Structural Asymmetries

The considerable body of data indicating that the human auditory cortices are specialized for different types of processing in the left and right hemispheres has led to the search for possible structural correlates of this lateralization. The region of the PT has been one focus of such research since many studies have indicated that it differs across the two cerebral hemispheres. This asymmetry has traditionally been characterized as one of size, with the left PT being larger than the right in approximately 65-80% of brains. It is also known that similar asymmetries exist in the brains of newborns. These findings have been widely interpreted as providing a basis for the specialization of left posterior STG regions for auditory language processing. However, there is only limited evidence directly linking structural to functional asymmetries.

Considerable evidence also exists for hemispheric differences in the shape of the superior temporal region. It is well-known that the right Sylvian fissure angles upward more steeply on the right side, and that the point at which it begins to ascend is more anterior than on the left. Therefore, the apparent size difference of the PT may at least partly be a consequence of hemispheric differences in morphology rather than size.

Evidence exists that certain cytoarchitectonic regions within the PT may be larger on the left, but because there is no established relationship between the cytoarchitecture and the gross anatomical features on which the PT is defined, it is difficult to determine how measurements of the PT are related to cytoarch-itectonically defined areas. The search for structure-function correlates is further complicated by the fact that the incidence of structural asymmetry in the PT is far lower than the incidence of left hemisphere language organization, which is estimated to be on the order of 95-98% of the population.

Other structural asymmetries in addition to the PT have also been described in auditory cortices. Recent evidence from both in vivo MRI studies and postmortem histological studies indicates that a greater volume of white matter underlies the left HG and surrounding cortex than the equivalent areas on the right (Fig. 2). In addition, there is evidence that myelination of axonal fibers on the left side is thicker, which would be compatible with the hypothesis that the left auditory cortices are specialized for faster neural transmission, as required for perception of speech signals.

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