Biomedical subjects

G K Edgar

Publications and source records attributed to G K Edgar.

6 recordsLinked to original sources

PubMed1991

The separability of temporal frequency and velocity.

McKee, Silverman and Nakayama (1986; Vision Research, 26, 609-619) have shown that velocity discrimination performance is little affected by quite large random changes in the spatial frequency, and hence temporal frequency, of the grating patterns to be discriminated. We show that the converse is also true: temporal frequency discrimination can be performed despite random changes in velocity. This casts doubt on McKee et al.'s conclusion that velocity is the fundamental dimension of the temporal mechanisms mediating discrimination and that temporal frequency has to be inferred indirectly from velocity and spatial frequency. We suggest instead that both velocity and temporal frequency are represented in the visual system but that the two dimensions are only partially separable at the perceptual level.

Discrimination, Psychological↗

PubMed1991

Perceived speed and direction of complex gratings and plaids.

Measurements of perceived speed were obtained for a variety of drifting simple and complex gratings, and measurements of perceived speed and direction were obtained for plaids. For sine gratings, perceived speed falls off at high spatial frequencies, the effect of spatial frequency being greatest at high speeds. Speed matches obtained from a variety of one-dimensional complex gratings are in some cases consistent with a simple averaging of the discrepant speeds signaled by their spatial Fourier components when seen alone. However, in other cases the results do not fit such an interpretation but suggest the involvement of a mechanism other than Fourier decomposition and recombination. Measurements of the perceived speed of plaids suggest that the observed spatial-frequency-dependent variations in encoded speed of gratings arise largely at a low level, before the aperture problem is solved. Measurements of the perceived direction of plaids whose components are of different spatial frequencies and hence have different perceived speeds show large deviations from the direction predicted by the intersection-of-velocity-constraints model [Nature 300, 523 (1982)] and are suggestive of a revised two-stage model in which the computation of pattern direction is based on the (sometimes disparate) perceived speeds of the components rather than their actual speeds.

Humans↗

PubMed1990

The influence of spatial frequency on perceived temporal frequency and perceived speed.

Speed matching experiments were conducted using drifting gratings of different spatial frequencies in order to assess the influence of spatial frequency on perceived speed. It was found that gratings of high spatial frequency appear to drift more slowly than low spatial frequency gratings of the same actual velocity. The perceived temporal frequency of a counterphase grating similarly declines as spatial frequency increases. The previously reported effect of temporal frequency on perceived spatial frequency probably does not contribute to these phenomena. Our results suggest that the motion sensors thought to operate within different spatial frequency ranges have different velocity transfer functions, a fact not incorporated in existing computational models of motion perception.

Contrast Sensitivity↗

PubMed1990

Hemifield differences in perceived spatial frequency.

Measurements of the perceived spatial frequency of stationary sinewave gratings were made with the gratings presented at the same eccentricity in the left, right, upper, and lower visual hemifields. Ten subjects performed the task binocularly with spatial frequencies of 1, 2, and 4 cycles deg-1. Two of these subjects also performed the task monocularly at 2 cycles deg-1. In the majority of cases, the spatial frequency of stimuli presented in the left and lower visual hemifields was overestimated relative to stimuli presented in the right and upper visual hemifields. The results were similar for all spatial frequencies tested, and the direction of the asymmetry was the same whether viewing was with the left eye, right eye or binocular, suggesting that the differences in perceived spatial frequency are not retinal in origin.

Adult↗

PubMed1990

Optic neuritis: variations in temporal modulation sensitivity with retinal eccentricity.

Temporal modulation sensitivity functions were measured centrally and at eccentricities of 2.5 degrees, 5 degrees and 10 degrees in the temporal visual field of 12 patients with recovered optic neuritis and in a group of matched normal controls. A circular, spatially uniform stimulus of 1 degree angular subtense was presented with sinusoidal modulation at 5, 8, 14 and 23 Hz. The general pattern of results in patients was a loss of sensitivity relative to normal controls at all temporal frequencies at 0 degree and 2.5 degrees eccentricity, with rather greater losses occurring at the medium-to-lower temporal frequencies. At 5 degrees eccentricity, the losses were confined to medium temporal frequencies only, and at 10 degrees eccentricity there was no significant loss at any temporal frequency. These findings may be explained by a greater vulnerability of optic nerve fibers of small diameters to the effects of demyelinating disease.

Adult↗

PubMed1990

Visual loss in multiple sclerosis and its relation to previous optic neuritis, disease duration and clinical classification.

Visual function was investigated in a group of 58 clinically classified cases of multiple sclerosis (MS). Psychophysical measures of luminance and chromatic threshold sensitivity and temporal contrast sensitivity were undertaken, together with visual evoked potentials and Bjerrum screen perimetry. The patient group was divided on the basis of optic neuritis (ON), clinical disease duration and clinical classification. A comparison of the results of all visual measures suggested a nonuniform loss of function in the patient group without ON and a more consistent loss of function in the group with ON. The various measures were equally efficient in detecting abnormal function, albeit from different areas of the central visual field. Clinical disease duration was not a significant independent factor in predicting visual dysfunction. In contrast, a comparison of clinical classification categories revealed significantly fewer abnormalities of visual function in the suspected MS category (31%) than in the ON, early probably and clinically definite categories (75-100%), a result which indicated the importance of clinical classification as a predictor of visual dysfunction.

Adult↗