Recording the human brainstem frequency-following-response in the free-field.

Background: The human auditory brainstem frequency-following response (FFR) is an objective measure used to investigate the brainstem's encoding ability of sounds. Traditionally, FFRs are recorded under close-field conditions (earphones), but free-field stimulations (loudspeaker) have yet to be attempted, which would increase the applications of FFRs by making this technique accessible to those who cannot wear inserted transducers. Here we test the feasibility and reliability of measuring speech ABRs across free and close-field.

New Method: The FFR was evoked by a 40-ms consonant-vowel (cv) /da/ syllable which was presented in the standard close-field conditions with insert earphones, and in a novel free-field condition via a loudspeaker.

Results: A well-defined FFR was observed for each stimulating method (free or close-field). We show that it is possible and reliable to elicit FFRs from a speaker and that these do not systematically differ from those elicited by conventional earphones.

Comparison With Existing Method: Neural responses were subjected to a comparative within-subjects analysis, using standard measures found in the literature in order to quantify and compare the intrinsic (amplitude, noise, consistency), acoustic (latency, spectral amplitude) and reliability properties (intraclass correlation coefficients and Bland and Altman limits of agreement) of the neural signal.

Conclusions: Reliable FFRs can be elicited using free-field presentation with comparable to acoustical, intrinsic and reliability properties as those elicited by standard close-field presentations.