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Abstract

Recent fMRI studies demonstrated negative BOLD response beyond the stimulated regions within retinotopic visual areas (Shmuel et al, 2002 Neuron 36 1195 - 1210). It has been suggested that this negative BOLD response is a reflection of automatic withdrawal of attention when a large stimulus is presented in the visual field (Smith et al, 2004 NeuroReport 11 271 - 277). We investigated psychophysically whether stimulating large parts of the visual field significantly impairs the detection of a small target stimulus in the non-stimulated parts of the visual field. In half of the trials subjects had to detect a Gabor patch (target) of full contrast, presented either centrally (2 - 4 deg) or peripherally (8 - 10 deg) relative to a rotating checkerboard ring (stimulus) with an eccentricity of 4.5 - 7.5 deg. In the other half of the trials, the subjects had to detect the target in the central or peripheral region in the absence of the stimulus. Fixation had to be maintained on a spot in the centre of the screen. The target was presented at random orientation, time, and location within the central or peripheral region. Subjects indicated the detection of the target by pressing a button. We observed that the reaction times in both the central and the peripheral region are higher when a stimulus is presented than when no stimulus is presented (centre: p < 0.001, two-tailed paired t-test; periphery: p < 0.001, two-tailed paired t-test). Our results suggest that stimulating a large part of the visual field might cause automatic withdrawal of attention from the non-stimulated parts of the visual field.