Effects of cryogenic blockade of visual cortex on the responses of lateral geniculate neurons in the monkey

Abstract

Striate cortex in unanesthetized, paralyzed rhesus monkeys was cooled to assess the effects of temporary blockade of corticogeniculate fibers on responses of lateral geniculate neurons. Most neurons recorded under the thermode became inexcitable on cooling. After the onset of cooling, superficial layers became inexcitable before deep layers, and at a time when superficial activity was blocked, deep neurons were still orientation specific and retained this property until they themselves became inexcitable. Twenty-eight percent of the geniculate neurons projecting to cortex under the thermode were judged to be affected by cortical blockade, generally showing increases in driven and spontaneous activity. Cooling produced changes in the scale of response magnitude, rather than in the timing of impulse discharge, as evaluated by response time histograms. Responses evoked by bars or edges or by flashing spots were equally affected. The changes of activity produced were generally small and frequently difficult to differentiate from spontaneous shifts in excitability. In contrast, a visually driven inferior pulvinar neuron with a receptive field in the cooled area was strongly affected by cortical blockage, and driving quickly and completely returned soon after the cortex was rewarmed. It therefore appears that the method of blockade was effective but incapable of producing large effects in geniculate neurons. This suggests that cortical control may be inherently weak in the immobilized animal or that the distribution of activity between excitation and inhibition is equally balanced.