ON AND OFF ACTIVITY GRADIENTS IN THE LATERAL GENICULATE-NUCLEUS OF THE CAT - A COMBINED C-14 2-DEOXYGLUCOSE AND D,L-2-AMINO-4-PHOSPHONOBUTYRIC ACID STUDY

Experiments on the lateral geniculate nucleus (LGN) of the cat based o n C-14 2-deoxyglucose (2-DG) autoradiography and intraocular injection s of 2-amino-4-phosphonobutyric acid (APB) provided evidence for gradi ents of metabolic activity in the ON and OFF pathways in layer A, but only very weakly, if at all, in layer Al. Alert and freely moving cats were exposed to square-wave gratings over a 45-min period after injec tion of the 2-DG. When one eye had been treated previously with APB, c ontralateral layer A showed a clear gradient of 2-DG label indicating that the remaining OFF pathway was most active ventrally in the layer and, by implication, that the ON pathway is normally most active dorsa lly. No gradient was apparent in layer Al ipsilateral to the APB eye. Control experiments based on binocular injections of tetrodotoxin (TTX ) demonstrated that no gradients were present in the baseline activity within the layers. Finally, monocular injections of TTX provided evid ence for gradients of nondominant eye activity in layers A and A1 that were maximal near the interlaminar zone between layers A and A1 and d eclined in mirror-symmetric fashion toward the dorsal border of A and the ventral border of A1. Combined with earlier anatomical studies sho wing depth-dependent patterns of geniculo-cortical projection, these r esults indicate that in the cat, as in several other species, the visu al input to striate cortex is partly organized around ON and OFF pathw ays. In addition, the results suggest that a systematic variation of b inocular interaction, perhaps related to ocular dominance, exists thro ugh the depths of the geniculate layers. Understanding how the ON and OFF pathways, and binocular interactions, are organized in the thalamu s may provide insight into the functional merging of these systems in the cortex.