Pure tone habituation gradients based on responsiveness to electrical stimuli

Previous study demonstrated that responsiveness to electrical stimulation of subcortical areas of cats was enhanced if preceded by a tone pip. Habituation occurred when the tone was repeatedly presented. In this study, the capacity for generalization of a habituated tone was measured by occasionally interpolating a tone of different frequency or intensity among a group of monotonously presented standard tones, measuring responsiveness in each case. The responses were: (1) the visual cortical evoked potential, (2) evoked EMG activity, and (3) evoked head movement. Frequency shifts and intensity increases produced recovery of responsiveness at latencies as short as 90 msec, while decreases had no effect on recovery. Responsiveness increased with degree of frequency shift up to 576 Hz, but showed no further increase for greater shifts up to 2304 Hz. A shift of 16 Hz was effective, comparable to behavioral thresholds for frequency discrimination. Thus, these gradients agreed with the form predicted on the basis of central fatigue models of habituation and the bandwidth of the eighth nerve fibers.