions of the first experiment. If the counting technique is responsible for the recovery, reducing the correlation should reduce the improvement. For example, if a number of random orders of varying minimal correlations were used, such exploitation should not be possible. Thus, at intervals greater than 50 msec, a recovery in performance should occur for left-tpright and for right-to-left presentations, but not for random presentations.
Method Subjects. Subjects were 18 volunteers from the graduate and undergraduate classes of Queen's University. Materials and Apparatus. Materials and apparatus were the same as in the first experiment. Procedure. The procedure was basically the same as in the first experiment. However, two differences should be noted. First, a random presentation condition was used in addition to the left-to-right and the right-to-left conditions of the first experiment. Second, each subject received only one of the three orders of presentation, i.e., the variable was administered between subjects. The task for all subjects was identical with that in thefirstexperiment. For the random presentation group, 24 pseudo-random orders of presentation were used. In generating the random orders, three precautions were observed. First, within any order, the spatial position did hot match the temporal position. Second, across the 24 orders, no more than three matches in spatial-temporal position were permitted for any pair of orders. Finally, orders which produce "blanking" (Mayzner & Tresselt, 1970) were excluded. Although the orders are not strictly random, the correlation
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100
75
50
25 LEFT-TO-RIGHT RIGHT-TO-LEFT RANDOM 50
100
200
INTER-LETTER INTERVAL (MSEC)
FIGURE 2. Percentage correct for each combination of interletter interval and direction of presentation (Experiment n ) . between spatial position and temporal position is minimal, and because 24 orders are involved, subjects should not be able to develop any strategy based on a spatiotemporal correlation. Design. The design was between-within subjects. Each S received 192 trials, 6 replications for the factorial combinations of 2 within-subjects variables, inter-letter interval, and stimulus position probed, for one level of the direction variable. Subjects were randomly assigned to direction of presentation groups. Within a group of subjects, both the pairing of letter sequences to conditions and the order in which conditions occurred were determined randomly. RESULTS
Responses were scored in the same manner as in thefirstexperiment. Figure 2 shows the percentage of correct responses for each combination of inter-letter interval and direction of presentation. Figure 2 indicates that, as the inter-letter intervals was increased from 0 to 50 msec, localization decreased for all conditions. However, as the interval was increased
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further, there was a sharp increase in performance for the left-to-right and the right-to-left conditions, but not for the random condition. Clearly, the pattern of results for the correlated conditions replicates that of the first experiment but is not similar to that for the random condition. An analysis of variance confirms the trends shown in the figure. In particular, the interaction of inter-letter interval with presentation condition was highly significant, F(6,45) = 9.57, p < .00005. Orthogonal comparisons were used to examine the components of the interaction. Specifically, the linear trend across the 50 to 200 msec inter-letter interval range showed a large interaction with a comparison of the random condition against the two correlated conditions, F(l,15) = 54.85. There was no interaction (linear or quadratic) across the same range with a comparison between the two correlated conditions, F(l,15) = 0.62 (linear), F(l,15) = 0.72 (quadratic). Thus, although all presentation conditions showed a steep decrease in performance as the inter-letter interval was increased from 0 to 50 msec, only the correlated conditions showed a recovery as the interval was increased further. As in the first experiment, an error analysis was conducted. The analysis is identical with that presented for the first experiment and is summarized in Table n. Again the bulk of the errors are concentrated at the positions adjacent to the correct item, and the pattern of errors reflects a failure of precise localization. An additional point deserves mention. In interviews after the experiment, subjects in the correlated conditions reported using a temporal strategy with longer intervals. Subjects in the random condition, however, reported only a spatial strategy. DISCUSSION
As the inter-letter interval was increased to 50 msec, spatial performance decreased for all directions of presentation. The decrease for the correlated conditions replicated that found in the first experiment. With further increments in inter-letter interval, however, there was a sharp improvement in performance for the correlated conditions, but not for the random, or uncorrelated, condition. These results show that, as the temporal separation between letters is increased, spatial localization decreases and does not recover. Thus, the apparent recovery in the correlated conditions, both here and in the first experiment, reflects the efficiency of a temporal strategy and not a genuine recovery in spatial localization. GENEBAL DISCUSSION
As noted earlier, Mewhort (1974) studied accuracy and order of report in a free-recall task using presentation conditions very similar to those
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TABLE II PROPORTION OF TOTAL ERRORS AS A FUNCTION OF DISTANCE (ROW 0 CONTAINS THE DATA OBTAINED, ROW C SHOWS THE CHANCE CALCULATION, AND ROW D SHOWS THE DIFFERENCE)
Distance Direction
ILI
LR
0
LR
50
LR
100
LR
200
RL
0
RL
50
RL
100
RL
200
RANDOM
0
RANDOM
50
RANDOM
100
RANDOM
200
O C D O C D O C D O C D O C D O C D O C D O C D O C D O C D O C D O C D
1
2
3
4
.537 .271 .266 .598 .275 .324 .845 .274 .571 .848 .264 .584 .567 .268 .299 .480 .264 .216 .836 .270 .566 .949 .271 .678 .630 .266 .365 .673 .248 .425 .731 .241 .490 .789 .256 .533
.250 .222 .028 .189 .235 -.045 .082 .231 -.149 .121 .242 -.121 .213 .232 -.019 .281 .226 .055 .131 .234 -.103 .034 .240 -.206 .109 .203 -.095 .167 .206 - .039 .218 .207 .011 .148 .226 -.078
.120 .180 - .060 .106 .196 - .090 .041 .197 - .156 .000 .199 - .199 .063 .186 - .123 .123 .187 - .064 .016 .191 - .174 .000 .194 - .194 .087 .168 - .081 .026 .171 - .146 .032 .168 - .136 .014 .177 - .163
.028 .143 -.115 .015 .143 -.128 .000 .143 -.143 .030 .143 -.113 .094 .143 -.048 .070 .143 -.073 .000 .143 -.143 .000 .143 -.143 .065 .143 -.078 .064 .143 - .079 .006 .143 -.136 .014 .143 -.129
5
6
.009 .056 .063 .106 -.050 -.054 .015 .061 .051 .090 -.029 -.036 .021 .010 .054 .088 -.078 -.034 .000 .000 .087 .043 -.087 -.043 .000 .039 .054 .100 -.061 -.054 .041 .006 .099 .060 -.058 -.054 .008 .008 .052 .095 -.087 -.043 .017 .000 .092 .046 -.075 -.046 .054 .054 .118 .082 -.064 -.028 .038 .026 .080 .114 -.076 -.054 .006 .006 .117 .079 -.111 -.072 .014 .014 .109 .059 -.095 -.045
7 .000 .015 --.015 .015 .011 .004 .000 .012 --.012 .000 .022 --.022 .024 .018 .006 .000 .022 --.022 .000 .015 --.015 .000 .015 --.015 .000 .020 --.020 .006 .038 - .031 .000 .045 - .045 .007 .030 - .023
of the present task. The data were presented in the context of scanning theory, and to explain the results, he suggested that increasing the temporal separation between successive letters reduces the spatial information available and deprives the scanning mechanism of a necessary spatial context. As a result, at longer inter-letter intervals the scan is disrupted, and the subjects are forced to organize the material in the order of presentation. The present experiments were designed to test one aspect of
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Mewhort's account, the suggestion that increasing the inter-letter interval reduces spatial information. The data, particularly the results for the random presentation condition, confirm the spatial loss idea. In addition, the present results show that subjects switch to a temporal strategy when both spatial information is reduced and such a strategy is possible, i.e., in the correlated cases with long inter-letter intervals. The switch in strategy is, of course, similar to that found by Mewhort (1974) for comparable conditions in a free-recall situation. For the free-recall case, he argued that the switch is forced because subjects must organize a sequential response, a process that required at least one dimension in addition to the identity of the material (Bryden, 1967). The remarkable feature of the switch in the present case is that it occurs even when scanning is not implicated, i.e., when a single location response - one involving neither a sequential report nor short-term storage - is required. Hill (1971) has studied temporal factors in spatial localization using non-alphabetic stimuli. He presented four or six lights for five msec in various positions of an 8 X 3 matrix. The order of presentation was not predictable, and accuracy of localization decreased as the inter-item interval was increased over a range of 5 to 45 msec, but did not recover with further increments. Thus, Hill's data are consistent with those reported here. In addition, however, Hill found that the magnitude of the reduction in localization over the critical range varied directly with the number of stimuli. Recently, Shiffrin and his colleagues (Shiffrin & Gardner, 1972; Shiffrin, Gardner, & Allmeyer, 1973) have presented data which appear to conflict with those reported here. In particular, they found no difference in localization between simultaneous and sequential displays, a result that implies little loss of spatial information in sequential cases. However, procedural differences between their work and the present experiments make precise comparisons difficult. Specifically, they used exposure times much longer than in the present case, the spatial uncertainty in their displays was considerably lower, and they permitted a correlation between space and time. RESUME
Deux experiences sur la capacity de loealiser des lettres presentees en succession dans une rangee horizontale. Les lettres sont presentees pendant 5 ms, l'intervalle entre les lettres pouvant s'etendre de 0 a 200 ms. La premiere experience montre que le rendement diminue a mesure que grandit I'intervalle jusqu'a 50 ms, pour s'ameliorer ensuite avec raccroissement subsequent de rintervalle. La seconde experience revele que I'amelioration observee pour des intervalles depassant 50 ms n'est pas authentique (elle disparait quand on minimise la correlation). Ces donnees sont interpretees suivant un modele d'organisation sequentielle du rapport qu'on demande au sujet.
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REFERENCES BRYDEN, M.P. Accuracy and order of report in tachistoscopic recognition. Canad. J. Psychol, 1966, 20, 262-272. BRYDEN, M.P. A model for the sequential organization of behaviour. Canad. J. Psychol, 1967, 21, 37-56. CORNETT, S. Tachistoscopic recognition of alphabetic and geometric sequences. Unpublished MA thesis, Queen's University at Kingston, 1972. HERON, W. Perception as a function of retinal locus and attention. Am. J. Psychol., 1957,70, 38-48. HILL, J.W. Processing of tactual and visual point stimuli sequentially presented at high rates. /. exp. Psychol, 1971, 88, 340-348. MCCRAHY, J.W., & HUNTER, W.S. Serial position curves in verbal learning. Science, 1953, 117, 131-134. MAYZNER, M.S., & TRESSELT, M.E. Visual information processing with sequential inputs: a general model for sequential blanking, displacement, and overprinting phenomena. Ann. N. Y. Acad. Sci., 1970,169, 599-618. MEWHORT, DJ.K. Sequential redundancy and letter spacing as determinants of tachistoscopic recognition. Canad, J. Psychol, 1966, 20, 435-444. MEWHORT, D.J.K. Retrieval tags and order of report in dichotic listening. Canad. J. Psychol, 1973, 27, 119-126. MEWHORT, D.J.K. Accuracy and order of report in tachistoscopic identification. Canad. J. Psychol, 1974, 28, 383-398. MEWHORT, D.J.K., & CORNETT, S. Scanning and the familiarity eBect in tachistoscopic recognition. Canad. J. Psychol, 1972,26, 181-189. SCHEERER, E. Order of report and order of scanning in tachistoscopic recognition. Canad. J. Psychol, 1972, 26, 382-390. SHIFFRIN, R.M., & GARDNER, G.T. Visual processing capacity and attentional control. /. exp. Psychol, 1972, 93, 72-82. SHIFFRIN, R.M., GARDNER, G.T., & ALLMEYER, D.M.
On the degree of attention and
capacity limitations in visual processing. Percept. Psychophys., 1973, 14, 231-236. (First received 29 July 1975) (Date accepted 2 October 1975)
Two experiments investigated the ability to locate letters presented successively along a horizontal row. The letters were displayed for 5 msec, and the inter-letter interval varied between 0 and 200 msec. In the first experiment, localization decreased as the inter-letter interval was increased to 50 msec. With further increments in inter-letter interval, performance improved. In the first experiment, however, there was a correlation between the positions of the letters in space and in time. The second experiment indicated that the recovery in spatial localization with inter-letter intervals greater than 50 msec is spurious, i.e., it does not occur if the correlation is minimized. The data were discussed in terms of a recent model for the sequential organization of
report. subjects are shown a number of letters or digits simultaneously and are required to report as many of the items as possible. Here, the subject is provided with a multi-element display in parallel but is required to construct a sequential report. When subjects in such a task are free to report in any order, they begin with the left item and proceed toward the right (Bryden, 1966; Heron, 1957; Mewhort, 1966), and accuracy of report is superior for items from the left side of the display (Heron, 1957). A number of attempts have been made to explain the accuracy and order of report data. Heron (1957) proposed a model in which a postexposural attentional mechanism is responsible simultaneously for both the left-to-right order of report and the left-side superiority. He suggested that the stimulus exposure activates neural traces which are subject to decay and that an attentional mechanism "scans" the traces from left-toright. Responses are produced in the order used by the scan. Thus, the model accounts for left-to-right reporting as a direct consequence of ordered processing; the left-side superiority emerges because right-side traces fade before responses can occur. When the order of report is constrained by the experimenter, the pattern of results is more complex. Suppose subjects are directed to report either from left to right or from right to left. When the cue specifying IN A SIMPLE TACHISTOSCOPIC TASK,
"This research was supported by a grant from the National Research Council to DJKM (Grant No. AP-318), and the paper is based on an unpublished MA thesis by PJH. Address for reprints: Paul J. Hearty, Department of Psychology, Queen's University, Kingston, Ontario K7L 3N6. 348 CANAD. J. PSYCHOL/REV. CANAD. PSYCHOL. 1975, 29 (4)
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direction of report is presented prior to, or immediately after, the stimulus exposure, accuracy is better for the side reported first. When the cue is delayed, however, a left-side superiority emerges regardless of the direction of report (Cornett, 1972; Scheerer, 1972). If the direction of the scan can be controlled by instructions, and if subjects use a left-to-right scan by default whenever the cue has been delayed too long, the data can be explained in terms of Heron's ideas (e.g., Scheerer, 1972). Here, the side superiorities with immediate instructions would reflect directed scanning, and the left-side superiority, which emerges with the delayed cue, would reflect the default condition. However, Mewhort and Cornett (1972) have shown that even with an immediate right-to-left cue, instructions do not control the direction of the scan. A satisfactory model must explain the effect of instructions without assuming that they alter the direction of the scan. Mewhort (1973; Mewhort & Cornett, 1972) proposed a two-stage model which distinguishes between the input scan and rehearsal or response organization processes within short-term memory. He argued that the scan operates only from left to right and that it orders spatially discrete, but simultaneous, items along a temporal dimension. Thus, the scan converts the spatial dimension to a temporal one in preparation for rehearsal in short-term memory. The rehearsal process involves an iterative refreshing of item representations and requires material which has been ordered on a temporal dimension. The model permits material to be sustained without rehearsal for only a brief time, and once rehearsal has been initiated, the organization for rehearsal is fixed. For a free-recall case, the model assumes that the scan loads short-term memory and that rehearsal follows the organization implied by the scan. Thus, left-to-right reporting and a left-side superiority are direct results of an interaction of iterative rehearsal with loss during report. For the ordered-recall case, particularly report from right to left, the situation is more complicated. Here, the scan loads memory by ordering items (from left to right) on a temporal dimension. If the cue is available, the subject can ignore the order implied by the scan, and rehearsal follows the order dictated by the cue. However, when the cue has been delayed, the subject must initiate rehearsal, and as a result, rehearsal follows the only organition available, that established by the scan. Consequently, the left-side superiority emerges. To test the model, Mewhort (1974) presented a row of letters sequentially starting either from the left or from the right. The time between successive letters was 0, 25, 50, or 100 msec. The materials consisted of either first-order or fourth-order approximations to English, and subjects were free to report in any order. For left-to-right presentations, the order
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of report was left to right at all inter-letter intervals. For right-to-left presentations, however, the order of report varied with the inter-letter interval. As the interval increased from 0 to 50 msec, order of report changed from a left-to-right direction to one matching the direction of presentation, i.e., from a spatial to a temporal organization. Mewhort argued that increasing the inter-letter interval disrupted the scan and forced the rehearsal mechanism to adopt the temporal organization implicit in the order of presentation. Accuracy data, particularly variation in the size of the familiarity effect, were consistent with die argument. The assumption that increasing the inter-letter interval disrupts the scan is crucial to Mewhort's (1974) test of the model. He recognized the importance of the assumption but was unable to support it empirically. Instead, he suggested that the manipulation deprives the scanning mechanism of necessary spatial information and thus, attempted to explain, on theoretical grounds, why it might disrupt the scan. The present experiments were designed to furnish empirical support for the latter position. Specifically, the experiments measure the usability of spatial information under conditions comparable to those of Mewhort (1974), and if his explanation is correct, increasing the inter-letter interval should reduce the subjects' ability to access spatial information. EXPERIMENT I
In the first experiment, 8 letters of 5 msec duration were presented sequentially along a horizontal row. The direction of presentation was either left to right or right to left. The inter-letter interval varied between 0 and 200 msec, and the subjects were required to locate a designated letter in the display. The presentation conditions are almost identical with those used by Mewhort (1974), and if his explanation of scan disruption is correct, increasing the inter-letter interval should result in lower spatial localization. Method Subjects. Eight volunteers from the undergraduate and graduate classes of Queen's University served as Ss. Materials and Apparatus. One hundred and ninety-two 8-letter sequences were generated. The sequences were random with the restriction that no letter was repeated within any sequence. The letters were displayed on a cathode-ray tube (CUT) supplied with fast-decay phosphor and slaved to a PDp/8e computer. A letter was presented by brightening the appropriate dots in a matrix of 7 rows and 5 colums on the CRT. A complete letter sequence, centered on the display, subtended a visual angle of approximately 4° by 28'.
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Procedure. On each trial, a fixation dot appeared on the centre of the display, and E called out a letter. The letter display followed, and the letter designated by E was always present in the display. The S was required to indicate the spatial position of the designated letter within the 8-Ietter display and responded with a digit 1 to 8. The response referred to the location of the designated letter counting from the left. The letters of the display were presented one at a time and in sequence starting from either the left or the right. Each letter was exposed for 5 msec, and the interletter interval was one of 0, 50, 100, or 200 msec. Before starting the experiment proper, 40 practice trials were administered. The latter were identical with the trials to follow. Instructions to Ss were illustrated with the practice trials and stressed that the letter designated by E was always present in the display but could take any of the 8 spatial positions. Design. The design was within-subjects. Each S received 192 trials, 3 replications for the factorial combination of three variables: direction of presentation, inter-letter interval, and spatial position of the critical letter. Each trial used a different letter sequence. Both the pairing of letter sequences to conditions and the order in which conditions occurred were determined randomly. RESULTS
An accuracy measure was taken by computing the number of responses which matched the position of the designated letter. In all cases, accuracy was slightly better for letters presented at the ends and near the fixation point, i.e., accuracy across the stimulus array took the symmetrical Wshape typical of partial-report studies. In Figure 1, the percentage of correct responses is shown for each combination of inter-letter interval and direction of presentation. As the inter-letter interval was increased from 0 to 50 msec, accuracy of localization decreased sharply. With further increases in the inter-letter interval, however, performance improved. An analysis of variance indicated that the effect of inter-letter interval was highly significant, F(3,21) = 16.88, p < .0001. In addition, at an interval of 0 msec, right-to-left presentations yielded slightly higher performance than those from left to right. However, at longer intervals, the relation reversed. Thus, although both directions of presentation showed essentially the same trend, there was a significant interaction of inter-letter interval with directions, F(3,21) = 4.95, p < .0096. Performance in the present task is necessarily limited by the subjects' ability to identify the items. If performance were affected seriously by the limitations, the task could not be used as a method for measuring spatial localization. In such a case, however, performance should tend to chance (12.52). It is clear from Figure 1 that performance is well above chance. Nevertheless, it is possible that subjects fail to identify the material on a small proportion of the trials. In particular, many of the errors may reflect a failure to identify the material rather than a failure to locate it correctly.
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100
75
O
I
50
25
LEFT-TO-RIGHT RIGHT-TO-LEFT 50
100
200
INTER-LETTER INTERVAL (MSEC)
FIGURE 1. Percentage correct for each combination of interletter interval and direction of presentation (Experiment i).
If this were so, incorrect responses should tend to be distributed evenly across the response alternatives. To consider the latter possibility, an error analysis was conducted. In the analysis, the number of errors was calculated as a function of the distance from the correct response. For example, suppose the correct position is 5 and that the response is either 3 or 7. In such a case, the distance is 2. Because the number of errors varies across conditions, the errors at each distance are expressed as a proportion of the total number of errors in the condition (see McCrary & Hunter, 1953). Table i shows the error distributions for each condition. In addition, the table shows the chance distribution for the condition and the deviation of the data from chance. The chance distributions were calculated on the assumption that all errors are equally likely, and because the total number of errors varies across the conditions, separate distributions were calculated for each. As is clear from the table, the bulk of the errors are concentrated close to the correct item. Thus, the errors indicate a failure of precise localization.
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TABLE I PROPORTION OF TOTAL ERRORS AS A FUNCTION OF DISTANCE (ROW 0 CONTAINS THE DATA OBTAINED, ROW C SHOWS THE CHANCE CALCULATION, AND ROW D SHOWS THE DIFFERENCE)
Distance Direction
ILI
LR
0
LR
50
LR
100
LR
200
RL
0
RL
50
RL
100
RL
200
0 C D 0 C D O C D O C D O C D O C D O C D O C D
1
2
.588 .273 .315 .646 .270 .377 .806 .267 .539 .833 .271 .562 .483 .262 .221 .566 .267 .299 .743 .272 .471 .922 .266 .655
.162 .212 -.050 .162 .227 -.065 .075 .226 -.151 .100 .243 -.143 .167 .205 -.038 .255 .229 .026 .135 .239 -.104 .039 .244 -.204
3
4
.059 .103 .143 .164 - . 0 6 1 —.084 .071 .071 .143 .188 -.117 -.072 .030 .045 .143 .200 -.171 -.098 .000 .033 .214 .143 -.214 -.110 .083 .133 .143 .162 -.029 -.060 .057 .066 .143 .185 -.119 -.086 .081 .027 .143 .201 -.174 -.062 .000 .039 .143 .207 -.168 -.143
5
6
7
.074 .122 - .048 .040 .098 - .058 .030 .085 - .055 .000 .071 - .071 .133 .124 .010 .038 .101 - .063 .014 .085 - .071 .000 .078 - .078
.015 .074 -.059 .010 .059 -.049 .015 .060 -.045 .033 .043 -.010 .000 .081 -.081 .019 .057 -.038 .000 .046 -.046 .000 .042 -.042
.000 .013 -.013 .000 .016 -.016 .000 .019 -.019 .000 .014 -.014 .000 .024 -.024 .000 .019 -.019 .000 .014 -.014 .000 .020 -.020
An additional point is of interest. During interviews after the experiment, Ss indicated that at shorter inter-letter intervals (especially 0 msec), they attempted to see all of the display and to estimate the spatial position of the designated letter. Thus, at the 0 msec condition, Ss attempted to follow the intent of the instructions. However, with longer inter-letter intervals (especially 100 and 200 msec), Ss reported a different strategy. Specifically, they noted the direction of presentation and counted the items until they arrived at the designated letter. If presentation was from left to right, Ss started at 1 for the first item; if presentation was from right to left, they started at 8 and counted backwards. In short, when dealing with the longer intervals, Ss used a temporal strategy. DISCUSSION
As the inter-letter interval was increased from 0 to 50 msec, accuracy of localization decreased sharply. Such a result is consistent with the loss
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of spatial information required by Mewhort's explanation of scan disruption. However, the data show that increasing the inter-letter interval beyond 50 msec resulted in a sharp recovery in performance. If the data at the longer temporal intervals reflect an increase in spatial resolution, they are inconsistent with the scan disruption idea. However, it is not clear that the data at these intervals do reflect spatial localization. The subjects' reports indicate that they adopted a temporal strategy at the longer intervals. If the subjects' reports represent an accurate description of their behaviour, performance at the longer inter-letter intervals does not reflect a recovery in spatial localization but rather reflects the use of a temporal strategy.
EXPERIMENT II
The second experiment was designed to determine if the recovery in performance with inter-letter intervals greater than 50 msec reflects a temporal strategy. The Ss in the first experiment reported a simple counting strategy. In effect, their strategy exploits the correlation between spatial position and temporal position inherent in the presentation conditions of the first experiment. If the counting technique is responsible for the recovery, reducing the correlation should reduce the improvement. For example, if a number of random orders of varying minimal correlations were used, such exploitation should not be possible. Thus, at intervals greater than 50 msec, a recovery in performance should occur for left-tpright and for right-to-left presentations, but not for random presentations.
Method Subjects. Subjects were 18 volunteers from the graduate and undergraduate classes of Queen's University. Materials and Apparatus. Materials and apparatus were the same as in the first experiment. Procedure. The procedure was basically the same as in the first experiment. However, two differences should be noted. First, a random presentation condition was used in addition to the left-to-right and the right-to-left conditions of the first experiment. Second, each subject received only one of the three orders of presentation, i.e., the variable was administered between subjects. The task for all subjects was identical with that in thefirstexperiment. For the random presentation group, 24 pseudo-random orders of presentation were used. In generating the random orders, three precautions were observed. First, within any order, the spatial position did hot match the temporal position. Second, across the 24 orders, no more than three matches in spatial-temporal position were permitted for any pair of orders. Finally, orders which produce "blanking" (Mayzner & Tresselt, 1970) were excluded. Although the orders are not strictly random, the correlation
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100
75
50
25 LEFT-TO-RIGHT RIGHT-TO-LEFT RANDOM 50
100
200
INTER-LETTER INTERVAL (MSEC)
FIGURE 2. Percentage correct for each combination of interletter interval and direction of presentation (Experiment n ) . between spatial position and temporal position is minimal, and because 24 orders are involved, subjects should not be able to develop any strategy based on a spatiotemporal correlation. Design. The design was between-within subjects. Each S received 192 trials, 6 replications for the factorial combinations of 2 within-subjects variables, inter-letter interval, and stimulus position probed, for one level of the direction variable. Subjects were randomly assigned to direction of presentation groups. Within a group of subjects, both the pairing of letter sequences to conditions and the order in which conditions occurred were determined randomly. RESULTS
Responses were scored in the same manner as in thefirstexperiment. Figure 2 shows the percentage of correct responses for each combination of inter-letter interval and direction of presentation. Figure 2 indicates that, as the inter-letter intervals was increased from 0 to 50 msec, localization decreased for all conditions. However, as the interval was increased
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further, there was a sharp increase in performance for the left-to-right and the right-to-left conditions, but not for the random condition. Clearly, the pattern of results for the correlated conditions replicates that of the first experiment but is not similar to that for the random condition. An analysis of variance confirms the trends shown in the figure. In particular, the interaction of inter-letter interval with presentation condition was highly significant, F(6,45) = 9.57, p < .00005. Orthogonal comparisons were used to examine the components of the interaction. Specifically, the linear trend across the 50 to 200 msec inter-letter interval range showed a large interaction with a comparison of the random condition against the two correlated conditions, F(l,15) = 54.85. There was no interaction (linear or quadratic) across the same range with a comparison between the two correlated conditions, F(l,15) = 0.62 (linear), F(l,15) = 0.72 (quadratic). Thus, although all presentation conditions showed a steep decrease in performance as the inter-letter interval was increased from 0 to 50 msec, only the correlated conditions showed a recovery as the interval was increased further. As in the first experiment, an error analysis was conducted. The analysis is identical with that presented for the first experiment and is summarized in Table n. Again the bulk of the errors are concentrated at the positions adjacent to the correct item, and the pattern of errors reflects a failure of precise localization. An additional point deserves mention. In interviews after the experiment, subjects in the correlated conditions reported using a temporal strategy with longer intervals. Subjects in the random condition, however, reported only a spatial strategy. DISCUSSION
As the inter-letter interval was increased to 50 msec, spatial performance decreased for all directions of presentation. The decrease for the correlated conditions replicated that found in the first experiment. With further increments in inter-letter interval, however, there was a sharp improvement in performance for the correlated conditions, but not for the random, or uncorrelated, condition. These results show that, as the temporal separation between letters is increased, spatial localization decreases and does not recover. Thus, the apparent recovery in the correlated conditions, both here and in the first experiment, reflects the efficiency of a temporal strategy and not a genuine recovery in spatial localization. GENEBAL DISCUSSION
As noted earlier, Mewhort (1974) studied accuracy and order of report in a free-recall task using presentation conditions very similar to those
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TABLE II PROPORTION OF TOTAL ERRORS AS A FUNCTION OF DISTANCE (ROW 0 CONTAINS THE DATA OBTAINED, ROW C SHOWS THE CHANCE CALCULATION, AND ROW D SHOWS THE DIFFERENCE)
Distance Direction
ILI
LR
0
LR
50
LR
100
LR
200
RL
0
RL
50
RL
100
RL
200
RANDOM
0
RANDOM
50
RANDOM
100
RANDOM
200
O C D O C D O C D O C D O C D O C D O C D O C D O C D O C D O C D O C D
1
2
3
4
.537 .271 .266 .598 .275 .324 .845 .274 .571 .848 .264 .584 .567 .268 .299 .480 .264 .216 .836 .270 .566 .949 .271 .678 .630 .266 .365 .673 .248 .425 .731 .241 .490 .789 .256 .533
.250 .222 .028 .189 .235 -.045 .082 .231 -.149 .121 .242 -.121 .213 .232 -.019 .281 .226 .055 .131 .234 -.103 .034 .240 -.206 .109 .203 -.095 .167 .206 - .039 .218 .207 .011 .148 .226 -.078
.120 .180 - .060 .106 .196 - .090 .041 .197 - .156 .000 .199 - .199 .063 .186 - .123 .123 .187 - .064 .016 .191 - .174 .000 .194 - .194 .087 .168 - .081 .026 .171 - .146 .032 .168 - .136 .014 .177 - .163
.028 .143 -.115 .015 .143 -.128 .000 .143 -.143 .030 .143 -.113 .094 .143 -.048 .070 .143 -.073 .000 .143 -.143 .000 .143 -.143 .065 .143 -.078 .064 .143 - .079 .006 .143 -.136 .014 .143 -.129
5
6
.009 .056 .063 .106 -.050 -.054 .015 .061 .051 .090 -.029 -.036 .021 .010 .054 .088 -.078 -.034 .000 .000 .087 .043 -.087 -.043 .000 .039 .054 .100 -.061 -.054 .041 .006 .099 .060 -.058 -.054 .008 .008 .052 .095 -.087 -.043 .017 .000 .092 .046 -.075 -.046 .054 .054 .118 .082 -.064 -.028 .038 .026 .080 .114 -.076 -.054 .006 .006 .117 .079 -.111 -.072 .014 .014 .109 .059 -.095 -.045
7 .000 .015 --.015 .015 .011 .004 .000 .012 --.012 .000 .022 --.022 .024 .018 .006 .000 .022 --.022 .000 .015 --.015 .000 .015 --.015 .000 .020 --.020 .006 .038 - .031 .000 .045 - .045 .007 .030 - .023
of the present task. The data were presented in the context of scanning theory, and to explain the results, he suggested that increasing the temporal separation between successive letters reduces the spatial information available and deprives the scanning mechanism of a necessary spatial context. As a result, at longer inter-letter intervals the scan is disrupted, and the subjects are forced to organize the material in the order of presentation. The present experiments were designed to test one aspect of
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Mewhort's account, the suggestion that increasing the inter-letter interval reduces spatial information. The data, particularly the results for the random presentation condition, confirm the spatial loss idea. In addition, the present results show that subjects switch to a temporal strategy when both spatial information is reduced and such a strategy is possible, i.e., in the correlated cases with long inter-letter intervals. The switch in strategy is, of course, similar to that found by Mewhort (1974) for comparable conditions in a free-recall situation. For the free-recall case, he argued that the switch is forced because subjects must organize a sequential response, a process that required at least one dimension in addition to the identity of the material (Bryden, 1967). The remarkable feature of the switch in the present case is that it occurs even when scanning is not implicated, i.e., when a single location response - one involving neither a sequential report nor short-term storage - is required. Hill (1971) has studied temporal factors in spatial localization using non-alphabetic stimuli. He presented four or six lights for five msec in various positions of an 8 X 3 matrix. The order of presentation was not predictable, and accuracy of localization decreased as the inter-item interval was increased over a range of 5 to 45 msec, but did not recover with further increments. Thus, Hill's data are consistent with those reported here. In addition, however, Hill found that the magnitude of the reduction in localization over the critical range varied directly with the number of stimuli. Recently, Shiffrin and his colleagues (Shiffrin & Gardner, 1972; Shiffrin, Gardner, & Allmeyer, 1973) have presented data which appear to conflict with those reported here. In particular, they found no difference in localization between simultaneous and sequential displays, a result that implies little loss of spatial information in sequential cases. However, procedural differences between their work and the present experiments make precise comparisons difficult. Specifically, they used exposure times much longer than in the present case, the spatial uncertainty in their displays was considerably lower, and they permitted a correlation between space and time. RESUME
Deux experiences sur la capacity de loealiser des lettres presentees en succession dans une rangee horizontale. Les lettres sont presentees pendant 5 ms, l'intervalle entre les lettres pouvant s'etendre de 0 a 200 ms. La premiere experience montre que le rendement diminue a mesure que grandit I'intervalle jusqu'a 50 ms, pour s'ameliorer ensuite avec raccroissement subsequent de rintervalle. La seconde experience revele que I'amelioration observee pour des intervalles depassant 50 ms n'est pas authentique (elle disparait quand on minimise la correlation). Ces donnees sont interpretees suivant un modele d'organisation sequentielle du rapport qu'on demande au sujet.
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capacity limitations in visual processing. Percept. Psychophys., 1973, 14, 231-236. (First received 29 July 1975) (Date accepted 2 October 1975)
