STATE DEPENDENT CONTROL OF RETINOGENICULATE TRANSMISSION

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DESCRIPTION: (from the investigator's abstract) It is now clear that the lateral geniculate nucleus (LGN) of the dorsal thalamus functions as more than a simple relay of retinal information to the visual cortex. Instead, this structure acts as a dynamic filter, determining what, when and how much retinal information gets passed on to visual cortex. The cellular factors that control this filtering are the complex innervation patterns and intrinsic membrane properties of LGN cells. These have been studied extensively in vivo using anesthetized, paralyzed animals, or in vitro with a thalamic slice preparation. The problem is that we lack information about how these cellular factors interact with more dynamic factors, such as behavioral state or eye movements, to control the efficacy of visual processing through the LGN. Dynamic factors can only be studied in an awake, behaving animal. The first objective of this proposal is to understand how cells in the LGN filter retinal signals during states of wakefulness. In awake, restrained cats, we plan to record the activity of single neurons in the LGN and examine how aspects of cellular excitability and visual responses are influenced by states such as drowsiness, orienting vigilance, and focal attention. We manipulated the awake state by training cats to perform various visuomotor tasks, We evaluate behavioral state by monitoring the animal s performance on these tasks and by recording EEG activity. The second objective of this proposal is to understand how eye movements and orbital position modulate the transfer characteristics of LGN neurons. We plan to monitor the activity of LGN cells before, during and after various fixation and saccade tasks. Recordings are conducted with the animal's head in a fixed position and gaze is monitored by using the scleral search coil technique. Finally, we will begin to explore some of the cellular mechanisms underlying the dynamic modulation of LGN activity. Specifically, we will determine the conditions (i.e., behavioral state, eye movement or orbital position). which cause LGN cells to respond in either a tonic or burst firing mode. Burst responses reflect the activation of a voltage-dependent low threshold Ca2+ conductance, and permit hyperpolarized LGN cells to respond vigorously when depolarizing events reach spike threshold. The proposed experiments address a major and enduring challenge for neuroscience which is to link cognitive behavior to the behavior of single neurons. More specifically, it is our hope that these studies provide a better understanding for the role played by thalamic circuitry in various aspects of arousal, visual attention, and eye movements. Another important aspect of this proposal is to understand how the brain regulates burst responses in LGN cells during retinogeniculate signal transmission. Burst firing in LGN cells serves a general experimental model for understanding the cellular bases of sleep-waking cycles and pathological thalamocortical rhythms.