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...

Communication is an inherently interactive process that weaves together the fabric of both human and nonhuman primate societies. To investigate the properties of the primate brain during active social signaling, we recorded the responses of frontal cortex neurons as freely moving marmosets engaged in conversational exchanges with a visually occluded virtual marmoset. We found that small changes in firing rate (ϳ1 Hz) occurred across a broadly distributed population of frontal cortex neurons when marmosets heard a conspecific vocalization, and that these changes corresponded to subjects' likelihood of producing or withholding a vocal reply. Although the contributions of individual neurons were relatively small, large populations of neurons were able to clearly distinguish between these social contexts. Most significantly, this social context-dependent change in firing rate was evident even before subjects heard the vocalization, indicating that the probability of a conversational exchange was determined by the state of the frontal cortex at the time a vocalization was heard, and not by a decision driven by acoustic characteristics of the vocalization. We found that changes in neural activity scaled with the length of the conversation, with greater changes in firing rate evident for longer conversations. These data reveal specific and important facets of this neural activity that constrain its possible roles in active social signaling, and we hypothesize that the close coupling between frontal cortex activity and this natural, active primate social-signaling behavior facilitates social-monitoring mechanisms critical to conversational exchanges.

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SummaryWe tested whether social signal processing in more traditional, head-restrained contexts is representative of the putative natural analog – social communication – by comparing responses to vocalizations within individual neurons in marmoset prefrontal cortex (PFC) across a series of behavioral contexts ranging from traditional to naturalistic. Although vocalization responsive neurons were evident in all contexts, cross-context consistency was notably limited. A response to these social signals when subjects were head-restrained was not predictive of a comparable neural response to the identical vocalizations during natural communication, even within the same neuron. Neural activity at the population level followed a similar pattern, as PFC activity could be reliably decoded for the context in which vocalizations were heard. This suggests that neural representations of social signals in primate PFC are not static, but highly flexible and likely reflect how nuances of the dynamic behavioral contexts affect the perception of these signals and what they communicate.