G K Beh | PubMed Health

PubMed1996

A comparison of sampling efficiency and internal noise level in young and old subjects.

Contrast detection in different levels of external visual noise allows a given loss in contrast sensitivity to be attributed to either an increase in the internal noise of the visual system, a decrease in sampling efficiency, or both. Sampling efficiency indicates how effectively the available stimulus information is utilized by the visual system. The aim of this study is to investigate the effect of normal ageing on sampling efficiency and internal noise. Contrast thresholds for sine-wave gratings of 6 c/deg were measured in the presence of four (including zero) levels of externally added visual noise in young and older healthy observers. Results showed that sampling efficiencies were significantly lower for the older group compared to the younger, while the internal noise showed no significant change. The implications of the data for the relative contribution of the optical and neural systems on visual function loss with ageing are discussed. Our results suggest that the neural system plays a major role in the loss of contrast sensitivity with ageing in normal, healthy eyes.

Adolescent↗

PubMed1993

Contrast detection in noise: a new method for assessing the visual function in cataract.

The detectability of a signal decreases in the presence of externally added visual noise. A linear relation exists between the energy of a signal at threshold and the spectral density of the added visual noise. This relation, represented by contrast detection in noise function, allows a given loss in contrast sensitivity (CS) to be attributed to an increase in the internal intrinsic noise and/or a decrease in the detection efficiency of the observer. Intrinsic noise gives a measure of the random background noise within the visual system, whereas detection efficiency is a measure of how effectively the observer utilizes the available stimulus information. Recent work shows that neural dysfunction produces a decrease in the detection efficiency, whereas optical deficits produce a change in the intrinsic noise only. We investigated whether the CS loss in cataract can be attributed to either a change in intrinsic noise, in detection efficiency, or both. Contrast detection in noise functions was measured for 10 uniocular cataract patients. Comparison between the two eyes showed no significant difference in detection efficiency, although the intrinsic noise increased significantly in the cataractous eye. The data suggest that detection efficiency may provide a measure of neural visual function behind a cataract and, conversely, that intrinsic noise provides a measure of the visual effect of the cataract itself. We discuss the implications of intrinsic noise as a measure of cataract for both clinical assessment and research.

Aged↗