Modeling interaural-delay sensitivity to frequency modulation at high frequencies

Confirming an antiphasic bicyclic pattern of forward entrainment in signal detection: A reanalysis of Sun et al. (2021)

Forward entrainment refers to that part of the entrainment process that persists after termination of an entraining stimulus. Hickok et al. (2015) reported forward entrainment in signal detection that lasted for two post-stimulus cycles. In a recent paper, Sun et al. (2021) reported new data which suggested an absence of entrainment effects (Eur. J. Neurosci, 1-18, doi.org/10.1111/ejn.15367). Here we show that when Sun et al.'s data are analysed using unbiased detection-theoretic measures, a clear antiphasic bicyclic pattern of entrainment is observed. We further show that the measure of entrainment strength used by Sun et al., the normalized Fourier transform of performance curves, is not only erroneously calculated but is also unreliable in estimating entrainment strength due to signal-processing artifacts.

A critical analysis of Lin et al.'s (2021) failure to observe forward entrainment in pitch discrimination

Forward entrainment refers to that part of the entrainment process that outlasts the entraining stimulus. Several studies have demonstrated psychophysical forward entrainment in a pitch-discrimination task. In a recent paper, Lin et al. (2021) challenged these findings by demonstrating that a sequence of 4 entraining pure tones does not affect the ability to determine whether a frequency modulated pulse, presented after termination of the entraining sequence, has swept up or down in frequency. They concluded that rhythmic sequences do not facilitate pitch discrimination. Here, we describe several methodological and stimulus design flaws in Lin et al.'s study that may explain their failure to observe forward entrainment in pitch discrimination.

Intracranial lateralization bias observed in the presence of symmetrical hearing thresholds

It is generally assumed that listeners with normal audiograms have relatively symmetric hearing, and more specifically that diotic stimuli (having zero interaural differences) are heard as centered in the head. While measuring intracranial lateralization with a visual pointing task for tones and 50-Hz-wide narrowband noises from 300 to 700 Hz, examples of systematic and large (>50% from midline to the ear) lateralization biases were found. In a group of ten listeners, five showed consistent lateralization bias to the right or left side at all or a subset of frequencies. Asymmetries in hearing, not apparent in audiometric thresholds, may explain these lateralization biases.

Observer weighting of interaural cues in positive and negative envelope slopes of amplitude-modulated waveforms

The auditory system can encode interaural delays in highpass-filtered complex sounds by phase locking to their slowly modulating envelopes. Spectrotemporal analysis of interaurally time-delayed highpass waveforms reveals the presence of a concomitant interaural level cue. The current study systematically investigated the contribution of time and concomitant level cues carried by positive and negative envelope slopes of a modified sinusoidally amplitude-modulated (SAM) high-frequency carrier. The waveforms were generated from concatenation of individual modulation cycles whose envelope peaks were extended by the desired interaural delay, allowing independent control of delays in the positive and negative modulation slopes. In experiment 1, thresholds were measured using a 2-interval forced-choice adaptive task for interaural delays in either the positive or negative modulation slopes. In a control condition, thresholds were measured for a standard SAM tone. In experiment 2, decision weights were estimated using a multiple-observation correlational method in a single-interval forced-choice task for interaural delays carried simultaneously by the positive, and independently, negative slopes of the modulation envelope. In experiment 3, decision weights were measured for groups of 3 modulation cycles at the start, middle, and end of the waveform to determine the influence of onset dominance or recency effects. Results were consistent across experiments: thresholds were equal for the positive and negative modulation slopes. Decision weights were positive and equal for the time cue in the positive and negative envelope slopes. Weights were also larger for modulations cycles near the waveform onset. Weights estimated for the concomitant interaural level cue were positive for the positive envelope slope and negative for the negative slope, consistent with exclusive use of time cues.

Cross-modulation interference with lateralization of mixed-modulated waveforms

PURPOSE

This study investigated the ability to use spatial information in mixed-modulated (MM) sounds containing concurrent frequency-modulated (FM) and amplitude-modulated (AM) sounds by exploring patterns of interference when different modulation types originated from different loci as may occur in a multisource acoustic field.

METHOD

Interaural delay thresholds were measured from 5 normal-hearing adults for an AM sound in the presence of interfering FM and vice versa as a function of interferer modulation rate. In addition, the effects of near versus remote interferer rates, and fixed versus randomized interferer interaural delay, were investigated.

RESULTS

AM interfered with lateralization of FM at all modulation rates. However, the FM interfered with AM lateralization only when the FM rate was higher than the AM rate. This rate asymmetry was surprising given the prevalence of low-frequency dominance in lateralization, but was predicted by a cross-correlation model of binaural interaction. Effects were similar for fixed and randomized interferer interaural delays.

CONCLUSIONS

The results suggest that in multisource environments, sources containing different modulation types significantly interfere with localization in complex ways that reveal interactions between modulation type and rate. These findings contribute to the understanding of auditory object formation and localization.

Detection of sinusoidal amplitude modulation in logarithmic frequency sweeps across wide regions of the spectrum

Many natural sounds such as speech contain concurrent amplitude and frequency modulation (AM and FM), with the FM components often in the form of directional frequency sweeps or glides. Most studies of modulation coding, however, have employed one modulation type in stationary carriers, and in cases where mixed-modulation sounds have been used, the FM component has typically been confined to an extremely narrow range within a critical band. The current study examined the ability to detect AM signals carried by broad logarithmic frequency sweeps using a 2-alternative forced-choice adaptive psychophysical design. AM-detection thresholds were measured as a function of signal modulation rate and carrier sweep frequency region. Thresholds for detection of AM in a sweep carrier ranged from -8 dB for an AM rate of 8 Hz to -30 dB at 128 Hz. Compared to thresholds obtained for stationary carriers (pure tones and filtered Gaussian noise), detection of AM carried by frequency sweeps substantially declined at low (12 dB at 8 Hz) but not high modulation rates. Several trends in the data, including sweep- versus stationary-carrier threshold patterns and effects of frequency region were predicted from a modulation filterbank model with an envelope-correlation decision statistic.

Perception of across-frequency interaural level differences

The interaural level difference (ILD) is an important cue for the localization of sound sources. Just noticeable differences (JND) in ILD were measured in 12 normal hearing subjects for uncorrelated noise bands with a bandwidth of 13 octave and a different center frequency in both ears. In one ear the center frequency was either 250, 500, 1000, or 4000 Hz. In the other ear, a frequency shift of 0, 16, 13, or 1 octave was introduced. JNDs in ILD for unshifted, uncorrelated noise bands of 13 octave width were 2.6, 2.6, 2.5, and 1.4 dB for 250, 500, 1000, and 4000 Hz, respectively. Averaged over all shifts, JNDs decreased significantly with increasing frequency. For the shifted conditions, JNDs increased significantly with increasing shift. Performance on average worsened by 0.5, 0.9, and 1.5 dB for shifts of 16, 13, and 1 octave. Though performance decreases, the just noticeable ILDs for the shifted conditions were still in a range usable for lateralization. This has implications for signal processing algorithms for bilateral bimodal hearing instruments and the fitting of bilateral cochlear implants.

Detection of large interaural delays and its implication for models of binaural interaction

The interaural time difference (ITD) is a major cue to sound localization along the horizontal plane. The maximum natural ITD occurs when a sound source is positioned opposite to one ear. We examined the ability of owls and humans to detect large ITDs in sounds presented through headphones. Stimuli consisted of either broad or narrow bands of Gaussian noise, 100 ms in duration. Using headphones allowed presentation of ITDs that are greater than the maximum natural ITD. Owls were able to discriminate a sound leading to the left ear from one leading to the right ear, for ITDs that are 5 times the maximum natural delay. Neural recordings from optic-tectum neurons, however, show that best ITDs are usually well within the natural range and are never as large as ITDs that are behaviorally discriminable. A model of binaural crosscorrelation with short delay lines is shown to explain behavioral detection of large ITDs. The model uses curved trajectories of a cross-correlation pattern as the basis for detection. These trajectories represent side peaks of neural ITD-tuning curves and successfully predict localization reversals by both owls and human subjects.

Multiple auditory steady-state responses to AM and FM stimuli

Multiple auditory steady-state responses were recorded using tonal stimuli that were amplitude-modulated (AM), frequency-modulated (FM) or modulated simultaneously in both amplitude and frequency (mixed modulation or MM). When MM stimuli combined 100% AM and 25% FM (12.5% above and below the carrier frequency) and the maximum frequency occurred simultaneously with maximum amplitude, the MM response was one third larger than the simple AM response. This enhancement occurred at intensities between 50 and 30 dB SPL and at carrier frequencies between 500 and 4000 Hz. The AM and FM components of a MM stimulus generate independent responses that add together to give the MM response. Since AM responses generally occur with a slightly later phase delay than FM responses, the largest MM response is recorded when the maximum frequency of the MM stimulus occurs just after the maximum amplitude.

Neural bases of an auditory illusion and its elimination in owls

Humans and owls localize sounds by detecting the arrival time disparity between the ears. Both species determine the interaural time difference by finding the delay necessary to match the leading signal with the lagging one. This method produces ambiguity with periodic signals, because the two signals can be matched by delaying either one or the other. As predicted, owls localized periodic signals in illusory directions, whereas they always perceived the real source when signal bandwidth exceeded a certain value. This bandwidth also enabled higher-order auditory neurons to discriminate between real and illusory sources.

Effects of interaural decorrelation on neural and behavioral detection of spatial cues

The detection of interaural time differences (ITDs) for sound localization critically depends on the similarity between the left and right ear signals (interaural correlation). We show that, like humans, owls can localize phantom sound sources well until the correlation declines to a very low value, below which their performance rapidly deteriorates. Decreasing interaural correlation also causes the response of the owl's tectal auditory neurons to decline nonlinearly, with a rapid drop followed by a more gradual reduction. A detection-theoretic analysis of the statistical properties of neuronal responses could account for the variance of behavioral responses as interaural correlation is decreased. Finally, cross-correlation analysis suggests that low interaural correlations cause misalignment of cross-correlation peaks across different frequencies, contributing heavily to the nonlinear decline in neural and ultimately behavioral performance.