Flicker adaptation and neural transmission speed in the human MC pathway

Abstract

INTRODUCTION: Flicker-induced adaptation of the human magnocelluar (MC) pathway increases contrast thresholds (Zhuang, Pokorny & Cao, IOVS 2015). This study tests whether flicker adaptation also reduces neural transmission speed for suprathreshold stimuli by measuring the response time to a suprathreshold disc. The working hypotheses are that (#1) flicker presented to a given eye can adapt the monocular pathway and also a subsequent binocular mechanism and (#2) flicker presented to both eyes causes greater adaptation at the binocular level than flicker to only one eye. METHODS: Adaptation was induced with a flickering 16x13 deg rectangular field modulated at 9.4 Hz with 100% Michelson contrast. Adaptation could be in neither eye (baseline using a steady adapting field), in only one eye or in both eyes. The test stimulus for response time, a foveal 0.5 deg diameter disc, could be presented to either eye alone or both eyes. Observers responded to the test stimulus as quickly as possible. RESULTS AND DISCUSSION: Response time was the slowest when a monocular test stimulus was presented to the same monocularly adapted eye. Surprisingly, with a monocular test the response time was significantly faster with binocular adaptation than with same-eye monocular adaptation. This suggests that when both eyes are adapted to flicker, the overall adapting effect for a monocular test is inhibited by flicker adaptation in the fellow eye. Overall, the results confirm that flicker adaptation reduces neural transmission speed for suprathreshold stimuli (consistent with working hypothesis #1), but also show that transmission speed for a stimulus presented to only one eye can be speeded up by adding adaptation in the fellow eye (contrary to #2). The latter finding corroborates a similar conclusion from measurements of monocular and binocular contrast thresholds (Zhuang & Shevell, Vision Research 2015). Meeting abstract presented at VSS 2016