Connections of the primary auditory cortex in the common marmoset, Callithrix jacchus jacchus

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Abstract
The afferent and efferent connections of the primary auditory cortex (AI) of common marmosets were traced following small injections of wheat germ agglutinin conjugated with horseradish peroxidase (WGA‐HRP) made at best frequency (BF)‐defined sites in the AI. After the injections the animals remained anesthetized for 15–23 hours; they were then perfused transcardially with fixative and the brains were processed for WGA‐HRP reaction product. Examination of the disposition of labelled material revealed the following results. First, patches of terminal labelling, and to a lesser extent retrograde labelling, were found outside the injection site in the ipsilateral cortex rostral and caudal to the AI. Second, the region of the contralateral cortex corresponding to the injection site contained labelled terminals throughout the depth of the cortex; labelled neurons were found in the middle layers. Third, in each experiment a discrete region of the medial geniculate body (MG) contained retrogradely labelled neurons interspersed with anterogradely labelled terminals. These regions had a banded appearance, were found in the dorsal and rostral half of the MG, and shifted in location progressively dorsal and as the injection site BF increased. The presence of projection zones rostral and caudal to the AI of marmosets, and the disposition of the MG sources of projection in relation to BF, are similar to observations made on other New World monkeys. The ipsilateral corticocortical projections confirm electrophysiological evidence suggesting the existence of auditory fields rostral and caudal to the AI. The thalamocortical auditory system of the marmoset appears relatively simple, with a comparatively undifferentiated MG projecting to a cortical auditory system dominated by a large AI.