Some Subcortical Determinants of Visual Cortical Excitability in the Cat

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Abstract
Alterations in visual cortical excitability due to tetanization of MRF, SC or PUL were assessed in terms of the amplitude of the cortical response to single, and to paired lateral geniculate (LGB) shocks at varying temporal intervals (the recovery cycle). Tetanization of any one of these subcortical structures could potentiate both the single evoked response and the recovery cycle. In addition, all three structures produced maximal effects on later components of the shock response, and reacted similarly to changes in tetanization intensity, LGB shock intensity and level of Nembutal anesthesia. MRF tetanization generally produced the greatest potentiation effect at lowest voltages, with SC and PUL requiring progressively higher voltages to produce an effect. In the case of PUL smaller effects on both the recovery cycle and single cortical responses were obtained. While there were no significant differences in the magnitude of recovery cycle potentiation following MRF and SC activation, the time course of single evoked response facilitation was different after stimulation of these two structures. Comparing single response potentiation to that of the recovery cycle, it was found that for all structures there were consistent differences in the time course of these two effects. Higher tetanizing intensities were also generally necessary to achieve maximal potentiation of the recovery cycle, and enhancement of the single evoked response could occur without concominant changes in the recovery cycle.