Comparison of the acoustic and neural distortion product at 2f1-f2 in normal-hearing adults

Abstract

Input/output functions of the simultaneously recorded acoustic distortion product otoacoustic emissions (DPOAE) and neural frequency following-response distortion products (FFR-DP) at 2f1-f2 were evaluated to determine if these two representations of cochlear nonlinearity exhibit similar response behavior, which would suggest shared cochlear generators. Responses were recorded from normal-hearing adults for a tone burst stimulus pair (F1: 500 Hz; F2: 612 Hz) at 40-70 dB nHL. DPOAE responses were recorded from the ear canal, and FFR responses were recorded differentially from scalp electrodes, representing a vertical configuration. The input/output function for FFR-DP revealed a compressive saturating nonlinearity, whereas the DPOAE input/output function exhibited a linear growth at higher intensities following a compressive behavior at moderate levels. Results appear to suggest that cochlear generators may be contributing differentially to the acoustic and the neural distortion products. Also, FFR-DP responses appeared more identifiable and less variable, particularly at lower stimulus levels, than the corresponding DPOAE. These findings may point to a potential benefit of applying FFR testing to complement DPOAE in evaluating cochlear function at low frequencies.

Selective Subcortical Enhancement of Musical Intervals in Musicians

“…Because of the presence of background noise in the ear canal, DPOAEs at frequencies Ͻ1000 Hz are difficult to measure. Therefore, this result suggests that the FFR could complement the less reliable DPOAE at low frequencies (Pandya and Krishnan, 2004;Elsisy and Krishnan, 2008).…”

“…We observed a neural correlate of this DPOAE at 2f 1 Ϫ f 2 in the spectra of the brainstem responses. There have been several studies demonstrating the presence of 2f 1 Ϫ f 2 in the FFRs to two pure tones (Chertoff and Hecox, 1990;Rickman et al, 1991;Chertoff et al, 1992;Pandya and Krishnan, 2004;Elsisy and Krishnan, 2008). However, we are the first to document this phenomenon using ecologically valid stimuli, namely musical intervals composed of complex tones.…”

“…Our current research explores this topic by measuring the brainstem responses to consonant and dissonant intervals in musically trained and control subjects. Previous studies have demonstrated the existence of nonlinear components (including f 2 Ϫ f 1 and 2f 1 Ϫ f 2 ) in the human brainstem response to intervals composed of pure tones ( f 1 and f 2 ) (Greenberg et al, 1987;Chertoff and Hecox, 1990;Rickman et al, 1991;Chertoff et al, 1992;Galbraith, 1994;Krishnan, 1999;Pandya and Krishnan, 2004;Elsisy and Krishnan, 2008). We, therefore, expected that distortion products would occur in response to harmonically rich musical intervals.…”

“…On the one hand, DPs measured in the FFR may be a response to DPs originally created by the mechanical properties of the cochlea (McAlpine, 2004;Pandya and Krishnan, 2004;Elsisy and Krishnan, 2008). Alternatively, the DPs may be created during neural processing of the signal.…”

Efferent modulation of pre-neural and neural distortion products

“…These “frequency following responses” (FFRs) represent cycle-to-cycle phase locking of auditory nerve and brainstem ensembles to primary tones (F1-FFR and F2-FFR) and multiple distortion product frequencies (Figure 1c). Observations in animals and humans suggest that the 2f1-f2 cubic distortion product FFR (CDP-FFR 1 ) is initiated by or tightly coupled to the same nonlinear mechanical cochlear processes that generate the DPOAE reflection source component (Elsisy & Krishnan, 2008). For example, CDP-FFRs may only occur with monotic presentation of probe tones with f2/f1 ratios consistent with those used to evoke DPOAEs (Rickman & Chertoff, 1991; Chertoff et al, 1992; Bhagat & Champlin, 2004).…”