1. The earliest electrical response detectable in foveal striate cortex of anaesthetized Rhesus monkeys following light stimulation is a graded potential which is positive at the cortical surface and negative in the grey matter and has a peak latency of about 60 msec. The response is similar at both the on- and the off-phase of a light stimulus.2. The relationship of this graded potential to the depth of the recording electrode, to the latency of extracellular impulses and to post-synaptic potentials suggests that it is generated by the depolarization of cortical cells.3. Action spectra obtained in the presence of strong selective chromatic adaptation indicate the participation of all three cone mechanisms in this response.4. Each cone mechanism contributes a similar potential to the response but antagonism between cone mechanisms is apparent. The proportion in which a cone mechanism contributes to the response varies from one area to another implying topographical differences in the representation of cone mechanisms in striate cortex.
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