Poster Session I: Binocular contrast integration: Cortical and behavioral signals reflect different computations.

Abstract

Although binocular contrast perception under dichoptic viewing conditions has been extensively characterized behaviorally, little is known about how the signals from each eye are combined in cortex. Here we compared simultaneously-collected behavioral and EEG measures of dichoptic contrast perception. Observers (n=16) dichoptically viewed a 2-cpd grating flickering at 7.5 Hz and rotating slowly (1º/s). The contrast of the grating shown to each eye modulated sinusoidally over time at independent rates (1/6 and 1/8 Hz). We recorded EEG activity while observers positioned a joystick lever to report perceived contrast as it changed over time. A multiband filter was used to isolate the SSVEP responses to 7.5 Hz signals from electrode Oz (occipital pole), and the standard deviation of this signal provided a measure of neural response amplitude as a function of contrast. A simple model was fit to VEP and behavioral responses, [(L^m+R^m)/2]^(1/m)], which essentially characterized whether responses were better fit by a mean (m≈1) or a max (m>1) model. VEP responses (m = 1.0; MSE = 0.013) were well fit by a mean model, suggesting the EEG signal may have been driven by the input layers of V1. In contrast, behavioral responses (m = 8.8; MSE = 0.011) were well fit by a model that was heavily shifted towards a max model, suggesting a significant non-linear transformation of the contrast signal between the input layers of V1 and conscious perception.