Biomedical subjects

A B Sekuler

Publications and source records attributed to A B Sekuler.

4 recordsLinked to original sources

PubMed1992

Simple-pooling of unidirectional motion predicts speed discrimination for looming stimuli.

Looming objects comprise many 2D unidirectional motion elements changing over time. However, observers assign a single 3D speed to looming objects, not many independent unidirectional speeds to different regions of the object. These experiments examined speed discrimination for looming stimuli to illuminate the mechanisms underlying the perception of 3D motion. Speed discrimination thresholds for looming displays were comparable to those of fronto-parallel translating and rotating displays (approximately 5%), and thresholds were predicted from the simple linear combination of 2D unidirectional thresholds. These results suggest that 2D unidirectional motion and looming motion are not independent: the simple pooling of unidirectional motion units limits sensitivity to looming stimuli. These results do not support the notion that the visual system directly encodes relative motion within a distinct channel.

Depth Perception↗

PubMed1991

Peripheral spatial vision: limits imposed by optics, photoreceptors, and receptor pooling.

We examined the contribution of optical and photoreceptor properties as well as receptor pooling to eccentricity-dependent variations in spatial vision by comparing the performance of ideal observers with that of human observers. We measured contrast sensitivity functions in human observers and calculated such functions in ideal observers for retinal eccentricities of 0-40 deg. Comparisons of human and ideal performance in a variety of tasks reveal that many aspects of the variation in spatial vision with eccentricity can be understood from an analysis of the discrimination information available at the retinal ganglion cells.

Contrast Sensitivity↗

PubMed1990

Motion segregation from speed differences: evidence for nonlinear processing.

This paper examines observers' ability to detect regions delimited by speed differences. Several types of translational motion stimuli, of varying task difficulty, were tested over a wide range of base speeds. The observer's task was to decide whether a region of dots moving at a different speed from the base speed was located to the left or right of the display's center. Both increments (dots within the test region moving faster than the base speed) and decrements (dots within the test region moving slower than the base speed) were examined. When the task was relatively difficult the following asymmetries were found: at slow base speeds, incremental thresholds were lower than decremental thresholds; at fast base speeds, the reverse pattern occurred. When the task was relatively easy, no consistent asymmetries were found. It is proposed that the visual system encodes speed through a sigmoidal nonlinear response function, and it is shown that this one nonlinearity can be used to explain results for both easy and difficult tasks.

Differential Threshold↗

PubMed1990

How the visual system detects changes in the direction of moving targets.

To determine how the visual system represents information about change in target direction, we studied the detection of such change under conditions of varying stimulus certainty. Target direction was either held constant over trials or was allowed to vary randomly. When target direction was constant the observer could be certain about that stimulus characteristic; randomizing the target direction rendered the observer uncertain. We measured response times (RTs) to changes in target direction following initial trajectories of varying time and distance. In different conditions, the observer was uncertain about either the direction of the initial trajectory, or the direction of change or both. With brief initial trajectories in random directions, uncertainty about initial direction elevated RTs by 50 ms or more. When the initial trajectories were at least 500 ms, this directional uncertainty ceased to affect RTs; then, only uncertainty about the direction of change affected RTs. We discuss the implications of these results for (i) schemes by which the visual system might code directional change; (ii) the visual integration time for directional information; and (iii) adaptational processes in motion perception.

Attention↗