Pattern evoked potentials from the cat's retina.

Abstract

We have studied electroretinograms (ERG) in the cat using phase-reversed sinusoidal gratings as a stimulus. Our purpose was to characterize response properties of this type of ERG. One basic question we addressed was whether the response to a grating stimulus is actually pattern specific. For the purpose of comparison, we used the same stimulus to investigate mass potentials from the lateral geniculate nucleus (LGN) and the visual cortex. The pattern ERG consists mainly of a vitreous negative after potential peaking shortly (120-200 ms) after reversal of the pattern. There is a notable absence in the pattern ERG of a b-wave that, however, can be elicited by a step increase of luminance over a uniform field. Pattern ERG amplitudes decrease monotonicaly with increasing spatial frequency and show no low-frequency attenuation when the pattern is phase reversed in square-wave fashion. This is markedly different than evoked potentials from the LGN and visual cortex that show band-pass characteristics. On the other hand, sinusoidal phase reversal reveals a clear attenuation of the pattern ERG amplitude at low spatial frequencies, whereas this type of stimulation produces very poor responses from LGN and visual cortex. The low spatial-frequency attenuation in the pattern ERG shows that the generating mechanism involves lateral interactions. There is thus a clear pattern-specific component in the pattern ERG. The pattern ERG has a surprisingly high contrast threshold relative to those estimated from cortical and LGN evoked potentials. Above threshold, pattern ERG response amplitude increases rapidly with contrast, but it often shows saturation at high contrast levels. These saturation points are generally high when contrast thresholds are high so that the rising portion of the contrast-response functions have fairly uniform slopes. Contrast-response curves from the LGN and cortical potentials are quite different from those for the retina in that amplitudes increase approximately linearly with log contrast over a 2-log-unit range (1 to 100%).