City Research Online

Abstract

This investigation has aimed to provide data required for a model of visual search performance, and to test models of visual processing.

Psychophysical thresholds have been measured over a range of visual field angles for luminance and colour contrast, motion displacement and speed discrimination, for small, uniform, briefly presented stimuli on a large uniform background. These are reported as threshold functions and as visual lobes, and show how these functions vary with target diameter, visual field position, target velocity and background luminance. Eye movements and pupil responses to similar stimuli were also examined, and threshold levels of stimulation, amplitudes and latencies of response were measured.

In this study foreknowledge of target position made little difference to detection performance, but the use of a suddenly appearing and disappearing stimulus gave very different thresholds to one that appeared gradually. For stimuli close to threshold, luminance contrast correlated well with visual lobe area, whereas chromatic contrast did not. The contrast required for saccadic response was similar to that for a threshold level of detection.

Speed discrimination thresholds were lowest for target velocities near 3°/sec, changing little with increasing- eccentricity. Displacement thresholds for detecting motion were proportional to target speed, for speeds from 5°/sec to 50°/sec, and were almost constant for speeds from 2°/ sec to 5°/sec, suggesting a model using a single time delay at high speeds, and a range of time delays at lower speeds.

A hemianope showed a normal pupil light reflex, but little response in his blind hemifield to a counterphase grating of the same mean luminance as the surround. These findings suggest that response to such a stimulus requires a direct projection to the striate cortex.

The same subject responded to flashed stimuli in his blind hemifield with saccades of normal latency and accurate mean amplitude, but with greater variability than to stimuli in his normal hemifield. His contrast threshold for saccades in his blind hemifield was higher than normal, but stray light has been virtually ruled out as an explanation of these results.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QH Natural history Q Science > QH Natural history > QH301 Biology Q Science > QM Human anatomy
Departments:	School of Health & Medical Sciences > Optometry & Visual Sciences School of Health & Medical Sciences > School of Health & Medical Sciences Doctoral Theses Doctoral Theses

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