Hock, Howard S.

A category effect of alphanumeric characters in a visual search paradigm
was examined to see whether the effect could be accounted for
by the number of memory comparisons or by an early stage of perceptual
processing. In Experiments 1 and 2, subjects searched for
targets in either a within-category or between-category condition.
The two experiments differed by the point in the trials where "catch
trials" were presented. Experiment 3 used new configurations based
on Gestalt principles which had been shown to influence target detection
times at an early level of perceptual processing. The results
of Experiments 1 and 2 favored a perceptual explanation mediating the
effect. In Experiment 3, the critical interaction of category and
Gestalt factors which would have demonstrated a competition in
perceptually parsing the display fell short of significance.

An array of four motion quartets (stimuli for which either horizontal or vertical motion is perceived depending on quartet aspect ratio) is arranged in a diamond configuration such that two global motion patterns are formed: (1) Rotation---alternating counterclockwise and clockwise motion is perceived, and (2) Parallel path motion---the perceived motions of all the elements are simultaneously horizontal or simultaneously vertical. The perception of rotation resulted in global feedback that biases the motion perceived for an individual component motion quartet to be more consistent with rotation than aspect ratio. Stronger rotation produced greater bias. Under certain conditions, the feedback-induced bias occurred even though global rotation was not perceived. The results were interpreted in the context of neurophysiological evidence regarding neurons in Areas MT and MSTd, and a dynamical theory of motion pattern formation (Hock, Schoner & Giese, 2003; Nichols, Hock & Schoner, 2006).

This study tested the hypothesis that the perception of 2-flash apparent motion (points of light are briefly presented in succession at a nearby locations) is the outcome of competition between two opposing motion directions activated by the stimulus. Experiment 1 replicated previous results obtained using 2-flash stimuli; motion was optimal for a non-zero inter-frame interval (Kolers, 1972; Wertheimer, 1912). In Experiment 2, stimuli were pared down to a single luminance change toward the background at one location, and a single luminance change away from the background at one location at another. Results were consistent with apparent motion being specified by the counter-changing luminance; motion was optimal for a non-zero inter-frame interval. A subtractive model based on counter-change stimulating opposing motion directions did not account for the results of the 2-flash experiment. An alternative model based on the combined transient responses of biphasic detectors is discussed.

When perceivers examine a visual scene, they can control the extent to which their attention is either narrowly focused or spread over a larger spatial area. The experiments reported in this dissertation explore the consequences of narrow vs. broad attention for simple spatial discriminations as well as more complex cooperative interactions that are the basis for the self-organization of coherent motion patterns. Subjects' attentional spread (narrow or broad) is manipulated by means of a primary, luminance detection task. In conjunction with the luminance detection task is a secondary, spatial discrimination or detection task, which differs in the four reported experiments. In Experiment 1, the discrimination of misalignment of two visual elements is enhanced by narrowly focused attention. In Experiment 2, discrimination of horizontal spatial separation of two visual elements is improved for small inter-element distances by narrow attention and for relatively large inter-element distances by broad attention. Experiment 3 shows that the inter-element distance among counterphase-presented visual elements for which unidirectional and oscillatory motion patterns are observed with equal frequency depends on subjects' attentional spread. Narrow attention favors the oscillatory pattern and broad attention favors the unidirectional pattern. Experiment 4 shows that attentional spread has a minimal effect on the detection of motion, and, additionally that attentional effects on simple spatial judgments (Experiments 1 and 2) are too small to account for the large shift in the equi-probable boundary of reported unidirectional and oscillatory motion patterns found in Experiment 3. Therefore, it is concluded in conjunction with Hock and Balz's (1994) differential gradient model, that attentional spread influences the self-organization of unidirectional and oscillatory motion patterns through its effects on the relative strength of facilitating and inhibiting interactions among directionally selective motion detectors.