Frequency following auditory brain stem responses in man.

The most interesting electrode positions for recording the FFR are the Vertex and the mastoid. The response is greatest at the Vertex. At the mastoid it comes earlier and probably in a different phase. As a combined result of these relations the responses are in opposite phase with a 500 Hz stimulus, and at 400 Hz they are in-phase. The Vertex-derived response is contaminated by the Jewett waves and at the mastoid there is in addition a CM-like activity. In order to simplify the interpretation we recommend use of the Vertex only as active electrode position, with reference electrodes on the neck, which is largely neutral. A very effective stimulus is a 500 Hz tone-burst with rise-fall times of 2 msec and a plateau of 6 msec. Shorter stimuli yield a smaller response, and with longer stimulus duration, adaption becomes a problem. The Jewett wave contamination can be eliminated by means of recording the response twice with opposite stimulus polarity and subtracting the two from each other.