Midlatency auditory evoked responses: differential recovery cycle characteristics.

Middle latency responses (MLRs), in the 10-100 msec latency range evoked by click stimuli, were examined in two sets of 7 adult subjects utilizing 5 randomly ordered rates of stimulus presentation: 0.5/sec, 1/sec, 5/sec, 8/sec and 10/sec. Evoked potentials were collected in 250 trial averages for each rate, and a replication across rates yielded 500 trial averages. Peak-to-peak measurements for Pa-Nb and P1-Nb components revealed that the P1 component was reduced in amplitude or absent at the faster rates, while the amplitude of the Pa component remained unchanged across rates. In addition, the latency of Pa was significantly longer for the faster rates of stimulation. These findings were similar across both mastoid and sternovertebral references. Taken together with previous work, these data suggest that the human Pa and P1 potentials reflect different generator systems. Moreover, the physiological similarities between the human P1 potential and the cat wave A suggest that in the human, as in the cat, this potential may be generated within the ascending reticular activating system, whereas the physiological similarities between the human Pa and the cat wave 7, as well as previous clinical data, suggest an auditory cortex origin of this component.