"Saturation" of the Dark-Adapted b-Wave

Abstract

Luminance-response functions of the dark-adapted ERG b-wave have been previously described by the Naka-Rushton equation R/R max = L n /(L n + K n ), in which R is the response to a stimulus of luminance L, R max is the maximum response amplitude, K is the luminance required to achieve a response one-half the amplitude of R max , and n is a dimensionless slope constant (1,2). However, when recording from normal observers and from patients with retinal disorders, we have observed that the Naka-Rushton equation does not represent the entire b-wave luminance-response function. Instead, our functions are composed of two distinct limbs. One limb appears to saturate at the luminance levels reported previously (1). However, as flash luminance increases above this level, a second limb appears. In the present report, we document the non-monotonic nature of the b-wave luminance-response function and examine possible explanations for the high luminance limb by recording ERGs from selected patients.