Different Inhibitory Synaptic Input Patterns in Excitatory and Inhibitory Layer 4 Neurons of Ferret Visual Cortex

The synaptic mechanisms underlying the generation of orientation and direction selectivity in layer 4 of the primary visual cortex are still largely unclear. Previous in vivo work has shown that intra-cortical inhibition plays a major role in generating the properties of orientation and direction selectivity. Excitatory and inhibitory cortical neurons differ in their receptive field properties: excitatory neurons tend to be orientation- and direction-selective, inhibitory neurons tend to be orientation-, but not direction-selective. Here we have compared the relationship between direction preference maps recorded in vivo and synaptic input maps recorded in vitro from excitatory and inhibitory stellate cells in layer 4 of ferret visual cortex. Our goal was to test whether the differences in direction tuning between these cell populations might result from different inhibitory connectivity patterns. We found that excitatory neurons, which are direction tuned in vivo, receive approximately 50% of their inhibitory inputs from cortical regions of opposite direction preference whereas inhibitory cells, which are not or poorly direction tuned, receive only very few inputs from regions of opposite direction preference. This confirms that inhibitory connections arising in cortical regions of opposite direction preference may be required to create or strengthen direction tuning in their target neurons. Thus, differences in intracortical inhibitory circuit patterns may underlie the differences in receptive field properties observed between excitatory and inhibitory neurons in vivo.