Processing Nominal Information during Visual Search*

that some focal processing is required for the development of a sufficiently detailed level of visual representation to allow for naming.

ROBERT T. SOLMAN University of New South Wales

GENERAL INTRODUCTION

ABSTRACT

Three experiments were designed to examine the processing of nominal information during a visual search. In Experiment I subjects searched for a single pre-specified target letter, and the stimulus exposure-time needed to yield a 50% (corrected for chance) level of accuracy was estimated using the PEST procedure. The results showed that the exposure-time was not influenced by the presence (in the irrelevant items) of the target's other case, and this suggested that there was no obligatory accessing of nominal information during preattentive (Neisser, 1967) processing. In Experiment n subjects searched in one condition for a single target which was specified as being one of a same-name pair of letters, and in a second condition for a target specified as one of a same-shape pair of letters. The exposure-time required for the same-name search was greater than that required for the same-shape search, and this suggested that even when nominal information might be expected to aid performance, it was either not accessed during the search, or if accessed, was relatively ineffective as a basis for selection. In Experiment m accuracy of search was compared in the sameshape and same-name conditions with a control condition. The target in this control condition was specified as one of a pair of letters not sharing a name or any special shape. The results confirmed the difference detected in Experiment 11, but they did not show any difference between performance in the same-name condition and in the control condition. This last finding indicated that subjects were unable to access the name of the target during the first stage of analysis. Consequently, it was suggested

When describing his two-stage theory of attention, Neisser (1967) emphasized that selection during the first stage of analysis was based on physical or formal information about the stimulus material, and the results of empirical investigations have generally supported this proposition (e.g., Eriksen & Collins, 1969; Von Wright, 1968). However, the results of studies by Brand (1971), Ingling (1972), and Jonides and Gleitman (1972), and those obtained by Henderson (1973) can be taken to imply that information which might be expected to depend on the development of fairly detailed visual structures may be accessed during first-stage processing (cf. Carr & Bacharach, 1976). That is, the studies by Brand, by Ingling, and by Jonides and Gleitman suggest that alpha-numeric characters may be categorized as letters or digits during the initial feature processing of the input, and the study by Henderson seems to suggest that the name of letters which are of different case may be accessed during this first stage of analysis. Dick (1974), after contrasting the Brand and the Ingling findings with results which showed that alpha-numeric characters could not be categorized prior to their identification (Dick, 1971; Nickerson, 1973) considered that (unexplained) differences between detection and identification tasks could account for the contradictory findings. That is, Brand (1971) and Ingling (1972) used a visual search task while Dick (1971) and Nickerson (1973) used an identification task. However, when one considers that the Brand, Ingling, Jonides and Gleitman, and Henderson studies can be taken to imply that first-stage processing builds up a fairly sophisticated visual rep-

*Requesls for reprints should be addressed to K.T. Solnian, c/o School of Education, University of New South Wales, P.O. Box 1, Kensington, N.S.W. 2033.

Canad. J. Psychol./Rev. Canad. Psychol., 1977,31 (4)

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specify the size and direction of the step which the experimenter must take in selecting the next exposure-time. These rules effect a bracketing and gradual convergence on the desired level of exposure-time, L,. The procedure used at any particular exposure-time to test whether the probability of a correct response is greater or less than P, was a Wald (1947) sequential likelihood-ratio test. That is, the subject is tested repeatedly, and a running total of correct reGENERAL METHOD sponses is maintained. At the completion of each trial this total is compared with the previously In Experiments i and u the PEST (parameter computed sequential test bounds (details below), estimation by sequential testing) procedure was and if it is equal to or lies outside either the used to estimate directly the stimulus exposureupper or lower bound, the probability of a cortime under each experimental condition at rect response differs from P, and a change in which the subject had a pre-specified probability exposure-time is called for. of successfully locating the target (Taylor & Creelman, 1967; Pollack, 1968). In Experiment The computation of a Wald test is tedious (see in the stimuli were exposed for a brief period of Wald, 1947, pp. 88-105), hut, as pointed out by constant duration, and the number of correct Taylor and Creelman, its application is simple. responses was recorded. In all three experi'If the current testing level were exactly L,, the ments the task was visual search, and throughout expected number of correct trials E(N(C)) = P, each a trial consisted of the following steps. After x T after T trials. The sequential test bounds are placing a stimulus card in the tachistoscope, the given by the expected number of events plus and experimenter gave a verbal ready signal upon minus a constant Nb(C) = E(N(C)) ± W, where which the subject fixated the cross and pressed a Nb(C) is the bounding number of events after T hand switch. The contact made by the switch trials, W is a constant, called the deviation limit of removed the cross and initiated the display. The the sequential test' (Taylor and Creelman, 1967, display remained for a pre-set period (constant p. 783.). The power of the test is directly related at 80 msec in Experiment in) and was replaced to the size of W. by a 50 msec mask. Subjects responded verbally, When applying PEST in Experiments 1 and n, and the experimenter recorded the response testing was begun with a level of stimulus and told the subject whether it was correct or exposure-time (100 msec) which (from previous incorrect. experience) yielded a greater proportion of correct responses than P,, and it was assumed that a The PEST Procedure Wald test had previously indicated that a hypothetical exposure-time of 1 16 msec be reParameter estimation by sequential testing (Taylor & Creelman, 1967; Pollack, 1968) is an duced by the maximum step size of 16 msec. The exposure-time was reduced in steps of 16 msec adaptive method for finding a desired level of an until a level was reached which yielded a smaller independent variable. In the present study, PEST proportion of correct responses than P,. The was used to find the level of stimulus exposure exposure-time was then increased by half the (L,) (as far as possible notation is taken from Taylor and Creelman) which yields a pre- previous step size (i.e. 8 msec). Testing at this new level could indicate that the exposure-time specified proportion of correct responses (referbe (a) reduced, or (b) increased further. In the red to as the target probability, P,). P, was in this former case the step size was again halved (i.e. case set at a value which, when corrected for the exposure-time was reduced by 4 msec), and chance performance, gave a probability of a corin the latter it remained the same (i.e. a further rect response of .500. To calculate the P, value it increase of 8 msec). The procedure was conwas assumed that guessing responses would be tinued in this manner, i.e. by halving the step size equally distributed over all response alternatives for reversals, and by leaving unchanged or (A), and as a consequence P, was set equal to .500 doubling (occasions when the step size should be + .500/A. doubled are specified by Taylor and Creelman, The PEST procedure requires testing at a series 1967) the step size for changes in the same direcof stimulus exposure-times. For each exposuretion, until a minimum step size of .5 msec was time a decision is made as to whether the probacalled for. At this point testing was terminated, bility of a correct response is greater or less than and the resulting level of stimulus exposureP,. The procedure provides a set of rules (see time was taken as the point estimate of L,, i.e. the Taylor & Creelman, 1967, for details) which

resentation, the whole notion of an initial feature processing stage followed by a stage of detailed representation may be under challenge, and the question of whether non-structural information is available during first-stage processing needs to be examined further.

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R.T. Solman

point estimate of the exposure-time required to yield the target probability, P,.

In the Henderson (1973) study subjects carried out a search through a list of items for a pre-specified target letter, and the results showed that, when the target's other case was contained in the list, search rate declined. The present experiment followed Henderson's procedure and examined the effect of the presence of a distractor item on visual search, but the stimuli in this case were displayed briefly, and accuracy (or more precisely, the exposure-time yielding a pre-specified accuracy level of 50%) rather than rate of search was the dependent measure. That is, subjects were required to search for a single target letter in a briefly displayed array, and performance was compared for conditions where the target's other case was either present or absent. Consistent with Henderson's results, it was predicted that subjects would be less accurate when the distractor was present, or with accuracy equated for the 'presence' and 'absence' conditions, that a greater stimulus exposure-time would be required in the presence condition.

2-hour practice sessions and two experimental sessions, each lasting a minimum of 1 hour and a maximum of 3 hours. One experimental session was devoted to presence and one to absence displays, and the order of these sessions was counterbalanced. The PEST procedure was used to obtain point estimates (L,) of the stimulus exposure-time required to yield the same level of accuracy (P,) for both presence and absence displays, and two of these estimates were obtained during each experimental session. Stimulus displays contained six items - presence displays contained one target, one distractor (target's other case), and four irrelevant items, and absence displays contained one target, and five irrelevant items. To simplify the search, irrelevant items were selected which were physically dissimilar from the target, i.e. for E as target, Q, q, G, g, n; and for e as target, H, h, T, t, N. The letters N and n appeared only on absence displays. The displays were constructed by mounting items at 6 of 12 clock positions (i.e. all clock positions were used, but only 6 were filled on a given trial), on an imaginary circle the diameter of which subtended a visual angle of 2.53°. Each item had a vacant position on either side, and the target appeared once in every position for both presence and absence displays, i.e. a total of 48 (2 x 2 x 12) cards was prepared. When constructing the presence displays the target and the distractor were positioned an equal number of times at the three possible separations. Subjects were instructed that it was a visual search task, and shown the appropriate target. They were not given any information about the distractor, and during practice presence and absence trials were randomly mixed. A response consisted of calling out a clock position and was taken as correct if the target position or one either side was reported. If forced to guess when specifying the position of the target, the subject had one chance in four of being correct. Therefore, a non-guessing accuracy level of 50% was represented by a performance level (Pt) of .500 + (.500/4) = .625. A deviation of ± . 125 on this target probability, and a deviation limit (W) of 2, specified a probability of a correct decision of .90 for each Wald (1947) test.

METHOD

RESULTS

Eight (6 of whom were experienced with visual search under tachistoscopic conditions) subjects searched circular arrays of items for a single target letter in the presence or absence of its other case. Four subjects searched for the target letter 'E and 4 for 'e.' Each subject attended two

Average values of the point estimates (Lt) of stimulus exposure-time required to yield an accuracy level of 62.5% (Pt) are shown in Table 1. The exposure-time for both targets 'E' and 'e' (Table 1) suggested that, if the

Apparatus

All stimuli were displayed in a Model GB Scientific Prototype Three-Channel Tachistoscope with channel luminance for fixation, stimulus, and mask of 10.8, 17.2, and 38.7 millilamberts respectively. Thefixationcard had a black cross mounted at its centre, and the mask consisted of a jumble of broken, distorted, and whole letraset letters. EXPERIMENT i: INFLUENCE OF A SAME-NAME DISTRACTOR DURING VISUAL SEARCH

Introduction

Processing nominal information during visual search

163

TABLE I

The average stimulus exposure-time (in msec) required to find the targets 'E' and 'e' in the presence or absence of their other case Different case letter Target

Present

Absent

E e

35.63 36.25

36.63 42.00

for one of two letters of different case (same-name). If nominal information can be utilized to direct the search during feature processing (first-stage analysis), and if this is an event of the same kind as target selection on the basis of shape, then the stimulus exposure-times should not differ for the two conditions.

METHOD

presence of the target's other case has any influence on the time required to find the target, it is to decrease it. However, an analysis of variance showed that the presence of the distractor had no effect on exposure-time (F(i,6) = 1.58, P > .25). These results suggest that if nominal information can be extracted during firststage processing, then either the development of the level of representation (of the stimulus material) necessary to do so is not obligatory, or (contra Henderson) the availability of this information does not interfere with the search. EXPERIMENT I i : SEARCHING FOR SAME-NAME AND SAME-SHAPE TARGETS

Introduction

The results of Experiment 1 (unlike those obtained by Henderson, 1973) failed to show any performance decrement when the target's other case was present in the irrelevant items. This indicated that the extraction of nominal information during first-stage processing, if it can occur, may not be obligatory. In Experiment 11 subjects were required to search in a condition where the availability of nominal information during the first stage of analysis might be expected to improve their performance. That is, in one condition they searched for one of two physically similar target letters (same-shape), and the stimulus exposuretime required to yield a pre-set level of accuracy was compared with that required for the same accuracy level when the search was

164

Six experienced subjects searched circular arrays of' items for a pre-specified target letter. Only one target appeared in each display, but for two of the four target conditions this target could be one of two different letters. That is, the four target conditions were F/E (same-shape), E/e (same-name), F, and e. Each subject attended two 2-hour practice sessions and four experimental sessions, each lasting a minimum of 1 hour and a maximum of 3 hours. One experimental session was devoted to each target condition, and the same-shape and same-name conditions were presented in counterbalanced order. The PEST procedure was used to obtain point estimates (L,) of the stimulus exposuretime required to yield the same level of accuracy (P,) for each of the four target conditions, and one point estimate was obtained during each experimental session. Stimulus displays contained one target and five irrelevant items. Irrelevant items were selected at random from the set O, Q, G, U, W, 1, i, t, y, j , with the restriction that each display contain three upper-case and three lower-case letters (including the target). Items were positioned at the clock positions 1,3,5, 7> 9>an .05) or for E/e and e (diff. (critical value 1.98) = 1.83, p > .05) (there was, of course, a significant difference between these groups). Since, as mentioned above, F-like and E-like features were equivalent in the context of the experiment, it was not surprising that the estimates for F/E and F did not differ. On the other hand, this was not the case in the E/e and e situation. A possible explanation of this finding is that visual confusions between e and the irrelevant items may have made a search based on e-like features more difficult than a search based on either F-like or E-like attributes. This would suggest that the significant difference found between processing-time estimates for same-name and same-shape conditions was due to the visual confusions between e and the irrelevant items' depressing performance in the E/e condition. In other words, the differences between F/E and E/e conditions might be completely accounted for in terms of the corresponding differences between the F condition and the e condition. The possibility that the performance differences observed in the shape and name conditions might simply reflect the relative difficulty of finding e as compared to F, was examined by first adjusting the data obtained in the former two conditions and then testing for a difference between the adjusted values. That is, the exposure-times obtained in the F condition were allowed to co-vary with those obtained in the F/E condition, and similarly for e with E/e. this adjustment increased the average stimulus 166

exposure-time in the shape condition from 23.00 to 23.55 msec and decreased the average in the name condition from 28.33 to 27.78 msec. A two-way analysis of variance (with shape and name conditions contrasted within subjects and their order of presentation varied between subjects) on the adjusted data, detected a difference between shape and name conditions (F( 1,3) = 13.15, P < .05). This suggests that the previously observed differences (Table 11) do, in fact, indicate an advantage of shape over name information as a basis for initial target selection. It does not, however, rule out the possibility that nominal information might be extracted during this feature processing stage. EXPERIMENT III: SEARCHING FOR SAME-NAME AND UNRELATED TARGETS

Introduction

The results of Experiment n demonstrated an advantage of information about shape over information about name as the basis for the initial selection of a target item. This finding indicated either that the name of the target letter was not accessed during the initial analysis of the input, or that if it was accessed, it provided a less efficient means of selection than the corresponding analysis of shape. In Experiment m, subjects were again required to search for targets defined by shape and by name. However, in this experiment the results obtained in the same-name condition were contrasted with results obtained in a control condition. The targets in this control condition did not share a name or any special shape. In other words, the two letters in both the name condition and the control condition were visually distinct from each other, and each target pair bore the same, or a closely similar, visual relationship to each of the irrelevant items. These conditions were considered appropriate for demonstrating an effect of initial selection using nominal information. Specifically, it was predicted than an analysis of name during feature

R.T. Solman

processing would lead to superior performance in the same-name condition. METHOD

In contrast to the procedure used in Experiments i and n where the exposure-time required to yield a 50% level of accuracy was the dependent measure, in the present study stimulus exposure-time was held constant at 80 msec and accuracy of performance was the dependent measure. Twenty-four subjects searched circular arrays of items for a pre-specified target letter. Twelve subjects searched 6-item displays and 12 searched 12-item displays. Only one target appeared in each display, but for each of the three target conditions this target could be one of two different letters. That is, the three target conditions were F/E (same-shape), E/e (same-name), and F/e (control). Each subject attended a single 2-hour session during which he/she practised for approximately 50 minutes, and after a 1 o-minute rest, searched for approximately 1 hour under experimental conditions. During practice the subject searched for 3 blocks of 48 trials with 1 block devoted to each target condition. A masked stimulus exposure-time was held constant at 200 msec for the first 24 trials in each block, and was reduced to the experimental value of 80 msec for the remaining 24. Experimental trials were administered in a similar manner, with each subject searching 3 blocks of 60 trials at a constant masked stimilus exposure-time of 80 msec. The three blocks of trials corresponding to the shape, name, and control conditions were ordered so that two subjects from each experimental group searched one of the six possible permutations. At the completion of the initial 2-hour session, subjects were asked to attend a further 1-hour session on the following day. Stimulus presentation procedures during this session were similar to those described for the previous experimental hour, but the target conditions differed. That is, performance was assessed during search for the single target letter F, the letter E, and the letter e. The 6-item stimulus displays were the same as those used in Experiment 11. The 12-item displays differed from these 6-item displays in the following ways: they contained 11 irrelevant items selected at random from the set O, Q, G, U, W, 1, i, t, y, j, with the restriction that each display contain 6 upper-case and 6 lower-case letters (including the target); and items were positioned at all 12 clock positions with the target letter (F, E, or e) appearing once at each position. Subjects were informed that it was a visual search task, that they must find the target and

position it on the clock face, and that if forced to guess, they should distribute their responses over the available clock positions. They were then familiarized with the target conditions F/E, E/e, and F/e, and told that they need not differentiate between the two members of a particular pair of targets. Before presenting a block of trials the two appropriate 12 card sets were randomly mixed, and the subjects were shown a prepared card identifying the target condition as well as being told that the other member of the target set would not be present in the irrelevant items (e.g., for V/K as target, they were told that e would not appear among the irrelevant items). A response consisted of calling out a clock position. It was taken as correct in the 6-item condition if the target position was reported, and was considered correct in the 12-item condition if the target position or one either side was reported. To aid the comparison of performance in these two conditions, the obtained data were then corrected for chance. That is, for each subject the total of incorrect responses was divided by the number of response alternatives minus one (since correct responses were obtained from one clock position in the 6-item condition and from three clock positions in the 12-item condition, there were effectively 6 and 4 response alternatives respectively), and the result was subtracted from the total of correct responses. This scoring procedure was also used during the 1-hour session in which subjects searched for the single target letters F, E, and e. When attending this session subjects were shown a card identifying the target, and they were told that the other members of the target set would not be present in the irrelevant items (e.g., for F as target, they were told that E and e would not appear as irrelevant items). RESULTS

The percentages of correct responses, for 6-item and 12-item stimulus displays in combination with the three target conditions, same-shape, same-name, and control, are illustrated in Figure 1. Observation of these results indicates (consistent with the findings in Experiment 11) that subjects found search for targets similar in shape easier than search for both the targets sharing a name and the targets sharing no name and no special shape. On the other hand, they do not appear to have found search for the targets snaring a name any easier than search for the targets sharing no name and

Processing nominal information during visual search

167

showed that performance was superior in the 6-item condition (/r(i,22) = 14.70,/*
.10). There is, therefore, no suggestion from these data that subjects are capable of using the name of the target as a means of selection during the initial analysis of the input. However, before accepting this conclusion, we mustfirstconsider the results obtained in the three single-target conditions F, E, and e. The average number of correct, responses for conditions F, E, and e are shown in Table HI. Observation of these values indicates that (consistent with a similar result in Experiment n) subjects found search for e more difficult than search for both the F and the E; that (consistent with the findings in the double-target conditions) they found search through 6-item displays easier than search through 12-item displays; and that they found the tasks of searching for Fand searching for E roughly equivalent in the 12-item displays, but in the 6-item displays searching for F appeared to be more difficult than searching for E. A two-way analysis of variance with appropriate planned comparisons confirmed the observation of poor performance in the search for e, and confirmed

R.T. Solman

TABLE III

The average number of correct responses (maximum value 60) for the combination of 6-item and 12item displays with the target conditions F, E, and e. All data have been corrected for guessing Target Identity Number of items

F

E

e

6 12

37.60 25.44

42.88 24.56

34.10 18.89

Total

31.52

33.72

26.49

the observed superior performance in the case of the 6-item displays. That is, a difference was detected between e and the average of F and E(F( 1,22) = 9.79,p < .01), and the difference between the 6-item and 12item displays was found to be significant (f(i,2 2) = 13.19, p < .01). On the other hand, however, the interaction contrast relevant to the pattern of differences in the case of F and E in the 6-item and 12-item displays failed to reach significance (F(i,22) = 3.86, .05 < p < .10). This suggested that both groups of subjects found the tasks of searching for F and searching for E roughly equivalent, and this suggestion was supported by the failure to detect a difference between these two target conditions (^(1,22) = i.g6,p > .10). This absence of a significant difference in the levels of performance observed during search for F and search for E, suggests that the similar levels of accuracy observed during search for E/e (name) and search for F/e (control), (F( 1,22) = 2.54, p > .10), do in fact indicate that subjects were unable to utilize the nominal information in the name sharing condition. However, the inferior performance observed in the target condition e does suggest (in the manner detailed in Experiment n), that the differences between F/E and the average of E/e and F/e might be completely accounted for in terms of the corresponding differences between the e condition and the average of the F and E conditions. This suggestion was examined, by first adjusting the data ob-

tained in the double-target conditions for the differences observed in the singletarget conditions, and then testing for differences between the adjusted values. That is, the average values obtained from the F and E conditions were allowed to co-vary with the data obtained in the F/E condition (i.e, (F + E)/2 with F/E), and similarly for (E + e)/2 with E/e and (F + e)/2 with F/e). As can be seen in Table iv, this adjustment reduced the difference between F/E and the average of E/e and F/e, and slightly altered the pattern of the results in the name and control conditions. When the adjusted data were analysed, however, the two-way analysis of variance showed that the differences replicated the original findings. That is, the difference between F/E and the average of E/e and F/e was found to be significant (F( 1,43) = 16.11 ,p < .01), no difference was detected between E/e and F/e (^(1,43) = .27,/) > .25), and (as was to be expected) the difference between the 6-item and 12-item displays was unchanged (F( 1,22) = 60.29,P < -oi). GENERAL DISCUSSION

The results of Experiment 1 showed that during a search for a pre-specified target letter, the presence of its other case as an irrelevant item did not interfere with the search; in Experiment 11 subjects found searching for one of a pair of target letters which shared a common shape easier than searching for one of a pair which shared a name; and the results in Experiment m replicated those obtained in Experiment 11, and in addition showed that, as a consequence of sharing a name, targets were no easier to find than when they shared no name and no special shape. There is no suggestion from the results of these three studies that subjects can access nominal information without some analysis of structure. In other words, these results support Neisser's (1967) suggestion that selection during the feature processing of the input in a visual search task is based on an analysis

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TABLE IV

The average number of correct responses (maximum value 60), for both original and adjusted data, for the 6-item and 12-item displays combined with the shape, name, and control target conditions Target Identity Number of items

F/E Original

Adjusted

Original

Adjusted

Original

Adjusted

6 12

40.00 22.00

38.38 20.39

31.40 15.00

33.26 Li. 64

32.40 17.11

32.16 18.10

Total

31.00

29.38

23.20

24.45

24.76

25.13

F/e

of physical or formal information about the stimulus material. Or, conversely, it should not be assumed (conta Carr & Bacharach, I 976) that either the results obtained in the Henderson (1973) study, or those obtained in the Brand (1971), Ingling (1972), and Jonides and Gleitman (1972) studies, indicate that the initial selection of the target can be directed by conceptual characteristics of the stimuli. How then can we explain what appears to be selection using letter name in the Henderson study, and selection using category membership in the Brand, Ingling, and Jonides and Gleitman studies? Selection Using Letter Name

Henderson found first that search rate declined if the target's other case was included in the irrelevant items, and secondly that, when search was for two targets of different case, search rate improved if the targets shared a name. He explained these findings by adopting Posner's (1969) notion of parallel processing. That is, Posner has argued that during the process of letter recognition, the analysis of shape and the analysis of name are carried out in parallel. There can be little argument with this account if it assumes that in visual search the simultaneous analyses of shape and of name occur after the initial selection of an item. Indeed, Henderson makes no attempt to specify the 'level' of abstraction of the name code, and it will, therefore, be argued that naming did

170

E/e

not take place until some level of structural representation had been achieved during the second stage of analysis. Under these circumstances, (a) the naming of the target's other case when it was present as an irrelevant item induced a competing response (see Hodge, 1973), thereby delaying its rejection as a non-target, and (b) the absence of response competition when the two different case targets shared a name facilitated the execution of the response. This is likely if we accept Neisser's (1967) description of visual search, and is supported by some aspects of Henderson's study. Thus, Neisser considered initial processing to be prone to error, and it must follow that in any search task where subjects make few errors, some irrelevent items are passing the first stage and being rejected during or after detailed analysis. Also, since subjects searched for all targets during any one experimental session, it might be expected that the probability of selecting one of these 'sometimes-relevant' irrelevant distractor items would be greater than the probability of selecting a 'never-relevant' irrelevant item (see Hodge, 1959, 1973). Consequently, (a) when the target's other case was used as a distractor, and on those occasions when it was incorrectly selected and named, delays in response would be quite likely, and (b) when search was for the target and its other case, the correct selection and naming of either letter would be more likely to facilitate a response than the correct selection and naming of either of

R.T. Solman

two different case targets not sharing a name. Selection Using Category Membership As previsouly mentioned, Brand (1971) and Ingling (1972) found a faster rate of search for letter targets in digit contexts than for the same targets in letter contexts (and vice versa for digits). They assumed that during the initial processing of the input, levels of representation were achieved which allowed alpha-numeric characters to be classified as letters or digits. However, as was pointed out by Jonides and Gleitman (1972), these results may, in fact, be explained by considering the structural features which distinguish digits as a group from letters as a group,'... symmetry about some axis is more prevalent among letters than among digits; on many type faces, letters tend to be wider than digits' (p. 457). Also, at least one feature description of the alpha-numeric characters (Sutcliffe, 1971) suggests that there are structural features which distinguish digits from letters, and it is possible that during the initial analysis or feature processing of the input, an item with these features may have been isolated when the target differed from the context. The plausible account of the category effect given above was examined and rejected by Jonides and Gleitman (1972). They found that when the target and field category differed, the measured reaction times were independent of the size of the display and that, more importantly, this category effect held for the ambiguous target character O. That is, when this character was specified as 'zero' in a context of letters, reaction times were independent of display size, but when it was specified as 'oh' reaction times increased with display size (equivalent results were obtained when the character was specified as first 'oh' and then 'zero' in a digit context). After a further study (Gleitman & Jonides, 1976), these investigators suggested that a notion of partial processing could account for the category effect. In essence, this hypothesis

suggests that when the target differs from the field items it may be selected after a crude or rudimentary analysis of the input. When target and field items are from the same category, on the other hand, a more extensive or detailed analysis of the input is required prior to the selection of the target. In support of this hypothesis they showed that less information was registered and/or retained in between-category search than in within-category search. They did, however, also point out that this did not necessarily imply that the selection of the target need be based on anything more than an analysis of structural information. Can a f eatural mechanism account for a category effect that occurs solely as a function of instruction? In principle it might. A given item could contain some features of two different categories in the sense in which a baseball belongs both to the class of white objects and of spherical ones. Given such an overlapping of feature membership, the item would be in this sense ambiguous. If the f eatural analysis isflexibleand can be guided by the subject's strategy, then instructions might by themselves produce a category-effect. If the subject looks for a zero among letters, he extracts those fewer (or perhaps easier) features that define the category, digit, processing partially and thus more quickly, (p. 287) While the results reported in this article do not bear directly upon the above version of the partial processing hypothesis, they are consistent with Neisser's (1967) view of visual search, and it will be argued that this view offers an economical alternative account of the category effect. First, it can be assumed that the probability of the target being correctly isolated during the errorprone pre-attentive (or first) stage of analysis, depends on its physical or structural relationship with the field items, and secondly, if we accept Hodge's (1959 and 1973) suggestion that the more closely an irrelevent item resembles the relevant item, the more likely it is that it will give rise to a competing response, then it is possible to explain the relatively poor performance observed in the within-category condition.

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These assumptions imply (a) that the probability of incorrectly selecting a field item is independent of whether the character O is specified as 'zero' or 'oh,' and (b) that when processed, an incorrectly selected field item is more likely to give rise to a competing response if it comes from the same category as the target. Consequently, the rejection of incorrectly selected field items will require more time in the within-category condition and reaction times will increase accordingly. Also, this description of the search process explains the superior post-search recognition observed in the within-category condition (Gleitman & Jonides, 1976). That is, it is plausible to suggest that the extra time required to reject an incorrect item from the same category as the target involves further processing. In conclusion, is should be pointed out that neither the partial processing hypothesis nor Neisser's two-stage view of the search process has adequately accounted for the fact that when the category of the field items differed from that of the target, increases in the number of field items were not accompanied by increases in reaction times (Jonides & Gleitman, 1972, Figure 1). One possible explanation of this finding is that it is a result of 'range effect' (Poulton, 1975). Specifically, the number of field items was varied randomly within subjects, and it is therefore possible that they may have adopted a constant rate of response for all displays. However, although there is some evidence to suggest that subjects do behave differently when the number of field items in a between-category search is blocked rather than varied within subjects, (Egeth, Jonides and Wall, 1972), it is not clear why a similar effect was not found in the within-category condition. Therefore, any detailed account of this aspect of the Jonides and Gleitman results will need to follow upon further empirical work, and an analysis of speed-accuracy trade-off functions (see Wickelgren, 1977) might be particularly useful.

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RESUME

Trois experiences sur le traitement de Finformation nominate pendant une recherche visuelle. Dans la premiere experience, les sujets ont a chercher une seule lettre cible predeterminee et Ton estime, avec la technique PEST, le temps d'exposition du stimulus necessaire a permettre un niveau d'exactitude de 50% (corrige pour le hasard). Les resultats montrent que ce temps d'exposition n'est pas influence par la presence (dans les items non pertinents) de I'autre cas de la cible, ce qui suggere qu'il n'y a pas necessite d'acceder a ('information nominate pendant le traitement preattentif (Neisser, 1967). Dans la seconde experience, les sujets ont a chercher une cible unique specifiee comme etant l'une de deux lettres de meme noin, dans une premiere condition, et specifiee comme etant l'une de deux lettres de meme forme dans une seconde condition. Le temps d'exposition requis est plus long dans la premiere que dans la seconde condition, ce qui suggere que meme lorsque l'information nominate serait susceptible d'aider la performance, le sujet n'y accede pas pendant sa recherche ou, s'il y accede, cette information lui sert relativement peu a faire son choix. Dans la troisieme experience, l'exactitude de la recherche observee dans les deux conditions precedentes est comparee a celle qu'on observe dans une condition de controle, ou la cible est specifiee comme etant l'une de deux lettres n'ayant ni de nom ni de forme en commun. Les resultats confirment la difference trouvee dans l'experience precedente, mais sans reveler de difference entre la condition de controle et la condition de letters de meme nom. Ce dernier resultat indique que les sujets sont incapables d'acceder au nom de la cible au cours du premier stade de leur analyse, ce qui suggererait done qu'il faut un traitement localise pour parvenir a un niveau de representation visuelle assez detaille pour pouvoir trouver le nom. REFERENCES BRAND, j . Classification without identification in visual search. Quart. J. exp. Psyrhoi, 1971, 83, 178-186 CARR, T.H., & BACHARACH, V.R. Perceptual tuning and

conscious attention: Systems of input regulation in visual information processing. Cognil. 1976,4, 281-302 DICK, A.o. Processing time for naming and categorization of letters and numbers. Percept. Psychophys., 1 97 '. 9.35°-353 DICK, A.o. Iconic memory and its relation to perceptual processing and other memory mechanisms. Percept. Psyclwphys., 1974,16, 575-596

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