The sensory neocortex of the brain is essential for sensation and perception, and this has led to an intense effort to understand the functional properties of this tissue. Pyramidal cells comprise about 80% of all neurons in the cortex, and a special type of pyramidal cell in layer-5 that shows intrinsic bursts of activity is one of the most anatomically and physiologically distinctive of pyramidal sub-populations. These neurons have especially thick apical dendrites and they project descending axons that branch to innervate multiple subcortical areas of the brain. The superior colliculus is one of the subcortical structures innervated by these bursting pyramidal neurons and is responsive to visual stimulation. Such "corticotectal" pyramidal neurons are known to strongly influence the response properties of neurons in the superior colliculus, but the mechanism of this influence is still unknown. It is not known, for example, whether receptive field properties are conveyed to superior colliculus neurons directly by corticotectal input, or whether the corticotectal influence modulates other input to the neurons of the superior colliculus. This proposal focuses on the means by which corticotectal neurons influence their targets in the superior colliculus in awake rabbits, and how EEG state (alert vs. drowsy state) may affect corticotectal communication. First, Dr. Swadlow will gain a comprehensive, global view of the synaptic impact of single corticotectal neurons on the superior colliculus, and the dynamics of this impact. He will do this by electrophysiological examination of monosynaptic currents generated within the superior colliculus by the impulses (bursts and single spikes) of single corticotectal neurons. Next, Dr. Swadlow will gain a local view of the synaptic impact of single corticotectal neurons on members of specific neural sub-populations within the superior colliculus, and the dynamics of this impact. To do this, he will obtain simultaneous extracellular electrophysiological recordings from corticotectal neurons and from specific sub-populations within the superior colliculus, and apply cross-correlation methods to infer synaptic connectivity. In addition, Dr. Swadlow will record intracellularly from corticotectal neurons and examine the sub-threshold sensory events related to bursting behavior. The proposed studies will provide important insights into both the manner in which sensory cortical information is processed in an awake, perceiving subject and how cortical neurons communicate with and influence neurons in distant regions of the brain. Such information is crucial if we are to understand the mechanisms of perception in awake, perceiving mammals. The proposed studies will also further the training of future neurobiologists through involvement of undergraduate students in the PI's research.
