Depth evoked potential and single unit correlates of vertex midlatency auditory evoked responses

The rostral brainstem and thalamus of awake cats were studied for depth correlates of a surface-recorded midlatency auditory evoked potential, wave "A', with a latency range of 17-25 ms. In response to clicks or pure tones, midlatency potentials were recorded from the level of the cuneiform nucleus (14-15 ms latencies) forward through the medial tegmentum to the level of the intralaminar thalamic nuclei centralis lateralis (CL) and center median (CM) (17-19 ms latencies). While this medial projection system to the thalamus involved primarily CL and CM, slightly longer latency responses were also found in nucleus ventralis anterior (VA) and ventralis lateralis (VL). A ventral diencephalic response was characterized by latencies averaging 0.5-1.2 ms less than those from the dorsal thalamic regions. Both surface and depth midlatency potentials showed comparable sensitivity to rate of stimulation. At click rates above 1/s, peak amplitudes diminished, and for rates greater than 10/s, both surface and depth midlatency responses were abolished. This rate sensitivity differs from that of the auditory brainstem responses (ABRs), which are essentially unchanged at rates of 20/s. Whereas ABRs are unaffected by surgical levels of sodium pentobarbital, the surface and depth midlatency potentials are replaced by a deep negativity within minutes following administration of anesthesia. Extracellular recordings of acoustically responsive single units in CL and CM demonstrated latencies comparable to the CL and CM field potential latencies. Both the units and field potentials were similarly rate sensitive. Each auditory unit showed a best frequency response, but none demonstrated somatosensory convergence. Bilateral aspiration of the inferior colliculus did not abolish the midlatency depth or surface responses. Rather, recordings in CL and CM suggested response enhancement over a two week postoperative period. Taken together these data suggest that the midlatency vertex potential, wave "A', reflects a generator system which projects from cuneiform nucleus, through the medial tegmentum to the medial thalamus, particularly to CL and CM. The functional significance of this medial auditory projection system remains to be determined. It could mediate physiological correlates of "state', modulate sensory input or motor output, or it could provide an integrative mechanism for the focusing of auditory attention.