Cellular Computation in Primary Visual Cortex

Individual neurons in the primary visual cortex respond selectively to different
features of visual stimuli, such as spatial orientation or direction of motion. A longstanding
goal in systems neuroscience has been to understand the transformations single
cells perform as they integrate synaptic inputs to generate spiking output. Recent
technological developments have facilitated these lines of investigation by enabling direct
measurement of the functional properties of single synaptic inputs to neurons in the
neocortex. It remains an outstanding question as to whether the tuning of single
neocortical neurons can be predicted by their excitatory synaptic inputs. Here, I show
that excitatory synaptic inputs exhibit significant functional diversity with respect to
orientation and direction selectivity. I show that cells can use at least two strategies to
overcome this functional diversity to achieve selective responses in the face of broadly
tuned excitatory input: enhancing responses to the preferred stimuli and suppressing
responses to the non-preferred stimuli. In the case of orientation selectivity, synaptic inputs cluster according to orientation preference and evoke local dendritic nonlinearities,
thereby enhancing somatic responses to the preferred direction. For direction selectivity,
cells receive excitatory synaptic inputs tuned to the preferred and null directions, but
selectively suppress inputs tuned for the null direction to enhance direction selectivity.
This suppression comes from direction-tuned GABAergic interneurons that make longrange,
intercolumnar projections to enhance direction selectivity.