Computer-Assisted Analysis of Clinical Electroretinographic Intensity-Response Functions

Abstract

The amplitude of the human dark-adapted electroretinogram, measured from the trough of the a-wave to the peak of the b-wave as a function of stimulus intensity can be described nominally by the Naka-Rushton equation which has three independent parameters: Rmax, K, and n. Rmax is the asymptotic value of the ERG amplitude as a function of intensity, K is the intensity that produces an ERG amplitude that is one-half Rmax and n is a dimensionless constant that controls the slope of the function. The advantage of using the Naka-Rushton equation is that the three empirical parameters can be interpreted clinically. For example, changes in Rmax may correspond to response compression owing to retinal ‘gain’ changes or to regional losses of retinal function; changes in K correspond to changes in retinal sensitivity or to changes in pre-retinal light absorption; and changes in n may be interpreted in terms of heterogeneous losses of retinal sensitivity. ERG intensity-response functions were obtained from normal subjects and retinal disease patients, and maximum likelihood values were computed for Rmax, K, and n for each individual’s data. The present paper describes these data and the computer system and analysis algorithm for recording and analyzing the ERG intensity-response function.