Specific components of amplitude modulation that enable detection in listeners with cochlear implants

Dissociation of psychophysical and EEG steady-state response measures of cross-modal temporal correspondence for amplitude modulated acoustic and vibrotactile stimulation

Recent studies on amplitude modulation (AM) detection for tones in noise reported that AM-detection thresholds improve when the AM stimulus is preceded by a noise precursor. The physiological mechanisms underlying this AM unmasking are unknown. One possibility is that adaptation to the level of the noise precursor facilitates AM encoding by causing a shift in neural rate-level functions to optimize level encoding around the precursor level. The aims of this study were to investigate whether such a dynamic-range adaptation is a plausible mechanism for the AM unmasking and whether frequency modulation (FM), thought to be encoded via AM, also exhibits the unmasking effect. Detection thresholds for AM and FM of tones in noise were measured with and without a fixed-level precursor. Listeners showing the unmasking effect were then tested with the precursor level roved over a wide range to modulate the effect of adaptation to the precursor level on the detection of the subsequent AM. It was found that FM detection benefits from a precursor and the magnitude of FM unmasking correlates with that of AM unmasking. Moreover, consistent with dynamic-range adaptation, the unmasking magnitude weakens as the level difference between the precursor and simultaneous masker of the tone increases.

According to the temporal principle of multisensory integration, cross-modal synchronisation of stimulus onset facilitates multisensory integration. This is typically observed as a greater response to multisensory stimulation relative to the sum of the constituent unisensory responses (i.e., superadditivity). The aim of the present study was to examine whether the temporal principle extends to the cross-modal synchrony of amplitude-modulation (AM) rate. It is well established that psychophysical sensitivity to AM stimulation is strongly influenced by AM rate where the optimum rate differs according to sensory modality. This rate-dependent sensitivity is also apparent from EEG steady-state response (SSR) activity, which becomes entrained to the stimulation rate and is thought to reflect neural processing of the temporal characteristics of AM stimulation. In this study we investigated whether cross-modal congruence of AM rate reveals both psychophysical and EEG evidence of enhanced multisensory integration. To achieve this, EEG SSR and psychophysical sensitivity to simultaneous acoustic and/or vibrotactile AM stimuli were measured at cross-modally congruent and incongruent AM rates. While the results provided no evidence of superadditive multisensory SSR activity or psychophysical sensitivity, the complex pattern of results did reveal a consistent correspondence between SSR activity and psychophysical sensitivity to AM stimulation. This indicates that entrained EEG activity may provide a direct measure of cortical activity underlying multisensory integration. Consistent with the temporal principle of multisensory integration, increased vibrotactile SSR responses and psychophysical sensitivity were found for cross-modally congruent relative to incongruent AM rate. However, no corresponding increase in auditory SSR or psychophysical sensitivity was observed for cross-modally congruent AM rates. This complex pattern of results can be understood in terms of the likely influence of the principle of inverse effectiveness where the temporal principle of multisensory integration was only evident in the context of reduced perceptual sensitivity for the vibrotactile but not the auditory modality.

Separable developmental trajectories for the abilities to detect auditory amplitude and frequency modulation

This study examined the auditory steady-state responses evoked by amplitude-modulated (AM), mixed-modulated (MM), exponentially-modulated (AM2), and frequency-modulated (FM) tones in 50 newborn infants (within 3 days of birth) and in 20 older infants (within 3-15 wk of birth). Our hypothesis was that MM and AM2 tonal stimuli would evoke larger responses than either the AM or FM tones, and that this increased size would make the responses more readily detectable. Multiple auditory steady-state responses were recorded to four tonal stimuli presented simultaneously to each ear at 50 dB SPL. The carrier frequencies of the stimuli were 500, 1000, 2000, and 4000 Hz and the modulation rates were between 78 and 95 Hz. Recordings lasting 12 minutes were obtained for each of the three types of modulation: 100% AM, MM (100% AM and 20% FM) and AM2. In six infants, responses to 20% FM were also recorded. In newborn infants, MM and AM2 stimuli produced responses that were on average 15% larger than AM stimuli. For AM, MM, and AM2 stimuli, the percentage of significant responses was 67%, 73%, 76%, respectively. Responses to FM stimuli were clearly evident in newborn infants and were about half the amplitude of the AM responses. Responses recorded in the older infants were 17% larger when evoked by MM and AM2 stimuli, rather than AM stimuli. Responses in the older infants were, on average, 32% larger and showed a higher incidence of significant responses than for infants in the first 3 days of life. For AM, MM, and AM2 stimuli, the percentage of significant responses was 82%, 82%, 84%, respectively. In both newborn and older infants, the overall percentage of significant responses was decreased by the 500 Hz results, which showed lower amplitudes and were less frequently detected than responses evoked by other frequencies. The responses to MM and AM2 tones were larger than those evoked by AM tones. Using these stimuli will increase the reliability and efficiency of evoked potential audiometry in infancy. Responses at 50 dB SPL are more easily detected at 3-15 wk of age than in the first few days after birth. Comprehensive frequency-specific testing of hearing using steady-state responses will likely be more accurate if postponed until after the immediate neonatal period.

This study tests the hypothesis that amplitude modulation (AM) detection will be better under conditions where basilar membrane (BM) response growth is expected to be linear rather than compressive. This hypothesis was tested by (1) comparing AM detection for a tonal carrier as a function of carrier level for subjects with and without cochlear hearing impairment (HI), and by (2) comparing AM detection for carriers presented with and without an ipsilateral notched-noise precursor, under the assumption that the precursor linearizes BM responses. Average AM detection thresholds were approximately 5 dB better for subjects with HI than for subjects with normal hearing (NH) at moderate-level carriers. Average AM detection for low-to-moderate level carriers was approximately 2 dB better with the precursor than without the precursor for subjects with NH, whereas precursor effects were absent or smaller for subjects with HI. Although effect sizes were small and individual differences were noted, group differences are consistent with better AM detection for conditions where BM responses are less compressive due to cochlear hearing loss or due to a reduction in cochlear gain. These findings suggest the auditory system may quickly adjust to the local soundscape to increase effective AM depth and improve signal-to-noise ratios.

Sid Bacon was a pioneer in studies of the extent to which the detection of target amplitude modulation (AM) of a carrier is affected by additional (masker) amplitude modulation applied to the same carrier (within-channel modulation masking) or to a different carrier (across-channel modulation masking). Here, these two types of modulation masking were compared for normal-hearing and hearing-impaired subjects. The target was either 4-Hz or 16-Hz sinusoidal AM of a 4000-Hz carrier. The target AM depth was fixed. The masker AM was applied either to the same carrier or to a carrier at 3179 or 2518 Hz. The masker AM rate was 0.25, 0.5, 1, 2, or 4 times the target rate. The masker AM depth was varied adaptively to determine the value needed just to mask the target AM. Preliminary results indicate that within-channel modulation-masking patterns are similar for normal-hearing and hearing-impaired subjects, suggesting that the hypothetical modulation filters are not affected by hearing loss. However, the amount of across-channel modulation masking is lower for normal-hearing than for hearing-impaired subjects, presumably because of the reduced frequency selectivity of the latter. The increased across-channel masking for the hearing-impaired subjects may contribute to their difficulties in understanding speech in background sounds.

This study investigated the efficiency of a multiplexed amplitude-modulated (AM) stimulus in eliciting auditory steady-state responses. The multiplexed AM stimulus was created by simultaneously modulating speech-shaped noise with three frequencies chosen to elicit different neural generators: 3.1, 40.1, and 102.1 Hz. For comparison, a single AM stimulus was created for each of these frequencies, resulting in three single AM conditions and one multiplex AM condition. Twenty-two bilaterally normal-hearing participants (18 females) listened for 8 minutes to each type of stimuli. The analysis compared the signal to noise ratios (SNRs) and amplitudes of the evoked responses to the single and multiplexed conditions. The results revealed that the SNRs elicited by single AM conditions were, on average, 1.61 dB higher than those evoked by the multiplexed AM condition ( p < 0.05). The single conditions consistently produced a significantly higher SNR when examining various stimulus durations ranging from 1 to 8 minutes. Despite these SNR differences, the frequency spectrum was very similar across and within subjects. In addition, the sensor space patterns across the scalp demonstrated similar trends between the single and multiplexed stimuli for both SNR and amplitudes. Both the single and multiplexed conditions evoked significant auditory steady-state responses within subjects. On average, the multiplexed AM stimulus took 31 minutes for the lower bound of the 95% prediction interval to cross the significance threshold across all three frequencies. In contrast, the single AM stimuli took 45 minutes and 42 seconds. These findings show that the multiplexed AM stimulus is a promising method to reduce the recording time when simultaneously obtaining information from various neural generators.

Results are presented for detection of sinusoidal amplitude modulation (AM) superimposed upon linear-frequency-modulated, 120-ms tone bursts. Thresholds for detection of AM were obtained using an adaptive 2IFC procedure over a range of AM frequencies from 25 to 400 Hz, and over a range of frequency modulation (FM) rates from 0 to 30 Hz/ms. The stimuli are a simplification of speech transitions, with both AM and FM, but consisting of only one signal component. These experiments explore the ability to detect modulations that are distributed across critical bands. For fast rates of FM, the stimuli will sweep through a critical band within one to two periods of the AM. A rapid frequency transition also broadens the spectrum of the signal, so that these stimuli do not possess the distinct sidebands of an amplitude-modulated, stationary tone. Thus, both temporal cues from periodicities within individual channels and place cues derived from the excitation pattern will weaken as the FM rate increases. Nonetheless, data indicate that listeners readily detect amplitude modulation for all modulation frequencies at the highest rates of FM. These data will be discussed in light of some current physiologically based models of auditory processing, wherein both within- and across-channel temporal information is utilized. [Work supported by the Virginia Merrill Bloedel Hearing Research Center, University of Washington.]

Adaptation effects on amplitude modulation detection: behavioral and neurophysiological assessment in the goldfish auditory system.

This study investigates the degree of similarity of three different auditory cortical areas with respect to the coding of periodic stimuli. Simultaneous single- and multiunit recordings in response to periodic stimuli were made from primary auditory cortex (AI), anterior auditory field (AAF), and secondary auditory cortex (AII) in the cat to addresses the following questions: is there, within each cortical area, a difference in the temporal coding of periodic click trains, amplitude-modulated (AM) noise bursts, and AM tone bursts? Is there a difference in this coding between the three cortical fields? Is the coding based on the temporal modulation transfer function (tMTF) and on the all-order interspike-interval (ISI) histogram the same? Is the perceptual distinction between rhythm and roughness for AM stimuli related to a temporal versus spatial representation of AM frequency in auditory cortex? Are interarea differences in temporal response properties related to differences in frequency tuning? The results showed that: 1) AM stimuli produce much higher best modulation frequencies (BMFs) and limiting rates than periodic click trains. 2) For periodic click trains and AM noise, the BMFs and limiting rates were not significantly different for the three areas. However, for AM tones the BMF and limiting rates were about a factor 2 lower in AAF compared with the other areas. 3) The representation of stimulus periodicity in ISIs resulted in significantly lower mean BMFs and limiting rates compared with those estimated from the tMTFs. The difference was relatively small for periodic click trains but quite large for both AM stimuli, especially in AI and AII. 4) Modulation frequencies <20 Hz were represented in the ISIs, suggesting that rhythm is coded in auditory cortex in temporal fashion. 5) In general only a modest interdependence of spectral- and temporal-response properties in AI and AII was found. The BMFs were correlated positively with characteristic frequency in AAF. The limiting rate was positively correlated with the frequency-tuning curve bandwidth in AI and AII but not in AAF. Only in AAF was a correlation between BMF and minimum latency was found. Thus whereas differences were found in the frequency-tuning curve bandwidth and minimum response latencies among the three areas, the coding of periodic stimuli in these areas was fairly similar with the exception of the very poor representation of AM tones in AII. This suggests a strong parallel processing organization in auditory cortex.

The acquisition of new auditory skills can be facilitated by experiencing a conspecific performing a well-defined behavior (i.e., social learning). Although the neural bases for auditory social learning remain uncertain, one plausible hypothesis is that social experience induces long-term changes to auditory cortex, thereby facilitating the subsequent acquisition of an auditory skill. To explore this idea, we developed a social learning paradigm in which naïve observer gerbils are exposed to a demonstrator that performs an amplitude modulation (AM) rate discrimination task on the other side of an opaque divider. Thus, observers have no access to visual cues. When exposed to a demonstrator for five successive days, observers subsequently acquire the AM task more rapidly than controls. Two experiments suggest that auditory cortex is necessary and sufficient for social learning. First, inactivating the observer’s auditory cortex during each social exposure session significantly delayed task acquisition. Second, recordings from the observer’s auditory cortex revealed that individual neurons displayed a significant improvement in AM stimulus discrimination across the five exposure sessions, and the magnitude of neural improvement correlated with an animal’s subsequent rate of task acquisition. Together, these findings suggest that auditory cortex plasticity plays a pivotal role in social learning.

1. In mammals with good low-frequency hearing, the medial superior olive (MSO) processes interaural time or phase differences that are important cues for sound localization. Its cells receive excitatory projections from both cochlear nuclei and are thought to function as coincidence detectors. The response patterns of MSO neurons in most mammals are predominantly sustained. In contrast, the MSO in the mustached bat is a monaural nucleus containing neurons with phasic discharge patterns. These neurons receive projections from the contralateral anteroventral cochlear nucleus (AVCN) and the ipsilateral medial nucleus of the trapezoid body (MNTB). 2. To further investigate the role of the MSO in the bat, the responses of 252 single units in the MSO to pure tones and sinusoidal amplitude-modulated (SAM) stimuli were recorded. The results confirmed that the MSO in the mustached bat is tonotopically organized, with low frequencies in the dorsal part and high frequencies in the ventral part. The 61-kHz region is overrepresented. Most neurons tested (88%) were monaural and discharged only in response to contralateral stimuli. Their response could not be influenced by stimulation of the ipsilateral ear. 3. Only 11% of all MSO neurons were spontaneously active. In these neurons the spontaneous discharge rate was suppressed during the stimulus presentation. 4. The majority of cells (85%) responded with a phasic discharge pattern. About one-half (51%) responded with a level-independent phasic ON response. Other phasic response patterns included phasic OFF or phasic ON-OFF, depending on the stimulus frequency. Neurons with ON-OFF discharge patterns were most common in the 61-kHz region and absent in the high-frequency region. 5. Double tone experiments showed that at short intertone intervals the ON response to the second stimulus or the OFF response to the first stimulus was inhibited. 6. In neuropharmacological experiments, glycine applied to MSO neurons (n = 71) inhibited any tone-evoked response. In the presence of the glycine antagonist strychnine the response patterns changed from phasic to sustained (n = 35) and the neurons responded to both tones presented in double tone experiments independent of the intertone interval (n = 5). The effects of strychnine were reversible. 7. Twenty of 21 neurons tested with sinusoidally amplitude-modulated (SAM) signals exhibited low-pass or band-pass filter characteristics. Tests with SAM signals also revealed a weak temporal summation of inhibition in 13 of the 21 cells tested.(ABSTRACT TRUNCATED AT 400 WORDS)

Temporal modulation transfer functions for AM and FM stimuli in cat auditory cortex. Effects of carrier type, modulating waveform and intensity

Amplitude modulation (AM) is a characteristic feature of wind farm noise and has the potential to contribute to annoyance and sleep disturbance. Detection, quantification and characterisation of AM is relevant for regulatory bodies that seek to reduce adverse impacts of wind farm noise and for researchers and wind farm developers that aim to understand and account for this phenomenon. We here present an approach to detect and characterise AM in a comprehensive and long-term wind farm noise data set using human scoring. We established benchmark AM characteristics, which are important for validation and calibration of results obtained using automated methods. We further proposed an advanced AM detection method, which has a predictive power close to the practical limit set by human scoring. Human-based approaches should be considered as benchmark methods for characterising and detecting unique noise features.