Abstract
The speech-evoked auditory brain stem response (speech ABR) is widely considered to provide an index of the quality of neural temporal encoding in the central auditory pathway. The aim of the present study was to evaluate the extent to which the speech ABR is shaped by spectral processing in the cochlea. High-pass noise masking was used to record speech ABRs from delimited octave-wide frequency bands between 0.5 and 8 kHz in normal-hearing young adults. The latency of the frequency-delimited responses decreased from the lowest to the highest frequency band by up to 3.6 ms. The observed frequency-latency function was compatible with model predictions based on wave V of the click ABR. The frequency-delimited speech ABR amplitude was largest in the 2- to 4-kHz frequency band and decreased toward both higher and lower frequency bands despite the predominance of low-frequency energy in the speech stimulus. We argue that the frequency dependence of speech ABR latency and amplitude results from the decrease in cochlear filter width with decreasing frequency. The results suggest that the amplitude and latency of the speech ABR may reflect interindividual differences in cochlear, as well as central, processing. The high-pass noise-masking technique provides a useful tool for differentiating between peripheral and central effects on the speech ABR. It can be used for further elucidating the neural basis of the perceptual speech deficits that have been associated with individual differences in speech ABR characteristics.
Highlights
DEFICITS IN TEMPORAL PROCESSING in the central auditory pathway are thought to contribute to difficulties in speech perception, in noise (Boets et al 2007; Pichora-Fuller and Souza 2003)
The findings of the present study show that both the amplitude and the latency of the periodic portion of the speech ABR are strongly dependent on the frequency region of origin in the cochlea
The latency of the response was found to increase by 3.6 ms as the cochlear place of origin moved from regions of high (5.7 kHz) to low (0.7 kHz) frequency
Summary
DEFICITS IN TEMPORAL PROCESSING in the central auditory pathway are thought to contribute to difficulties in speech perception, in noise (Boets et al 2007; Pichora-Fuller and Souza 2003). The onset peak of the speech ABR is generally thought to share common neural generators with the click ABR wave V These latter generators are presumed to comprise onset- or primary-like units located in the lateral lemniscus as it enters the inferior colliculus (Melcher and Kiang 1996; Møller and Jannetta 1983). It has been suggested that, in these populations, the speech ABR abnormalities reflect reduced precision of phase locking in central neurons. It can be questioned, whether normal audiometric thresholds guarantee normal suprathreshold cochlear function. Of particular importance for the click ABR wave V is the increase in cochlear response time from high-frequency (basal) to lowfrequency (apical) regions. This increase results in part from the travelling wave delay, which is determined by the passive mechanical properties of the cochlear partition, but to a larger extent results from the increase in filter build-up time due to the narrowing of cochlear filters, the tuning of which is determined www.jn.org
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