No abstract

There is a lack of physiological data pertaining to how listening humans process auditory information. Functional magnetic resonance imaging (fMRI) has provided some data for the auditory cortex in awake humans, but there is still a paucity of comparable data for subcortical auditory areas where the early stages of processing take place, as amply demonstrated by single-unit studies in animals. It is unclear why fMRI has been unsuccessful in imaging auditory brain-stem activity, but one problem may be cardiac-related, pulsatile brain-stem motion. To examine this, a method eliminating such motion (using cardiac gating) was applied to map sound-related activity in the auditory cortices and inferior colliculi in the brain stem. Activation in both the colliculi and cortex became more discernible when gating was used. In contrast with the cortex, the improvement in the colliculi resulted from a reduction in signal variability, rather than from an increase in percent signal change. This reduction is consistent with the hypothesis that motion or pulsatile flow is a major factor in brain-stem imaging. The way now seems clear to studying activity throughout the human auditory pathway in listening humans.

Architecture of auditory areas of the superior temporal region (STR) in the human was analyzed in Nissl-stained material to see whether auditory cortex is organized according to principles that have been described in the rhesus monkey. Based on shared architectonic features, the auditory cortex in human and monkey is organized into three lines: areas in the cortex of the circular sulcus (root), areas on the supratemporal plane (core), and areas on the superior temporal gyrus (belt). The cytoarchitecture of the auditory area changes in a stepwise manner toward the koniocortical area, both from the direction of the temporal polar proisocortex as well as from the caudal temporal cortex. This architectonic dichotomy is consistent with differences in cortical and subcortical connections of STR and may be related to different functions of the rostral and caudal temporal cortices. There are some differences between rhesus monkey and human auditory anatomy. For instance, the koniocortex, root area PaI, and belt area PaA show further differentiation into subareas in the human brain. The relative volume of the core area is larger than that of the belt area in the human, but the reverse is true in the monkey. The functional significance of these differences across species is not known but may relate to speech and language functions.