FUNCTIONAL-MORPHOLOGY OF THE FEEDBACK PATHWAY FROM AREA-17 OF THE CATVISUAL-CORTEX TO THE LATERAL GENICULATE-NUCLEUS

Two approaches were adopted to study the pattern of connectivity betwe en the cat visual cortex and lateral geniculate nucleus. Fourteen indi vidual cortico-geniculate axons were labeled and reconstructed after i ntracellular or extracellular injection of biocytin into regions of kn own receptive-field position and ocular dominance preference, and the distribution of boutons from multi-axon clusters was mapped in three d imensions and compared with the locations of strategically placed geni culate recordings made in the same tissue. The results show that the f eedback has an accurate retinotopic component but that individual axon s are both more extensive and more selective than described previously . Area 17 feedback axons terminate primarily in layers A and A1, but t he distribution of terminal boutons is strongly biased (3:1 ratio) tow ard the layer that matches their eye preference. Thus, those driven by the contralateral eye preferentially target layer A, and those driven by the ipsilateral eye target layer A1. Each axon also innervates the perigeniculate nucleus (PGN), but the pattern is otherwise variable, suggesting that there are different axonal classes. The terminal field s of individual axons are much larger than described previously, with a maximum spread of 500-1500 mu m. Nevertheless, the projection from a given location in area 17 has a center of maximum terminal density 40 0-500 mu m across, which is in retinotopic correspondence with the agg regate receptive field of the cortical cells of origin. The surroundin g zone of relatively sparse boutons, however, must permit corticofugal cells to influence visual processing well beyond the regions over whi ch their own responses summate. It follows that any geniculate cell re ceives corticofugal input covering an equally extensive area of visual space.