Mammalian Auditory Cortex Structure as the Basis of Cortical Sound Processing

Gleb Khorunzhii, Marina Egorova
2018 Journal of Behavioral and Brain Science  
The basic morphological aspects of auditory cortex organization in different orders of eutherian mammals are considered in the present review. The modern data describing a partitioning of mammalian auditory cortex into subfields are presented. A detailed observation of the structural organization of primary auditory cortex is given, as well as a review of recent morphological data about secondary auditory areas. Another section describes the system of auditory cortical projections. The data are
more » ... tions. The data are considered from the perspective of possible homologies existing between the auditory cortices in different mammalian species. Figure 1. Representation of the cat auditory cortical area, suggested by Woolsey [22] . AI-primary auditory field (black filling). Dashed and dotted lines indicate approximate borders of areas. Ep-posterior ectosylvian gyrus, AII-secondary auditory field, SF-suprasylvian peripheral area, Ins-insular area, AIII-temporal auditory field, Assoc-associative cortex. MI-precentral motor cortical area, Late VII-visual cortex. In tonotopic auditory fields the direction of frequency increase is indicated (from A to B). d-dorsal direction, v-ventral direction, r-rostral direction, c-caudal direction. Modified from: [49] . developed cortical layers II and IV receiving a large pool of both specific and non-specific thalamocortical projections. Primary auditory fields have a coniocortical cellular structure, receive direct inputs from MGB and differ from other auditory fields by a number of biochemical properties. Coniocortical cells in the neocortex usually demonstrate a small percentage of large pyramidal neurons and are predominated by densely distributed small stellate cells. The basic source of the primary auditory cortex afferents is a main lemniscal auditory pathway-the system of ascending projections rising from the auditory brainstem and reaching MGB as part of the lateral lemniscus fibres. The secondary auditory belt is usually located on the periphery of the primary auditory core. Due to its cytoarchitectonics, the auditory belt is defined as agranular neocortex with the thick cortical layer III (pyramidal) where there is a high spatial density of cells, including large pyramidal neurons. The extensive development of cortical layer III is provided by many associative and commissural fibres, located within this layer. Secondary auditory fields receive a large set of afferents from different parts of MGB and cortical primary auditory fields. The associative region of the temporal cortex is usually classified as the auditory parabelt. This part of auditory cortex receives inputs from non-lemniscal sources [37] [38] [39] . This so-called "diffuse" or "additional to the lemniscal" auditory system has significant convergent inputs from sensory systems of other modalities but demonstrates a strong response to acoustic stimuli [25] . It seems that the diffuse auditory system doesn't have direct connections with primary auditory areas but receives rich afferentation from the secondary auditory cortex, medial pulvinar, thalamic suprageniculate nucleus and nucleus limitans. Weak afferent inputs to the auditory parabelt rising from dorsal and medial parts of MGB have also been described [40] . Currently, the non-lemniscal auditory pathway is known as a part of a higher order stage of processing, constituting a secondary system capable of processing more complex aspects of auditory scene analysis [1]. On a cortical level, this integrative system of sound processing is closely associated with the auditory belt and parabelt [1] [37] [38]. Thus, as the main integrative centre of the mammalian auditory system, auditory cortex provides the basement for hierarchical (serial) cortical sound processing [15] [16], being involved in the different aspects of acoustic information analysis, which become more complex and more integrative from the primary auditory cortex up to auditory parabelt.
doi:10.4236/jbbs.2018.812040 fatcat:uor57lzjvnegbn4cfclcogsj64