Spatially mediated capacity limits in attentive visual perception

Modern theories conceptualize visual selective attention as a competition between objects for the control of cortical receptive fields (RFs). Implicit in this framework is the suggestion that spatially proximal objects, which draw from overlapping pools of RFs, should be more difficult to represent in parallel and with excess capacity than spatially separated objects. The present experiments tested this prediction using analysis of response time distributions in a redundant-targets letter identification task. Data revealed that excess-capacity parallel processing is possible when redundant targets are widely separated within the visual field, but that capacity is near fixed when targets are adjacent. Even at the largest separations tested, however, processing capacity remained strongly limited.     

Hair Pulling Behavior Reported by African American and Non-African American College Students

Of the few attempts to determine the prevalence rate of trichotillomania, virtually none have investigated potential ethnic differences. The present study provides data on the prevalence of hair twirling and hair pulling behavior among 176 African American and 422 non-African American students and systematically explores differences between these 2 groups. Fully 10.2% of the sample reported hair pulling that resulted in noticeable hair loss, with African American women reporting the highest rate (15.7%). Overall, 2% of participants responded consistently with Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR; American Psychiatric Association, 2000) criteria for trichotillomania; rates did not differ significantly among genders or ethnic backgrounds. African Americans, and especially African American women, were more likely than other participants to report hair pulling in response to skin irritation. Results are discussed in the context of previous research, the importance of hair care in the African American community, and directions for future research.     

ePsych: Interactive demonstrations and experiments in psychology

ePsych (http://epsych.msstate.edu), a new Web site currently under active development, is intended to teach students about the discipline of psychology. The site presumes little prior knowledge about the field and so may be used in introductory classes, but it incorporates sufficient depth of coverage to be useful in more advanced classes as well. Numerous interactive and dynamic elements are incorporated into various modules, orientations, and guidebooks. These elements include Java-based experiments and demonstrations, video clips, and animated diagrams. Rapid access to all material is provided through a layer-based navigation system that allows users to visit various “Worlds of the Mind.” Active learning is encouraged, by challenging students with puzzles and problems and by providing the opportunity to “dig deeper” to learn more about the phenomena at hand.     

Sequential priming of 3-D perceptual organization

In four experiments, the effects of sequential priming on the perceptual organization of complex three-dimensional (3-D) displays were examined. Observers were asked to view stereoscopic arrays and to search an embedded subset of items for an odd-colored target while 3-D orientation of the stimuli was varied randomly between trials. Search times decreased reliably when 3-D stimulus orientation was unchanged on consecutive trials, indicating substantial sequential priming by 3-D stimulus layout. The priming was nonsensory and was independent of priming by additional stimulus characteristics. Finally, priming by 3-D layout was unaffected by observers' foreknowledge of display orientation. Results indicate that perceptual organization of 3-D stimuli is guided by a short-term trace of 3-D spatial relationships between stimuli.     

Executive working memory load does not compromise perceptual processing during visual search: Evidence from additive factors analysis

Executive working memory (WM) load reduces the efficiency of visual search, but the mechanisms by which this occurs are not fully known. In the present study, we assessed the effect of executive load on perceptual processing during search. Participants performed a serial oculomotor search task, looking for a circle target among gapped-circle distractors. The participants performed the task under high and low executive WM load, and the visual quality (Experiment 1) or discriminability of targets and distractors (Experiment 2) was manipulated across trials. By the logic of the additive factors method (Sternberg, 1969, 1998), if WM load compromises the quality of perceptual processing during visual search, manipulations of WM load and perceptual processing difficulty should produce nonadditive effects. Contrary to this prediction, the effects of WM load and perceptual difficulty were additive. The results imply that executive WM load does not degrade perceptual analysis during visual search.     

Voluntary and reflexive eye movements to illusory lengths

Considerable debate surrounds the extent and manner that motor control is, like perception, susceptible to visual illusions. Using the Brentano version of the Müller-Lyer illusion, we measured the accuracy of voluntary and reflexive eye movements to the endpoints of equal length line segments that appeared different (Experiment 1) and different length line segments that appeared equal (Experiment 3). Voluntary and reflexive saccades were both influenced by the illusion, but the former were more strongly biased and closer to the subjective percept. Experiment 2 demonstrated that these data were the results of the illusion and not centre-of-gravity effects. The representations underlying perception and action interact and this interaction produces biases for actions, particularly voluntary actions.     

Asymmetry in 3-D perceptual organization: ground-like surface superior to ceiling-like surface

Attneave (1954) and Barlow (1961) proposed that the visual system might increase efficiency of representation by preferentially encoding spatiotemporally redundant patterns of the external world. The present experiments tested the application of this principle to three-dimensional (3-D) perceptual organization, capitalizing on the ecological constraint that human observers must frequently interact with objects arranged on the ground or on a surface parallel to it (Gibson, 1950). Observers performed a task that required them to perceptually segregate and search multiple items distributed in depth and embedded within a larger, 3-D array of distractors. Stimulus displays were organized to globally recede top-away in depth, as if attached to an underlying ground-like surface, or bottom-away, as if attached to an overhanging ceiling-like surface; ground-like and ceiling-like displays differed only in the direction of disparity gradient within the displays. Primary findings revealed superior performance with ground-like displays, suggesting that spatially and stereoscopically distributed items are more easily organized to represent an ecologically representative pattern, even when no inherent physical regularities favor that pattern.     

An oblique effect of chromatic gratings measured by color-mixture thresholds

Contrast sensitivity is lower for obliquely oriented achromatic gratings than for vertical or horizontal gratings at high spatial and low temporal frequencies. Although this response is suggestive of mediation by P-pathway cortical correlates, no clear sensory (i.e. class 1) oblique effect has been demonstrated with isoluminant chromatic stimuli. In the present experiment, a two-alternative forced-choice detection task was used to measure observers' sensitivity to spatiotemporal sinusoids varying in orientation and color contrast. A maximum-likelihood method fit ellipses to the thresholds, with the length of each ellipse taken as a measure of chromatic contrast sensitivity at isoluminance, and the width as luminance contrast threshold. A chromatic oblique effect was observed at about 3 cycles deg-1 suggesting an orientation bias within the cortical stream conveying P-cell activity.     

How much memory does oculomotor search have?

Research has demonstrated that oculomotor visual search is guided by memory for which items or locations within a display have already been inspected. In the study reported here, we used a gaze-contingent search paradigm to examine properties of this memory. Data revealed a memory buffer for search history of three to four items. This buffer was effected in part by a space-based trace attached to a location independently of whether the object that had been seen at that position remained visible, and was subject to interference from other stimuli seen in the course of a trial.     

Visual search has memory

By monitoring subjects' eye movements during a visual search task, we examined the possibility that the mechanism responsible for guiding attention during visual search has no memory for which locations have already been examined. Subjects did reexamine some items during their search, but the pattern of revisitations did not fit the predictions of the memoryless search model. In addition, a large proportion of the refixations were directed at the target, suggesting that the revisitations were due to subjects' remembering which items had not been adequately identified. We also examined the patterns of fixations and compared them with the predictions of a memoryless search model. Subjects' fixation patterns showed an increasing hazard function, whereas the memoryless model predicts a flat function. Lastly, we found no evidence suggesting that fixations were guided by amnesic covert scans that scouted the environment for new items during fixations. Results do not support the claims of the memoryless search model, and instead suggest that visual search does have memory.