Spatial characteristics of motion-sensitive mechanisms change with age and stimulus spatial frequency
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Contrast-dependent interactions between classical (CRF) and non-classical regions (nCRF) of visual neuron receptive fields are well documented in primate visual cortex. Physiological models that describe CRF and nCRF interactions in single neurons have recently been applied to psychophysical measures of spatial summation and suppression in motion perception of young adults (Tadin & Lappin, 2005). We wished to determine whether such models could account for the reduction in spatial suppression that occurs in normal aging (Betts et al., 2005). We applied three models to duration thresholds obtained in a simple motion discrimination task using drifting Gabor stimuli that ranged in spatial frequency from 0.5 to 4c/deg. We found that a model in which the center CRF and surrounding nCRF are represented as spatially-overlapping excitatory and inhibitory 2D Gaussians with independent contrast response functions, which we call the Gain model, could account for the effects of aging simply by increasing the spatial extent of the CRF. Two additional models were evaluated. The Size model, which varied the size of the CRF as a function of contrast, produced CRF and nCRF size constants that departed significantly from physiological estimates of receptive field sizes. The Drive model, which yoked the activation of the suppressive nCRF to the CRF response, yielded reasonable fits to the data and suggested an age-related decline in the strength of suppression from the nCRF. However, the Drive model estimated the CRF size parameter to be equal to, or even slightly larger than, the nCRF size parameter, which is inconsistent with the physiological literature. Our findings therefore suggest that the Gain model provides the most plausible estimates of receptive field sizes. Based on this model, age-related increases in the size of central excitatory receptive fields relative to the inhibitory surrounds may contribute to behavioral measures of reduced spatial suppression found in older observers.
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