Spatial organization of sensitivity regulation in rod vision

Abstract

To investigate whether scotopic sensitivity is set locally or in neural "pools", we have tested the spatial variation in sensitivity after bleaching with gratings using 3 different methods. One experiment circumvented the influence of involuntary eye movements by deliberately randomizing the horizontal position of a fine test line on the area bleached by the vertical gratings. The spatial variation of threshold across the bleached area is reflected in the width of the frequency-of-seeing curve. A clear difference between the probability-of-seeing curves following a grating bleach and a uniform bleach was seen only up to between 4.2 and 6.3 c/deg, suggesting that adaptation signals are pooled so as to almost obliterate the contrast in finer gratings than this. In a second experiment the lowest bleaching-grating contrast (for a space-averaged initial rhodopsin bleach of 10%) that produced a patterned afterimage stayed close to the scotopic threshold contrast for frequencies from 1 to 6.4 c/deg, but it rose above the contrast threshold at high spatial frequencies. This slight loss of sensitivity at the high frequencies is more evidence for pooling in adaptation. A third experiment assessed the sensitivity profile at the adapting site without any influence of later stages of neural integration. Bleaching and test gratings of slightly different spatial frequency were flashed successively. If the effect of bleaching is restricted to the bleached rods, the observer will effectively be looking at the test grating through a grid of sensitive and insensitive stripes in his own retina. The two gratings come in and out of register at the difference frequency, and a corresponding low-frequency grating should be visible even when the test and bleaching gratings are not themselves resolved by the later stages. We could not see the difference frequency unless the test and bleach gratings were themselves coarse enough to be resolvable in rod vision. This is very strong evidence against any model in which each rod has its own sensitivity-regulating mechanism, and instead supports (for these conditions) Rushton's view that adaptation is entirely the work of a neural pool. The estimated pool size is about 10 min arc of visual angle.