Auditory system

Rebert CS, Boyes WK, Pryor GT, et al. 1993. Combined effects of solvents on the rat s auditory system styrene and trichloroethylene. Int J Psychophysiol 14 49-59. 

Hall DB Michigan State University, Chicago, IL After describing the properties of the young spiral ganglion cells within the mammalian auditory system then investigate the neurotoxic effects of lead on the sodium current National Institute on Deafness Other Communication Disorders... 

Laughlin NK University of Wisconsin, Madison, Wl Examine auditory system function and attention in the rhesus monkey as a model of childhood lead exposure National Center For Research Resources... 

Robinson GS, Keith RW, Bomschein RL, et al. 1987. Effects of environmental lead exposure on the developing auditory system as indexed by the brainstem auditory evoked potential and pure tone hearing evaluations in young children. In Lindberg SE, Hutchinson TC. eds. International Conference on Heavy Metals in the Environment, Vol. 1, New Orleans, LA. September. Edinburgh, UK CEP Consultants, Ltd., 223-225. 

The deafness gene approach has proven fruitful for finding genes important to the auditory system 839 A variety of genes have been identified, including a set of genes that apparently interact with each other in the auditory and visual systems 839... 

The endolymphatic compartment of the auditory system is at an elevated potential (about +80 mV) this endocochlear potential increases the driving force on K+ yet more, producing additional transduction current. Some mutations that cause deafness affect either K+ levels in the endolymph or the endolymphatic potential itself. 

About 1 in 1,000 children are born deaf, and another 1 in 1,000 develop deafness by adolescence. Although some deafness can arise during development due to external sources, much deafness in children is due to mutations in specific genes. As many forms of deafness are nonsyn-dromic — there is no other associated abnormality — identification of the responsible genes can lead to characterization of proteins that are essential for the auditory system (and often the vestibular system, as well). 

The deafness gene approach has proven fruitful for finding genes important to the auditory system. The... 

In specialized cells of the auditory system, there are also receptors that are gated by pressure changes. 

Toxicity/symptoms nervous system, developmental effects include cerebral palsy-like symptoms with involvement of the visual, sensory, and auditory systems, tingling around lips and mouth, tingling in fingers and toes, vision and hearing loss... 

EXTENSIONS AND COMMENTARY Most psychedelic drugs affect, primarily, the visual sense, but here is one that shows its effects primarily in the auditory system. And it screws it up in a most unlinear manner, in that there in not just a simple decrease in pitch which would be as if someone had his thumb against the LP record and made everything come out at a 3/4 speed text, or a 1/2 speed text. Actual roportionality is lost, so there is complete harmonic distortion. 

Dodson, H.C., Charalabapoulou, M., 2001, PMCA2 mutation causes structural changes in the auditory system in deafwaddler mice. J Neurocytol 30, 281-292. 

Before one can start predicting MOS scores several problems have to be solved, The first one is that different subjects have different auditory systems leading to a large range of possible models. If one wants to determine the quality of telephone-band speech codecs (300-3400 Hz) differences between subjects are only of minor importance. In the determination of the quality of wideband music codecs (compact disc quality, 20-20000 Hz) differences between subjects are a major problem, especially if the codec shows dynamic band limiting in the range of 10-20 kHz. Should an objective... 

In thinking about how to calculate the internal representation of a signal one could dream of a method where all the transformation characteristics of the individual elements of the human auditory system would be measured and modelled. In this exact approach one would have the, next to impossible, task of modelling the ear, the transduction mechanism and the neural processing at a number of different abstraction levels. 

One can doubt whether it is necessary to have an exact model of the lower abstraction levels of the auditory system (outer-, middle-, inner ear, transduction). Because audio quality judgements are, in the end, a cognitive process a crude approximation of the internal representation followed by a crude cognitive interpretation may be more appropriate then having an exact internal representation without cognitive interpretation of the differences. 

The composite operation, smearing followed by compression, results in partial masking (see Fig. 1.5). The advantage of this method is that the model automatically gives a prediction of the behavior of the auditory system when distortions are above masked threshold. 

Different from source coding, in perceptual coding the emphasis is on the removal of only the data which are irrelevant to the auditory system, i.e. to the ear. The signal is coded in a way which minimizes noise audibility. This can lead to increased noise as measured by Signal-to-Noise-Ratio (SNR) or similar measures. The rest of the chapter describes how knowledge about perception can be applied to code generic audio in a very efficient way. 

Research on the hearing process carried out by many people (see [Scharf, 1970]) led to a frequency analysis model of the human auditory system. The scale that the ear appears to use is called the critical band scale. The critical bands can be defined in various ways that lead to subdivisions of the frequency domain similar to the one shown in table 2.1. A critical band corresponds to both a constant distance on the cochlea and the bandwidth within which signal intensities are added to decide whether the combined signal exceeds a masked threshold or not. The frequency scale that is derived by mapping frequencies to critical band numbers is called the Bark scale. The critical band model is most useful for steady-state tones and noise. 

Looking for stereo irrelevancy we find that the ability of the human auditory system to discriminate the exact location of audio sources decreases at high frequencies [Blauert, 1983], The cues to get spatial impression are mainly taken from the energy maxima in space at each frequency. 

The most widespread opinion is that the phase need not be modified because of the properties of the human auditory system [Lim and Oppenheim, 1979]. Strictly speaking however, the assertion that the ear is insensitive to the phase was highlighted by psychoacoustic findings only in the case of stationary sounds and for the phase of the Fourier transform [Moore, 1997]. Moreover, it is well known that in the case of STFT, phase variations between successive short-time frames can give rise to audible effects (such as frequency modulation) [Vary, 1985]. 

The nature of the signal processing, and its potential effectiveness, depends on the characteristics of the auditory system. The ear transforms the incoming acoustic signal into mechanical motion, and this motion ultimately triggers neural pulses that carry the auditory information to the brain. The essential components of the ear are shown in Fig 6.1. 

Figure 6.1 Major features of the human auditory system...

Krasner, 1979] Krasner, M. A. (1979). Digital encoding of speech and audio signals based on the perceptual requirements of the auditory system. Technical Report 535, Massachusetts Institute of Technology, Lincoln Laboratory, Lexington. 

Zwicker and Zwicker, 1991] Zwicker, E. and Zwicker, U. T. (1991). Audio engineering and psychoacoustics Matching signals to the final receiver, the human auditory system. J. Audio Eng. Soc., 39(3) 115-126. 

Auditory scene analysis, 24, 26 Auditory system, 8, 10, 16 Auralization, 87 Automatic gain control, 255 Autoregressive (AR) model, 135, 142, 159, 164 Autoregressive (AR) model, Interpolation (See Restoration,Interpolation)... 

For systems that are nearly linear or time-variant, the concept of the impulse (complex frequency) response is still applicable. For weakly non-linear systems the characterization can be extended by including measurements of the non-linearity (noise, distortion, clipping point). For time-variant systems the characterization can be extended by including measurements of the time dependency of the impulse response. Some of the additional measurements incorporate knowledge of the human auditory system which lead to system characterizations that have a direct link to the perceived audio quality (e.g. the perceptually weighted signal to noise ratio). 

If the perceptual approach is used for the prediction of subjectively perceived audio quality of the output of a linear, time-invariant system then the system characterization approach and the perceptual approach must lead to the same answer, In the system characterization approach one will first characterize the system and then interpret the results using knowledge of both the auditory system and the input signal for which one wants to determine the quality. In the perceptual approach one will characterize the perceptual quality of the output signals with the input signals as a reference. 

The compression that is used to calculate the internal representation consists of a transformation rule from the excitation density to the compressed Sone density as formulated by Zwicker [Zwicker and Feldtkeller, 1967], The smearing of energy is mostly the result of peripheral processes [Viergever, 1986) while compression is a more central process [Pickles, 1988], With the two simple mathematical operations, smearing and compression, it is possible to model the masking properties of the auditory system not only at the masked threshold, but also the partial masking [Scharf, 1964] above masked threshold (see Fig. 1.5). 

Our lives are for the most part spent in reverberant environments. Whether we are enjoying a musical performance in a concert hall, speaking to colleagues in the office, walking outdoors on a city street, or even in the woods, the sounds we hear are invariably accompanied by delayed reflections from many different directions. Rather than causing confusion, these reflections often go unnoticed, because our auditory system is well equipped to deal with them. If the reflections occur soon after the initial sound, the result is not perceived as separate sound events. Instead, the reflections modify the perception of the sound, changing the loudness, timbre, and most importantly, the spatial characteristics of the sound. Late reflections, common in very reverberant... 

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