Evoked potential decrements in auditory cortex. II. Critical test for habituation.

The goal of the present experiment was to rule out the hypothesis that evoked potential (EP) decrements during repetitive stimulation are due to a change in the subject's state; i.e., that the decrements are part of general, nonselective, non-specific decrement in all EPs as a result of a change in state during the course of the experiment. To this end, we obtained average evoked potentials (AEPs) to tone pips of two different frequencies; pips of one frequency were the repetitive stimuli, one and pips of the other frequency served as "test" stimuli. Before and again after a 15 min series of repetitive pips, AEPs to the repetitive stimuli and to the test stimuli were obtained. We found that from the beginning to the end of the 15 min repetitive-pip series, certain components of the AEPs to the repetitive stimuli decreased significantly in amplitude. However, comparable components in the AEPs to the test stimuli did not demonstrate significant decrements. These results suggested that the decrements were not part of a non-selective, non-specific, state-related decrement in all AEPs. However, the selective decrements could have been attributable to either of two aspects of the repetitive tone pips; their repetitive aspect, or their frequency. Because there were two independent variables, a counterbalanced design was required. We found that regardless of which frequency was repetitive, there were significant decrements in the AEPs to the repetitive stimuli and smaller decrements (if any) in the AEPs to the test stimuli. This ruled out the frequencies of the tone pips as critical factors in the decrements; by the process of elimination the repetitive aspect of the stimuli remained as the critical variable. Only decrements that were specific to the repetitive stimulus were considered to be habituatory. All of the habituatory decrements were in components subsequent to the initial AEP components and had latencies in the range of 17--105 msec. In several cases, when subjects participated in a supplemental experiment, the same AEP components demonstrated habituatory decrements in both experiments.