Jessica de Boer

Neural Correlates of Perceptual Learning in the Auditory Brainstem: Efferent Activity Predicts and Reflects Improvement at a Speech-in-Noise Discrimination Task

An extensive corticofugal system extends from the auditory cortex toward subcortical nuclei along the auditory pathway. Corticofugal influences reach even into the inner ear via the efferents of the olivocochlear bundle, the medial branch of which modulates preneural sound amplification gain. This corticofugal system is thought to contribute to neuroplasticity underlying auditory perceptual learning. In the present study, we investigated the involvement of the medial olivocochlear bundle (MOCB) in perceptual learning as a result of auditory training. MOCB activity was monitored in normal-hearing adult listeners during a 5 d training regimen on a consonant–vowel phoneme-in-noise discrimination task. The results show significant group learning, with great inter-individual variability in initial performance and improvement. As observed in previous auditory training studies, poor initial performers tended to show greater learning. Strikingly, MOCB activity measured on the first training day strongly predicted the subsequent amount of improvement, such that weaker initial MOCB activity was associated with greater improvement. Moreover, in listeners that improved significantly, an increase in MOCB activity was observed after training. Thus, as discrimination thresholds of listeners converged over the course of training, differences in MOCB activity between listeners decreased. Additional analysis showed that MOCB activity did not explain variation in performance between listeners on any training day but rather reflected an individual listeners performance relative to their personal optimal range. The findings suggest an MOCB-mediated listening strategy that facilitates speech-in-noise perception. The operation of this strategy is flexible and susceptible to training, presumably because of task-related adaptation of descending control from the cortex.

Effect of subject task on contralateral suppression of click evoked otoacoustic emissions

Contralateral suppression of click evoked otoacoustic emissions (CEOAEs) is widely used as a non-invasive measure of the activity of the (uncrossed) medial olivocochlear bundle (MOCB). There is evidence that the uMOCB receives descending input from the cortex, potentially mediating top-down control during higher order processing. This study investigated whether the contralateral suppression measure is affected by top-down influences during different tasks performed by the participants during recording. Suppression of CEOAEs evoked at 50 and 60dB SPL was measured under four different task conditions: (1) no task; (2) passive visual (watching a silent subtitled DVD); (3) active visual (responding to visually presented sums); (4) active auditory (detecting tone pips embedded in the evoking click train). The most significant effect of task was found on the recording noise, with both the passive visual and the active auditory task producing significantly lower noise levels than the no task condition. In the passive visual task, this was associated with a reduced inter-subject variability, which enhanced the effect size relative to the no task condition. A main effect of subject task was also found on the change in CEOAE I/O slope due to contralateral noise. This effect reflected a significantly smaller suppression during the active auditory task compared to the no task condition, leading to a reduced effect size. No significant difference in suppression strength between the no task condition and the two non-auditory tasks was observed, suggesting that the main effect of task reflects a specific effect of auditory attention. The data suggest that MOCB activity is inhibited due to top-down influences when selective attention is focussed on the ipsilateral ear.

Click-evoked otoacoustic emissions (CEOAEs) recorded from neonates under 13 hours old using conventional and maximum length sequence (MLS) stimulation

Maximum length sequence (MLS) stimulation allows click evoked otoacoustic emissions (CEOAEs) to be averaged at very high stimulation rates. This enables a faster reduction of noise contamination of the response, and has been shown to improve the signal-to-noise ratio (SNR) of CEOAEs recorded from adult subjects. This study set out to investigate whether MLS averaging can enhance the SNR of CEOAEs recorded in newborns within the first day after birth, and so improve the pass rates for OAE screening in this period, when false alarm rates are very high. CEOAEs were recorded in a neonatal ward from 57 ears in 37 newborns ranging from 6 to 13h old, using both conventional (50/s) and high rate (5000/s) MLS averaging. SNR values and pass rates were compared for responses obtained within equal recording times at both rates. MLS averaging produced an SNR improvement of up to 3.8dB, with the greatest improvement found in higher frequency bands. This SNR advantage resulted in pass rate improvement between 5% and 10%, depending on pass criterion. A significant effect of age was found on both SNR and pass rate, with newborns between 6 and 10h old showing significantly lower values than those tested between 10 and 13h after birth, as well as a much greater improvement due to MLS averaging. The findings show that MLS averaging can reduce false alarm rates by up to 15% in very young neonates in a neonatal ward setting.

Is Off-Frequency Overshoot Caused by Adaptation of Suppression?

This study is concerned with the mechanism of off-frequency overshoot. Overshoot refers to the phenomenon whereby a brief signal presented at the onset of a masker is easier to detect when the masker is preceded by a “precursor” sound (which is often the same as the masker). Overshoot is most prominent when the masker and precursor have a different frequency than the signal (henceforth referred to as “off-frequency overshoot”). It has been suggested that off-frequency overshoot is based on a similar mechanism as “enhancement,” which refers to the perceptual pop-out of a signal after presentation of a precursor that contains a spectral notch at the signal frequency; both have been proposed to be caused by a reduction in the suppressive masking of the signal as a result of the adaptive effect of the precursor (“adaptation of suppression”). In this study, we measured overshoot, suppression, and adaptation of suppression for a 4-kHz sinusoidal signal and a 4.75-kHz sinusoidal masker and precursor, using the same set of participants. We show that, while the precursor yielded strong overshoot and the masker produced strong suppression, the precursor did not appear to cause any reduction (adaptation) of suppression. Predictions based on an established model of the cochlear input–output function indicate that our failure to obtain any adaptation of suppression is unlikely to represent a false negative outcome. Our results indicate that off-frequency overshoot and enhancement are likely caused by different mechanisms. We argue that overshoot may be due to higher-order perceptual factors such as transient masking or attentional diversion, whereas enhancement may be based on mechanisms similar to those that generate the Zwicker tone.

The influence of cochlear spectral processing on the timing and amplitude of the speech-evoked auditory brain stem response

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.

Auditory Attention Causes Gain Enhancement and Frequency Sharpening at Successive Stages of Cortical Processing—Evidence from Human Electroencephalography

Previous findings have suggested that auditory attention causes not only enhancement in neural processing gain, but also sharpening in neural frequency tuning in human auditory cortex. The current study was aimed to reexamine these findings. Specifically, we aimed to investigate whether attentional gain enhancement and frequency sharpening emerge at the same or different processing levels and whether they represent independent or cooperative effects. For that, we examined the pattern of attentional modulation effects on early, sensory-driven cortical auditory-evoked potentials occurring at different latencies. Attention was manipulated using a dichotic listening task and was thus not selectively directed to specific frequency values. Possible attention-related changes in frequency tuning selectivity were measured with an adaptation paradigm. Our results show marked disparities in attention effects between the earlier N1 deflection and the subsequent P2 deflection, with the N1 showing a strong gain enhancement effect, but no sharpening, and the P2 showing clear evidence of sharpening, but no independent gain effect. They suggest that gain enhancement and frequency sharpening represent successive stages of a cooperative attentional modulation mechanism that increases the representational bandwidth of attended versus unattended sounds.

Does reduction in cochlear gain explain the overshoot effect

Under certain conditions, detection of a masked tone is improved by a preceding sound (a precursor); this is the overshoot effect. Despite over half a century of research, its underlying mechanisms remain unknown. A popular hypothesis links overshoot to reduction in cochlear gain by the medial olivocochlear reflex. This is thought to reduce excitatory masking when the masker is at the signal frequency (within‐channel effect) and suppressive masking when the masker is remote from the signal in frequency (across‐channel effect). This hypothesis was examined in the first of the two experiments presented in this study. The results found no within‐channel overshoot, indicating that the effect must be due to factors other than gain reduction at the signal frequency. While there was substantial across‐frequency overshoot, the pattern of results was inconsistent with reduction in suppressive masking. Interpretation of results from overshoot experiments is often complicated by the possibility that the precursor itself might have a masking effect on the signal. The second experiment presented in this study was designed to overcome this problem.