Far-field responses of acoustic brain stem potentials in the thalamus and the subthalamic area.

Short latency evoked potentials in response to acoustic stimulation can be recorded from the thalamus and the subthalamus during stereotactic surgery for relief of intractable pain or tremor. The wave shape, polarity, amplitude and latencies of such responses do not change in wave I-V (wave VI and VII are exceptions) with different electrode positions from the cortical surface to the subthalamic area. Wave I-V may be generated at the midbrain stem, but wave VI-VII must be generated at the thalmic level. The amplitude of the potentials recorded at the thalamus and the subthalamic area, on the other hand, is enhanced after producing a lesion at the recording site. The results also indicate that short latency evoked potentials to clicks recorded from electrodes in the thalamus and the subthalamic area, or from an electrode at vertex, are volume-conducted far field potentials emanating from the brain stem.