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Contrast induction and visual cortex


Our goal was to determine where in the visual pathway the coding of contrast changes from a veridical signal to a representation of the illusory percept associated with Contrast Induction. We recorded Contrast Discrimination Thresholds for targets in a 5% and a 95% contrast surround. Thresholds in the 95% surround were significantly higher than for the 5% surround. We also demonstrated the attenuating effect of Contrast Induction on perception. We then recorded Contrast Response Functions for each surround using fMRI. BOLD activity in V1 shows no difference between the surrounds. V2 & V3 show the 95% contrast surround reduced BOLD activity to a greater degree than the 5% contrast surround. A Contrast Matching experiment was performed simultaneously to the fMRI experiment. The 95% surround reduced the appearance of contrast more than the 5% surround. To determine if any early visual areas respond in a way reflecting the psychophysical effect of Contrast Induction on Contrast Discrimination and Contrast Matching we used Heeger's Normalization model. V1 activity can reasonably predict the overall shape of the empirical TvCs, but cannot show the subtle differences between the surrounds. V1 cannot predict the Matching data. V2 has shown to under predict, accurately predict, and over predict the Contrast Induction effect on perception. V3 activity can accurately predict or over predict the effect. The activity of V2 and V3 in this study cannot predict Contrast Discrimination Thresholds. This evidence suggests that Contrast Discrimination and Contrast Matching are not performed by the same mechanism and are not executed in the same area of cortex. It appears that Contrast Discrimination can be predicted from cortex as early as V1. Our V2 and V3 predictions for Contrast Discrimination are inconclusive. We found no evidence that V1 can predict the altered perceptual effect of Contrast Induction. We suggest that the process of Contrast Discrimination is carried out as early as V1, but that Contrast Matching is not possible in V1 and must be achieved in V2 or higher. Accordingly, V1 contains the veridical representation of the target stimulus, while V2 and above represent the illusory percept associated with Contrast Induction

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