Differential specialization of the cerebral hemispheres for the perception of sameness and difference

The Johns Hopkins University, Baltimore, Maryland 21218 Pairs of letters were presented 4 deg left or right of fixation, and Ss were asked to indicate as quickly as possible whether the letters in a pair were the same or different. Reaction times for “different” responses were faster when stimuli were in the left visual field than in the right, but reaction times for “same” responses were faster when stimuli were in the right visual field than in the left. These results may indicate that the right hemisphere is better specialized for difference detection, while the left hemisphere is better specialized for sameness detection.     

Verbal interference in a perceptual comparison task

When Ss had to indicate if the colors of pairs of items from the Stroop test were the same or different, the usual interference effect was completely eliminated. However, when the verbal information consisted of SAME and DIFF rather than color names, interference was reestablished. This indicates that the perceptual comparison task does not eliminate interference simply by rendering Ss insensitive to the meanings of words.     

Detecting conjunctions of color and form in parallel

Certain theories of visual attention assume that at least one processing stage must be serial when the target of search is defined as the conjunction of two or more separable features. To explain why conjunction-search response times do not always form linearly increasing functions of display size, recent versions of this general model have posited the existence of an early parallel process that guides the serial stage toward display elements that are likely targets. Other models have relaxed the seriality assumption, allowing for a limited number of parallel decisions. In the three experiments reported here, a redundant-target detection task was used with conjunctively defined targets and display sizes of two (Experiment 1), one or two (Experiment 2), and six (Experiment 3). In all three experiments, strong evidence for parallel processing was observed. The implications for models of elementary visual processes are discussed.     

Are attentional dwell times inconsistent with serial visual search?

Duncan, Ward, and Shapiro (1994) estimated that attention must remain focused on an object for several hundred milliseconds before being shifted to another object, and they referred to this period as theattentional dwell time. An important implication of these long estimates of the dwell time for models of visual search is that the search process must not involve an item-by-item serial scanning mechanism. If it did, then searching through an array of items would require enormous amounts of time, which—based on data from visual search experiments—it does not. The present report, however, provides evidence that the long estimates of attentional dwell time were caused, at least in part, by the use of masked targets. Implications of these variable estimates of the attentional dwell time for models of visual search are discussed.     

On the nonautomaticity of “automatic” activation: Evidence of selective seeing

Early perceptual selection refers to a sequence of perceptual processing in which attentional selection of stimuli occurs before perceptual processing is complete. By contrast, late perceptual selection refers to a sequence in which all stimuli receive complete perceptual processing before selection takes place. An attempt was made to determine if early selection is possible. Three experiments were conducted in which subjects were presented with arrays containing a variable number of red items and a variable number of black items; all array items were randomly positioned around the circumference of an imaginary circle. The subjects task was to report the number of red items. Response time increased with the number of red items, but was essentially independent of the number of black items. The nature of the red and black items was also varied. When the to-be-attended red items were given “Stroop-like” quality (the symbols that were used were directly inconsistent with the correct response for that trial), mean reaction time was increased compared to a baseline condition; when the to-be-ignored black items were given “Stroop-like” quality, mean reaction time was unaffected. These results imply that the relevant (red) and irrelevant (black) items were processed differentially. We conclude that early selection is possible; late-selection models are not supported.     

Selective attention in the speeded classification and comparison of multidimensional stimuli

Selective attention was studied when subjects were required to make either speeded classifications of single stimuli or comparisons of pairs of multidimensional stimuli. Experiment 1 established that subjects were able to attend selectively to form when the irrelevant dimension of size or shading varied in a speeded-classification (card-sorting) task. Experiment 2 confirmed this finding in a discrete-trials task. However, subjects werenot able to filter out irrelevant dimensional disparity in a comparably designed simultaneous-comparison (i.e., “same”-“different”) task. Mean “same” reaction time increased monotonically with increases in disparity between the two stimuli on the irrelevant dimension. Experiment 3 also revealed a monotonie increase in “same” RT as a function of irrelevant disparity in a successive-comparison task. These results were discussed in terms of a normalization model proposed by Dixon and Just (1978) in which it is assumed that a subject equates the two stimuli on the irrelevant dimension before deciding that they are the same along the relevant dimension. It was concluded that: (1) although subjects can efficiently filter out irrelevant disparity in a speeded-classification task, interference due to irrelevant disparity is obtained in the comparison tasks, (2) a common process such as normalization does not necessarily underlie performance in the speeded-classification and comparison tasks, (3) the ability to attend selectively to a stimulus dimension may be task determined as well as stimulus determined, and (4) contrary to the Dixon and Just proposal, normalization of irrelevant disparity occurs in a comparison task, even when the relevant dimension is represented as a separate encoding feature.     

Right-left confusion in the adult: A verbal labeling effect

Directional judgments are typically slower when relative location is described by the words “east” and “west” or “right” and “left” than when described by the words “north” and “south” or “up” and “down.” A series of experiments are reported that disentangle verbal from perceptual encoding explanations for right-left difficulty. Overall, our results support a verbal encoding explanation for right-left confusion in the adult. Experiments 1-3 demonstrate that in a response-differentiation task, it is response to the labels “north,” “east,” “south,” and “west” that is responsible for right-left confusion. In addition, Experiments 4-6 demonstrate that right-left difficulty in a mirror image discrimination task is contingent on the use of directional labels. (The data also suggest that it may be more difficult to deal with “up,” “down,” “left,” and “right” than with “north,” “south,” “east,” and “west”) The data are interpreted as inconsistent with a bilateral symmetry explanation for right-left confusion.     

Parallel versus serial processing in visual search: further evidence from subadditive effects of visual quality.

The authors propose a diagnostic for distinguishing between serial and parallel processing in visual search; it is based on testing for subadditive effects of a within-trial visual quality manipulation on target-absent trials. It was evaluated in 2 experiments wherein parallel and serial processing might be expected on the basis of previous work and was then applied to a more uncertain situation in a third experiment. The diagnostic indicates parallel processing of stimuli that differ from each other on a featural basis (Xs and Os) and canonical letters that differ in line arrangement (Ts and Ls) but serial processing when Ts and Ls are randomly rotated. These results form a coherent pattern that is understandable in terms of the literature on visual search, and thus they suggest that the diagnostic may be a useful addition to the methodology used to distinguish between serial and parallel processes.     

Mixed reference frames for dynamic inhibition of return

Reference frames mediating inhibition of return (IOR) in dynamic displays were investigated by comparing the effects of a cue in simple versus extended objects. Experiment 1 replicated S. P. Tipper, B. Weaver, L. M. Jerreat, and A. L. Burak's (1994) finding of location- and object-based IOR for boxes rotating about fixation. In Experiments 2 and 3, the boxes were replaced by two boomerang-shaped objects. Response times were slowed to all target locations across a cued object. They were also slowed to locations vacated by the uncued region of the cued object-evidence for a representation the authors call the "perceptual footprint." Whereas an object served as the medium for location-based IOR for B. S. Gibson and H. Egeth (1994), the environment served as the medium for object- and location-based IOR in Experiments 2 and 3. This suggests that (a) frames of reference may be considered hierarchically and (b) nesting of frames is context dependent.