Biomedical subjects

S Shamma

Publications and source records attributed to S Shamma.

5 recordsLinked to original sources

PubMed2001

On the role of space and time in auditory processing.

Unlike visual and tactile stimuli, auditory signals that allow perception of timbre, pitch and localization are temporal. To process these, the auditory nervous system must either possess specialized neural machinery for analyzing temporal input, or transform the initial responses into patterns that are spatially distributed across its sensory epithelium. The former hypothesis, which postulates the existence of structures that facilitate temporal processing, is most popular. However, I argue that the cochlea transforms sound into spatiotemporal response patterns on the auditory nerve and central auditory stages; and that a unified computational framework exists for central auditory, visual and other sensory processing. Specifically, I explain how four fundamental concepts in visual processing play analogous roles in auditory processing.

Journal Article↗

PubMed2000

The case of the missing pitch templates: how harmonic templates emerge in the early auditory system.

Periodicity pitch is the most salient and important of all pitch percepts. Psychoacoustical models of this percept have long postulated the existence of internalized harmonic templates against which incoming resolved spectra can be compared, and pitch determined according to the best matching templates [J. Goldstein, J. Acoust. Soc. Am. 54, 1496-1516 (1973)]. However, it has been a mystery where and how such harmonic templates can come about. We present here a biologically plausible model for how such templates can form in the early stages of the auditory system. The model demonstrates that any broadband stimulus, including noise and random click trains, suffices for generating the templates, and that there is no need for any delay lines, oscillators, or other neural temporal structures. The model consists of two key stages: cochlear filtering followed by coincidence detection. The cochlear stage provides responses analogous to those recorded in the auditory nerve and cochlear nucleus. Specifically, it performs moderately sharp frequency analysis via a filterbank with tonotopically ordered center frequencies (CFs); the rectified and phase-locked filter responses are further enhanced temporally to resemble the synchronized responses of cells in the cochlear nucleus. The second stage is a matrix of coincidence detectors that compute the average pairwise instantaneous correlation (or product) between responses from all CFs across the channels. Model simulations show that for any broadband stimulus, a degree of high coincidence occurs among cochlear channels that are spaced precisely at harmonic intervals. Accumulating coincidences over time results in the formation of harmonic templates for all fundamental frequencies in the phase-locking frequency range. The model accounts for the critical role played by three subtle but important factors in cochlear function: the nonlinear transformations following the filtering stage, the rapid phase shifts of the traveling wave near its resonance, and the spectral resolution of the cochlear filters. Finally, we discuss the physiological correlates and location of such a process and its resulting templates.

Cochlea↗

PubMed1999

Detection of modulation in spectral envelopes and linear-rippled noises by budgerigars (Melopsittacus undulatus).

Budgerigars were trained to discriminate complex sounds with two different types of spectral profiles from flat-spectrum, wideband noise. In one case, complex sounds with a sinusoidal ripple in (log) amplitude across (log) frequency bandwidth were generated by combining 201 logarithmically spaced tones covering the frequency region from 500 Hz to 10 kHz. A second type of rippled stimulus was generated by delaying broadband noise and adding it to the original noise in an iterative fashion. In each case, thresholds for modulation depth (i.e., peak-to-valley in dB) were measured at several different ripple frequencies (i.e., cycles/octave for logarithmic profiles) or different repetition pitches (i.e., delay for ripple noises). Budgerigars were similar to humans in detecting ripple at low spatial frequencies, but were considerably more sensitive than humans in detecting ripples in log ripple spectra at high spatial frequencies. Budgerigars were also similar to humans in detecting linear ripple in broadband noise over a wide range of repetition pitches. Taken together, these data show that the avian auditory system is at least as good, if not better, than the human auditory system at detecting spectral ripples in noise despite gross anatomical differences in both the peripheral and central auditory nervous systems.

Animals↗

PubMed1999

Spectro-temporal modulation transfer functions and speech intelligibility.

Detection thresholds for spectral and temporal modulations are measured using broadband spectra with sinusoidally rippled profiles that drift up or down the log-frequency axis at constant velocities. Spectro-temporal modulation transfer functions (MTFs) are derived as a function of ripple peak density (omega cycles/octave) and drifting velocity (omega Hz). The MTFs exhibit a low-pass function with respect to both dimensions, with 50% bandwidths of about 16 Hz and 2 cycles/octave. The data replicate (as special cases) previously measured purely temporal MTFs (omega = 0) [Viemeister, J. Acoust. Soc. Am. 66, 1364-1380 (1979)] and purely spectral MTFs (omega = 0) [Green, in Auditory Frequency Selectivity (Plenum, Cambridge, 1986), pp. 351-359]. A computational auditory model is presented that exhibits spectro-temporal MTFs consistent with the salient trends in the data. The model is used to demonstrate the potential relevance of these MTFs to the assessment of speech intelligibility in noise and reverberant conditions.

Auditory Threshold↗

PubMed1983

A proposed neural network for the integrator of the oculomotor system.

Single-unit recordings, stimulation studies, and eye movement measurements all indicate that the firing patterns of many oculomotor neurons in the brain stem encode eye-velocity commands in premotor circuits while the firing patterns of extraocular motoneurons contain both eye-velocity and eye-position components. It is necessary to propose that the eye-position component is generated from the eye-velocity signal by a leaky hold element or temporal integrator. Prior models of this integrator suffer from two important problems. Since cells appear to have a steady, background signal when eye position and velocity are zero, how does the integrator avoid integrating this background rate? Most models employ some form of lumped, positive feedback the gain of which must be kept within totally unreasonable limits for proper operation. We propose a lateral inhibitory network of homogeneous neurons as a model for the neural integrator that solves both problems. Parameter sensitivity studies and lesion simulations are presented to demonstrate robustness of the model with respect to both the choice of parameter values and the consequences of pathological changes in a portion of the neural integrator pool.

Animals↗