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 corte x of the supratemporal plane (STP) and is surrounded by auditory assoc iation areas in circular sulcus and superior temporal gyrus (STG). Arc hitectonic studies have parcellated auditory areas into a number of su bregions. Beginning from the temporal polar proisocortex up to the par ietal cortex, these areas shows progressive laminar differentiation, a nd are arranged into three parallel lines. The most medial line occupi es the cortex of the circular sulcus. The regions of this line maintai ns limbic features and is termed as root line. Another line is located in STG. The regions of this line show progressive emphasis in the thi rd and fourth layer neurons and is termed as belt line. Interposed bet ween root and belt line is a core line located in STP. In this line th ere is greater accumulation of fourth layer neurons. Recent physiologi cal studies have outlined several auditory representations surrounding AI. These auditory representations correspond to above mentioned arch itectonic subregions of STR. The subregions within each line have bidi rectional connectional laminar specificity. The feedforward connection s originate from the supragranular layer Ill and terminate in and arou nd layer IV of the rostrally adjacent region. Feedback projections in constrast stem from the infragranular layers and terminate in layer I. The long association connections of auditory areas are with the prefr ontal cortex (PFC), the multimodal areas and the limbic regions, and a re derived from belt and root line areas of STR. These projections fol low the rostro-caudal architectonic differentiation of STR. Thus the r ostral STG areas are mainly connected with orbital and medial PFC area s whereas the caudal STG areas are connected with caudal PFC. The inte rmediate STG areas are preferentially related to the lateral PFC regio ns. It seems that STG-PFC connections are betwen the areas with simila r 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) o f medial geniculate nucleus (MGN) whereas the root and belt line areas are connected respectively to magnocellular (MGmc) and dorsal (MGd) s ubdivisions of MGN. The root and belt areas share some connections and are also related to pulvinar, suprageniculate, dorsomedial and intral aminar nuclei. It seems therefore that progressive laminar and tripart rate organization of auditory regions of STR is reflected in intrinsic , association and thalamic connections. The feedforward connections ma y be engaged in analysis of external environmental cues whereas feedba ck connections may have a role in matching learned or stored informati on with incoming auditory signals. The preferential core line connecti vity with MGv may be involved in spectral analysis of sound whereas th e connections of the belt and root areas with MGmc, MGd, and pulvinar may have role in sound pattern recognition, auditory memory, the local ization of sound in space as well as matching auditory information wit h other modalities.