But, while the auditory system is so deeply dependent on timing, there are still many instances where adaptation processes take place. This faithful encoding of auditory information is maintained along the ascending auditory pathway up to the auditory cortex (AC), whose neurons are capable of maintaining millisecond precision in the encoding of auditory stimuli ( Kayser et al., 2010). In other cells, the synchronization of their responses is enhanced thanks to the convergence of a few auditory nerve fibers through large endbulbs ( Ryugo and Sento, 1991 Joris et al., 1994). The auditory nerve fibers that innervate the anterior ventral cochlear nucleus in mammals have large, specialized calyceal endings, also known as endbulbs, that surround the soma of the target neuron (for a review, see Ryugo and Parks, 2003). The requirement of a precise and faithful transmission of timing information has given rise to the development of certain cellular specializations.
#Auditory nerve definition psychology code#
For instance, responses of low-frequency auditory nerve fibers are locked to a particular phase of the stimulus waveform ( Kiang et al., 1965 Johnson, 1980 Palmer and Russell, 1986), and thus carry a temporal code for sound frequency. The timing of action potentials, conveyed with the precision of microseconds, carries acoustic information in all higher vertebrates. The detection of the minimum change for sound localization in the horizontal plane in humans requires comparing the arrival time at both ears with a precision of a few microseconds ( Hafter et al., 1979 Kollmeier et al., 2008). Timing information is also essential for the localization of sounds, since it requires a precise encoding of the time at which sounds arrive at each ear. In order to process these very rapid variations of the signal, the auditory system requires fast and reliable responses from its elements. For instance, mice can hear sounds over 80 kHz ( Heffner and Masterton, 1980) and some bats up to 120 kHz ( Koay et al., 1997). Some animals have good low frequency hearing, similar to humans, like the guinea pig ( Heffner et al., 1971), but other animals can hear much higher frequencies. Humans typically can hear sounds from 20 Hz to 20 kHz. The range of frequencies that each animal is sensitive to varies greatly.
One of the basic features of the components of all sounds is their frequency (or how fast the sound waves change) and the auditory brain must be able to extract it very precisely. A complication lies in the nature of sound, which consists of rapid variations of the pressure in an elastic medium, usually air for most mammals. The challenging task that the auditory system faces is to process naturally occurring sounds, so that they can be identified, characterized, and localized, in order to be able to respond accordingly and in a timely manner. Introduction: The Auditory System Needs to Preserve the Timing of the Signal Here we review the multiple types of adaptation that occur in the auditory system, which are part of the pool of resources that the neurons employ to process the auditory scene, and are critical to a proper understanding of the neuronal mechanisms that govern auditory perception. In the auditory cortex, adaptation takes more elaborate forms, and contributes to the processing of complex sequences, auditory scene analysis and attention. For example, stimulus-specific adaptation, where neurons show adaptation to frequent, repetitive stimuli, but maintain their responsiveness to stimuli with different physical characteristics, thus representing a distinct kind of processing that may play a role in change and deviance detection. However, it is at higher levels in the auditory hierarchy where more sophisticated types of neuronal processing take place.
For instance, auditory nerve fiber responses already experience adaptation of their firing rates, a type of response that can be found in many other auditory nuclei and may be useful for emphasizing the onset of the stimuli. At the same time, different processes of neuronal adaptation occur at several levels to further process the auditory information. The early stages of the auditory system need to preserve the timing information of sounds in order to extract the basic features of acoustic stimuli. 2Department of Cell Biology and Pathology, Faculty of Medicine, University of Salamanca, Salamanca, Spain.1Auditory Neurophysiology Laboratory (Lab 1), Institute of Neuroscience of Castilla y León, University of Salamanca, Salamanca, Spain.
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