Surround suppression and facilitation in the fovea: very long-range spatial interactions in contrast perception.

Surround modulation of perceived contrast has been almost exclusively studied in short-range conditions, i.e., in situations where a tiny gap, at most, separates center from surround.

Area summation in human visual system: psychophysics, fMRI, and modeling.

Contextual modulation is a fundamental feature of sensory processing, both on perceptual and on single-neuron level. When the diameter of a visual stimulus is increased, the firing rate of a cell typically first increases (summation field) and then decreases (surround field). Such an area summation function draws a comprehensive profile of the receptive field structure of a neuron, including areas outside the classical receptive field.

Low-level features determine brightness in White's and Benary's illusions.

We masked White's and Benary's brightness illusions and simultaneous contrast with narrowband visual noise and measured detection thresholds and brightness. The noise was either isotropic or orientation filtered. A narrow spatial frequency tuning was found for detection and brightness for every stimulus.

Time course of suppression by surround gratings: highly contrast-dependent, but consistently fast.

Timing is critical for the effectiveness of a modulating surround signal. In this study, the optimal timing of a suppressing surround signal was measured psychophysically in human subjects. The perceived contrast of a fixated 1-deg circular patch of vertical sinusoidal grating (the target: 4 cpd, Michelson contrast 0.

Brightness processing in the visual cortex.

Single cell recordings have shown that some cells in the primary visual cortex (V1) signal surface brightness. However, fMRI experiments have found brightness related activation only in the higher cortical areas. In a psychophysical setup, we were able to dissociate the reduction of brightness caused by Gabor flankers, similar to the receptive fields in V1, from the edge induced brightness change.