Predicting the path of a changing sound: velocity tracking and auditory continuity

Adaptive Response Behavior in the Pursuit of Unpredictably Moving Sounds

Although moving sound-sources abound in natural auditory scenes, it is not clear how the human brain processes auditory motion. Previous studies have indicated that, although ocular localization responses to stationary sounds are quite accurate, ocular smooth pursuit of moving sounds is very poor. We here demonstrate that human subjects faithfully track a sound’s unpredictable movements in the horizontal plane with smooth-pursuit responses of the head. Our analysis revealed that the stimulus–response relation was well described by an under-damped passive, second-order low-pass filter in series with an idiosyncratic, fixed, pure delay. The model contained only two free parameters: the system’s damping coefficient, and its central (resonance) frequency. We found that the latter remained constant at ∼0.6 Hz throughout the experiment for all subjects. Interestingly, the damping coefficient systematically increased with trial number, suggesting the presence of an adaptive mechanism in the auditory pursuit system (APS). This mechanism functions even for unpredictable sound-motion trajectories endowed with fixed, but covert, frequency characteristics in open-loop tracking conditions. We conjecture that the APS optimizes a trade-off between response speed and effort. Taken together, our data support the existence of a pursuit system for auditory head-tracking, which would suggest the presence of a neural representation of a spatial auditory fovea (AF).

The Interaction Between Timescale and Pitch Contour at Pre-attentive Processing of Frequency-Modulated Sweeps

Speech comprehension across languages depends on encoding the pitch variations in frequency-modulated (FM) sweeps at different timescales and frequency ranges. While timescale and spectral contour of FM sweeps play important roles in differentiating acoustic speech units, relatively little work has been done to understand the interaction between the two acoustic dimensions at early cortical processing. An auditory oddball paradigm was employed to examine the interaction of timescale and pitch contour at pre-attentive processing of FM sweeps. Event-related potentials to frequency sweeps that vary in linguistically relevant pitch contour (fundamental frequency F0 vs. first formant frequency F1) and timescale (local vs. global) in Mandarin Chinese were recorded. Mismatch negativities (MMNs) were elicited by all types of sweep deviants. For local timescale, FM sweeps with F0 contours yielded larger MMN amplitudes than F1 contours. A reversed MMN amplitude pattern was obtained with respect to F0/F1 contours for global timescale stimuli. An interhemispheric asymmetry of MMN topography was observed corresponding to local and global-timescale contours. Falling but not rising frequency difference waveforms sweep contours elicited right hemispheric dominance. Results showed that timescale and pitch contour interacts with each other in pre-attentive auditory processing of FM sweeps. Findings suggest that FM sweeps, a type of non-speech signal, is processed at an early stage with reference to its linguistic function. That the dynamic interaction between timescale and spectral pattern is processed during early cortical processing of non-speech frequency sweep signal may be critical to facilitate speech encoding at a later stage.

Noise occlusion in discrete tone sequences as a tool towards auditory predictive processing?

The notion of predictive coding is a common feature of many theories of auditory information processing. Experimental demonstrations of predictive auditory processing often rest on omitting predictable input in order to uncover the prediction made by the brain. Findings show that auditory cortical activity elicited by the omission of a predictable tone resembles the activity elicited by the actual tone. Here we attempted to extend this approach towards using noises instead of omissions in order to capture a more prevalent case of degraded sensory input. By applying a subtraction approach to remove ERP effects of the noise itself, auditory cortical activity elicited "behind" the noise was uncovered. We hypothesized that ERPs elicited behind noise stimuli covering predictable tones should be more similar to ERPs elicited by the actual tones than when the same comparison is made for unpredictable tones. ERP results during passive listening partly confirm this hypothesis, but also point towards some methodological caveats in this particular approach towards studying neural correlates of predictive auditory processing due to contributions from predictability-unrelated factors. A follow-up active listening condition indicated that participants were not more likely to perceive the tone sequence as continuous when a predictable tone was covered with noise than when this pertained to an unpredictable tone. Overall, the noise-based paradigm in its present form was not shown to be successful in revealing predictive processing in perceptual judgments or early neural correlates of sound processing. We discuss these findings in the contexts of predictive processing and illusory auditory continuity. This article is part of a Special Issue entitled SI: Prediction and Attention.

Perceived tonal continuity through two noise bursts separated by silence

Three experiments measured the perceived continuity of two pure tones "flankers" through a masker containing a silence. Experiment 1 used a 2I-2AFC procedure; one interval contained two noise bursts separated by a silent gap, and the other contained two noise bursts separated by a tone of the same duration as the silence. Discrimination between masker conditions was very accurate when the flankers were absent but was impaired substantially when the flankers were present. This was taken as evidence that illusory flanker continuity during the silent gap was heard as similar to the physical presence of a tone in the gap. In experiment 2, performance remained poor when the flankers were frequency glides aligned along a common trajectory. Performance improved significantly when the flankers were misaligned in trajectory. In experiment 3, listeners rated directly perceived flanker continuity. Strong continuity was reported in the silent gap conditions for which poor performance had been observed in experiments 1 and 2. These findings show that continuity may be heard through a masker that cannot mask a physically continuous tone but can mask the flankers' offset and onset. The results are explained in terms of the perceptual grouping of onsets and offsets of the flankers.

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.

Relative priming of temporal local--global levels in auditory hierarchical stimuli

Priming is a useful tool for ascertaining the circumstances under which previous experiences influence behavior. Previously, using hierarchical stimuli, we demonstrated (Justus & List, 2005) that selectively attending to one temporal scale of an auditory stimulus improved subsequent attention to a repeated (vs. changed) temporal scale; that is, we demonstrated intertrial auditory temporal level priming. Here, we have extended those results to address whether level priming relied on absolute or relative temporal information. Both relative and absolute temporal information are important in auditory perception: Speech and music can be recognized over various temporal scales but become uninterpretable to a listener when presented too quickly or slowly. We first confirmed that temporal level priming generalized over new temporal scales. Second, in the context of multiple temporal scales, we found that temporal level priming operates predominantly on the basis of relative, rather than absolute, temporal information. These findings are discussed in the context of expectancies and relational invariance in audition.

Perception of complete and incomplete formant transitions in vowels

In everyday speech, formant transitions rarely reach the canonical frequencies of a target vowel. The perceptual system often compensates for such production undershoots, called vowel reduction (VR), by a perceptual overshoot of the final transition frequencies. The present investigation explored the perceptual parameters and existence region of VR. In a series of experiments a 100-ms steady-state vowel V(1) was followed by a formant transition toward a target vowel V(2). By manipulating both its duration and velocity, in most stimuli the transition was truncated and only seldom reached the target. After being presented with the vowel V(2) before each block of trials, listeners were asked to rate their confidence that the transition actually reached the V(2) target. Transitions along six trajectories connecting the three cardinal vowels /a/, /i/, and /u/ in both directions as well as the transition /ie/ (halfway along the trajectory /ia/) were examined in experiments in which either the duration of the transition was fixed and its velocity was varied or vice-versa. Results confirmed the existence of perceptual overshoot and showed that, at the point a transition short of reaching the vowel V(2) was just perceived as if it had reached the target, transition duration and transition velocity were inversely related. The amount of overshoot was found to be larger for larger V(1)-V(2) distances and shorter trajectory durations. The overshoot could be reliably predicted by a linear model based on three parameters--the extent of V(1)-V(2) distance, transition velocity, and transition acceleration. These findings suggest that the perceptual dynamics of speech relies on mechanisms that estimate the rate of change in the resonant characteristics of the vocal tract.