Abstract
Sensitivity to luminance modulation is greatly enhanced at low temporal frequencies ([[lt]]2 Hz) if a target is set in a surround matched to its mean luminance rather than in a dark surround. Kelly (1979) proposed this was due to fixational eye movements across the edge, but experiments with stabilized images (Tulunay-Keesey, 1973) led this explanation to be discounted, and some kind of local contrast effect was assumed (Spehar & Zaidi, 1997). We have examined the neurophysiological signals which might be associated with this effect. Responses of macaque ganglion cells to slowly modulated targets (0.5 Hz) were recorded as a function of the luminance of the surround (dark, equiluminant, and bright), and as a function of the location of the stimulus-surround border relative to the receptive field. Responses of magnocellular (MC) ganglion cells were little affected by surround luminance and edge effects were weak. Responsivity to such stimuli was low, and neurometric analysis indicated a high contrast threshold. Responsivity of parvocellular cells were of even lower amplitude. Introduction of small target movements dramatically improved neurometric sensitivity of MC cells near the target edge, provided cell type (on- or off-center), movement direction and modulation phase was appropriate. We conclude that fixational eye movements (which may be difficult to entirely eliminate by image stabilization) do indeed form the basis of this psychophysical result. This is consistent with recent evidence concerning the importance of fixational eye movements in spatial tasks (Donner & Hemilä, 2007).
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SpeharB.ZaidiQ. (1997). Surround effects on the shape of the temporal contrast-sensitivity function. J. Opt. Soc. Am., 14, 2517–2525.
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Supported by NIH EY013112.