The frequency following response for dichotic pitch stimuli: No evidence for pitch encoding.

Abstract

The frequency following response (FFR) was measured for five subjects during the last 350 ms of a 450-ms three-tone complex with a 244-Hz F0. All components were ramped on together in both ears; some were turned off gradually so that in the last 400 ms the harmonics presented were (i) 2+3+4 to the left ear (“mono”), (ii) 2+4 to the left and 3 to the right (“dichotic”), (iii) 2+4 to the left (“2+4”), (iv) 3 right (“3”). Stimuli mono and dichotic had the same pitch. A “vertical” montage (+=Fz, —=C7, ground=mid-forehead) was used. FFR waveforms to alternating-polarity stimuli were averaged for each polarity and added, to enhance envelope, or subtracted, to enhance temporal fine structure information. In both cases, FFR magnitude spectra for the dichotic condition were similar to the sum of the spectra for conditions 2+4 and 3, indicating that information from both ears was present, but lacked peaks at 244 Hz and other distortion products visible for condition mono. Furthermore, in both cases, autocorrelation functions for the dichotic condition were similar to those for 2+4 and dissimilar to those for mono. Thus, the FFR does not reflect similarity of pitch for dichotic and monaural presentation [Work supported by Wellcome Trust Grant No. 088263.]