HESP Seminar Series: Saradha Ananthakrishnan (Towson)

The Frequency Following Response: A Subcortical Marker of Speech Perception in Normal Hearing and Hearing Loss

Abstract: The objective of my research is to investigate feasibility, and provide a systematic evaluation of subcortical (brainstem) neural processing in the healthy and impaired human auditory systems using the Frequency Following Response (FFR). The FFR is a scalp recorded sustained evoked potential that reflects neural activity in an ensemble of neural elements in the rostral brainstem, synchronized to the individual cycles of the stimulus waveform (phase-locking). Overall, findings from my research thus far suggest that the FFR is indeed a viable technique for indexing brainstem representation of special acoustic features in speech such as the fundamental ("envelope") and formant ("temporal fine structure" (TFS)) frequencies in listeners with hearing impairment (HI). Further, sensorineural hearing loss appears to have a differential effect on subcortical representation of envelope and TFS cues in HI, consistent with established findings in behavioral and animal physiology studies. Specifically, subcortical representation of higher frequency TFS cues in HI appear to reflect a reduced phase-locking ability of brainstem neurons, whereas lower-frequency envelope cue representation seems linked more to high frequency stimulus audibility. These conclusions were reiterated in another study where FFRs were recorded in normal-hearing listeners (NH) to low-and high-pass filtered stimuli. When the amount of high frequency information in the stimulus was reduced, brainstem representation of envelope cues was reduced whereas neural representation of TFS cues was not significantly affected. The findings from these studies have led to experiments examining brainstem representation of envelope and TFS cues in NH listeners in response to vocoded speech signals, which are generated using techniques similar to those used in cochlear implant speech processors. The FFR offers an excellent platform to study neural representation of these acoustically-complex stimuli when the amount of temporal and spectral cues is altered through various parameters of vocoding such as envelope cutoff frequency and number of vocoder channels. Overall, the FFR offers an objective, non-invasive window to study brainstem representation of the acoustic cues in listeners with normal hearing and hearing loss.