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Abstract

We studied the relationship between brain activity and conscious visual experience of motion in area V5, the activity of which 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 and also
increase in strength with increases in the directional coherence of the perceived dot movies. Over
half 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, characterizing 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‘ neurons or areas in the brain.
It is rather the activation of the same neuronal assemblies, responsible for the processing of a
certain visual feature, that can also lead to its perception.