Quarterly Journal of Experimental Psychology
ISSN: 0033-555X (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/pqje19
Processing order in visual perception Philip M. Merikle & Marcia J. Glick To cite this article: Philip M. Merikle & Marcia J. Glick (1976) Processing order in visual perception, Quarterly Journal of Experimental Psychology, 28:1, 17-26, DOI: 10.1080/14640747608400535 To link to this article: http://dx.doi.org/10.1080/14640747608400535
Published online: 29 May 2007.
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Citing articles: 9 View citing articles
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Date: 05 November 2015, At: 19:00
Quarterly Journal of Experimental Psychology (1976) 28, 17-26
PROCESSING ORDER I N VISUAL PERCEPTION PHILIP M. MERIKLE AND MARCIA J. GLICK
Quarterly Journal of Experimental Psychology 1976.28:17-26.
Department of Psychology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada Initial increases in the availability of items for report following tachistoscopic presentation of centrally-hated rows of seven random letters were directly evaluated by measuring report accuracy following exposure durations of 10,20, 40, 80 and 160ms. A partial-report technique was used, and each presentation of a letter row was immediately followed by the presentation of a masking stimulus. Each of 10subjects received 840 trials which reflected 24 trials for each exposure-duration by position-probed combination. The letters at both ends of a row became available for report prior to the centre letters. In addition, report of the left-most letter was consistently better than report of the right-most letter, and report of the centre item at hation improved at a more rapid rate with increased exposure duration than report of the other centre letters. This pattern of results provides support for certain components of several different previous proposals concerning the order in which individual items from multi-element displays become available for report.
Introduction One widely used technique for the study of visual information processing has been the tachistoscopic presentation of centrally-hated, multi-element displays containing alphabetic materials. A prime question to emerge from this research concerns the initial order in which the individual elements become available for report. T o date, there have been several quite different answers proposed. An influential proposal stemming originally from the work of Heron (1957) suggests that letter rows are processed in a left-to-right sequential order based upon a post-exposural perceptual mechanism which scans the visual input. The most basic finding in support of this position is that when subjects are requested to report as many letters as possible from a centrally-fixated display, it is usually found that accuracy is higher for material presented to the left of fixation than for material presented in the right visual field (e.g. Bryden, 1960, 1966, 1968; Heron, 1957; Merikle, Lowe and Coltheart, 1971; Smith and Ramunas, 1971). This left-to-right scanning model, or closely related variants, has also been shown to be consistent with a wide variety of findings related to the perception of multielement displays (e.g. Bryden, 1967; Harcum, 1967; Mewhort, 1974; Mewhort, Merikle and Bryden, 1969). Despite the large amount of evidence consistent with the scanning model, a number of investigators have asked whether the evidence in favour of an initial left-to-right scanning process may actually reflect a much later operating left77
Quarterly Journal of Experimental Psychology 1976.28:17-26.
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P. M. MERIKLE AND M. J. GLICK
to-right, report-order bias (e.g. Ayres, 1966; Merikle, Lowe and Coltheart, 1971; Smith and Ramunas, 1971; Winnick and Bruder, 1968). T o circumvent possible biases introduced by requesting subjects to report as many items from a display as possible, several investigators (e.g. Merikle, Lowe and Coltheart, 1971; Smith and Ramunas, 1971)have used partial-report techniques which only require report of a single probed item. Under such conditions, it has been repeatedly found that accuracy across position can be best described as a W-shaped function; there is relatively good report for end and centre letters but much poorer report of the other items in a row (e.g. Averbach and Coriell, 1961; Haber and Standing, 1969; Merikle, Lowe and Coltheart, 1971; Smith and Ramunas, 1971). T h e general symmetry of these accuracy functions has led to the suggestion that the initial availability of items for report is determined primarily by the quality of stimulus information. Such information is best for the centre items due to retinal acuity and the end items due to reduced lateral masking from adjacent items (e.g. Haber and Standing, 1969). T h e partial-report studies have led some current proponents of the scanning model to abandon its strong form in favour of a weaker version. When first proposed (Heron, 1957), the scanning model assumed that a left-to-right scan was necessary for the initial perception of letter sequences. T h e recent weak versions of the scanning model imply that a left-to-right scan may only occur when multiple-item report is requested (Mewhort, 1974; Mewhort and Cornett, 1971; Scheerer, 1972). Thus one is left in the potential position of stating that the initial availability of items for report is due to scanning when multiple-item report or free recall is required but is due to stimulus factors when only a single item is probed following the presentation of a display. Yet another proposal concerning initial availability has been derived from experiments involving backward masking of multi-element displays. When a display is followed immediately by a patterned masking stimulus, there is a selective effect under both free-recall and single-item probe conditions ; report accuracy for centre letters is lowered much more by the masking stimulus than report of the items at both ends of a display (Butler and Merikle, 1973; Henderson and Park, 1973 ; Merikle, 1974; Merikle and Coltheart, 1972; Merikle, Coltheart and Lowe, 1971). Under the assumption that the items which are processed first should be least affected by a masking stimulus, it has been argued that selective masking indicates that end items become available for report prior to centre items (Merikle, 1974; Merikle and Coltheart, 1972; Merikle, Coltheart and Lowe, 1971) and that this ends-first availability reflects a processing strategy employed whenever task demands require analysis of the entire display (Butler and Merikle, 1973). One aspect common to all studies designed to determine the initial order in which items become available for report is that initial availability has been inferred from accuracy functions obtained following relatively long exposure durations (i.e. IOO to 150 ms). Given these long exposure durations and the variety of experimental techniques, it is perhaps not too surprising that there is considerable disagreement as to whether left-to-right scanning, ends-first processing, or factors determining the quality of stimulus information provides the best explanation of the initial availability of items for report.
Quarterly Journal of Experimental Psychology 1976.28:17-26.
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T h e present study was designed to assess initial processing order directly. The exposure duration for linear multi-element displays was systematically varied and each presentation of a display was immediately followed by the presentation of a masking stimulus. As in previous studies (Mewhort el al., 1969), it was assumed that initial processing order is reflected by the report of items coded into a nonmaskable state. T o avoid possible ordering differences attributable to the organization of a multiple-item report, a partial-report procedure was used. Thus the merits of the various proposals concerning the order in which items are initially processed were evaluated by measuring the increases in accuracy at the different positions in a display that occur with increased exposure duration. If certain items are always processed prior to others, accuracy of report for these items should begin to increase at shorter exposure durations than are necessary to increase accuracy for the other items in a display.
Method Materials and design A set of 210 sequences of seven random letters was prepared using all letters except I , 0, Q, W, and X. The only constraints imposed in generating the set were that no letter was repeated in a sequence and each of the 21 letters was used 10times in each of the seven positions. The sequences in the set were arranged into four different random orders. Five different exposure durations, which represented equal intervals on a logarithmic scale, were used. These were 10,20,40,80 and 160ms. When the five exposure durations were combined with the seven different positions in the displays, 35 different exposureduration by position-probed combinations were obtained. Each of the 35 combinations was used six times to form a random ordered sequence of 210 trials, and four different random orders of the 210 trials were generated. Each subject received a total of 840 presentations. For each subject the four letter-sequence orders and the four exposure-probe orders were paired randomly within the restrictions that each subject received all orders and each possible combination of letter-sequence and exposure-probe orders was used either two or three times in the entire experiment.
Procedure The stimuli were presented on a Tektronix monitor (Model 602) with a P-4 phosphor. The subjects viewed the screen by looking through a binocular hood attached to a translucent tube affixed to the monitor's screen. The letter sequences subtended a visual angle of approximately 4'30 degrees horizontal and 0.48 degrees vertical. The presented letters resembled those of the Hewlett-Packard light emitting diode set, except for slight alteration in the letters C and D. On each trial, the subjects first viewed a pre-exposure field containing two small fixation dots located above and below the position of the centre letter. The fixation dots were presented for I s, and they were followed by the presentation of a letter sequence. A masking stimulus consisting of seven ampersands (Mewhort et al., 1969) located in each of the positions occupied by a letter was presented immediately following the presentation of each letter sequence. The partial-report probe consisted of bars located above and below one of the ampersands. The ampersands and the bar markers remained on the screen until the subjects attempted to report the letter which had appeared in the position indicated by the probe. The experimenter entered each response on a teletype machine and the noise of the machine served as a ready signal for the presentation of the new pair of fixation dots. Since the experimenter and a subject were in separate rooms, all communication, except for
P. M. MERIKLE AND M. J. GLICK
20
the initial instructions, was via an intercom system. The subjects were not given any information concerning accuracy until all trials were completed. Each subject was given 420 trials on each of two separate days. An experimental session on a given day lasted approximately 1.5 h, including an approximate ro-min rest after the first 210 trials.
Subjects The 10 subjects (6 male) were undergraduate students at the University of Waterloo. They were paid $6.00 for their participation.
Quarterly Journal of Experimental Psychology 1976.28:17-26.
Results Scores for each subject were determined by summing performance across the 24 trials for each condition. These scores were submitted to a 5 x 7 analysis of variance to assess the effects of exposure duration and position in the display.
"
+
t
8
m W e
W c
2 a C
P
5
Position in display
.-.,
FIGURE I . Mean percentage correct at each position in the displays as a function of exposure 10; 0 -0, 20; .-B, 40; 0-0, 80; 160. duration. A-A,
T h e mean percentages correct at each position for the five exposure durations are presented in Figure I. As an inspection of this figure suggests, there were highly significant overall effects due to both the improvements in report which occurred with increased exposure duration, F = I 19.5,df = 4,36,P < 0.001,and the large differences in accuracy across the seven positions, F = 63.1,df = 6,54, P < 0.001. More importantly, the presence of a significant exposure-duration by position interaction confirms the suggestion given in Figure I that increases in exposure duration lead to differential effects upon accuracy at the various positions in the displays, F = 17-9,df = 24, 216,P < 0.001. At the shorter exposure durations (10,20 and 40 ms) it is primarily report of the end positions ( I and 7) which benefits from the increased presentation time. Report of the
PROCESSING ORDER IN VISUAL PERCEPTION
21
centre positions (2-6) does not improve appreciably until exposure duration is increased to 80 and 160 ms. Thus the characteristic W-shaped accuracy function across position which has been shown in numerous previous studies (e.g. Averbach and Coriell, 1961; Haber and Standing, 1969; Merikle, Lowe and Coltheart, 1971) is seen in the present data not to emerge until after a display has been presented for a considerable period of time.
Quarterly Journal of Experimental Psychology 1976.28:17-26.
70
Exposure dumtion Ims)
FIGURE 2. Mean percentage correct at each exposure duration for each of the seven positions in the displays ( I = left-most item; 7 = right-most item). I ; M-M, 2 ; 0-0, 3; 4-4, 4; A-A, 5 ; A-A, 6 ; 0-0, 7.
These differential rates of increase in report accuracy for the different display positions can be seen somewhat more easily in Figure 2 which shows the same data presented in Figure I but with accuracy at each position plotted as a function of exposure duration. It is evident from an inspection of Figure 2 that report of the end positions is always better than report of the centre positions. More importantly, at the shorter exposure durations (10 and 20 ms), only report of the end positions is much above the chance level of performance (4.8 f 3.6 for the 1% confidence interval). However, Figure 2 also indicates that report of the left-most item (I) was always superior to report of the right-most item (7). For the centre positions (2-6), all positions appear to be above the chance level of performance at the 40-ms exposure, but thereafter report of the item located at the point of fixation improves at a faster rate than report of the other centre items (2, 3, 5 and 6). It also appears that report of the centre items to the right of fixation ( 5 and 6) improves more rapidly with increases in exposure duration than does report of the centre items to the left of fixation ( 2 and 3). The above differences were supported by the results of a set of orthogonal comparisons of the linear and quadratic components of the position functions shown in Figure 2. T h e most meaningful comparisons involve the linear components since interactions are indicative of differential rates of increase in report
Quarterly Journal of Experimental Psychology 1976.28:17-26.
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P. M. MERIKLE AND M. J. GLICK
probability across exposure duration. The four most important contrasts are the following: (a) ends (I and 7) vs. centres (2-6), (b) left-end (I) vs. right-end (7), (c) middle (4) vs. sides (2, 3, 5 and 6), (d) left-side (zand 3) vs. right-side ( 5 and 6). For the linear components, the comparisons revealed a significantly more rapid increase for end than centre positions, F = 178.1, df = I, 9, P < 0.001, and this interaction accounted for approximately 65 yoof the total variance in the exposureduration by position interaction. I n addition, report of the item located at fixation (4)showed a greater rate of improvement than the other centre positions, F = 19.6, df = I , 9, P < 0.01, and report of the centre items to the right of fixation ( 5 and 6) improved at a marginally more rapid rate than report of the centre items to the left of fixation (z and 3), F = 4-79, df = I , 9, P < 0.10. Given that report of both end positions is near the chance level of performance at the 10-ms exposure duration and is approaching the asymptotic level of performance at the 160-ms exposure, it is not surprising that there was no significant difference in the linear trends for the left-end vs. right-end comparison, F < I, df = I , 9. However, there was a significant difference in the quadratic components, F = 26.5, df = I, 9, P < 0.001, which supports the suggestion in Figure z that at the middle exposure durations, where performance is constrained by neither floor nor ceiling effects, report of the left-most item improves at a more rapid rate than report of the right-most item. Of the remaining possible comparisons, the analysis only revealed significant differences in the quadratic components for the end vs. centres, F = 51.4, df = I, 9, p < 0.01, and the middle vs. sides, F = 11.5, df = I, 9, P < 0.01, comparisons. There were no other significant differences in linear or quadratic components.
Exposure duration
FIGURE 3. Estimated total number of letters available for report following each exposure duration.
Figure 3 presents estimates of the total number of letters available for report following each exposure duration. Each estimate was derived from the overall mean percentage correct for each exposure duration. Availability varied from 0.6 letters at the 10-ms exposure to 3.2 letters at the 160-ms exposure duration. These estimates are considerably lower than have been reported in other studies (e.g. Sperling, 1963), and procedural differences probably account for these lower absolute values. T h e negatively accelerated function, however, is similar to those found in other tachistoscopic-recognition experiments (cf. Coltheart, 1972).
PROCESSING ORDER IN VISUAL PERCEPTION
23
Quarterly Journal of Experimental Psychology 1976.28:17-26.
Discussion T h e present results show clear differences concerning the order in which items from linear multi-element displays become available for report with increases in exposure duration. T h e end positions become available prior to the centre positions, and within the present range of exposure durations, report of the end items is always superior to report of the centre items. In addition, there are ordering differences for both the end and centre positions. T h e left-most item is always better reported than the right-most item, and report of the item at fixation (4)improves at a more rapid rate than report of the other centre items. The data also suggest that the centre items to the right of fixation become available for report somewhat more rapidly than the centre items to the left of fixation. Since a masking stimulus followed each presentation of a target sequence, these differences in initial availability must reflect a process whereby the visual information is encoded into a non-maskable state. Perhaps what is most interesting about the present results is that they provide some support for several of the previous proposals concerning the initial order in which items become available for report. T h e inference drawn from previous studies of selective masking that the end items become available for report prior to the centre items (e.g. Butler and Merikle, 1973; Merikle, Coltheart and Lowe, 1971) is certainly well supported, T h e present data also extend these previous observations by showing that “end items” are only the left-most and right-most items in a linear display, and not the two most peripheral items to either side of fixation as has sometimes been assumed (Butler and Merikle, 1973; Coltheart, 1972). T h e existence of a “left” primacy effect is also supported by the present data. While it is clear that both ends become available for report prior to the centre items, the left-most item does have an advantage over the right-most item. I n previous studies involving relatively long exposure durations, this left-end, rightend difference has been less obvious because the availability of both end positions has been at a high level when report was measured. T h e presence of a left-end, right-end difference is consistent with previous findings showing that inverted letters in words are more easily detected at the left end than at the right end or centre positions (Bruner and O’Dowd, 1958) and the recent results reported by Sekuler, Tynan and Levinson (1973) showing that when two letters are presented in rapid sequence, the one to the left of fixation is more likely to be reported as occurring first regardless of the actual order of presentation. T h e present data, however, do not provide any additional information concerning the possible reasons for the existence of the left-end primacy effect. T h e present pattern of results also provides certain suggestions concerning the role of stimulus factors in determining the initial encoding order. Previous partial-report studies showing fairly symmetric W-shaped accuracy functions across position (e.g. Averbach and Coriell, 1961; Haber and Standing, 1969; Smith and Ramunas, 1971) suggest that the two most important stimulus factors are retinal acuity, which aids report of the centre items, and reduced lateral masking from adjacent items, which benefits report of the end items. Several aspects of
Quarterly Journal of Experimental Psychology 1976.28:17-26.
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P. M. MERIKLE AND M. J. GLICK
the present data, however, indicate that any explanation of initial encoding base solely on these factors would be inadequate. By only considering stimulus factors, there is no apparent explanation for the asymmetries in report accuracy for items at comparable locations to the left and right of fixation. I n other words, it would be difficult to explain why report of the left end is superior to report of the right end and why report of the centre items to the left of fixation is poorer than report of the centre items to the right of fixation. An explanation based solely on the stimulus factors of retinal acuity and reduced lateral masking would also predict the presence of a small W-shaped accuracy function at the shortest exposure duration necessary for report to be above the chance level of performance. T h e present data indicate that this is not the case in that the W-shaped function takes considerable time to develop and it is not present until a display has been presented for 80-160 ms. Initially, the accuracy functions are U-shaped indicating that only the end items are available for report. Given that accuracy differences across position consistent with those found at the 160-ms exposure duration in the present study occur even when there is no time limit on the processing of information from a linear display (Townsend, Taylor and Brown, 1971), it appears that the W-shaped function reflects the upper limit for report from the various positions rather than the initial encoding order. What the present data suggest is that initial encoding is based upon the manner in which the quality of stimulus information across a display develops with increases in exposure duration. T h e ends may be encoded prior to the centre items because initially the best stimulus information is available at the ends due to the reduced lateral masking from adjacent items. Any benefits attributable to greater retinal acuity may take somewhat more time to develop, especially when there is considerable lateral masking from adjacent items as there was for the centre items in the present study. Thus stimulus factors are probably quite intimately involved in determining both the order of initial encoding and the upper limits for report from the various positions, but because of different exposure durations necessary for different factors to become effective, the maximal contribution of the various stimulus factors may occur at different times following the onset of a target sequence. One proposal which the present data do not support is that initial encoding is determined by a sequential scan which proceeds serially from left to right. Although there is definitely a primacy effect for the left-most item, report of the other items to the left of fixation does not show a similar advantage. I n agreement with other recent data (Mewhort, 1974; Scheerer, 197z), the present results suggest that the overall better report of items to the left of fixation observed under free-recall instructions must reflect a process which occurs after initial encoding. Scheerer (1972) has provided a clear demonstration of this latter point by showing that even with free-recall instructions, the better report of items to the left than to the right of fixation does not occur until 1000ms following the offset of a display. T h e present data are neutral with respect to whether initial encoding into a nonmaskable state is best conceptualized as a visual or a verbal code. However, the different accuracy functions across position found with free-recall and partialreport procedures (e.g. Merikle, Lowe and Coltheart, 1971; Smith and Ramanus,
Quarterly Journal of Experimental Psychology 1976.28:17-26.
PROCESSING ORDER I N VISUAL PERCEPTION
25
1971) may be explained by assuming that initial encoding is into a non-maskable visual code. Recent findings reported by Mitchell (1972) certainly support the existence of such a visual code and show that it may persist for several seconds following the presentation of a masking stimulus. It is conceivable that partialreport procedures emphasize the maintenance of this visual code while free-recall instructions encourage subjects to transform this visual code into an auditoryverbal code. Both codes would be dependent upon the quality of stimulus information, but the auditory-verbal code would also reflect an additional left-toright ordering bias due to the strategy used to transform the visual code and/or the rehearsal strategy used to maintain the auditory-verbal code. Such a conceptualization provides an explanation of why phenomena like selective masking (Merikle, Coltheart and Lowe, I 971) and W-shaped accuracy functions (Mewhort, 1966) are found with both free-recall and partial-report procedures, and it is primarily accuracy of report for items to the left and right of fixation that varies with differences in report instructions. This report is based on an honours thesis by the second author done under the supervision of the first author. It is a pleasure to acknowledge the hospitality of the Department of Psychology at the University of Reading received by the first author during the preparation of the manuscript. Financial support was provided by Grant APA-23 I from the National Research Council of Canada and a Canada Council Leave Fellowship to the first author.
References AVERBACH, E. and CORIELL, A. S. (1961). Short-term memory in vision. Bell Systems Technical Journal, 40, 359-28. AYRES,J. J. B. (1966). Some artifactual causes of perceptual primacy. Journal of Experimental Psychology, 71, 896-901.
BRUNER, J . S. and O'DOWD, D. (1958). A note on the informativeness of words. Language and Speech,
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98-101.
BRYDEN, M. P. (1960). Tachistoscopic recognition of non-alphabetic material. Canadian Journal of Psychology, 14,78-86. BRYDEN, M. P. (1966). Accuracy and order of report in tachistoscopic recognition. Canadian Journal of Psychology,
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BRYDEN, M. P. (1967). A model for the sequential organization of behaviour. Canadian Journal of Psychology, 21,37-46.
BRYDEN,M. P. (1968). Symmetry of letters as a factor in tachistoscopic recognition. American Journal of Psychology, 81, 513-24. BUTLER,B. E. and MERIKLE, P. M. (1973). Selective masking and processing strategy. Quarterly Journal of Experimental Psychology, 25, 542-8.
COLTHEART, M. (1972). Visual information processing. In P. C. DODWELL (Ed.), Nee0 Horizons in Psychology 2 . Harmondsworth : Penguin Books. HABER, R. N. and STANDING, L. (1969). Location of errors with a post-stimulus indicator. Psychonomic Science, 17,345-6. HARCUM, E. R. (1967). Parallel functions of serial learning and tachistoscopic pattern perception. Psychological Review, 74, 51-62. HENDERSON, L. and PARK,N. (1973). Are the ends of tachistoscopic arrays processed first? Canadian Journal of Psychology, 27, 178-83. HERON,W. (1957). Perception as a function of retinal locus and attention. American Journal of Psychology, 70, 38-48.
MERIKLE, P. M. (1974). Selective backward masking with an unpredictable mask. Journal of Experimental Psychology, 103,589-91.
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MFXIKLE,P. M. and COLTHEART, M. (1972). Selective forward masking. Canadian Journal of Psychology, 26, 296-302. MERIKLE, P.M., COLTHEART, M. and LOWE,D. G. (1971). On the selective effects of a patterned masking stimulus.
Canadian Journal of Psychology, 25, 264-79.
MERIKLE, P. M., LOWE,D. G. and COLTHEART, M. (1971). Familiarity and method of report as determinants of tachistoscopic recognition. Canadian Journal of Psychology,
25, 167-74. MEWHORT, D. J. K. (1966). Sequential redundancy and letter spacing as determinants
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of tachistoscopic recognition.
Canadian Journal of Psychology,
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435-44.
MEWHORT, D. J. K. (1974). Accuracy and order of report in tachistoscopic identification. Canadian Journal of Psychology, 28, 383-97. MEWHORT, D.J. K. and CORNETT, S. (1972). Scanning and the familiarity effect in tachistoscopic recognition. Canadian Journal of Psychology, 26, I 81-9. MEWHORT, D. J. K., MERIKLE, P. M., and BRYDEN, M. P. (1969). On the transfer from iconic to short-term memory. Journal of Experimental Psychology, 81,89-94. MITCHELL, D.C. (1972). Short-term visual memory and pattern masking. Quarterly Journal of Experimental Psychology, 24, 394-405. SCHEERER, E. (1972). Order of report and order of scanning in tachistoscopic recognition. Canadian Journal of Psychology, 26, 383-90. SEKULER, R.,TYNAN, P. and LEVINSON, E. (1973). Visual temporal order: A new illusion. Science (Washington), 180,210-12. SMITH,M. C. and RAMUNAS,S. (1971). Elimination of visual field effects by use of a single report technique : evidence for order of report artifact. Journal of Experimental Psychology, 87, 23-8. SPERLING, G. (1963). A model for visual memory tasks. Human Factors, 5 , 19-31. TOWNSEND, J. T., TAYLOR, S. G. and BROWN,D. R. (1971). Lateral masking for letters with unlimited viewing time. Perception and Psychophysics,
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WINNICK, W. A. and BRUDER, G. W. (1968). Signal detection approach to the study of retinal locus in tachistoscopic recognition. Journal of Experimental Psychology, 78, 528-3
I.
Received 9 January 1975
ISSN: 0033-555X (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/pqje19
Processing order in visual perception Philip M. Merikle & Marcia J. Glick To cite this article: Philip M. Merikle & Marcia J. Glick (1976) Processing order in visual perception, Quarterly Journal of Experimental Psychology, 28:1, 17-26, DOI: 10.1080/14640747608400535 To link to this article: http://dx.doi.org/10.1080/14640747608400535
Published online: 29 May 2007.
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Article views: 15
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Citing articles: 9 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=pqje19 Download by: [University of Florida]
Date: 05 November 2015, At: 19:00
Quarterly Journal of Experimental Psychology (1976) 28, 17-26
PROCESSING ORDER I N VISUAL PERCEPTION PHILIP M. MERIKLE AND MARCIA J. GLICK
Quarterly Journal of Experimental Psychology 1976.28:17-26.
Department of Psychology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada Initial increases in the availability of items for report following tachistoscopic presentation of centrally-hated rows of seven random letters were directly evaluated by measuring report accuracy following exposure durations of 10,20, 40, 80 and 160ms. A partial-report technique was used, and each presentation of a letter row was immediately followed by the presentation of a masking stimulus. Each of 10subjects received 840 trials which reflected 24 trials for each exposure-duration by position-probed combination. The letters at both ends of a row became available for report prior to the centre letters. In addition, report of the left-most letter was consistently better than report of the right-most letter, and report of the centre item at hation improved at a more rapid rate with increased exposure duration than report of the other centre letters. This pattern of results provides support for certain components of several different previous proposals concerning the order in which individual items from multi-element displays become available for report.
Introduction One widely used technique for the study of visual information processing has been the tachistoscopic presentation of centrally-hated, multi-element displays containing alphabetic materials. A prime question to emerge from this research concerns the initial order in which the individual elements become available for report. T o date, there have been several quite different answers proposed. An influential proposal stemming originally from the work of Heron (1957) suggests that letter rows are processed in a left-to-right sequential order based upon a post-exposural perceptual mechanism which scans the visual input. The most basic finding in support of this position is that when subjects are requested to report as many letters as possible from a centrally-fixated display, it is usually found that accuracy is higher for material presented to the left of fixation than for material presented in the right visual field (e.g. Bryden, 1960, 1966, 1968; Heron, 1957; Merikle, Lowe and Coltheart, 1971; Smith and Ramunas, 1971). This left-to-right scanning model, or closely related variants, has also been shown to be consistent with a wide variety of findings related to the perception of multielement displays (e.g. Bryden, 1967; Harcum, 1967; Mewhort, 1974; Mewhort, Merikle and Bryden, 1969). Despite the large amount of evidence consistent with the scanning model, a number of investigators have asked whether the evidence in favour of an initial left-to-right scanning process may actually reflect a much later operating left77
Quarterly Journal of Experimental Psychology 1976.28:17-26.
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P. M. MERIKLE AND M. J. GLICK
to-right, report-order bias (e.g. Ayres, 1966; Merikle, Lowe and Coltheart, 1971; Smith and Ramunas, 1971; Winnick and Bruder, 1968). T o circumvent possible biases introduced by requesting subjects to report as many items from a display as possible, several investigators (e.g. Merikle, Lowe and Coltheart, 1971; Smith and Ramunas, 1971)have used partial-report techniques which only require report of a single probed item. Under such conditions, it has been repeatedly found that accuracy across position can be best described as a W-shaped function; there is relatively good report for end and centre letters but much poorer report of the other items in a row (e.g. Averbach and Coriell, 1961; Haber and Standing, 1969; Merikle, Lowe and Coltheart, 1971; Smith and Ramunas, 1971). T h e general symmetry of these accuracy functions has led to the suggestion that the initial availability of items for report is determined primarily by the quality of stimulus information. Such information is best for the centre items due to retinal acuity and the end items due to reduced lateral masking from adjacent items (e.g. Haber and Standing, 1969). T h e partial-report studies have led some current proponents of the scanning model to abandon its strong form in favour of a weaker version. When first proposed (Heron, 1957), the scanning model assumed that a left-to-right scan was necessary for the initial perception of letter sequences. T h e recent weak versions of the scanning model imply that a left-to-right scan may only occur when multiple-item report is requested (Mewhort, 1974; Mewhort and Cornett, 1971; Scheerer, 1972). Thus one is left in the potential position of stating that the initial availability of items for report is due to scanning when multiple-item report or free recall is required but is due to stimulus factors when only a single item is probed following the presentation of a display. Yet another proposal concerning initial availability has been derived from experiments involving backward masking of multi-element displays. When a display is followed immediately by a patterned masking stimulus, there is a selective effect under both free-recall and single-item probe conditions ; report accuracy for centre letters is lowered much more by the masking stimulus than report of the items at both ends of a display (Butler and Merikle, 1973; Henderson and Park, 1973 ; Merikle, 1974; Merikle and Coltheart, 1972; Merikle, Coltheart and Lowe, 1971). Under the assumption that the items which are processed first should be least affected by a masking stimulus, it has been argued that selective masking indicates that end items become available for report prior to centre items (Merikle, 1974; Merikle and Coltheart, 1972; Merikle, Coltheart and Lowe, 1971) and that this ends-first availability reflects a processing strategy employed whenever task demands require analysis of the entire display (Butler and Merikle, 1973). One aspect common to all studies designed to determine the initial order in which items become available for report is that initial availability has been inferred from accuracy functions obtained following relatively long exposure durations (i.e. IOO to 150 ms). Given these long exposure durations and the variety of experimental techniques, it is perhaps not too surprising that there is considerable disagreement as to whether left-to-right scanning, ends-first processing, or factors determining the quality of stimulus information provides the best explanation of the initial availability of items for report.
Quarterly Journal of Experimental Psychology 1976.28:17-26.
PROCESSING ORDER IN VISUAL PERCEPTION
'9
T h e present study was designed to assess initial processing order directly. The exposure duration for linear multi-element displays was systematically varied and each presentation of a display was immediately followed by the presentation of a masking stimulus. As in previous studies (Mewhort el al., 1969), it was assumed that initial processing order is reflected by the report of items coded into a nonmaskable state. T o avoid possible ordering differences attributable to the organization of a multiple-item report, a partial-report procedure was used. Thus the merits of the various proposals concerning the order in which items are initially processed were evaluated by measuring the increases in accuracy at the different positions in a display that occur with increased exposure duration. If certain items are always processed prior to others, accuracy of report for these items should begin to increase at shorter exposure durations than are necessary to increase accuracy for the other items in a display.
Method Materials and design A set of 210 sequences of seven random letters was prepared using all letters except I , 0, Q, W, and X. The only constraints imposed in generating the set were that no letter was repeated in a sequence and each of the 21 letters was used 10times in each of the seven positions. The sequences in the set were arranged into four different random orders. Five different exposure durations, which represented equal intervals on a logarithmic scale, were used. These were 10,20,40,80 and 160ms. When the five exposure durations were combined with the seven different positions in the displays, 35 different exposureduration by position-probed combinations were obtained. Each of the 35 combinations was used six times to form a random ordered sequence of 210 trials, and four different random orders of the 210 trials were generated. Each subject received a total of 840 presentations. For each subject the four letter-sequence orders and the four exposure-probe orders were paired randomly within the restrictions that each subject received all orders and each possible combination of letter-sequence and exposure-probe orders was used either two or three times in the entire experiment.
Procedure The stimuli were presented on a Tektronix monitor (Model 602) with a P-4 phosphor. The subjects viewed the screen by looking through a binocular hood attached to a translucent tube affixed to the monitor's screen. The letter sequences subtended a visual angle of approximately 4'30 degrees horizontal and 0.48 degrees vertical. The presented letters resembled those of the Hewlett-Packard light emitting diode set, except for slight alteration in the letters C and D. On each trial, the subjects first viewed a pre-exposure field containing two small fixation dots located above and below the position of the centre letter. The fixation dots were presented for I s, and they were followed by the presentation of a letter sequence. A masking stimulus consisting of seven ampersands (Mewhort et al., 1969) located in each of the positions occupied by a letter was presented immediately following the presentation of each letter sequence. The partial-report probe consisted of bars located above and below one of the ampersands. The ampersands and the bar markers remained on the screen until the subjects attempted to report the letter which had appeared in the position indicated by the probe. The experimenter entered each response on a teletype machine and the noise of the machine served as a ready signal for the presentation of the new pair of fixation dots. Since the experimenter and a subject were in separate rooms, all communication, except for
P. M. MERIKLE AND M. J. GLICK
20
the initial instructions, was via an intercom system. The subjects were not given any information concerning accuracy until all trials were completed. Each subject was given 420 trials on each of two separate days. An experimental session on a given day lasted approximately 1.5 h, including an approximate ro-min rest after the first 210 trials.
Subjects The 10 subjects (6 male) were undergraduate students at the University of Waterloo. They were paid $6.00 for their participation.
Quarterly Journal of Experimental Psychology 1976.28:17-26.
Results Scores for each subject were determined by summing performance across the 24 trials for each condition. These scores were submitted to a 5 x 7 analysis of variance to assess the effects of exposure duration and position in the display.
"
+
t
8
m W e
W c
2 a C
P
5
Position in display
.-.,
FIGURE I . Mean percentage correct at each position in the displays as a function of exposure 10; 0 -0, 20; .-B, 40; 0-0, 80; 160. duration. A-A,
T h e mean percentages correct at each position for the five exposure durations are presented in Figure I. As an inspection of this figure suggests, there were highly significant overall effects due to both the improvements in report which occurred with increased exposure duration, F = I 19.5,df = 4,36,P < 0.001,and the large differences in accuracy across the seven positions, F = 63.1,df = 6,54, P < 0.001. More importantly, the presence of a significant exposure-duration by position interaction confirms the suggestion given in Figure I that increases in exposure duration lead to differential effects upon accuracy at the various positions in the displays, F = 17-9,df = 24, 216,P < 0.001. At the shorter exposure durations (10,20 and 40 ms) it is primarily report of the end positions ( I and 7) which benefits from the increased presentation time. Report of the
PROCESSING ORDER IN VISUAL PERCEPTION
21
centre positions (2-6) does not improve appreciably until exposure duration is increased to 80 and 160 ms. Thus the characteristic W-shaped accuracy function across position which has been shown in numerous previous studies (e.g. Averbach and Coriell, 1961; Haber and Standing, 1969; Merikle, Lowe and Coltheart, 1971) is seen in the present data not to emerge until after a display has been presented for a considerable period of time.
Quarterly Journal of Experimental Psychology 1976.28:17-26.
70
Exposure dumtion Ims)
FIGURE 2. Mean percentage correct at each exposure duration for each of the seven positions in the displays ( I = left-most item; 7 = right-most item). I ; M-M, 2 ; 0-0, 3; 4-4, 4; A-A, 5 ; A-A, 6 ; 0-0, 7.
These differential rates of increase in report accuracy for the different display positions can be seen somewhat more easily in Figure 2 which shows the same data presented in Figure I but with accuracy at each position plotted as a function of exposure duration. It is evident from an inspection of Figure 2 that report of the end positions is always better than report of the centre positions. More importantly, at the shorter exposure durations (10 and 20 ms), only report of the end positions is much above the chance level of performance (4.8 f 3.6 for the 1% confidence interval). However, Figure 2 also indicates that report of the left-most item (I) was always superior to report of the right-most item (7). For the centre positions (2-6), all positions appear to be above the chance level of performance at the 40-ms exposure, but thereafter report of the item located at the point of fixation improves at a faster rate than report of the other centre items (2, 3, 5 and 6). It also appears that report of the centre items to the right of fixation ( 5 and 6) improves more rapidly with increases in exposure duration than does report of the centre items to the left of fixation ( 2 and 3). The above differences were supported by the results of a set of orthogonal comparisons of the linear and quadratic components of the position functions shown in Figure 2. T h e most meaningful comparisons involve the linear components since interactions are indicative of differential rates of increase in report
Quarterly Journal of Experimental Psychology 1976.28:17-26.
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P. M. MERIKLE AND M. J. GLICK
probability across exposure duration. The four most important contrasts are the following: (a) ends (I and 7) vs. centres (2-6), (b) left-end (I) vs. right-end (7), (c) middle (4) vs. sides (2, 3, 5 and 6), (d) left-side (zand 3) vs. right-side ( 5 and 6). For the linear components, the comparisons revealed a significantly more rapid increase for end than centre positions, F = 178.1, df = I, 9, P < 0.001, and this interaction accounted for approximately 65 yoof the total variance in the exposureduration by position interaction. I n addition, report of the item located at fixation (4)showed a greater rate of improvement than the other centre positions, F = 19.6, df = I , 9, P < 0.01, and report of the centre items to the right of fixation ( 5 and 6) improved at a marginally more rapid rate than report of the centre items to the left of fixation (z and 3), F = 4-79, df = I , 9, P < 0.10. Given that report of both end positions is near the chance level of performance at the 10-ms exposure duration and is approaching the asymptotic level of performance at the 160-ms exposure, it is not surprising that there was no significant difference in the linear trends for the left-end vs. right-end comparison, F < I, df = I , 9. However, there was a significant difference in the quadratic components, F = 26.5, df = I, 9, P < 0.001, which supports the suggestion in Figure z that at the middle exposure durations, where performance is constrained by neither floor nor ceiling effects, report of the left-most item improves at a more rapid rate than report of the right-most item. Of the remaining possible comparisons, the analysis only revealed significant differences in the quadratic components for the end vs. centres, F = 51.4, df = I, 9, p < 0.01, and the middle vs. sides, F = 11.5, df = I, 9, P < 0.01, comparisons. There were no other significant differences in linear or quadratic components.
Exposure duration
FIGURE 3. Estimated total number of letters available for report following each exposure duration.
Figure 3 presents estimates of the total number of letters available for report following each exposure duration. Each estimate was derived from the overall mean percentage correct for each exposure duration. Availability varied from 0.6 letters at the 10-ms exposure to 3.2 letters at the 160-ms exposure duration. These estimates are considerably lower than have been reported in other studies (e.g. Sperling, 1963), and procedural differences probably account for these lower absolute values. T h e negatively accelerated function, however, is similar to those found in other tachistoscopic-recognition experiments (cf. Coltheart, 1972).
PROCESSING ORDER IN VISUAL PERCEPTION
23
Quarterly Journal of Experimental Psychology 1976.28:17-26.
Discussion T h e present results show clear differences concerning the order in which items from linear multi-element displays become available for report with increases in exposure duration. T h e end positions become available prior to the centre positions, and within the present range of exposure durations, report of the end items is always superior to report of the centre items. In addition, there are ordering differences for both the end and centre positions. T h e left-most item is always better reported than the right-most item, and report of the item at fixation (4)improves at a more rapid rate than report of the other centre items. The data also suggest that the centre items to the right of fixation become available for report somewhat more rapidly than the centre items to the left of fixation. Since a masking stimulus followed each presentation of a target sequence, these differences in initial availability must reflect a process whereby the visual information is encoded into a non-maskable state. Perhaps what is most interesting about the present results is that they provide some support for several of the previous proposals concerning the initial order in which items become available for report. T h e inference drawn from previous studies of selective masking that the end items become available for report prior to the centre items (e.g. Butler and Merikle, 1973; Merikle, Coltheart and Lowe, 1971) is certainly well supported, T h e present data also extend these previous observations by showing that “end items” are only the left-most and right-most items in a linear display, and not the two most peripheral items to either side of fixation as has sometimes been assumed (Butler and Merikle, 1973; Coltheart, 1972). T h e existence of a “left” primacy effect is also supported by the present data. While it is clear that both ends become available for report prior to the centre items, the left-most item does have an advantage over the right-most item. I n previous studies involving relatively long exposure durations, this left-end, rightend difference has been less obvious because the availability of both end positions has been at a high level when report was measured. T h e presence of a left-end, right-end difference is consistent with previous findings showing that inverted letters in words are more easily detected at the left end than at the right end or centre positions (Bruner and O’Dowd, 1958) and the recent results reported by Sekuler, Tynan and Levinson (1973) showing that when two letters are presented in rapid sequence, the one to the left of fixation is more likely to be reported as occurring first regardless of the actual order of presentation. T h e present data, however, do not provide any additional information concerning the possible reasons for the existence of the left-end primacy effect. T h e present pattern of results also provides certain suggestions concerning the role of stimulus factors in determining the initial encoding order. Previous partial-report studies showing fairly symmetric W-shaped accuracy functions across position (e.g. Averbach and Coriell, 1961; Haber and Standing, 1969; Smith and Ramunas, 1971) suggest that the two most important stimulus factors are retinal acuity, which aids report of the centre items, and reduced lateral masking from adjacent items, which benefits report of the end items. Several aspects of
Quarterly Journal of Experimental Psychology 1976.28:17-26.
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P. M. MERIKLE AND M. J. GLICK
the present data, however, indicate that any explanation of initial encoding base solely on these factors would be inadequate. By only considering stimulus factors, there is no apparent explanation for the asymmetries in report accuracy for items at comparable locations to the left and right of fixation. I n other words, it would be difficult to explain why report of the left end is superior to report of the right end and why report of the centre items to the left of fixation is poorer than report of the centre items to the right of fixation. An explanation based solely on the stimulus factors of retinal acuity and reduced lateral masking would also predict the presence of a small W-shaped accuracy function at the shortest exposure duration necessary for report to be above the chance level of performance. T h e present data indicate that this is not the case in that the W-shaped function takes considerable time to develop and it is not present until a display has been presented for 80-160 ms. Initially, the accuracy functions are U-shaped indicating that only the end items are available for report. Given that accuracy differences across position consistent with those found at the 160-ms exposure duration in the present study occur even when there is no time limit on the processing of information from a linear display (Townsend, Taylor and Brown, 1971), it appears that the W-shaped function reflects the upper limit for report from the various positions rather than the initial encoding order. What the present data suggest is that initial encoding is based upon the manner in which the quality of stimulus information across a display develops with increases in exposure duration. T h e ends may be encoded prior to the centre items because initially the best stimulus information is available at the ends due to the reduced lateral masking from adjacent items. Any benefits attributable to greater retinal acuity may take somewhat more time to develop, especially when there is considerable lateral masking from adjacent items as there was for the centre items in the present study. Thus stimulus factors are probably quite intimately involved in determining both the order of initial encoding and the upper limits for report from the various positions, but because of different exposure durations necessary for different factors to become effective, the maximal contribution of the various stimulus factors may occur at different times following the onset of a target sequence. One proposal which the present data do not support is that initial encoding is determined by a sequential scan which proceeds serially from left to right. Although there is definitely a primacy effect for the left-most item, report of the other items to the left of fixation does not show a similar advantage. I n agreement with other recent data (Mewhort, 1974; Scheerer, 197z), the present results suggest that the overall better report of items to the left of fixation observed under free-recall instructions must reflect a process which occurs after initial encoding. Scheerer (1972) has provided a clear demonstration of this latter point by showing that even with free-recall instructions, the better report of items to the left than to the right of fixation does not occur until 1000ms following the offset of a display. T h e present data are neutral with respect to whether initial encoding into a nonmaskable state is best conceptualized as a visual or a verbal code. However, the different accuracy functions across position found with free-recall and partialreport procedures (e.g. Merikle, Lowe and Coltheart, 1971; Smith and Ramanus,
Quarterly Journal of Experimental Psychology 1976.28:17-26.
PROCESSING ORDER I N VISUAL PERCEPTION
25
1971) may be explained by assuming that initial encoding is into a non-maskable visual code. Recent findings reported by Mitchell (1972) certainly support the existence of such a visual code and show that it may persist for several seconds following the presentation of a masking stimulus. It is conceivable that partialreport procedures emphasize the maintenance of this visual code while free-recall instructions encourage subjects to transform this visual code into an auditoryverbal code. Both codes would be dependent upon the quality of stimulus information, but the auditory-verbal code would also reflect an additional left-toright ordering bias due to the strategy used to transform the visual code and/or the rehearsal strategy used to maintain the auditory-verbal code. Such a conceptualization provides an explanation of why phenomena like selective masking (Merikle, Coltheart and Lowe, I 971) and W-shaped accuracy functions (Mewhort, 1966) are found with both free-recall and partial-report procedures, and it is primarily accuracy of report for items to the left and right of fixation that varies with differences in report instructions. This report is based on an honours thesis by the second author done under the supervision of the first author. It is a pleasure to acknowledge the hospitality of the Department of Psychology at the University of Reading received by the first author during the preparation of the manuscript. Financial support was provided by Grant APA-23 I from the National Research Council of Canada and a Canada Council Leave Fellowship to the first author.
References AVERBACH, E. and CORIELL, A. S. (1961). Short-term memory in vision. Bell Systems Technical Journal, 40, 359-28. AYRES,J. J. B. (1966). Some artifactual causes of perceptual primacy. Journal of Experimental Psychology, 71, 896-901.
BRUNER, J . S. and O'DOWD, D. (1958). A note on the informativeness of words. Language and Speech,
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BRYDEN, M. P. (1960). Tachistoscopic recognition of non-alphabetic material. Canadian Journal of Psychology, 14,78-86. BRYDEN, M. P. (1966). Accuracy and order of report in tachistoscopic recognition. Canadian Journal of Psychology,
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BRYDEN, M. P. (1967). A model for the sequential organization of behaviour. Canadian Journal of Psychology, 21,37-46.
BRYDEN,M. P. (1968). Symmetry of letters as a factor in tachistoscopic recognition. American Journal of Psychology, 81, 513-24. BUTLER,B. E. and MERIKLE, P. M. (1973). Selective masking and processing strategy. Quarterly Journal of Experimental Psychology, 25, 542-8.
COLTHEART, M. (1972). Visual information processing. In P. C. DODWELL (Ed.), Nee0 Horizons in Psychology 2 . Harmondsworth : Penguin Books. HABER, R. N. and STANDING, L. (1969). Location of errors with a post-stimulus indicator. Psychonomic Science, 17,345-6. HARCUM, E. R. (1967). Parallel functions of serial learning and tachistoscopic pattern perception. Psychological Review, 74, 51-62. HENDERSON, L. and PARK,N. (1973). Are the ends of tachistoscopic arrays processed first? Canadian Journal of Psychology, 27, 178-83. HERON,W. (1957). Perception as a function of retinal locus and attention. American Journal of Psychology, 70, 38-48.
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MFXIKLE,P. M. and COLTHEART, M. (1972). Selective forward masking. Canadian Journal of Psychology, 26, 296-302. MERIKLE, P.M., COLTHEART, M. and LOWE,D. G. (1971). On the selective effects of a patterned masking stimulus.
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MERIKLE, P. M., LOWE,D. G. and COLTHEART, M. (1971). Familiarity and method of report as determinants of tachistoscopic recognition. Canadian Journal of Psychology,
25, 167-74. MEWHORT, D. J. K. (1966). Sequential redundancy and letter spacing as determinants
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MEWHORT, D. J. K. (1974). Accuracy and order of report in tachistoscopic identification. Canadian Journal of Psychology, 28, 383-97. MEWHORT, D.J. K. and CORNETT, S. (1972). Scanning and the familiarity effect in tachistoscopic recognition. Canadian Journal of Psychology, 26, I 81-9. MEWHORT, D. J. K., MERIKLE, P. M., and BRYDEN, M. P. (1969). On the transfer from iconic to short-term memory. Journal of Experimental Psychology, 81,89-94. MITCHELL, D.C. (1972). Short-term visual memory and pattern masking. Quarterly Journal of Experimental Psychology, 24, 394-405. SCHEERER, E. (1972). Order of report and order of scanning in tachistoscopic recognition. Canadian Journal of Psychology, 26, 383-90. SEKULER, R.,TYNAN, P. and LEVINSON, E. (1973). Visual temporal order: A new illusion. Science (Washington), 180,210-12. SMITH,M. C. and RAMUNAS,S. (1971). Elimination of visual field effects by use of a single report technique : evidence for order of report artifact. Journal of Experimental Psychology, 87, 23-8. SPERLING, G. (1963). A model for visual memory tasks. Human Factors, 5 , 19-31. TOWNSEND, J. T., TAYLOR, S. G. and BROWN,D. R. (1971). Lateral masking for letters with unlimited viewing time. Perception and Psychophysics,
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WINNICK, W. A. and BRUDER, G. W. (1968). Signal detection approach to the study of retinal locus in tachistoscopic recognition. Journal of Experimental Psychology, 78, 528-3
I.
Received 9 January 1975
