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A recent fMRI study has shown that the BOLD response to visual stimuli in human V1 is spatially nonlinear near the edge of the stimulated region (Shmuel et al, 2005, in Proceedings of the 11th Annual Meeting of the Organization for Human Brain Mapping, Toronto, 12 - 16 June). In particular, the activation extends significantly (ie about 1 mm) beyond the expected retinotopic mapping of the stimulus edge. However, it is unclear whether the corresponding increase in neuronal activity shows a similar nonlinearity and whether this nonlinearity biases our perception. Here, we have studied whether a visual stimulus that is spatially constrained by additional surrounding stimuli leads to a different percept than a stimulus that is presented on its own. We compared the perceived widths of bars with and without abutting flanks. High-contrast checker-bars flickering at 5 Hz were presented to four naive subjects. One bar flanked by bars flickering in counterphase and one non-flanked bar were shown simultaneously for 200 ms. The widths of both bars and flanks were varied randomly and the position jittered around an eccentricity of 6.8 deg. Subjects were instructed to fixate and to judge which one of the two bars had the larger width. A psychometric function was fit to the behavioural responses. Estimation of the point of subjective equality revealed that all subjects perceived the non-flanked stimulus as having a larger width. Over all subjects, the effect was about 0.3 deg of visual space, which corresponds to a cortical distance of about 1.3 mm (Brewer et al, 2002 Journal of Neuroscience 22 10416 - 10426). We conclude that the perceived dimensions of bars depend on the context of the visual stimulus. Whether this perceptual effect resembles previously found spatial nonlinearities in the BOLD response needs to be further studied.
