TRANSGENICULATE SIGNAL TRANSMISSION TO MIDDLE SUPRASYLVIAN CORTEX IN INTACT CATS AND FOLLOWING EARLY REMOVAL OF AREA-17 AND AREA-18 - A MORPHOLOGICAL-STUDY

Removal of cat areas 17 and 18 early, but not late, in postnatal devel opment results in the sparing of certain reflexive and nonreflexive vi sually guided behaviors. These spared behaviors are accompanied by an expansion of geniculocortical projections to middle supra sylvian (MS) cortex. However, little is known about the types of visual signals re layed along these pathways. The purpose of our study was to reveal the morphologies of the neurons participating in the rewired circuits and , by relating them to the morphologies of functionally characterized n eurons described by others, infer the types of visual signals transmit ted via the lateral geniculate nucleus (LGN) to MS cortex. To do this, we retrogradely labeled LGN neurons from MS cortex with fluorescent m icrospheres, and subsequently intracellularly filled them with Lucifer Yellow. We then classified well-filled neurons according to a battery of morphological parameters (such as soma size and shape, and dendrit ic field-form and specializations), and compared them with already def ined structure/function relationships. By doing this, we found that th e large majority of visual thalamic relay neurons to MS cortex of both normal cats and cats that incurred removal of areas 17 and 18 were ty pes I and IV. These results indicate that visual Y and W signals, resp ectively, are relayed directly from LGN to MS cortex in both types of cats. Following the early lesions, some of the MS-projecting type I ne urons were found in layers A and Al, where they are never found in int act cats. Thus, some layer A and Al type I neurons redirect axons to M S cortex following early removal of areas 17 and 18. For the type IV M S-projecting neurons in early lesioned cats, the somas were hypertroph ied and they had more profuse and broader dendritic arbors than equiva lent neurons in intact cats. These results suggest that dynamic intera ctions take place between inputs and outputs of LGN neurons during dev elopment that influence final LGN neuron morphology. Moreover, they su ggest that signals transferred to MS cortex by type IV neurons may be modified by early lesions of areas 17 and 18. Overall, these results c ontribute to our understanding of the types of behaviors that may be s pared by early lesions of areas 17 and 18.