Journal of Vision - Cross- and Iso- oriented surrounds modulate the contrast response function: The effect of surround contrast, by Yu, Klein, & Levi

Cong Yu	School of Optometry, University of California at Berkeley, Berkeley, CA, USA
Stanley A. Klein	School of Optometry, University of California at Berkeley, Berkeley, CA, USA
Dennis M. Levi	School of Optometry, University of California at Berkeley, Berkeley, CA, USA

The detectability and appearance of visual targets can be modulated by surround stimuli. In this study we asked how cross- and iso-oriented surrounds modulate contrast detection and discrimination in foveal vision. We systematically measured the Threshold-versus-Contrast (TvC) functions over a wide range of pedestal and surround contrasts. Our results show that cross-oriented surrounds lower the contrast threshold over the entire range of pedestal and surround contrasts, but iso-surround modulation of the TvC function is dependant on the relative contrast, being facilitative when the surround/pedestal contrast ratio Csur/Cped < 1 and suppressive when Csur/Cped > 1. Data fitting indicates that cross-surround modulation (facilitation) is mainly due to improved gain, except at very low and high surround contrasts. Iso-surround modulation on the other hand is more complicated, probably reflecting more than one surround process as determined by the relative contrast.