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WIREs Cogn Sci
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The biological basis of audition

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Abstract Audition is one of the fundamental extrasensory percepts in mammals. Two of the primary objectives of audition are to determine where sounds originate from in space and what those sounds are. Neural processing of acoustic signals, which are commonly quite complex under natural conditions, is extensive in the brainstem, midbrain, and thalamus. This processing extracts multiple salient features that are then transmitted to the cerebral cortex. The cerebral cortex is a necessary neural structure for audition, or the perception of acoustic auditory objects and/or events. This entry will review the early processing along the ascending auditory central nervous system from the cochlea to the cerebral cortex. The neural mechanisms of audition will then be explored for spatial and non‐spatial perception, drawing largely on examples from non‐human primates, but insights gained from other mammalian species will also be covered. How these models relate to current studies in human subjects, using both functional imaging and invasive techniques, will also be explored as well as the types of future studies that will enable us to better understand the neural mechanisms of audition. WIREs Cogni Sci 2011 2 408–418 DOI: 10.1002/wcs.118 This article is categorized under: Neuroscience > Physiology

Generation of head‐related transfer functions. Schematic representation of how the pinna can influence the stimulus spectrum from the sound in air to the sound in the ear canal. A stimulus with a flat frequency spectrum (broadband noise) is composed of multiple different frequency components (left). These frequencies will be reflected off of different portions of the pinna (middle) depending on the stimulus frequency, causing delays when that frequency energy reaches the ear canal. This results in the amplification and attenuation of certain frequencies (right) known as spectral peaks and notches, depending on the spatial location of the stimulus. Not shown are similar influences of the head and body.

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Model accuracy as a function of cortical area and hemifield. The error computed from the model based on the firing rates of the population of neurons is shown for ipsilateral locations (gray) and contralateral locations (black). Dashed line represents the average error for humans localizing the same stimuli. Only estimates in contralateral space based on the population of neurons in CL are accurate, with estimates based on neural responses of MM neurons being the least accurate. (Based on data from Ref 47.)

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Population response to spatial location in three cortical areas. Each line represents the normalized response as a function of location for populations of neurons recorded from alert macaque monkeys, taken from each of three different cortical areas. Zero degrees is directly in front of the animal, with negative numbers denoting leftward locations. The population of MM neurons (blue) do not show very much modulation as a function of spatial location, whereas the population of CL neurons (green) have a much greater overall response to rightward locations compared to leftward locations (corresponding to contralateral and ipsilateral space, respectively), with the population of A1 neurons (red) somewhere in between. (Based on data from Ref 34.)

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Dual‐stream hypothesis based on the intracortical connections of the auditory cortex. Schematic representation of the interconnections of the core with the belt, parabelt, and other cortical areas. These fields are strongly interconnected with their neighboring cortical fields, but not with fields beyond their immediate neighbors. Red arrows denote pathways that are hypothesized to represent spatial processing, whereas green arrows denote the non‐spatial pathways based on the dual‐stream hypothesis proposed in Ref 16. The multiple arrows from the parabelt fields denote serial connections to multiple cortical areas including the prefrontal areas associated with the spatial and non‐spatial processing of visual information.

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Anatomical organization of the primate auditory cortex. Schematic representation of the left auditory cortex in a primate modeled after a macaque monkey. The core region is represented by areas A1, R, and RT. The belt cortex surrounds the core, with the rostral and caudal parabelt located laterally to the belt cortical areas. The core region has a frequency representation from high frequencies (H) to low frequencies (L) that reversed at the A1–R and R–RT borders. AI: primary auditory cortex; R: rostral field; RT: rostrotemporal field; CM: caudomedial field; MM: middle medial field; RM: rostromedial field; RTM rostrotemporal medial field; RTL: rostrotemporal lateral field; AL: anterolateral field; ML: middle lateral field; CL: caudolateral field. (Based on data from Refs 8 and 9.)

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Isofrequency laminae of the inferior colliculus. Schematic representation of the central nucleus of the inferior colliculus. Inputs from the brainstem converge in specific laminae, which then project to the medial geniculate nucleus of the thalamus. Frequency tuning curves of hypothetical neurons are shown in each isofrequency lamina (color conventions as in Figure 2) DCN: dorsal cochlear nucleus; SOC: superior olivary complex; NLL: nucleus of the lateral lemniscus.

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Spiral ganglion cell projections from the cochlea to the cochlear nucleus. The left shows a schematic of a mammalian cochlea, with the high frequencies represented at the base (lower right) and progressing to the low frequencies in the apex (center). Approximately 20 spiral ganglion cells contact each hair cell and then project centrally where each axon branches to innervate neurons in each of the three divisions of the cochlear nucleus. These projections generate frequency‐specific lamina in each of the divisions. Frequency tuning curves of hypothetical single neurons in each lamina are shown to the far right. DCN: dorsal cochlear nucleus; AVCN: anterior ventral cochlear nucleus; PVCN: posterior ventral cochlear nucleus.

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