Anatomy of the auditory cortex.

Cortical auditory areas located in the superior temporal region (STR) in monkey and human. The primary auditory area (AI) occupies the cortex of the supratemporal plane (STP) and is surrounded by auditory association areas in circular sulcus and superior temporal gyrus (STG). Architectonic studies have parcellated auditory areas into a number of subregions. Beginning from the temporal polar proisocortex up to the parietal cortex, these areas shows progressive laminar differentiation, and are arranged into three parallel lines. The most medial line occupies the cortex of the circular sulcus. The regions of this line maintains limbic features and is termed as root line. Another line is located in STG. The regions of this line show progressive emphasis in the third and fourth layer neurons and is termed as belt line. Interposed between root and belt line is a core line located in STP. In this line there is greater accumulation of fourth layer neurons. Recent physiological studies have outlined several auditory representations surrounding AI. These auditory representations correspond to above mentioned architectonic subregions of STR. The subregions within each line have bidirectional connectional laminar specificity. The feedforward connections originate from the supragranular layer III and terminate in the around layer IV of the rostrally adjacent region. Feedback projections in contrast stem from the infragranular layers and terminate in layer I. The long association connections of auditory areas are with the prefrontal cortex (PFC), the multimodal areas and the limbic regions, and are derived from belt and root line areas of STR. These projections follow the rostro-caudal architectonic differentiation of STR. Thus the rostral STG areas are mainly connected with orbital and medial PFC areas whereas the caudal STG areas are connected with caudal PFC. The intermediate STG areas are preferentially related to the lateral PFC regions. It seems that STG-PFC connections are between the areas with similar level of architectonic differentiation. The thalamic connections of the subregions of STR also follow the architectonic organizations. The core line areas are preferentially related to ventral nucleus (MGv) of medial geniculate nucleus (MGN) whereas the root and belt line areas are connected respectively to magnocellular (MGmc) and dorsal (MGd) subdivisions of MGN. The root and belt areas share some connections and are also related to pulvinar, suprageniculate, dorsomedial and intralaminar nuclei. It seems therefore that progressive laminar and tripartrate organization of auditory regions of STR is reflected in intrinsic, association and thalamic connections. The feedforward connections may be engaged in analysis of external environmental cues whereas feedback connections may have a role in matching learned or stored information with incoming auditory signals. The preferential core line connectivity with MGv may be involved in spectral analysis of sound whereas the connections of the belt and root areas with MGmc, MGd, and pulvinar may have role in sound pattern recognition, auditory memory, the localization of sound in space as well as matching auditory information with other modalities.