Browsing by Author "Shamma, Shihab A."
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Item A Biophysical Model of Cochlear Processing: Intensity Dependence of Pure Tone Responses.(1986) Shamma, Shihab A.; Chadwik, Richard S.; Wilbur, W. John; Morrish, Kathleen A.; Rinzel, John; ISRA mathematical model of cochlear processing is developed to account for the nonlinear dependence of frequency selectivity on intensity in inner hair cell and auditory nerve fiber responses. The model describes the transformation from acoustic stimulus to intracellular hair cell potentials in the cochlea. It incorporates a linear formulation of basilar membrane mechanics and subtectorial fluid-cilia displacement coupling, and simplified description of the inner hair cell nonlinear transduction process. The analysis at this stage is restricted to low-frequency single tones. The computed responses to single tone inputs exhibit the experimentally observed nonlinear effects of increasing intensity such as the increase in the bandwidth of frequency selectivity and the downward shift of the best frequency. In the model, the first effect is primarily due to the saturating effect of the hair cell nonlinearity. The second results from the combined effects of both the nonlinearity and the inner hair cell low-pass transfer function. In contrast to these shifts along the frequency axis, the model does not exhibit intensity dependent shifts of the spatial location alone the cochlea of the peak response for a given single tone. The observed shifts do not contradict an intensity invariant tonotropic code.Item A Functional Model of Primary Auditory Cortex: Spectral Orientation Columns(1990) Shamma, Shihab A.; Chettair, Geeth M.; ISRA functional model of the primary auditory cortex is proposed based on physiological maps of the receptive field organization in ferret AI (Shamma, Fleshman and Wiser [1990]). Systematic changes in the excitatory and inhibitory portions of the receptive fields along the isofrequency planes are approximated by a difference of gaussians function with spatially changing parameters. We consider here only the response properties to stationary stimuli, i.e. those with non-varying spectra. The fundamental functional principle that emerges from the analysis of the model is that the primary auditory cortex encodes the shape of the acoustic spectrum in the distribution of its responses along the isofrequency planes. Specifically, it maps to each isofrequency plane a normalized measure of the locally averaged gradient of the input spectrum at that frequency. Physiological and psychoacoustical correlates and implications of these findings are explained. Parallels to the functional organization of the visual cortex are also discussed.Item Linear stimulus-invariant processing and spectrotemporal reverse correlation in primary auditory cortex(2003) Klein, David J.; Simon, Jonathan Z.; Depireux, Didier A.; Shamma, Shihab A.; Shamma, Shihab A.; ISR; CAARThe spectrotemporal receptive field (STRF) provides a versatile and integrated (spectral and temporal) functional characterization of single cells in primary auditory cortex (AI). We explore in this paper the origin and relationship between several different ways of measuring and analyzing the STRF. Specifically, we demonstrate that STRFs measured using a spectrotemporally diverse array of broadband stimuli --- such as dynamic ripples, spectrotemporally white noise (STWN), and temporally orthogonal ripple combinations (TORCs) --- are very similar, confirming earlier findings that the STRF is a robust linear descriptor of the cell. We also present a new deterministic analysis framework that employs the Fourier series to describe the spectrotemporal modulation frequency content of the stimuli and responses. Additional insights into the STRF measurements, including the nature and interpretation of measurement errors, is presented using the Fourier transform, coupled to singular-value decomposition (SVD), and variability analyses including bootstrap. The results promote the utility of the STRF as a core functional descriptor of neurons in AI.Item Organization of Response Areas in Ferret Primary Auditory Cortex(1992) Shamma, Shihab A.; Fleshman, James W.; Wiser, Philip R.; Versnel, Huib; ISRWe studied the topographic organization of the response areas obtained from single - and multi-unit recordings along the isofrequency planes of the primary auditory cortex (AI) in the barbiturate-anesthertized ferret. Using a two-tone stimulus, the excitatory and inhibitory portions of the response areas were determined and then parameterized in terms of an asymmetry index. The index measures the balance of excitatory and inhibitory influences around the best frequency (BF). The sensitivity of responses to the direction of a frequency-modulated (FM) tone was tested and found to correlate strongly with the asymmetry index of the response areas. Specifically, cells with strong inhibition from frequencies above (below) the BF preferred upward (downward) sweeps. Responses to spectrally shaped noise were also consistent with the asymmetry of the response areas: cells that were strongly inhibited by frequencies higher than the BF responded best to stimuli that contained least spectral energy above the BF. In a local region, most cells exhibited similar response area types and other response features. the distribution of the asymmetry index values along the isofrequency planes revealed systematic changes in the symmetry of the response areas. At the center, response areas with narrow and symmetric inhibitory sidebands predominated. These gave way to asymmetric inhibition, with high frequency inhibition (relative to the BF ) becoming more effective caudally, and low frequency inhibition more effective rostrally. These response types tended to cluster along repeated bands that paralleled the tonotopic asix. One functionaly implication of the response area organization is that cortical responses encode the locally averaged gradient of the acoustic spectrum by their differential distribution along the isofrequency planes. This enhances the representation of such features as the symmetry of spectral peaks, edges, and the spectral envelope. This scheme can be viewed as the one- dimensional analogue of spatial phase sensitivity in simple cells of the primary visual cortex, which there gives rise to spatial frequency channels and orientation columns.Item Receptive Field Organization in Ferret Primary Auditory Cortex: Spectral Orientation Columns(1990) Shamma, Shihab A.; Fleshman, James W.; Wiser, Philip R.; ISRAbstract not available.Item Ripple Analysis in Ferret Primary Auditory Cortex. I. Response Characteristics of Single Units to Sinusoidally Rippled Spectra(1994) Shamma, Shihab A.; Versnel, Huib; Kowalski, Nina; ISRWe compared the response properties of single units to tones and sinusoidally, rippled spectral stimuli in the primary auditory cortex (AI) of the, barbiturate-anesthetized ferret. Using two- tone stimuli, we determined the response area of each cell and parameterized it in terms of best frequency (BF), the bandwidth of the excitatory responses at 20dB above threshold (BW20), and an asymmetry index measuring the balance of inhibition and excitation around the BF.Using frequency-modulated (FM) tones, we also determined a directional sensitivity index for the cell. Using broadband stimuli (1-20 kHz) with sinusoidally modulated spectral envelopes (ripples), we measured the response magnitude of each cell as a function of ripple frequency (W) and ripple phase (F), and then reconstructed the magnitude and phase of a ripple transfer function. Most cells (approximately 90 %) were tuned to a specific ripple frequency, denoted as a characteristic ripple frequency (Wo) . Most cells also exhibited a linear ripple phase as a function of W. The intercept of the phase function defined as the characteristic ripple phase (Fo), and is interpreted as the best ripple phase to drive the cell; the slope of the phase function reflects the location of the response area of the cell along the tonotopic axis. By inverse Fourier transforming the transfer function, we obtain the response field (RF) of the cell, an analogue of the response area measured with tonal stimuli. Like the response area, the RF was parametrized by the following measures: BFRF, which is the location of the maximum of the RF along the tonotopic axis, Wo , which is roughly inversely proportional to the width of the RF, and Fo which reflects the asymmetry of the RF. In the ferret Wo , ranges from 0.2 to 3 cycles/octave, with the average of the distribution around 1.0. Fo , ranges over the full cycle in a Gaussian-like distribution around 0o. For a subgroup of cells the sinusoidal modulations of the spectrum were presented both on linear and logarithmic amplitude scale. The responses were not notably different. The effect of the variations of amplitude of the sinusoidal modulation was studied. The largest effect was observed for the magnitude transfer function, which increased with amplitude and then saturated. The parameters Wo and Fo did not vary significantly with ripple amplitude. Typically, cells respond best to intermediate sound levels of the ripple stimulus, i.e., the magnitude transfer function shows a nonmonotonic dependence on overall stimulus level. The phase function and Wo do not depend much on level. The effects of a few nonlinearities on the responses are examined briefly. Effects of nonlinearities as threshold and saturation of the neural firing rates are examined. It is found that (non)monoticity of the rate level function of a cell could be distinguished from its ripple response characteristics. The RF of a cell closely corresponds to the response area measured with tone stimuli. Regression analysis shows that: (A) BFRF is, very similar to the tonal BF; (B) Wo is inversely correlated to the excitatory bandwidth; (C) Fo is correlated to the asymmetry of the response area.
Responses to rippled spectra in AI resemble closely the response properties to sinusoidal gratings in the primary visual cortex (VI). This provides a unified framework within which to interpret the functional organization of both corticies. Basic differences between the two systems, however, are also evident as the lack in AI of a substantial simple/complex distinction in the responses.
It is hypothesized that AI effectively analyzes an arbitrary input spectrum into a weighted sum of ripple components of different ripple frequencies and phases. This analysis is performed locally around each BF by a two-dimensional bank of filters tuned to different Wo and Fo values. Psychophysical support and implications of this hypothesis are also discussed in relation to the perception of timbre and other auditory tasks.
Item Stereausis: Binaural Processing without Neural Delays.(1987) Shamma, Shihab A.; Shen, Naiming; Gopalaswamy, Preetham; ISRA biologically realistic neural network model is proposed for the binaural processing of interaural-time and level cues that closely resembles computational schemes suggested for atereopsis in vision. The network requires no neural delay lines to generate such attributes of binaural hearing as the lateralization of all frequencies, and the detection and enhancement of noisy signals. The two-dimensional network measures interaural differences by detecting the spatial disparities between the instantaneous outputs of the two ears. It achieves this by comparing systematically at various horizontal shifts, the spatiotemporal responses of the tonotopically ordered array of auditory-nerve- fibers. An alternative view of the network operation is that it computes approximately the cross-correlation between the responses of the two cochlei by combining an ipsilateral input at a given CF with contralateral inputs from locally off-CF locations. Thus, the network utilizes the delays already present in the travelling waves of the basilar membrane to extract the correlation function. Simulations of the network operation with various signals are presented; Physiological arguments in support of this scheme are also discussed.