Abstract: PURPOSE: To explore information encoding strategies in the visual cortex of ascending hierarchy during visual contrast detection.
METHODS: We measured cats’ behavioral contrast sensitivity versus spatial frequency (CSF) and contrast threshold versus external noise contrast (TvC) functions using staircase method, and constructed neuronal CSF and TvC functions through ROC analysis of gamma power and theta-gamma phase-amplitude coupling (PAC) intensity based on local field potentials recorded in cats’ primary visual cortex (V1) and the area 21a, a higher-order visual cortex homologous to V4 of primates.
RESULTS: The neuronal CSFs and TvCs measured by either gamma power or theta-gamma PAC in the V1 and V4 cortex were highly correlated with their behavioral counterparts. However, the neuronal contrast sensitivity (CS) on CSFs and the contrast threshold (TC) on TvCs in V1 cortex were closer to behavioral counterparts than in V4 cortex if neuronal CSFs and TvCs were measured with gamma power, whereas the neuronal CS and TC in V4 were closer to behavioral counterparts than in V1 if neuronal CSFs and TvCs were assessed with theta-gamma PAC intensity.
CONCLUSIONS: V1 and V4 cortex may contribute to visual perception through different strategies of information processing. The low-level V1 cortex may encode stimulus contrast depending primarily on the high signal-to-noise activity of gamma oscillation whereas the higher-level V4 may encode stimulus contrast depending primarily on the modulation of gamma by low frequency of theta oscillation.

Key words: Visual contrast detection, V1 cortex, V4 cortex, Gamma oscillation, theta-gamma phase-amplitude coupling