A psychophysical study of the achromatic watercolor effect and computational modeling of brightness-related responses in visual cortex
Cao, Bo Cloud
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We investigate the neural mechanisms of brightness perception. The first of two projects concerns the effects of luminance contrast intensity and polarity on the achromatic watercolor effect (WCE), which is a filling-in phenomenon in a region demarcated by two thin abutting lines that induce illusory brightness changes in human observers. Our main finding is that the relationship between the strength of the achromatic WCE and the luminance contrast between the two lines is not monotonic but like an inverted-U curve. This result indicates that non-linear cortical processing of contour information affects the lightness of the enclosed region. In the second project, we develop models with a small set of parameters to explain the brightness-related responses in cat visual cortex (Rossi et al., 1996; Rossi and Paradiso, 1999). These studies reported a cut-off at 4 Hz in the modulation amplitude of neural responses to large (up to 14 degrees) simultaneous contrast stimuli in the striate cortex of cats, as the temporal frequency of the luminance of flanking patches increased, while the luminance of a central patch covering the neurons' classical receptive fields (CRFs) was held constant. We find that at least three models with the following mechanisms can fit the data: 1) slow local inhibition (Slow Inhibition Model); 2) slow excitation of the model neurons (nodes) in the second of two layers representing primary visual cortex (V1) and extrastriate cortex. These nodes project to the inhibitory nodes in the first model layer (Slow Excitation Model); 3) conduction delays along lateral connections (Delay Model). However, the Slow Inhibition Model predicts that neurons in extrastriate cortex show similar response modulations as neurons in V1, while the Slow Excitation Model predicts that, unlike the modulations of V1 neurons shown in the experimental data, neurons in extrastriate cortex show slow modulations of responses to both the direct luminance change and the simultaneous contrast stimuli. The Delay Model predicts that the cut-off frequency of the response modulations depends on the distance from the flanker to the CRFs of the neurons. Thus, although the three suggested models fit the current electrophysiological findings, they have distinct predictions that can be further tested experimentally.
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