Thalamocortical Architecture of a Visual Cortex Without Orientation Columns

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DESCRIPTION (provided by applicant): A major goal of the proposed work is to better understand how thalamocortical inputs to rabbit visual cortical (V1) neurons contribute to the synthesis of the diverse receptive fields properties seen in the input layers of V1. In both layers 4 and 6, these include (a) "simple" cells with highly oriented and directional receptive fields composed of separate parallel ON and/or OFF sub-zones, and (b) putative fast spike inhibitory interneurons (SINs) with overlapping ON-OFF, receptive fields that lack significant orientation/direction selectivity. While considerable work has been done on these questions for simple cells in layer 4 of feline V1, little is known about receptive field synthesis in V1 of rabbts and rodents, and the nature of the thalamocortical contribution to this synthesis. V1 of rabbits and rodents is similar to that of carnivores and primates in having many "simple" cells that are highly selective for the orientation of a visual stimulus. However, unlike carnivores and primates, rabbits and rodents lack orientation columns. Moreover, their visual thalamus (LGN) contains neurons that provide directional and orientation information directly to V1. Whereas in cats and primates, orientation and directional selectivity are thought to be synthesized within V1, in rabbits and rodents these properties could potentially be inherited from the thalamus. The proposed experiments will address this question directly, in both layers 4 and 6. We will record from different cell types in the LGN of awake rabbits, some of which have directional and or orientation selective receptive fields. We will make simultaneous recordings from simple cells and SINs in the input layers (4 and 6), the simple cells being highly tuned for orientation/direction, the SINs being poorly tuned. We will use cross-correlation methods to see which LGN cells make functional contacts with which cortical cells. Our preliminary results suggest that layer 4 simple cells do not inherit their orientation/directional preference from LGN directional selective neurons. Therefore, the orientation preference of layer 4 simple cells may be largely created from the receptive field arrangement of ON and OFF LGN afferents, as in the cat. Alternatively, it may be inherited from LGN inputs with concentric receptive fields that show an orientation bias. By measuring the receptive field properties of the LGN inputs to a L4 cortical simple cell, our experiments will be able to distinguish between these two possible mechanisms.