Distortion of spatial selectivity by pattern onset stimulation.

Spatial selectivity of pattern evoked potentials has been thought to provide evidence of lateral inhibition. However, spatial tuning functions may be distorted by pattern onset stimulation that is applied repeatedly to the same area of retina so an after-image is formed. This only applies at low spatial frequencies because of the randomizing effects of eye movements. Low-frequency attenuation may therefore be exaggerated. Pattern reversal stimulation has the opposite effect and this is reflected in the literature by fewer reports of bandpass functions. A new method has therefore been devised to provide the true spatial response function. The spatial phase is reversed after every two consecutive presentations. By combining this paradigm with a correction for the optical transfer function of the eye, the true neural response function is obtained. Ten subjects participated in this study to evaluate the distortion of spatial selectivity in the pattern electroretinogram. The new stimulus paradigm reduced the low spatial frequency attenuation to a barely significant level giving an almost flat amplitude response for the + ve and - ve transients of the pattern electroretinogram for check sizes from 222' to 7' angular subtense. However, correction for optical degradation produces bandpass curves, which closely correspond to those predicted from recent data on receptive fields of primate retinal ganglion cells.