CONTRAST DETECTION IN NOISE - A NEW METHOD FAR 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 s ignal at threshold and the spectral density of the added visual noise. This relation, represented by contrast detection in noise function, a llows a given loss in contrast sensitivity (CS) to be attributed to an increase in the internal intrinsic noise and/or a decrease in the det ection efficiency of the observer. Intrinsic noise gives a measure of the random background noise within the visual system, whereas detectio n efficiency is a measure of how effectively the observer utilizes the available stimulus information. Recent work shows that neural dysfunc tion 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 detecti on in noise functions was measured for 10 uniocular cataract patients. Comparison between the two eyes showed no significant difference in d etection efficiency, although the intrinsic noise increased significan tly 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 ef fect of the cataract itself. We discuss the implications of intrinsic noise as a measure of cataract for both clinical assessment and resear ch.