Intracranial and extracranial recordings of the auditory middle latency response

Simultaneous epidural and cortical depth recordings of the auditory middle latency response (MLR) were obtained from 18 anesthetized guinea pigs. Microelectrodes were advanced at a right angle to the cortical surface at sites shown to be optimal for recording surface MLRs. Transcortical polarity reversals of waves A (14 msec) and B (24 msec) of the MLR were recorded in depth penetrations initiated at sites on the temporal lobe with large amplitude surface potentials. In 6 of 18 penetrations yielding phase inversions, wave polarities changed abruptly as microelectrodes were advanced into the cortex. In the remaining penetrations, the reversals were preceded by gradual decreases in wave latencies at progressively deep sites. As electrodes were advanced beyond the depth at which polarity reversals were encountered, decreases in amplitude and only minor changes in latency were observed. Surface and depth MLR activity were temporarily eliminated immediately after electrolytic lesions were made at polarity reversal sites. Recovery of responses occurred within 30-60 min. Lesions produced in penetrations initiated at sites with no surface MLR activity had no effect. Histologic examination confirmed the location of the phase reversal sites as being within grey matter of the temporal lobe. These results are consistent with previous investigations in experimental animals which demonstrated transcortical polarity reversals, and provide evidence for dipolar generating systems of the early components of the MLR at the cortical level.