Visual Neuroscience

Table of Contents - 2001 - Volume 18, Issue 01  


Processing of scotopic increments and decrements

TODD J.  PURKISS a1, ALAN  HUGHES a1 and PAUL J.  DEMARCO  Jr. a1a2c1
a1 Department of Psychological and Brain Sciences, University of Louisville, Louisville
a2 Louisville VA Medical Center, Louisville

Abstract

Rod and cone photoreceptors send their signals to ON- and OFF-retinal ganglion cells through different pathways in the primate retina. We hypothesized that increments and decrements of light may be processed differently by the rod-bipolar pathway because of the funneling of the rod signal through the rod bipolar cell. We tested this hypothesis using a psychophysical adaptation paradigm, which has provided evidence that photopic increments and decrements of light are processed by ON- and OFF-pathways in the human visual system. We had observers adapt to either a rapid-on or rapid-off sawtooth waveform, under both photopic and scotopic conditions. We then measured detection thresholds for one cycle of a rapid-on or rapid-off sawtooth stimulus. For photopic stimuli, sawtooth adaptation asymmetrically raised thresholds for test stimuli in a manner that depended on the polarity of the adaptation stimulus. For scotopic stimuli, thresholds were raised, but no significant selective adaptation effect was found. By repeating the photopic condition with sawtooth stimuli which had been filtered using an impulse response function derived for the rod system, we demonstrated that the lack of selective adaptation was not a consequence of the sluggish temporal response of the rod-bipolar pathway. We conclude instead that the reduced effectiveness of sawtooth adaptation is due to channeling of rod photoreceptor signals through the rod bipolar cell before reaching ON- and OFF-ganglion cells.

(Received May 25 2000)
(Accepted October 10 2000)


Key Words: Scotopic pathways; Photopic pathways; ON- and OFF-pathways; Temporal adaptation.

Correspondence:
c1 Address correspondence and reprint requests to: Paul J. DeMarco, Department of Psychological and Brain Sciences, 317 Life Sciences Bldg., University of Louisville, Louisville, KY 40292, USA. E-mail: paul.demarco@louisville.edu