The auditory cortex of primates contains a core region of three primary areas surrounded by a belt region of secondary areas. Recent neurophysiological studies suggest that the belt areas medial to the core have unique functional roles, including multisensory properties, but little is known about their connections. In this study and its companion, the cortical and subcortical connections of the core and medial belt regions of marmoset monkeys were compared to account for functional differences between areas and refine our working model of the primate auditory cortex. Anatomical tracer injections targeted two core areas (A1 and R) and two medial belt areas (rostromedial [RM] and caudomedial [CM]). RM and CM had topographically weighted connections with all other areas of the auditory cortex ipsilaterally, but these were less widespread contralaterally. CM was densely connected with caudal auditory fields, the retroinsular (Ri) area of the somatosensory cortex, the superior temporal sulcus (STS), and the posterior parietal and entorhinal cortex. The connections of RM favored rostral auditory areas, with no clear somatosensory inputs. RM also projected to the lateral nucleus of the amygdala and tail of the caudate nucleus. A1 and R had topographically weighted connections with medial and lateral belt regions, infragranular inputs from the parabelt, and weak connections with fields outside the auditory cortex. The results indicated that RM and CM are distinct areas of the medial belt region with direct inputs from the core. CM also has somatosensory input and may correspond to an area on the posteromedial transverse gyrus of humans and the anterior auditory field of other mammals.

In this study and its companion, the cortical and subcortical connections of the medial belt region of marmoset monkey auditory cortex were compared with the core region. The main objective was to document anatomical features that account for functional differences observed between areas. Injections of retrograde and bi-directional anatomical tracers targeted two core areas (A1 and R), and two medial belt areas (RM, rostromedial; CM, caudomedial). Topographically distinct patterns of connections were revealed among subdivisions of the medial geniculate complex (MGC) and multisensory thalamic nuclei, including the suprageniculate (Sg), limitans (Lim), medial pulvinar (PM), and posterior nucleus (Po). The dominant thalamic projection to CM was the anterior dorsal division (MGad) of the MGC, whereas the posterior dorsal division (MGpd) targeted RM. CM also had substantial input from multisensory nuclei, especially the magnocellular division (MGm) of the MGC. RM had weak multisensory connections. Corticotectal projections of both RM and CM targeted the dorsomedial quadrant of the inferior colliculus, while the CM projection also included a pericentral extension around the ventromedial and lateral portion of the central nucleus. Areas A1 and R were characterized by focal topographic connections within the ventral division (MGv) of the MGC, reflecting the tonotopic organization of both core areas. The results indicate that parallel subcortical pathways target the core and medial belt regions, and that RM and CM represent functionally-distinct areas within the medial belt auditory cortex.

Kajikawa, Yoshinao, Lisa de la Mothe, Suzanne Blumell, and Troy A. Hackett. A comparison of neuron response properties in areas A1 and CM of the marmoset monkey auditory cortex: tones and broadband noise. J Neurophysiol 93: [22][23][24][25][26][27][28][29][30][31][32][33][34] 2005. First published September 1, 2004; doi:10.1152/jn.00248.2004. The purpose of this study was to compare response properties of two adjacent areas of the marmoset monkey auditory cortex. Multiunit responses to 50 ms tones and broadband noise bursts (BBN) were recorded in the core area, A1, and the caudomedial belt area, CM, of ketamine-anesthetized animals. Neurons in A1 and CM exhibited robust low-threshold short-latency responses to BBN and tones, whereas neurons in adjoining lateral belt areas were poorly responsive or unresponsive to tones and noise. Except for a population of broadly tuned units in CM, the characteristic frequency (CF) could be determined for all recording sites in A1 and CM. Both areas were tonotopically organized and shared a high CF border. Whereas the tonotopic gradient in A1 was smooth and continuous across the field, the gradient in CM was discontinuous, and the intermediate CF range was underrepresented. For BBN stimuli, rate level functions were largely monotonic in A1 and CM. Response profiles were also similar in both areas. As a population, neurons in CM were distinguished from A1 by significantly shorter response latencies, lower thresholds, and broader tuning bandwidth at higher intensities. The results indicated that, while A1 and CM represent anatomically and physiologically distinct areas, their response profiles under anesthesia overlapped considerably compared with the lateral belt areas. Therefore refinements of current models of the primate auditory cortex may be needed to account for differences in organization among the auditory belt areas. I N T R O D U C T I O NIn recent years, we have adopted a model of auditory cortical organization in primates based on the collective findings of the field (Hackett 2002; Hackett and Kaas 2004; Hackett 1998, 2000). The model defines auditory cortex as the corpus of cortical areas that are the preferential targets of neurons in either the ventral (MGv) or dorsal (MGd) divisions of the medial geniculate complex (MGC). By this definition, three regions of the superior temporal cortex comprise the auditory cortex in Old World and New World primates: core, belt, and parabelt. The core region includes two or three tonotopically organized subdivisions that receive thalamic inputs from MGv. In the belt region, surrounding the core, as many as seven distinct areas have been proposed. These fields receive inputs from the core and MGd. At least some of the lateral belt areas are tonotopically organized in a manner that parallels the adjacent core area (Kosaki et al. 1997;Merzenich and Brugge 1973;Rauschecker and Tian 2004;Rauschecker et al. 1995). The belt areas project topographically to the caudal and rostral divisions of the parabelt region, which receive inputs f...

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The primate auditory cortex is comprised of a core region of three primary areas, surrounded by a belt region of secondary areas and a parabelt region lateral to the belt. The main sources of thalamocortical inputs to the auditory cortex are the medial geniculate complex (MGC), medial pulvinar (PM), and several adjoining nuclei in the posterior thalamus. The distribution of inputs varies topographically by cortical area and thalamic nucleus, but in a manner that has not been fully characterized in primates. In this study, the thalamocortical connections of the lateral belt and parabelt were determined by placing retrograde tracer injections into various areas of these regions in the marmoset monkey. Both regions received projections from the medial (MGm) and posterodorsal (MGpd) divisions of the medial geniculate complex (MGC); however, labeled cells in the anterodorsal (MGad) division were present only from injections into the caudal belt. Thalamic inputs to the lateral belt appeared to come mainly from the MGC, whereas the parabelt also received a strong projection from the PM, consistent with its position as a later stage of auditory cortical processing. The results of this study also indicate that