Auditory Bra in stem Responses from Human Adults and Infants: Influence of Stimulus Onset: Réponses évoqués du tronc cérébral chez l'adulte et chez 1'enfant: Influence du temps de montée du stimulus

Effects of stimulus cosine onset properties on bottlenose dolphin (Tursiops truncatus) auditory brainstem responses

Although the auditory brainstem response (ABR) is known to be an onset response, the specific relationship between stimulus onset properties and the resulting ABR is not well understood. In this study, the effects of stimulus onset on dolphin ABR were examined by measuring ABRs in six bottlenose dolphins while systematically manipulating rise time and plateau sound pressure of cosine-enveloped noise bursts. Noise bursts were spectrally "pink" with frequency content from 10 to 160 kHz, rise times from 32 μs to 4 ms, and plateau sound pressure levels from 102 to 138 dB re 1 μPa. Envelope rise time and plateau sound pressure alone were found to be poor predictors for ABR peak amplitudes and latencies. Peak amplitudes were well described by the envelope sound pressure at the end of a 260-μs window; however, best-fits to the data across ABR peaks were obtained when the window start time was allowed to vary. Peak latencies were best described by the maximum value of the second derivative of the pressure envelope. These results are consistent with single-unit and nearfield response data for terrestrial mammals and indicate that stimuli with rise times greater than 260 μs are non-optimal with respect to maximizing ABR amplitudes.

Effects of presentation rate and onset time on auditory brainstem responses in Northern saw-whet owls (Aegolius acadicus)

Monitoring auditory brainstem responses (ABRs) is a common method of assessing auditory processing in non-model species. Although ABRs are widely used to compare auditory abilities across taxa, the extent to which different features of acoustic stimuli affect the ABR is largely unknown in most non-mammalian species. The authors investigated the effects of varying presentation rate and onset time to determine how different features of acoustic stimuli influence the ABR in Northern saw-whet owls (Aegolius acadicus), a species known for their unique auditory adaptations and hunting abilities. At presentation rates ranging from 21.1 to 51.1 s^-1, there were no differences in the size or synchrony of ABRs, suggesting that stimuli can be presented at a relatively rapid rate to maximize the number of observations recorded for analysis. While increasing onset time was associated with a decrement in response size and synchrony, tonebursts with 1 ms onset times produced overgeneralized neural responses as a result of spectral splatter. This suggests that 2 to 3 ms onset times may balance the trade-off between response synchrony and frequency specificity when comparing relative neural recruitment across frequencies. These findings highlight the importance of considering stimulus parameters when interpreting ABR data.

Effects of noise burst rise time and level on bottlenose dolphin (Tursiops truncatus) auditory brainstem responses

Although the auditory brainstem response (ABR) is known to be an onset response, specific features of acoustic stimuli that affect the morphology of the ABR are not well understood. In this study, the effects of stimulus onset properties were investigated by measuring ABRs in seven bottlenose dolphins while systematically manipulating stimulus rise time and the amplitude of the sound pressure temporal envelope plateau. Stimuli consisted of spectrally pink (i.e., equal mean-square pressure in proportional frequency bands) noise bursts with linear rise (and fall) envelopes and frequency content from 10 to 160 kHz. Noise burst rise times varied from 32 μs to 4 ms and plateau sound pressure levels varied from 96 to 150 dB re 1 μPa. ABR peak latency was found to be a function of the rate of change of the sound pressure envelope, while ABR peak amplitude was a function of the envelope sound pressure at the end of a fixed integration window. The data support previous single-unit and nearfield response data from terrestrial mammals and a model where the rate of change of envelope sound pressure is integrated across a time window aligned with stimulus onset.

Rising-frequency chirps and earphones with an extended high-frequency response enhance the post-auricular muscle response

The purpose of this study was to determine whether rising-frequency chirps presented via earphones with an extended high-frequency response would optimize the post-auricular muscle response (PAMR). The PAMR was recorded in adults using three different stimuli (a click, a rising-frequency chirp, and a truncated speech stimulus, /t/). Conventional ER-3A insert earphones were compared to ER-2 insert earphones to determine whether the PAMR is enhanced by the ER-2's extended highfrequency response. There were significant stimulus and earphone effects on PAMR amplitudes. The PAMR was largest for the chirp stimulus and the ER-2 earphones. The poorest responses were obtained using the /t/ stimulus and conventional ER-3A earphones. The results support previous ABR studies that have demonstrated a significant advantage of chirps over clicks for evoked response audiometry, and indicate that the PAMR is enhanced by inclusion of additional high-frequency stimulus energy.

Gap duration discrimination for frequency-asymmetric gap markers: psychophysical and electrophysiological findings

This study investigated gap duration discrimination (GDD) for frequency-asymmetric gap markers, where one marker was a two-tone complex consisting of a primary tone and a secondary tone, and the other marker was the primary tone alone. Three experiments were undertaken to examine the order effect wherein performance is better when the two-tone marker is the leading marker than when it is the trailing marker. Experiment 1 demonstrated that GDD for frequency-asymmetric markers is intermediate between the boundaries of within-frequency-channel versus across-frequency-channel processing. Experiment 2 compared psychophysical performance with auditory brainstem responses (ABRs) elicited by the same stimuli. Whereas GDD thresholds were elevated for a complex trailing marker relative to a within-frequency-channel baseline, ABRs elicited by the complex marker were more robust. Experiment 3 tested the hypothesis that poor GDD performance with frequency-asymmetric markers is due to some form of nonenergetic, or informational, masking. The results did not support a role for informational masking conferred by synthetic listening; however, informational masking conferred by the occurrence of novel spectral events provided a parsimonious account. One possible interpretation is that the capacity to accurately encode a gap is undermined by the occurrence of novel spectral events that engage limited attentional resources.

Parallels between timing of onset responses of single neurons in cat and of evoked magnetic fields in human auditory cortex

Sound onsets constitute particularly salient transients and evoke strong responses from neurons of the auditory system, but in the past, such onset responses have often been analyzed with respect to steady-state features of sounds, like the sound pressure level. Recent electrophysiological studies of single neurons from the auditory cortex of anesthetized cats have revealed that the timing and strength of onset responses are shaped by dynamic stimulus properties at their very onsets. Here we demonstrate with magnetoencephalography that stimulus-response relationships very similar to those of the single neurons are observed in two onset components, N100m and P50m, of auditory evoked magnetic fields (AEFs) from the auditory cortex of awake humans. In response to tones shaped with cosine-squared rise functions, N100m and P50m peak latencies vary systematically with tone level and rise time but form a rather invariant function of the acceleration of the envelope at tone onset. Hence N100m and P50m peak latencies, as well as peak amplitudes, are determined by dynamic properties of the stimuli within the first few milliseconds, though not necessarily by acceleration. The changes of N100m and P50m peak latencies with rise time and level are incompatible with a fixed-amplitude threshold model. The direct comparison of the neuromagnetic and single-neuron data shows that, on average, the variance of the neuromagnetic data is larger by one to two orders of magnitude but that favorable measurements can yield variances as low as those derived from neurons with mediocre precision of response timing. The striking parallels between the response timing of single cortical neurons and of AEFs provides a stronger link between single neuron and population activity.

Time-intensity trading in the late auditory evoked potential

The present study investigated physiological correlates of the time-intensity trading relationship in late components (N1, P2) of the auditory evoked potential. Late-potential and behavioral thresholds were estimated in five normal-hearing, young adult participants for 1000- and 4000-Hz tone bursts having durations of 8, 16, 32, 64, and 128 ms. The results showed that late-potential thresholds decreased by an average of 24 dB for 1000-Hz conditions and 18 dB for 4000-Hz conditions. Behavioral thresholds also improved by about 22 dB and 18 dB for 1000-Hz and 4000-Hz conditions, respectively. The slope of improvement for both late-potential and behavioral thresholds was on the order of -4 to -6 dB per doubling of stimulus duration, depending on stimulus frequency. Stimulus duration also influenced latency and amplitude measures of the N1 and P2 components such that response latency decreased and amplitude increased as stimulus duration increased. The present results demonstrate a time-intensity trading relationship in components of the late potentials that is consistent with previous psychophysical and physiological data.

Response magnitude and timing of auditory response initiation in the inferior colliculus of the awake chinchilla

Recent single-unit studies in anesthetized cats have revealed that the latency and strength of transient responses to tone burst stimuli are determined largely by stimulus events in the first few ms of the signal. The present study sought to extend these findings by studying the inferior colliculus potential (ICP) in unanesthetized chinchillas. The ICP magnitude and latency were studied as a function of the plateau amplitude and rise time of noise burst stimuli. ICP amplitude increased with stimulus amplitude and decreased with stimulus rise time. ICP latency decreased with stimulus amplitude and increased with stimulus rise time. The absolute values of the ICP latencies confirmed that it is only the first few ms of the stimulus which determine the timing of response initiation, and therefore, that it is not the plateau level of the stimulus that directly determines the latent period. These data constitute a direct link between earlier single-unit studies in anesthetized animals and brainstem-evoked potential data in animals and man.

The relationship between auditory brainstem response latencies and behavioral thresholds in normal hearing infants and adults

The relationship between behavioral thresholds and auditory brainstem response (ABR) latencies for 4 and 8 kHz tone pips were examined in normal-hearing 3-month-olds, 6-month-olds and adults. The latencies of waves I and V and the I-V interval of the ABR were analyzed. A linear latency-intensity function was also fit to each subject's latencies for each wave at several levels. The y-intercept of the latency-intensity function was used as a summary measure of latency to examine behavior-ABR correlations. The pattern of age-related change in behavioral threshold was not closely matched by age-related latency reduction for Wave I, Wave V or the I-V interval. However, 3-month-olds with higher behavioral thresholds had longer Wave V latencies and longer I-V intervals than 3-month-olds with lower behavioral thresholds. There was no significant difference in latency between 6-month-olds or adults with higher thresholds and 6-month-olds or adults with lower thresholds. There was also a significant correlation between the Wave V-Wave I latency-intensity intercept difference and behavioral threshold at both 4 and 8 kHz among 3-month-olds. The correlation was not significant among 6-month-olds or adults. These findings suggest that one of the factors responsible for immature behavioral thresholds at 3 months is related to transmission through the auditory brainstem. Because variability in hearing threshold among normal-hearing adults is low, it is not surprising that behavioral threshold is unrelated to ABR latency in this group. However, the lack of such a relationship among 6-month-olds implies that structures central to the auditory brainstem, either sensory or nonsensory, or both, must be responsible for immature behavioral thresholds after 6 months of age.

Effects of noise burst rise time and level on the human brainstem auditory evoked response

An experiment was conducted to examine the effects of noise burst rise time and level on the human BAER. Noise burst levels included 15, 30, 45 and 60 dB nHL, with linear rise times of 0, 0.5, 1.25 and 2.5 ms. With increasing noise burst level, there is a decrease in wave V latency and an increase in peak amplitude. With increasing noise burst rise time, there is an increase in wave V latency and a decrease in wave V amplitude. The slope of the latency/intensity function increases with increasing rise time. The slope of the latency/rise time function increases with decreasing noise burst level. The change in wave V latency associated with changing rise time is less than the change in rise time for all experimental conditions.

The relationship between auditory brainstem response and behavioral thresholds in normal hearing infants and adults

The nature of age-related improvements in auditory sensitivity was explored by comparing behavioral and auditory brainstem response (ABR) thresholds in 3- and 6-month-old infants and in adults. Thresholds were estimated for tone pips at 1, 4, and 8 kHz, presented at a rate of 13.3/s. The time course of development of the two response measures was compared, and the correlation between thresholds for individual subjects was examined. Infant ABR threshold was adultlike at all frequencies, even among 3-month-olds. Infant behavioral thresholds were elevated relative to adult thresholds. Between 3 and 6 months, significant improvement occurred in the 8-kHz behavioral threshold, but no improvement occurred at other frequencies. This difference between ABR and behavioral measures in developmental time course suggests that peripheral sensitivity is not a major determinant of behavioral threshold elevation during infancy. The correlation between behavioral and ABR thresholds was significant at 4 kHz for 3-month-olds and at 8 kHz for adults. This suggests that variability in sensory function at these frequencies contributes to both behavioral and ABR thresholds, although other factors are likely to be involved as well.

Effects of noiseburst rise time and level on the gerbil brainstem auditory evoked response

The effects of noiseburst rise time and level on the latency and amplitude of waves i and v of the gerbil brainstem auditory evoked response (BAER) were evaluated. For tonal carriers, changes in rise time produce changes in stimulus spectrum. The use of a noise carrier eliminates such confounding spectral changes. Noiseburst levels included 40, 55, 70 and 85 dB SPL. Noiseburst rise times included 0, 0.5, 1.25 and 2.5 ms. With increasing noiseburst level, there is a decrease in peak latency, an increase in peak amplitude and a small increase in the i-v interval. With increasing noiseburst rise time, there is an increase in peak latency, a decrease in peak amplitude and a small increase in the i-v interval. The slopes of wave i and v latency/intensity functions increase with increasing rise time. The changes in peak latencies are always less than the increase in rise time. The slopes of these functions increase with decreasing noiseburst level and are greatest for the change in rise time from 0 to 0.5 ms. These data demonstrate changes in the BAER with rise time manipulation, even in the absence of stimulus spectral changes. It was hypothesized that changes in the BAER with increasing noiseburst rise time were due to a decrease in the effective amplitude of the stimulus. It was found that this hypothesis does not, in isolation, adequately account for the peak latency and amplitude changes found in the present investigation.