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Abstract

We studied the relationship between brain activity and conscious visual experience of motion in area V5 whose activity correlates directly with perceived motion in monkey and man. We used flash-suppression to dissociate the constant stimulus from the varying percept and showed that neurons in V5 modulate their firing rate according to whether a random-dot motion stimulus is perceived or not. In all cases, these changes reflect the ones produced under physical stimulus alternations, i.e. are higher for the motion compared to the static checkerboard stimulus; they increase in strength with increases in the directional coherence of the perceived dot movies. 70 of these perceptually-modulating neurons showed a further modulation when the strength of the suppressed motion stimulus was varied. These modulations also reflected the ones observed under physical stimulation - an increase in firing rate with an increase in the coherence of the motion stimulus. V5 neurons were thus shown not only to modulate their responses with respect to perceptual modulations under constant stimulation conditions, but also with respect to physical stimulus modulations under constant perceptual conditions. Therefore, characterising neurons as 'perceptually modulating' or 'stimulus modulating' is not straightforward. Furthermore, activity in V5, which correlates with the perceived motion, also modulates in response to changes in the physical stimulus that never reach perceptual awareness. Our results suggest that there is no clear distinction between purely 'processing' and purely 'perceptual' areas in the motion system. It is rather the activation of the same areas responsible for the processing of a certain visual feature that, under certain circumstances, can also lead to its perception. We conclude that, as in the human (Moutoussis and Zeki 2002), areas involved in the processing of a specific visual attribute are also responsible for its perception.