Two-channel brain-stem frequency-following responses to pure tone and missing fundamental stimuli.

In 2 separate experiments the brain-stem frequency-following response (FFR) was recorded to a pure tone (200 Hz) and complex "missing fundamental" (MF) stimuli differing in temporal fine structure and envelope modulation depth. FFRs were simultaneously recorded in 2 channels with horizontal and vertical dipole orientations. Horizontal electrodes were identical in both experiments (right-left ear), but the vertical configuration was varied (vertex-left ear; vertex-linked mastoids). The horizontal channel yielded a well defined FFR to tone stimulation at a latency consistent with an origin along the auditory nerve. However, there was no horizontal response to MF stimulation. This latter finding provides electrophysiological support for the conclusion that MFs are not directly coded in the peripheral neural response. Vertical recordings, however, showed equally well defined FFRs to tone and MF stimuli. Thus, a representation of the missing fundamental frequency is registered in the brain-stem. Vertical latencies were consistent with a source at the level of the lateral lemniscus. The FFR is well suited to elucidate certain brain-stem mechanisms of auditory information processing. Important additional information results when responses are compared in horizontal and vertical dipole orientations. Thus, the present results provide the first evoked response demonstration of a peripheral-brain-stem dichotomy of MF coding.