Sensory constraints on auditory identification of the material and geometric properties of struck bars

Human Auditory Ecology: Extending Hearing Research to the Perception of Natural Soundscapes by Humans in Rapidly Changing Environments

Research in hearing sciences has provided extensive knowledge about how the human auditory system processes speech and assists communication. In contrast, little is known about how this system processes "natural soundscapes," that is the complex arrangements of biological and geophysical sounds shaped by sound propagation through non-anthropogenic habitats [Grinfeder et al. (2022). Frontiers in Ecology and Evolution. 10: 894232]. This is surprising given that, for many species, the capacity to process natural soundscapes determines survival and reproduction through the ability to represent and monitor the immediate environment. Here we propose a framework to encourage research programmes in the field of "human auditory ecology," focusing on the study of human auditory perception of ecological processes at work in natural habitats. Based on large acoustic databases with high ecological validity, these programmes should investigate the extent to which this presumably ancestral monitoring function of the human auditory system is adapted to specific information conveyed by natural soundscapes, whether it operate throughout the life span or whether it emerges through individual learning or cultural transmission. Beyond fundamental knowledge of human hearing, these programmes should yield a better understanding of how normal-hearing and hearing-impaired listeners monitor rural and city green and blue spaces and benefit from them, and whether rehabilitation devices (hearing aids and cochlear implants) restore natural soundscape perception and emotional responses back to normal. Importantly, they should also reveal whether and how humans hear the rapid changes in the environment brought about by human activity.

Response characteristics of primary auditory cortex neurons underlying perceptual asymmetry of ramped and damped sounds

Sound envelope plays a crucial role in perception: ramped sounds (slow attack and quick decay) are louder in strength and longer in subjective duration than damped sounds (quick attack and slow decay) even if they are equal in intensity and physical duration. To explain the asymmetrical perception, the perceptual constancy hypothesis supposes that the listener eliminates the slow decay of damped sounds from the judgment of perception, while the persistence of perception hypothesis supposes asymmetrical neural responses after the source has stopped. To understand neural mechanisms underlying the perceptual asymmetry, we explored response properties of the primary auditory cortex (A1) neurons during ramped and damped stimuli in awake cats. We found two distinct types of cells tuned to specific features of the sound envelope: edge cells sensitive to the temporal edge, such as quick attack and decay, while slope cells sensitive to slow attack and decay. The former needs a short (<2.5 ms) period of stimulus duration for evoking maximal peak responses, while the latter needs a long (20 ms) period, suggesting that the timescale of processing underlies differential sensitivity between the cell types. The findings suggest that perceptual constancy is not yet be executed at A1 because the specific cells distinguishing the direction of amplitude change (attack or decay) are lacking in A1. On the other hand, there is evidence of persistence of perception: overall response duration during ramped sound reached 1.4 times longer than that during damped sound, originating mainly from the response asymmetry of the edge cell (sensitive to the quick decay of ramped sounds but not to the slow decay of damped sounds), and neuronal persistence of excitation after the termination of ramped sounds was substantially longer than that of damped sounds, corresponding to the psychological evidence of persistence of perception.

A new approach to sound source segregation

We rely critically on our ability to 'hear out' (segregate) individual sound sources in a mixture. Yet, despite its importance, little is known regarding this -ability. Perturbation analysis is a psychophysical method that has been successfully applied to related problems in vision [Murray, R.F. 2011. J. of Vision 11, 1-25]. Here the approach is adapted to audition. The application proceeds in three stages: First, simple speech and environmental sounds are synthesized according to a generative model of the sound--producing source. Second, listener decision strategy in segregating target from non--target (noise) sources is determined from decision weights (regression coefficients) relating listener judgments regarding the target to lawful perturbations in acoustic parameters, as dictated by the generative model. Third, factors limiting segregation are identified by comparing the obtained weights and residuals to those of a maximum-likelihood (ML) observer that optimizes segregation based on the equations of motion of the generating source. Here, the approach is applied to test between the two major models of sound source segregation; target enhancement versus noise cancellation. The results indicate a tendency of noise segregation to preempt target enhancement when the noise source is unchanging. However, the results also show individual differences in segregation strategy that are not evident in the measures of performance accuracy alone.

Auditory perception of material is fragile while action is strikingly robust

While many psychoacoustic studies have found that listeners can recover some causal properties of sound-generating objects (such as the material), comparatively little is known about the causal properties of the sound-generating actions and how they are perceived. This article reports on a study comparing the performance of listeners required to identify either the actions or the materials used to generate sound stimuli. Stimuli were recordings of a set of cylinders of two sizes and four materials (wood, plastic, glass, metal) undergoing four different actions (scraping, rolling, hitting, bouncing). Experiment 1 tested how well each sound conveyed that it was generated with a different action or material. Experiment 2 measured both accuracy and reaction times for the identification of actions and materials. Listeners were faster and more accurate at identifying the action than the material. Even for the subset of sounds for which actions and materials were equivalently well identified, listeners were faster at identifying the action than the material. These results suggest that the auditory system is well-suited to extract information about sound-generating actions.

Discrimination of the spectral density of multitone complexes

Spectral density (D), defined as the number of partials comprising a sound divided by its bandwidth, has been suggested as cue for the identification of the size and shape of sound sources. Few data are available, however, on the ability of listeners to discriminate differences in spectral density. In a cue-comparison, forced-choice procedure with feedback, three highly practiced listeners discriminated differences in the spectral density of multitone complexes varying in bandwidth (W = 500-1500 Hz), center frequency (f(c) = 500-2000 Hz), and number of tones (N = 6-31). To reduce extraneous cues for discrimination, the overall level of the complexes was roved, and the frequencies were drawn at random uniformly over a fixed bandwidth and center frequency for each presentation. Psychometric functions were obtained relating percent correct discrimination to ΔD in each condition. For D < 0.02 Hz(-1), the steepness of the functions remained constant across conditions, but for D > 0.02 Hz(-1), they increased with D. The increase, moreover, was accompanied by a reduction in the upper asymptote of the functions. The data were well fit by a model in which spectral density discrimination is determined by the frequency separation of components on an equivalent rectangular bandwidth scale, yielding a roughly constant Weber fraction of ΔD/D = 0.3.

Auditory discrimination of force of impact

The auditory discrimination of force of impact was measured for three groups of listeners using sounds synthesized according to first-order equations of motion for the homogenous, isotropic bar [Morse and Ingard (1968). Theoretical Acoustics pp. 175-191]. The three groups were professional percussionists, nonmusicians, and individuals recruited from the general population without regard to musical background. In the two-interval, forced-choice procedure, listeners chose the sound corresponding to the greater force of impact as the length of the bar varied from one presentation to the next. From the equations of motion, a maximum-likelihood test for the task was determined to be of the form Δlog A + αΔ log f > 0, where A and f are the amplitude and frequency of any one partial and α = 0.5. Relative decision weights on Δ log f were obtained from the trial-by-trial responses of listeners and compared to α. Percussionists generally outperformed the other groups; however, the obtained decision weights of all listeners deviated significantly from α and showed variability within groups far in excess of the variability associated with replication. Providing correct feedback after each trial had little effect on the decision weights. The variability in these measures was comparable to that seen in studies involving the auditory discrimination of other source attributes.