A stimulus located outside the classic receptive field (CRF) of a
striate cortical neuron can markedly influence its behavior. To study
this phenomenon, we recorded from two cortical sites, recorded and
peripheral, with separate electrodes in cats anesthetized with Propofol
and nitrous oxide. The receptive fields of each site were discrete
(2–7.3 deg between centers). A control orientation tuning (OT)
curve was measured for a single recorded cell with a drifting grating.
The OT curve was then remeasured while stimulating simultaneously the
cell's CRF as well as the peripheral site with a stimulus
optimized for that location. For 22/60 cells, the peripheral
stimulus suppressed the peak response and/or shifted the center of
mass of the OT curve. For 19 of these 22 cells, we then reversibly
blocked stimulus-driven activity at the peripheral site by
iontophoretic application of GABA (0.5 M). For 6/19 cells, the
response returned to control levels, implying that for these cells the
inhibitory influence arose from the blocked site. The responses of nine
cells remained reduced during inactivation of the peripheral site,
suggesting that influence was generated outside the region of local
block in area 17. This is consistent with earlier findings suggesting
that modulatory influences can originate from higher cortical areas.
Three cells had mixed results, suggesting multiple origins of
influence. The response of each cell returned to suppressed levels
after dissipation of the GABA and returned to baseline values when the
peripheral stimulus was removed. These findings support a cortical
model in which a cell's response is modulated by an inhibitory
network originating from beyond the receptive field that supplants
convergence of excitatory lateral geniculate neurons.