Retinal mechanisms of visual latency

Abstract

The time-course of light-induced potential changes was measured transretinally as a function of flash intensity in the eyecup preparation of dark-adapted Bufo marinus. The electroretinogram (ERG) as indexed by the peak amplitude of the a-wave. and the latency of the a-wave onset yielded action spectra matching that of the red rod pigment (λ max at 502 nm) at low intensities near threshold, but at higher intensities there was evidence of intrusion from the single and principal short-latency, longwave cones (λ max at 575 nm). With uniform illumination of the retina with monochromatic (λ = 502 nm) flashes to isolate the red rod system over a range of 5 log units from threshold to saturation of the a-wave. the latency of the b-wave onset and the latency of the a-wave peak varied linearly with the latency of the a-wave onset. All three measures could be described by linear functions of the inverse cube root of flash intensity, or equivalently by the equation: t min/ t= I 1/3( I 1/3 + σ 1/3). For the mass receptor potential (MRP) isolated with excess Mg 2+. the initial segment and functional form of the time-course did not differ from that obtained for the a-wave at the same saturating intensity. The results are consistent with the hypothesis that the dominant mechanism controlling visual latency lies in the photoreceptors and that subsequent proximal transformations of the time-course are linear.