Segregation of on- and off-center aJ

In the lateral geniculate nucleus of the mink, on-center and off-center neurons occupy separate layers (LeVay, S. & McConnell, S. K. (1982) Nature (London) 300, 350-351). To study the mode of termination of geniculate af- ferents in area 17, we recorded from their terminal arboriza- tions in layer IV after the destruction of cortical neurons by injection of kainic acid. At the majority of recording sites, multifiber responses were entirely or predominantly of one type: on-center or off-center. Responses obtained during per- pendicular penetrations showed the same preferred sign of contrast throughout the thickness of layer IV. During tangen- tial penetrations through the layer, we encountered sequences of on- and off-center activity separated by stretches of mixed responses. We conclude that on- and off-center afferents ter- minate in separate, alternating patches that occupy the full thickness of layer IV. These coexist with another set of patches in which the same afferents are segregated by eye of origin. Most retinal ganglion cells have on-center or off-center re- ceptive fields. An on-center cell is excited by an increase in light intensity in the center of its receptive field; an off-cen- ter cell responds to a decrease in intensity (1). Neurons in the lateral geniculate nucleus, the main target of the optic nerve, also have on-center or off-center receptive fields; there appears to be little convergence of on- and off-center inputs at this level (2-4). Geniculate neurons project in turn to the visual cortex. How cortical receptive fields are elabo- rated from geniculate inputs is not yet clear, but there is evi- dence for at least partial convergence of the on and off chan-