| Description |
The primary visual cortex (V1) receives feedback projections from extrastriate visual cortical areas of a magnitude comparable to the feedforward projections it sends to them. Classically, this input has been thought to subserve "top-down" cognitive functions such as visual attention. Recently, it has been suggested that feedback could play role in more basic computations like surround modulation - the modulation of neuronal response by stimuli outside its receptive field (RF). The neural circuits and mechanisms generating such modulation remain unknown. A guiding hypothesis of this work is that feedback connections play an important role in this contextual computation. Using single unit electrophysiological recordings we have investigated the modulatory effects arising from the surround region of V1 neurons thought to be subserved by feedback connections, a region we term the ‘far' surround. We find that such modulation can be very specifically influence the response of the neuron to a stimulus inside its RF. First, by recording V1 neuron responses to an optimally oriented grating inside the neuron's RF while changing the orientation of a grating in the surround, we find that far-surround modulation is orientation-tuned, suggesting orientation specificity of the feedback connections themselves. Second, using grating stimuli confined to the ‘near' or ‘far' surround region, we find that the far surround is more broadly orientation-tuned than the near surround, perhaps indicating lesser specificity of feedback than horizontal connections. Third, using center and surround iv gratings both changing in orientation, we find that the surround is tuned to the stimulus presented to the V1 cells' receptive field, rather than to the neuron's preferred orientation. Specifically, the response of the neuron becomes maximally suppressed when the stimuli in the center and the surround are of the same orientation, irrespective of the preferred orientation of the neuron, and becomes less suppressed as the orientation difference increases, switching to facilitation in some instances. Thus, V1 neurons adapt their response to represent the orientation difference between the stimuli in the RF and the surround, and this is true across the range of the neuron's orientation tuning curve. Using computational modeling, we find that this mechanism can emerge from the dynamic interaction of orientation-specific horizontal and/or feedback connections with strong local recurrent connections. We suggest that such a mechanism may serve to enhance V1 responses to local orientation contrast, therefore increasing visual target saliency. |
