Andrew Brughera

Human interaural time difference thresholds for sine tones: The high-frequency limit

The smallest detectable interaural time difference (ITD) for sine tones was measured for four human listeners to determine the dependence on tone frequency. At low frequencies, 250-700 Hz, threshold ITDs were approximately inversely proportional to tone frequency. At mid-frequencies, 700-1000 Hz, threshold ITDs were smallest. At high frequencies, above 1000 Hz, thresholds increased faster than exponentially with increasing frequency becoming unmeasurably high just above 1400 Hz. A model for ITD detection began with a biophysically based computational model for a medial superior olive (MSO) neuron that produced robust ITD responses up to 1000 Hz, and demonstrated a dramatic reduction in ITD-dependence from 1000 to 1500 Hz. Rate-ITD functions from the MSO model became inputs to binaural display models-both place based and rate-difference based. A place-based, centroid model with a rigid internal threshold reproduced almost all features of the human data. A signal-detection version of this model reproduced the high-frequency divergence but badly underestimated low-frequency thresholds. A rate-difference model incorporating fast contralateral inhibition reproduced the major features of the human threshold data except for the divergence. A combined, hybrid model could reproduce all the threshold data.

Models of Brainstem Responses to Bilateral Electrical Stimulation

A simple, biophysically specified cell model is used to predict responses of binaurally sensitive neurons to patterns of input spikes that represent stimulation by acoustic and electric waveforms. Specifically, the effects of changes in parameters of input spike trains on model responses to interaural time difference (ITD) were studied for low-frequency periodic stimuli, with or without amplitude modulation. Simulations were limited to purely excitatory, bilaterally driven cell models with basic ionic currents and multiple input fibers. Parameters explored include average firing rate, synchrony index, modulation frequency, and latency dispersion of the input trains as well as the excitatory conductance and time constant of individual synapses in the cell model. Results are compared to physiological recordings from the inferior colliculus (IC) and discussed in terms of ITD-discrimination abilities of listeners with cochlear implants. Several empirically observed aspects of ITD sensitivity were simulated without evoking complex neural processing. Specifically, our results show saturation effects in rate–ITD curves, the absence of sustained responses to high-rate unmodulated pulse trains, the renewal of sensitivity to ITD in high-rate trains when inputs are amplitude-modulated, and interactions between envelope and fine-structure delays for some modulation frequencies.

Physiological and Psychophysical Modeling of the Precedence Effect

Many past studies of sound localization explored the precedence effect (PE), in which a pair of brief, temporally close sounds from different directions is perceived as coming from a location near that of the first-arriving sound. Here, a computational model of low-frequency inferior colliculus (IC) neurons accounts for both physiological and psychophysical responses to PE click stimuli. In the model, IC neurons have physiologically plausible inputs, receiving excitation from the ipsilateral medial superior olive (MSO) and long-lasting inhibition from both ipsilateral and contralateral MSOs, relayed through the dorsal nucleus of the lateral lemniscus. In this model, physiological suppression of the lagging response depends on the inter-stimulus delay (ISD) between the lead and lag as well as their relative locations. Psychophysical predictions are generated from a population of model neurons. At all ISDs, predicted lead localization is good. At short ISDs, the estimated location of the lag is near that of the lead, consistent with subjects perceiving both lead and lag from the lead location. As ISD increases, the estimated lag location moves closer to the true lag location, consistent with listeners’ perception of two sounds from separate locations. Together, these simulations suggest that location-dependent suppression in IC neurons can explain the behavioral phenomenon known as the precedence effect.

The role of reverberation in release from masking due to spatial separation of sources in speech recognition

Arbogast et al. [ARO Mtg. (2002)] found a large release from masking obtained by spatial separation of a target talker and competing speech masker. Both stimuli were sentences from the Coordinate Response Measure corpus [Bolia et al., J. Acoust. Soc. Am. (2000)] processed by extracting the envelopes of 15 narrow frequency bands and using the envelopes to modulate carrier tones at the center of each band. By playing nonoverlapping subsets (6–8) of bands from signal and masker they minimized the energetic component while maximizing the informational component of masking. This study extends that work to determine the interaction between reverberation, masker type, and spatial release from masking. Stimuli were processed and presented as above. The target sentence was played at 0‐deg azimuth while the masker sentence was played at 0 or 90‐deg azimuth. Noise–masker controls were also tested. The listening environment was an IAC booth having dimensions of 12 ft×13 ft. Acoustic extremes were achieved using Plexi...

The ESME Workbench: Simulating the Impact of Anthropogenic Sound on Marine Mammals

The Effects of Sound in the Marine Environment (ESME) Workbench (http://esme.bu.edu) is a software tool designed to predict the impact of anthropogenic sounds on marine mammals. The ESME Workbench allows the user to select site-specific environmental data such as bathymetry, sound-speed profiles, sediment type, and average wind speed to predict sound propagation in a wide range of scenarios and to record the sound exposures received by virtual animals. The Workbench provides access to raw exposure information as well as summarized exposure information at the end of the simulation run. These data are made available in formats suitable for postprocessing utilizing a variety of data analysis tools.

Threshold interaural time differences and the centroid model of sound localization

The centroid display model of sound lateralization hypothesizes a two-dimensional array of brain-stem cells with wide ranges of best frequencies (

Modeling Physiological and Psychophysical Responses to Precedence Effect Stimuli

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Short‐term adaptation to novel combinations of acoustic spatial cues

) and best interaural time delays (ITD,

Simulation model for interaural time difference discrimination for tones

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Modeling responses of brainstem neurons to electrical stimuli

). The cells are distributed according to a cell population density function