Excitation of visual cortex neurons by local intracortical microstimulation

The threshold current required for the excitation of visual cortex neurons in the vicinity (approximately 1 mm) of an intracortical stimulating electrode was measured as a function of the stimulus pulse duration in the anesthetized cat. For cortical neurons with latencies of activation from 0.4 to 3.4 ms and for stimulus pulse durations from 0.02 to 0.7 ms, the threshold current for all neurons tested decreased in an exponential fashion as the pulse width was increased. Rheobase current values (ampere-threshold) were 1.2 to 516 muA (mean 160 +/- 24 muA, N = 24) and chronaxie values were 0.07 to 0.79 ms (mean 0.217 +/- 0.036 ms, N = 24). When the quantity of charge required for neuronal excitation was calculated, a quasilinear relationship was found between threshold charge and stimulus pulse width. The minimum threshold charge (coulomb-threshold) occurred for the briefest pulse widths tested and were 2 to 86 nC (mean 36.4 +/- 4.4 nC, N = 24). When the pulse energy index was calculated (threshold current squared multiplied by the pulse width), the minimum pulse energy capable of generating an evoked response (a single action potential) occurred when the pulse width was approximately 80% greater than the chronaxie. These studies demonstrate that the predictions derived from A. V. Hill's classical theory of nerve excitation are to a first approximation obeyed by visual cortex neurons. For the three parameters analyzed as a function of stimulus pulse width, the pulse current is minimized at long pulse durations, the pulse charge is minimized at short pulse durations, and the pulse energy is minimized at pulse widths of intermediate value.