Visual readaptation after flash exposure under scotopic conditions. A study using optokinetic nystagmus as an indicator of visual perception.

Abstract

The purpose of the present study was to establish a method for objective measurements of visual readaptation after flash exposures and to define a model for measurements. Influences of target direction, luminance and velocity on optokinetic nystagmus (OKN) were investigated under scotopic conditions. Visual readaptation was measured using OKN as an indicator of visual perception after exposure to a flash. The interval between the triggering of the flash and the reoccurrence of OKN was defined as the visual readaptation time (RAT). A Goldmann perimeter hemisphere was used for flash stimulation. A horizontally moving vertical grating projected inside the hemisphere was used as the OKN stimulus. Eye movements were recorded by DC electrooculography (EOG). The dependence of RAT on the dose of the flash, the wavelength of the flash and the luminance of the OKN target were investigated. The precision of the measurement method was studied. This includes the analysis of the variance due to the experimental occasions, the repeated exposures, the sexes of the subjects, the methods for recognition of OKN and the ways of visual adaptation before measurements. The contributions of retinal receptor and the neural activity to RAT were investigated by electroretinography (ERG). The influences of target direction and luminance on binocular motion perception and OKN as well as monocular OKN were examined at various target velocities. The dependence of the frequency and amplitude of eye jerks during monocular OKN on target luminance and velocity were also examined. It was found that RAT increases with increasing doses of the flash or decreasing luminance of the grating. RAT is most extended after flashes near 520 nm. RAT does not differ between experimental occasions, between a manual and a semi-automatic method for recognition of OKN, between the sexes and between goggle adaptation and ordinary dark adaptation. There is a reduction of RAT due to repeated flash exposures. The data collected indicate that a well-defined model is crucial for measurements of RAT. The measurement of ERG showed that RAT is mediated by both retinal receptor and the neural activities. The receptor component depends on the wavelength of the flash while the neural component is wavelength-independent. Moreover, it was found that motion perception and OKN gain does not differ between right and left target directions. For a given target velocity, motion perception and OKN gain under both binocular and monocular viewing conditions increase with increasing luminance of the target with an exponential decay. The maximum OKN gain decreases as target velocity increases.(ABSTRACT TRUNCATED AT 400 WORDS)