Layer 4 in Primary Visual Cortex: Response Properties and Effects of Awake Brain States

In mammalian primary visual cortex (V1), layer 4 is the major input layer. It receives direct thalamocortical inputs, performs some of the earliest cortical computations and provides the main entrance of sensory information to the cortex. The first part of the current project investigated the response properties of two major cells classes in layer 4 of the awake rabbit V1: putative fastspike inhibitory interneurons (suspected inhibitory interneurons, SINs) and putative excitatory cells with simple receptive fields. The results show that the properties of these layer 4 populations are markedly distinct. SINs are far less linear, more broadly tuned to orientation, direction, spatial/temporal frequency, are more sensitive to contrast, have much higher spontaneous and stimulus-driven activity than simple cells, and they always had spatially overlapping ON/OFF receptive subfields. Furthermore, cross correlation between lateral geniculate nucleus (LGN) and SIN spike trains confirmed a fast and precisely timed monosynaptic connectivity. The second part of the project investigated the effects of alert/nonalert awake brain states on the response properties of layer 4 SINs and simple cells. It was previously shown that different brain states profoundly affect the receptive field properties, spontaneous firing rates, and spike-train statistics of neurons in the LGN. The results of the current project show that, in layer 4, alertness increases the strength and reliability of visual responses in layer 4 simple cells; it broadens their temporal frequency tuning but preserves their contrast sensitivity and selectivity for orientation, direction of motion and spatial frequency. Alertness also increased the amplitude and reliability of visual responses in layer 4 SINs but reduced their high spontaneous rate. Therefore, the net effect on SINs could be either enhancement or reduction of mean firing rate. Finally, alertness selectively suppressed the responses of layer 4 simple cells to stimuli moving orthogonal to the preferred direction and high contrast stimuli, effectively enhancing their responses to mid-contrast moving borders. Simulations of a direction/orientation detection model further demonstrate that enhanced response to preferred stimulation, increased reliability and stimulus-dependent suppression seen in layer 4 simple cells during the alertness can each contribute to increased cortical computational speed.