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Neurophysiological Processing Of Am Essay

2077 words - 9 pages

When an input leaves the cochlea, the afferent nerve fibers will fire in response to that input, sending an electric signal down the length of the auditory nerve. The term used to refer to this pathway is central auditory system, which includes the pathways and nuclei beyond the auditory nerve that ascend within the brainstem to the auditory cortex. Along this ascending pathway, the inputs from the auditory nerve fibers project to several processing stations. These several projections show the complexity of the connections in the central auditory system. This pathway to the auditory cortex can be summarized as follow: an acoustic input will go from the auditory nerve fibers, to the cochlear nucleus, to superior olivary complex in the brain stem, then to the inferior colliculus in the midbrain, to the medial geniculate body in the thalamus, and finally reaching the auditory cortex in the superior temporal gyrus.
Measuring neurophysiological responses to AM stimuli have led to a basic and crucial characterization of how temporal envelope of an acoustic signal is represented and processed at several levels of the auditory pathway. Similar to psychophiscal experiments, AM has received a tremendous attention in neurophysiological studies due to their fundamental role in shaping the natural speech signal. However, there are differences in the use of AM in both experimental fields. In psychophysical studies, the modulation detection thresholds are measured as a function of modulation frequencies using adaptive measures; meaning that the modulation depth is varied until a threshold criterion is met. In contrast, the majority of neurophysiological studies record the neural responses to100% sinusoidal AM signal as a function of modulation frequency, with few studies systematically varied the modulation depth (e.g., Sayles et al., 2013; Joris and Yin, 1992). Neurophysiological researchers investigate the response to an AM stimulus by analyzing phase-locking to the carrier frequency or the envelope of waveform; however, most of the available evidence is reported in relation to phase-locking to the modulation frequency (Joris et al. 2004). Particularly, neurophysiological studies analyze the response to AM stimuli either in terms of the spike rate and it is called rate-based (i.e., the number of spikes evoked over several modulation cycles) or synchronization (R) of the timing of actions potentials to the envelope waveform (i.e., temporal-based). These two metrics are usually reported as a function of the stimulus modulation frequency, resulting in a physiological rate modulation transfer function (rate-MTF, average rate versus modulation frequency) or a synchrony MTF (sync-MTF, synchrony versus modulation frequency) (Joris et al., 2004). The synchronization is often reported as the modulation gain (in dB), and it is defined as 20 log (2R/m) where m is the modulation depth. In psychophysical experiment, the modulation frequency at which the TMTF roll off is...

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