FIG. 4.

Electrode-to-axon distance greatly affects threshold, particularly for small distances. Threshold levels for two stimuli (single pulses and 5,000 pulse/s trains) are plotted for electrode-to-axon distances ranging from 0.235 to 2.025 mm. Single-pulse thresholds were defined by the level evoking a firing efficiency of 50%, while a criterion of 300 spike/s within an onset window (i.e., 0–12 ms) was used for the pulse-train stimuli. The arbitrariness of these thresholds advises against comparisons of the two curves' absolute values. The distances are defined from the center of the 0.45 mm diameter spherical electrode and the axon surface. To obtain measures of the space between electrode and neuron, a radius (0.225 mm) should be subtracted from the x-axis values.