Information flow in the auditory cortical network

Specialization of binaural responses in ventral auditory cortices

Accurate orientation to sound under challenging conditions requires auditory cortex, but it is unclear how spatial attributes of the auditory scene are represented at this level. Current organization schemes follow a functional division whereby dorsal and ventral auditory cortices specialize to encode spatial and object features of sound source, respectively. However, few studies have examined spatial cue sensitivities in ventral cortices to support or reject such schemes. Here Fourier optical imaging was used to quantify best frequency responses and corresponding gradient organization in primary (A1), anterior, posterior, ventral (VAF), and suprarhinal (SRAF) auditory fields of the rat. Spike rate sensitivities to binaural interaural level difference (ILD) and average binaural level cues were probed in A1 and two ventral cortices, VAF and SRAF. Continuous distributions of best ILDs and ILD tuning metrics were observed in all cortices, suggesting this horizontal position cue is well covered. VAF and caudal SRAF in the right cerebral hemisphere responded maximally to midline horizontal position cues, whereas A1 and rostral SRAF responded maximally to ILD cues favoring more eccentric positions in the contralateral sound hemifield. SRAF had the highest incidence of binaural facilitation for ILD cues corresponding to midline positions, supporting current theories that auditory cortices have specialized and hierarchical functional organization.

VGLUT1 and VGLUT2 mRNA expression in the primate auditory pathway

The vesicular glutamate transporters (VGLUTs) regulate the storage and release of glutamate in the brain. In adult animals, the VGLUT1 and VGLUT2 isoforms are widely expressed and differentially distributed, suggesting that neural circuits exhibit distinct modes of glutamate regulation. Studies in rodents suggest that VGLUT1 and VGLUT2 mRNA expression patterns are partly complementary, with VGLUT1 expressed at higher levels in the cortex and VGLUT2 prominent subcortically, but with overlapping distributions in some nuclei. In primates, VGLUT gene expression has not been previously studied in any part of the brain. The purposes of the present study were to document the regional expression of VGLUT1 and VGLUT2 mRNA in the auditory pathway through A1 in the cortex, and to determine whether their distributions are comparable to rodents. In situ hybridization with antisense riboprobes revealed that VGLUT2 was strongly expressed by neurons in the cerebellum and most major auditory nuclei, including the dorsal and ventral cochlear nuclei, medial and lateral superior olivary nuclei, central nucleus of the inferior colliculus, sagulum, and all divisions of the medial geniculate. VGLUT1 was densely expressed in the hippocampus and ventral cochlear nuclei, and at reduced levels in other auditory nuclei. In the auditory cortex, neurons expressing VGLUT1 were widely distributed in layers II-VI of the core, belt and parabelt regions. VGLUT2 was expressed most strongly by neurons in layers IIIb and IV, weakly by neurons in layers II-IIIa, and at very low levels in layers V-VI. The findings indicate that VGLUT2 is strongly expressed by neurons at all levels of the subcortical auditory pathway, and by neurons in the middle layers of the cortex, whereas VGLUT1 is strongly expressed by most if not all glutamatergic neurons in the auditory cortex and at variable levels among auditory subcortical nuclei. These patterns imply that VGLUT2 is the main vesicular glutamate transporter in subcortical and thalamocortical (TC) circuits, whereas VGLUT1 is dominant in corticocortical (CC) and corticothalamic (CT) systems of projections. The results also suggest that VGLUT mRNA expression patterns in primates are similar to rodents, and establish a baseline for detailed studies of these transporters in selected circuits of the auditory system.

Anatomical pathways for auditory memory in primates

Episodic memory or the ability to store context-rich information about everyday events depends on the hippocampal formation (entorhinal cortex, subiculum, presubiculum, parasubiculum, hippocampus proper, and dentate gyrus). A substantial amount of behavioral-lesion and anatomical studies have contributed to our understanding of the organization of how visual stimuli are retained in episodic memory. However, whether auditory memory is organized similarly is still unclear. One hypothesis is that, like the “visual ventral stream” for which the connections of the inferior temporal gyrus with the perirhinal cortex are necessary for visual recognition in monkeys, direct connections between the auditory association areas of the superior temporal gyrus and the hippocampal formation and with the parahippocampal region (temporal pole, perhirinal, and posterior parahippocampal cortices) might also underlie recognition memory for sounds. Alternatively, the anatomical organization of memory could be different in audition. This alternative “indirect stream” hypothesis posits that, unlike the visual association cortex, the majority of auditory information makes one or more synapses in intermediate, polymodal areas, where they may integrate information from other sensory modalities, before reaching the medial temporal memory system. This review considers anatomical studies that can support either one or both hypotheses – focusing on anatomical studies on the primate brain, primarily in macaque monkeys, that have reported not only direct auditory association connections with medial temporal areas, but, importantly, also possible indirect pathways for auditory information to reach the medial temporal lobe memory system.

An expanded role for the dorsal auditory pathway in sensorimotor control and integration

The dual-pathway model of auditory cortical processing assumes that two largely segregated processing streams originating in the lateral belt subserve the two main functions of hearing: identification of auditory "objects", including speech; and localization of sounds in space (Rauschecker and Tian, 2000). Evidence has accumulated, chiefly from work in humans and nonhuman primates, that an antero-ventral pathway supports the former function, whereas a postero-dorsal stream supports the latter, i.e processing of space and motion-in-space. In addition, the postero-dorsal stream has also been postulated to subserve some functions of speech and language in humans. A recent review (Rauschecker and Scott, 2009) has proposed the possibility that both functions of the postero-dorsal pathway can be subsumed under the same structural forward model: an efference copy sent from prefrontal and premotor cortex provides the basis for "optimal state estimation" in the inferior parietal lobe and in sensory areas of the posterior auditory cortex. The current article corroborates this model by adding and discussing recent evidence.

Spectral integration in primary auditory cortex attributable to temporally precise convergence of thalamocortical and intracortical input

Primary sensory cortex integrates sensory information from afferent feedforward thalamocortical projection systems and convergent intracortical microcircuits. Both input systems have been demonstrated to provide different aspects of sensory information. Here we have used high-density recordings of laminar current source density (CSD) distributions in primary auditory cortex of Mongolian gerbils in combination with pharmacological silencing of cortical activity and analysis of the residual CSD, to dissociate the feedforward thalamocortical contribution and the intracortical contribution to spectral integration. We found a temporally highly precise integration of both types of inputs when the stimulation frequency was in close spectral neighborhood of the best frequency of the measurement site, in which the overlap between both inputs is maximal. Local intracortical connections provide both directly feedforward excitatory and modulatory input from adjacent cortical sites, which determine how concurrent afferent inputs are integrated. Through separate excitatory horizontal projections, terminating in cortical layers II/III, information about stimulus energy in greater spectral distance is provided even over long cortical distances. These projections effectively broaden spectral tuning width. Based on these data, we suggest a mechanism of spectral integration in primary auditory cortex that is based on temporally precise interactions of afferent thalamocortical inputs and different short- and long-range intracortical networks. The proposed conceptual framework allows integration of different and partly controversial anatomical and physiological models of spectral integration in the literature.

Convergence of thalamic and cortical pathways in cat auditory cortex

Cat auditory cortex (AC) receives input from many thalamic nuclei and cortical areas. Previous connectional studies often focused on one connectional system in isolation, limiting perspectives on AC computational processes. Here we review the convergent thalamic, commissural, and corticocortical projections to thirteen AC areas in the cat. Each input differs in strength and may thus serve unique roles. We compared the convergent intrinsic and extrinsic input to each area quantitatively. The intrinsic input was almost half the total. Among extrinsic projections, ipsilateral cortical sources contributed 75%, thalamic input contributed 15%, and contralateral sources contributed 10%. The patterns of distribution support the division of AC areas into families of tonotopic, non-tonotopic, multisensory, and limbic-related areas, each with convergent input arising primarily from within its group. The connections within these areal families suggest a form of processing in which convergence of input to an area could enable new forms of integration. In contrast, the lateral connections between families could subserve integration between categorical representations, allowing otherwise independent streams to communicate and thereby coordinating operations over wide spatial and functional scales. These patterns of serial and interfamilial cooperation challenge more classical models of organization that underestimate the diversity and complexity of AC connectivity.