Physiological bases of the encoding of speech evoked frequency following responses

Abstract

Many studies have shown that fundamental frequency (F0) is represented in the speech evoked Frequency Following Response (FFR), however, it is not clear as to what aspect of the stimulus is the basis for the F0 coding. The energy at the Fo alone is less likely to be the basis, as our ear is less sensitive to very low frequencies which is evident from the very high RETSPL. Thus, the present study was taken up to analyse the independent role of high frequency harmonics and stimulus envelope in the encoding of the speech evoked FFRs. In the Experiment 1 of the present study, FFRs were elicited with a high-pass filtered syllable and compared with that of the unfiltered syllable. Results showed that the FFRs elicited for the 2 stimuli were not different in spectrum. This finding implies that the FFRs are primarily coded by the higher harmonics (frequencies beyond F2), and the lower harmonics contribute less for the coding of F0 and the first formants. However, as the envelope was same in both the stimuli, it cast a doubt that the responses obtained were because of the envelope and not the lower harmonics. To verify this, Experiment 2 was carried out, wherein FFRs were recorded to stimuli with envelope of the vowel portion removed without altering the fine structure. The FFRs elicited by the fine-structure stimulus revealed that the F0 amplitude was significantly lower compared to the original stimulus which implies that envelope is the key parameter for the coding of FFRs.