Some evidence for a limited capacity parallel selfterminating process in simple visual search tasks.

An experiment was carried out in order to investigate the questions of exhaustive vs selfterminating and serial vs parallel processing in a simple visual search task. In the experiment, 1, 2 or 3 letters were placed on an imaginary circle round the fixation point. Two different letters were used, one of which was defined as the ‘signal’. Ss had to respond ‘yes’ when one or more signals were in the display, ‘no’ otherwise. The number of signals in the display was varied from ‘no signals’ to ‘all signals’.A decrease in latencies with increasing number of signals for a fixed number of elements presented was observed indicating a selfterminating search. A decrease in latencies with increasing number of elements when only signals were presented was taken as evidence for a parallel selfterminating process. A further analysis of the error data showed that a limited capacity system had to be assumed. It was shown that it was possible to construct the overall pattern of latencies over conditions from the error data obtained.     

Response latency in visual search with redundancy in the visual display

Two experiments were run to investigate the effects of redundant display items upon response latency in a visual search task. In the first study, Ss searched five-letter displays for a predesignated critical letter. Both critical and noncritical letters could be repeated in the displays. Mean response latency decreased markedly with increasing redundancy in the critical letter and was affected to a lesser extent by redundancy in the noncritical letters. In the second study, Ss were required to detect the presence of redundant letters in displays of from two to five letters, first with no information as to what letter might be repeated, then with knowledge of which letter would be repeated if the display contained a redundant letter. Response latencies in the former case were much slower than in the latter. The implications of these findings for current views of visual information processing were discussed.     

Response effects in the perception of conjunctions of colour and form

Summary Two experiments addressed the question whether visual search for a target defined by a conjunction of colour and form requires a central, serial, attentional process, but detection of a single feature, such as colour, is preattentive, as proposed by the feature-integration theory of attention. Experiment 1 investigated conjunction and feature search using small array sizes of up to five elements, under conditions which precluded eye-movements, in contrast to previous studies. The results were consistent with the theory. Conjunction search showed the effect of adding distractors to the display, the slopes of the curves relating RT to array size were in the approximate ratio of 2:1, consistent with a central, serial search process, exhaustive for absence responses and self-terminating for presence responses. Feature search showed no significant effect of distractors for presence responses. Experiment 2 manipulated the response requirements in conjunction search, using vocal responses in a GO-NO GO procedure, in contrast to Experiment 1, which used key-press responses in a YES-NO procedure. Strikingly, presence-response RT was not affected significantly by the number of distractors in the array. The slope relating RT to array size was 3.92. The absence RT slope was 30.56, producing a slope ratio of approximately 8:1. There was no interaction of errors with array size and the presence and absence conditions, implying that RT-error trade-offs did not produce this slope ratio. This result suggests that feature-integration theory is at least incomplete.     

Visual search in a forced-choice paradigm

The processing of visual information was investigated in the context of two visual search tasks. The first was a forced-choice task in which one of two alternative letters appeared in a visual display of from one to five letters. The second task included trials on which neither of the two alternatives was present in the display. Search rates were estimated from the slopes of best linear fits to response latencies plotted as a function of the number of items in the visual display. These rates were found to be much slower than those estimated in yes-no search tasks. This result was interpreted as indicating that the processes underlying visual search in yes-no and forced-choice tasks are not the same.     

Age and the selectivity of visual information processing

Two experiments were conducted to assess age differences in the selectivity of visual information processing. Selectivity was measured by the amount of interference caused by nontarget letters when subjects detected a target letter in a visual display. In both experiments, young and elderly groups participated in search and nonsearch conditions; in the search condition targets appeared anywhere in the display, whereas in the nonsearch condition targets were confined to the center position of the display. In the first experiment, subjects were assigned to either condition for two sessions of testing, and in the second experiment each subject participated in both conditions. In both experiments nontargets produced larger interference effects for old compared to young adults in the search condition but not in the nonsearch condition. The obtained pattern of age effects could not be explained by age-related reductions in parafoveal acuity. The findings indicate that the magnitude of divided-attention deficit increases with age, whereas focused-attention deficits are unaffected by aging.     

Visual and verbal coding in the interhemispheric transfer of information

A manual response was used to record a decision of ‘same’ or ‘differ’ on two visual signals which were presented simultaneously, either both to the same hemisphere or each to different hemispheres. The signals were drawn from the set of letters A, B, a, b. Eight subjects were required to match on the basis of the names of the letters (irrespective of whether they were capitals or lower case) and eight subjects were required to match on the basis of the visual identity of the signals.Reaction times were measured for ‘same’ and ‘differ’ decisions.This experiment extends a previous study by the same authors (Davis and Schmit 1971). The previous finding, that when two signals are presented each to different hemispheres, reaction times are shorter than when both are presented to the same hemisphere, was confirmed for both visual and name matching. New evidence is provided on the role of each hemisphere in the analysis and comparison of signals, on either a visual or a verbal basis, for ‘same’ and ‘differ’ decisions. A model to explain the results in terms of the different functions of the two hemispheres is proposed.     

Selective search for conjunctively defined targets by children and young adults

Two experiments were conducted to investigate age-related differences in visual search for targets defined by the conjunction of two features. In the experiments, 7- and 10-year-old children and young adults searched visual displays for a black circle among distractors consisting of gray circles and black squares. In Experiment 1 (N = 60), we compared performance in the standard search task (where an equal number of each type of distractor appeared across all display sizes) with performance in a modified search task (where the number of black squares was fixed at two and the number of gray circles increased as the display size increased). In Experiment 2 (N = 60), the ratio of black stimuli to gray stimuli was varied systematically as the display size increased. Results of both experiments indicated that all participants were able to restrict search to an appropriate subset of the display rather than conduct an exhaustive search. However, the young adults were more efficient in their ability to do so than were either the 7- or 10-year-old participants. The 10-year-olds were as efficient as the young adults when the number of black stimuli in the display was relatively small. However, these children became relatively less able to restrict search effectively as the number of black stimuli increased. Discussion focused on possible preattentive and attentive processes that may change systematically with age.     

Response latencies in human matching-to-sample

Optimum characterization of individual information-processing skills requires isolation of assessable components. In matching-to-sample, three elementary processes might be separated if a proposed model of serial, selfterminating search were supported: registration of the stimulus in short-term memory, comparison of the two registered stimuli, and execution of the identifying response. Support for the model was obtained when response latencies were examined as a function of left-to-right position of the target stimulus in a display, but quantitative estimates of components could not be made because of interactions between tasks and the linearity of the scanning process. A second experiment, which varied the number of comparison stimuli, yielded highly linear functions whose slopes and intercepts violated certain aspects of the model. Data on eye movements obtained in a third experiment again supported the basic model but indicated that median response latencies represent a confounding of optimal and suboptimal performances.     

Modulation of selective attention by sequential effects in visual search tasks

In visual search detection of a target on one display facilitates its subsequent recognition on the next (Rabbitt, Cumming and Vyas, 1977). Experiment I shows that facilitation also occurs when a different display intervenes between two displays containing targets. Two further experiments show that detection of target letters among background letters is also facilitated if the same background letters recur on successive displays. Facilitation is greatest if background letters reappear in identical left-to-right spatial locations, but is also evident when the same background letters recur in different locations on successive displays. The results suggest modifications to models for the ways in which selective attention is continuously modulated by successive events during serial search.     

Intertrial priming stemming from ambiguity: A new account of priming in visual search

Sequential effects are ubiquitous in experimental psychology. Within visual search, performance is often speeded when participants search for the same target twice in a row, as opposed to two different targets. Here, we investigate such intertrial priming. Two experiments show that factors influencing search processes affect the presence and size of intertrial priming: It is larger when there are fewer elements in the visual display, and larger when there is a salient distractor present than when the target is the only salient element in the display. A control experiment showed that these increased priming effects were not due to longer baseline RTs. These findings, it is argued, are inconsistent with theories that explain intertrial priming as resulting from either only faster visual selection, or from episodic retrieval of responses. Instead, we propose that ambiguity in the stimulus or task underlies the occurrence of intertrial priming.     

Interactions of signal and background variables in visual processing

Three variables which determine the opportunities for signal-noise confusions, display size (D), number of redundant signals per display (N), and number of alternative signals (A) were studied in relation to nature of the noise elements, confusable or nonconfusable with signals. Data were obtained in a forced-choice visual detection situation, the displays being linear arrays of letters on a CRT screen. For all three performance measures used, frequency of correct detections and correct and error latencies, strong interactions were obtained between all of the other variables and signal-noise confusability. The functions obtained, together with other data bearing on the role of confusions and on spatial relations among characters within the display, suggest a model whose initial phase is a parallel feature extraction process involving inhibitory relations among input channels.     

The effect of a visual indicator on rate of visual search: Evidence for processing control

Search rates were estirnated from response latencies in a visual search task of the type used by Atkinson, Holmgren, and Juola (1969), in which a S searches a small set of letters to determine the presence or absence of a predesignated target. Half of the visual displays contained a marker above one of the letters. The marked letter was the only one that had to be checked to determine whether or not the display contained the target. The presence of a marker in a display significantly increased the estimated rate of search, but the data clearly indicated that Ss did not restrict processing to the marked item. Letters in the vicinity of the marker were also processed. These results were interpreted as showing that Ss are able to exercise some degree of control over the search process in this type of task.     

Concurrent visual search and time reproduction with cross-talk

Previous experiments indicate that humans can reproduce a time interval while concurrently searching a visual display presented near the beginning of the reproduced interval (Fortin, C., Rousseau, R., Bourque, P. & Kirouac, E. (1993). Time estimation and concurrent nontemporal processing: Specific interference from short-term-memory demands. Perception & Psychophysics, 53, 536-548.). In the earlier experiments, reproduced intervals were either insensitive to or shortened with increasing duration of visual search, suggesting that reproduction and search proceeded concurrently but sometimes with cross-talk. Here we test concurrency of time reproduction and visual search using a different methodology than before. Assuming the underlying mental processes are in a critical path network, factors selectively influencing concurrent processes are predicted to produce certain patterns in response time means and, through recent developments, certain patterns in cumulative distribution functions. Processes in an experiment were influenced by manipulating four factors, the time interval to be produced, stimulus onset asynchrony, display size, and target presence/absence. Patterns in response times provide evidence for concurrent time reproduction and visual search. However, a prediction based on selective influence failed in a manner suggesting the presence of cross-talk produced by a filled interval illusion.     

The interfering effect of word perception on letter identification

Subjects identified a single lowercase letter in a visual display by pressing one of two buttons. Two letters were assigned to each response. Groups received one of three context conditions: word, nonword, or single-letter displays. In words and nonwords, the flanking letters adjacent to the target varied as to whether they were response compatible or incompatible with the target. Single letters produced faster response latencies than either multi-letter condition, and words yielded slower latencies than did nonwords. Items Containing an incompatible-response flanking letter produced longer latencies than items containing a compatible flanking letter. Subgroups of subjects with different characteristic processing patterns were identified with a separate test. These subgroups were differentially affected by the context conditions in the letter-identification task. A greater subgroup difference was found in nonwords than in words.     

Age similarities in the inertial properties of attention

Adult age differences in the mode of allocation of visual attention were investigated, using a visual search task with a circular display containing one target letter and seven distractor letters. In two experiments, a total of 56 younger adults (M=20 years) and 56 older adults (M=66 years) searched for a target appearing with equal probability at one of two cued locations. The first cue appeared 115 msec before display onset, and the second cue appeared with display onset; distance between the two cued locations was varied. Target identification performance indicated that attention was inertial, in that reaction time for second-cued targets was related either to the area of the portion of the visual field containing possible target locations or to the mean path length of a serial self-tenminating search. There were no age-related decrements in the allocation of visual attention.     

Visual search and foraging compared in a large-scale search task

It has been argued that visual search is a valid model for human foraging. However, the two tasks differ greatly in terms of the coding of space and the effort required to search. Here we describe a direct comparison between visually guided searches (as studied in visual search tasks) and foraging that is not based upon a visually distinct target, within the same context. The experiment was conducted in a novel apparatus, where search locations were indicated by an array of lights embedded in the floor. In visually guided conditions participants searched for a target defined by the presence of a feature (red target amongst green distractors) or the absence of a feature (green target amongst red and green distractors). Despite the expanded search scale and the different response requirements, these conditions followed the pattern found in conventional visual search paradigms: feature-present search latencies were not linearly related to display size, whereas feature-absent searches were longer as the number of distractors increased. In a non-visually guided foraging condition, participants searched for a target that was only visible once the switch was activated. This resulted in far longer latencies that rose markedly with display size. Compared to eye-movements in previous visual search studies, there were few revisit errors to previously inspected locations in this condition. This demonstrates the important distinction between visually guided and non-visually guided foraging processes, and shows that the visual search paradigm is an equivocal model for general search in any context. We suggest a comprehensive model of human spatial search behaviour needs to include search at a small and large scale as well as visually guided and non-visually guided search.     

Dilution, not load, affects distractor processing

Lavie and Tsal (1994) proposed that spare attentional capacity is allocated involuntarily to the processing of irrelevant stimuli, thereby enabling interference. Under this view, when task demands increase, spare capacity should decrease and distractor interference should decrease. In support, Lavie and Cox (1997) found that increasing perceptual load by increasing search set size decreased interference from an irrelevant distractor. In three experiments, we manipulated the cue set size (number of cued locations) independently of the display set size (number of letters presented). Increasing the display set size reduced distractor interference regardless of whether the additional letters were relevant to the task. In contrast, increasing the cue set size increased distractor interference. Both findings are inconsistent with the load explanation, but are consistent with a proposed two-stage dilution account.     

On the tangibility of letters and braille

Braille characters and letters were presented to the senses of touch and vision of sighted observers. The optical stimuli were subjected to low-pass spatial filtering to the degree that visual spatial resolution closely matched tactile spatial resolution(re the size of characters presented). In one experiment, three sets of braille characters and five sets of Roman letters, varying in typography, were presented as stimuli. The variations in tangibility and legibility across the eight character sets were highly correlated (r =.98). In a second experiment, braille characters and letters were varied in size. The psychometric functions relating tangibility and legibility to character size were similar in both shape and position along the abscissa. The congruence of tangibility and legibility for these conditions, in which the senses were matched in spatial resolution, supports the idea that braille characters are more tangible by virtue of their greater distinctiveness after the low-pass spatial filtering of cutaneous processing.     

Some evidence for unequal loss of location and feature information as a function of retinal eccentricity in visual search

Abstract

Several psychophysical studies suggest that the quality of feature and positional information deteriorates differentially as a function of retinal eccentricity. Nevertheless, current models on visual search do not deal with such a difference. Two experiments have been executed in order to investigate whether eccentricity of target location affects the slope of the function relating reaction time (RT) to display size differently in a feature-search task compared to a task in which subjects search for the absence of a feature–that is, an absence-search task. The results showed that in the latter search rates strongly decreased when display elements were presented further from central fixation. In feature search there was no such effect. It is suggested that slope differences are related to the extent to which the attentional system is involved in compensating for inferior positional information in eccentric vision.     

Searching for a pre-specified target letter in briefly displayed words and unpronounceable non-words

Two experiments were designed in which subjects searched briefly displayed and masked arrays of letters for a single pre-specified target. The target was randomly located at one of twelve positions, its identity remained constant throughout, and it was embedded either in the context of words or in a context of unpronounceable strings of letters. In experiment 1 the letters were arranged in two six-item rows, and word and non-waord displays were mixed and presented to a single group of subjects. Accuracy of search was superior when the letters formed words, both for responses scored for correct position and for responses scored for correct row. In experiment 2 word and non-word displays were presented to different groups of subjects, and for both groups the twelve characters were either arranged as before in two rows or were positioned at loci representing the clock positions. Accuracy of search was again superior when the letters formed words and were arranged in rows, but there was no advantage for words when the letters formed a circle. These findings, firstly, indicated that the “word-superiority effect” obtained in experiment 1 was not a result of subjects adopting a word processing strategy for all stimuli, and secondly, they suggested that the word information could not be used to advantage in the absence of spatial cues delineating the words and/or when the letters were not positioned in the usual horizontal arrangement. It was concluded that the results of both studies were consistent with a perceptual account of the word superiority effect.