PERIODICITY ENVELOPE ENCODING: EVIDENCE FOR TWO SITES OF INTRODUCTION
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The Envelope Following Response (EFR) has been under investigation as part of the ASSR evoked potential but it is unclear where exactly this periodicity envelope originates. The commonly accepted theory holds that the periodicity envelope is introduced due to cochlear interactions and non-linearities but this does not account for measurable EFR responses to resolved stimuli (i.e. stimuli that should not interact on the basilar membrane; Korczak et al. 2012). Laroche et al. (2013) therefore proposed that the EFR to resolved stimuli arises centrally. To investigate this theory, EFR were measured to stimuli of different modulation rates, component frequencies, and phase relationships in normal hearing individuals. When stimuli were unresolved, response amplitude was seen to increase with decreasing degree of resolution and decrease when the envelope was minimized in the stimulus via phase manipulation, supporting the accepted hypothesis. When stimuli were resolved, response amplitude decreased with increasing component frequency (suggesting that phase locking is required) and responses were unaffected by phase-based minimization of the stimulus envelope. This evidence supports the theory that EFR to resolved harmonics is introduced centrally for it suggests that temporal encoding of the stimulus components by auditory nerve fibers is required before an EFR can be reliably measured. We therefore conclude that the periodicity envelope measured in the EFR is introduced at different levels along the auditory system depending on the resolution of its components.