A computational study of brightness-related responses in visual cortex

Rossi, Rittenhouse, and Paradiso (1996) reported a cut-off at 4 Hz in the modulation amplitude of neural responses to large (up to 14°) 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. This indicates that the modulation may involve slow processing of information in visual cortex. We develop models with a small set of parameters to explain these brightness-related responses in visual cortex. A model with any of the following mechanisms can fit the data: (a) slow local inhibition (Slow Inhibition Model); (b) slow excitation of the nodes in the second of two layers, which feed back to the inhibitory nodes in the first layer (Slow Excitation Model); and (c) 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. Further physiological experiments could clarify which mechanism plays an important role in the brightness-related responses in visual cortex.