Anatomy A Gross Morphology

The auditory cortex consists of several distinct fields that differ in terms of their cytoarchitecture, connectivity, and physiological response properties. In primates, three major subdivisions may be identified, which are usually referred to as core, belt, and parabelt regions. The correspondences and homologies between these regions in human and other primates have not been completely worked out. However, it is well established that in humans the primary auditory cortex is located within Heschl's gyrus (HG), a relatively well-defined gyrus located transversely on the superior temporal gyrus, deep within the Sylvian fissure (Figs. 1 and 2). It is not uncommon for more than one HG to be present in one or both hemispheres, but there is no consistent pattern of asymmetry, as had been once thought. The right HG is shifted anteriorly by approximately 6 mm compared to the left. The primary area is confined mostly to the medial half to two-thirds of HG (the first HG when there is more than one), although in some individuals it may extend into Heschl's sulcus, which forms the posterior border of HG.

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Heschel Convolutions

Figure 1 Anatomical specimen showing a lateral view of the right cerebral hemisphere with the lateral (Sylvian) fissure opened to reveal the hidden surface of the superior temporal gyrus. Heschl's gyrus is visible as the prominent transverse convolution, marked 41. The planum temporale is located posterior to Heschl's gyrus and is marked 42. Numbers correspond to cytoarchitectonic definitions according to Brodmann (reproduced courtesy of Charles C. Thomas, Publisher, Ltd., Springfield, Illinois from E. A. Kahn et al, 1969, Correlative Neurosurgery).

Figure 1 Anatomical specimen showing a lateral view of the right cerebral hemisphere with the lateral (Sylvian) fissure opened to reveal the hidden surface of the superior temporal gyrus. Heschl's gyrus is visible as the prominent transverse convolution, marked 41. The planum temporale is located posterior to Heschl's gyrus and is marked 42. Numbers correspond to cytoarchitectonic definitions according to Brodmann (reproduced courtesy of Charles C. Thomas, Publisher, Ltd., Springfield, Illinois from E. A. Kahn et al, 1969, Correlative Neurosurgery).

The precise boundaries of the belt and parabelt fields of the human auditory cortex have not been established, but it appears that auditory regions extend from HG both posteriorly and anteriorly along the superior temporal gyrus (STG) as well as onto the lateral aspect of the gyrus and into the superior temporal sulcus, as occurs in the macaque (Fig. 3). There is also evidence that auditory cortex may be found within the temporoparietal opercular area and even in some portions of the posterior insula bordering the medial portion of HG. However, there are no clearly established sulcal or gyral boundaries that correspond to these fields.

Posterior to HG is the planum temporale (PT), an approximately triangular region along the STG, bordering the Sylvian fissure (Fig. 1). This area, defined on the basis of its gross morphology, has been proposed to be important in language processes. Anterior to HG along the STG is an area sometimes referred to as the planum polare, which is likely to contain unimodal auditory cortex as well. The lateral aspect of the STG and the upper bank of the superior temporal sulcus may be considered as part of the parabelt auditory cortex as well. Beyond these regions lie areas that may have multimodal functions, notably cortex within the lower bank of the superior temporal sulcus and/or the middle temporal gyrus that may integrate auditory and visual information.

B. Cytoarchitecture

The core auditory region within HG, which in turn may be further subdivided, consists of koniocortex. Its laminar organization is characteristic of primary regions in other modalities; it is granular, with a thick, well-defined layer IV containing pyramidal cells. It is also densely myelinated and has high reactivity to cytochrome oxidase and acetylcholinesterase. This area is approximately coextensive with area 41 in Brodmann's cytoarchitectonic map of the human brain and with area TC according to the parcellation of von Economo and Koskinas. It is also sometimes referred to as A1 in the neurophysiological literature.

The cytoarchitectonic organization of the rest of the human auditory cortex is not well known, but studies in monkeys have helped to provide a model to which human data may be compared. The core, belt, and parabelt areas mentioned previously have been best defined in the macaque (Fig. 3). In that species the core region may be further subdivided into at least two and likely three separate areas based on the slightly different characteristics of their neuronal organization as well as on physiological response properties. It is likely, but not proven, that a similar organization exists in the human brain.

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