J Neurophysiol (May 1, 2003). 10.1152/jn.00820.2002
Submitted on Submitted 17 September 2002; accepted in final form 1 January 2003

Neuromagnetic Correlates of Perceived Contrast in Primary Visual Cortex

 ¹Department of Neurology II, Otto-von-Guericke University, D-39120 Magdeburg, Germany;  ²Institute of Neuroscience, University of Plymouth, Plymouth PL4 8AA, United Kingdom; and  ³Brain Research Institute,  ⁴Institute for Psychology and Cognition Research, and  ⁵Department of Neuropsychology and Behavioral Neurobiology, University of Bremen, D-28334 Bremen, Germany

Haynes, J. D., G. Roth, M. Stadler, and H. J. Heinze. Neuromagnetic Correlates of Perceived Contrast in Primary Visual Cortex. J. Neurophysiol. 89: 2655-2666, 2003. When a target grating is flashed into a larger, surrounding grating, its contrast is perceived to be lower when both gratings are oriented collinearly rather than orthogonally. This effect can be used to dissociate the perceived contrast from the physical contrast of a target grating. We recorded the transient electric potentials and magnetic fields evoked by flashed target gratings and compared them with psychophysical judgments of perceived contrast. Both early (100 ms) and late (150 ms) transients were reduced in amplitude when targets were flashed into a collinear rather than orthogonal surround, thus paralleling the reduction in perceived contrast. Although targets in orthogonal backgrounds required 40% lower physical contrast to match the perceived contrast of collinear targets, the amplitudes of electrophysiological transients of matching stimuli were almost identical. Thus the responses correlated better with perceived than with physical target contrast. This holds especially for the late transient response. Source localization indicated that the transients in question may originate in primary visual cortex. Our results therefore identify the activity of primary visual cortex as one possible neural correlate of perceived contrast.