Habituation and rate effect in the auditory cortical potentials evoked by trains of stimuli

The effects of the stimulus repetition rate over the habituated auditory cortical evoked responses were studied. The stimulation pattern consisted of trains of pure tone bursts with interstimulus interval (ISI) of 1 s, and intertrain interval (ITI) of 5 s, delivered with constant time and intensity parameters during 93 min. The analysis of the responses was based upon across averaging of the trains, each single response being evaluated in the latency and amplitude parameters. Two time-dependent factors affected the responses in a distinct way: the habituation throughout the whole stimulation and the rate effect within the train. The linear regressions of the time/amplitude functions of the responses were calculated in relation to the duration of ISI and ITI. By introducing a correction factor depending on the repetition rate it was possible to evaluate the relationships between habituation and repetition rate. Changes in the repetition rate do not have any effect on the habituation process. The two phenomena are completely distinct, and they probably have neurophysiologic substances corresponding to different levels in the central nervous system (CNS).