Visual neuroscience: illuminating the dark corners.
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Recent experiments suggest that our perception of lightness involves a sophisticated interpretation of illumination and shadow. This finding challenges common notions about hierarchical processing and the neural basis of perception. (+info)
Relationships of admissions data and measurements of psychological constructs with psychomotor performance of dental technology students.
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Abstract: The psychomotor skills required in dental laboratory technology and dentistry are similar. Dental educators have recognized the problems in selecting from among dental school applicants those with potential psychomotor skills. The purpose of this study was to examine the relationships of admissions data and measurements of psychological constructs of dental technology students with their psychomotor performance in first-semester dental laboratory courses. The dependent variables selected for the study were grades from three laboratory courses. Significant positive correlations (p<.05) were noted between all laboratory grades and previous college hours, previous college GPA, interview scores, field dependence-independence scores, block counting, trust, straightforwardness, and dutifulness. These data indicate that individual differentiation in learning ability, visual or spatial perception, and personality do affect psychomotor learning and should be taken into consideration in the design and execution of teaching and training curricula. (+info)
Triangular backgrounds shift the bias of line bisection performance in hemispatial neglect.
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OBJECTIVE: Patients with left neglect on line bisection show normal implicit sensitivity to manipulations of both the stimulus and the visual background. Three experiments were designed to define this sensitivity more exactly. METHODS: Normal controls and patients with left neglect performed a series of horizontal line bisection tasks. Independent variables were the configurations of the backgrounds for the line-rectangle, square, circle, left and right pointing isosceles triangles-and whether the background was the shape of the piece of paper or an outline drawn on a standard piece of paper. In a separate experiment different components of the triangle were outlined on a piece of paper. Deviation from true midpoint was calculated. RESULTS: Simply placing the target lines in a symmetric background such as a square or circle did not reliably reduce neglect. A triangle asymmetric in the horizontal plane caused a shift in bisection away from the triangle's vertex. With right pointing triangles the perceived midpoint shifted to the left of true centre (crossed over). The effects of the triangles were comparable in the patients and the controls when controlled for baseline bisection bias. The critical components of the triangles were the angular legs. This effect of background was not influenced by lesion site or by hemianopia. CONCLUSIONS: Patients with left visual neglect remain sensitive to covert manipulations of the visual background that implicitly shift the perceived midpoint of a horizontal line. This effect is strong enough to eliminate neglect on a bisection task. The mechanism of this effect is expressed through preattentive visual capacities. (+info)
The decay and interruption of interactions between search mechanisms.
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The attentional mechanisms in the brain responsible for fast pop-out search and slower difficult search have been shown to interact. Even if pop-out search is interrupted, by the addition of extra distractors to an initially simple search display, the partial computations calculated by the mechanisms responsible for pop-out can facilitate subsequent difficult search ("search assistance"; Psychon. Bull. Rev. 7 (2000) 292; Vision Res. 40 (2000) 891). With the present experiments, we aimed to discover whether search assistance is disrupted when the display that affords pop-out search disappears before the appearance of the display that must be examined by difficult search. Search assistance was not disrupted by the insertion of a blank screen in between the first and second portions of the display (Experiment 1). Search assistance for target-present trials was not disrupted by the insertion of black disks in between the first and second portions of the display (Experiment 2), but this manipulation did disrupt search assistance for target-absent trials. Implications for the relationship between search assistance and visual marking of distractors (Psychol. Rev. 104 (1997) 90) are discussed. (+info)
Uniformity and asymmetry of rapid curved-line detection explained by parallel categorical coding of contour curvature.
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The aim of this work was to elucidate several characteristic phenomena associated with rapid curved-line detection in multi-element arrays and to provide a unified account of the underlying curvature-sensitive mechanisms. To this end, a parametric experiment was performed in which the detectability of a curved-line target in a briefly presented planar array of curved-line distractors was measured for a range of target and distractor curvatures and distractor numbers. For both vertically oriented and randomly oriented curved lines, it was found that (1) the dependence of target detectability on target curvature was independent of distractor number for small distractor curvatures but not for medium-to-large distractor curvatures; (2) an asymmetry in target detectability with respect to interchange of target and distractor curvatures occurred only with large distractor numbers; and (3) with small distractor numbers, target detectability depended only on the difference between target and distractor curvatures. These properties of spatial parallelism, asymmetry, and uniformity were explained quantitatively by a minimal model of rapid curved-line detection in which contour curvature was coded in terms of just two or three curvature categories, depending on curved-line orientation. (+info)
Latency effects in orientation popout.
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A target that differs in orientation from neighboring lines and "pops out" has been found to evoke larger responses in cortical V1 cells than lines in the uniform texture surround which do not popout (e.g., Journal of Neurophysiology 67 (1992) 961). If this is more than a coincidence of observations, physiological properties of contextual modulation should be reflected in the perception of salience. In particular, as the differential suppression from texture surround has been reported to be delayed, target salience may be affected by the history of surrounding lines, i.e. by their orientation before the target was presented. This was tested using a feature flicker paradigm in which target and background lines changed their orientations (Experiment 2). All subjects (N = 4) indicated a benefit in target detection when target orientation was not previously present in the surround. A control experiment showed that this effect was not caused by the purely temporal aspects of asynchronous stimulus presentation (Experiment 3). To distinguish this effect from other sources of delayed processing, Experiment 1 compared the performance in target detection and target identification tasks, for single-lines and popout targets. All subjects required longer stimulus presentation time to identify the orientation of a single line than to detect the line itself, indicating that orientation coding needs longer processing than encoding stimulus onset. However, most subjects needed even longer presentations to detect popout, suggesting that the processing of orientation contrast adds to this delay. In an appendix, putative response variations of V1 cells to asynchronous flicker are computed. (+info)
The extent of crowding in peripheral vision does not scale with target size.
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Identifying a target is more difficult when distracters are present within a zone of interaction around the target. We investigated whether the spatial extent of the zone of interaction scales with the size of the target. Our target was a letter T in one-of-four orientations. Our distracters were four squared-thetas in one-of-two orientations, presented one in each of the four cardinal directions, equidistant from the target. Target-distracter separation was varied and the proportion of correct responses at each separation was determined. From these the extent of interaction was estimated. This procedure was repeated for different target sizes spread over a 5-fold range. In each case, the contrast of the target was adjusted so that its visibility was constant across target sizes. The experiment was performed in the luminance domain (grey targets on grey background) and in the chromatic domain (green target on equiluminant grey background). In the luminance domain, target size had only a small effect on the extent of interaction; these interactions did not scale with target size. The extents of interaction for chromatic stimuli were similar to those for luminance stimuli. For a fixed target visibility, decreasing the duration of the stimulus resulted in an increase in the extent of interaction. The relevance of our findings is discussed with regard to a variety of proposed explanations for crowding. Our results are consistent with an attention-based explanation for crowding. (+info)
Segmentation of objects from backgrounds in visual search tasks.
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In most visual search experiments in the laboratory, objects are presented on an isolated, blank background. In most real world search tasks, however, the background is continuous and can be complex. In six experiments, we examine the ability of the visual system to separate search items from a background. The results support a view in which objects are separated from backgrounds in a single, preattentive step. This is followed by a limited-capacity search process that selects objects that might be targets for further identification. Identity information regarding the object's status (target or distractor) then accumulates through a limited capacity parallel process. The main effect of background complexity is to slow the accumulation of information in this later recognition stage. It may be that recognition is slowed because background noise causes the preattentive segmentation stage to deliver less effectively segmented objects to later stages. Only when backgrounds become nearly identical to the search objects does the background have the effect of slowing item-by-item selection. (+info)