Robust Subthreshold Cross-modal Modulation of Auditory Response by Cutaneous Electrical Stimulation in First- and Higher-order Auditory Thalamic Nuclei

Integration of somatosensory and motor-related information in the auditory system

An ability to integrate information provided by different sensory modalities is a fundamental feature of neurons in many brain areas. Because visual and auditory inputs often originate from the same external object, which may be located some distance away from the observer, the synthesis of these cues can improve localization accuracy and speed up behavioral responses. By contrast, multisensory interactions occurring close to the body typically involve a combination of tactile stimuli with other sensory modalities. Moreover, most activities involving active touch generate sound, indicating that stimuli in these modalities are frequently experienced together. In this review, we examine the basis for determining sound-source distance and the contribution of auditory inputs to the neural encoding of space around the body. We then consider the perceptual consequences of combining auditory and tactile inputs in humans and discuss recent evidence from animal studies demonstrating how cortical and subcortical areas work together to mediate communication between these senses. This research has shown that somatosensory inputs interface with and modulate sound processing at multiple levels of the auditory pathway, from the cochlear nucleus in the brainstem to the cortex. Circuits involving inputs from the primary somatosensory cortex to the auditory midbrain have been identified that mediate suppressive effects of whisker stimulation on auditory thalamocortical processing, providing a possible basis for prioritizing the processing of tactile cues from nearby objects. Close links also exist between audition and movement, and auditory responses are typically suppressed by locomotion and other actions. These movement-related signals are thought to cancel out self-generated sounds, but they may also affect auditory responses via the associated somatosensory stimulation or as a result of changes in brain state. Together, these studies highlight the importance of considering both multisensory context and movement-related activity in order to understand how the auditory cortex operates during natural behaviors, paving the way for future work to investigate auditory-somatosensory interactions in more ecological situations.

Sound Intensity-dependent Multiple Tonotopic Organizations and Complex Sub-threshold Alterations of Auditory Response Across Sound Frequencies in the Thalamic Reticular Nucleus

The thalamic reticular nucleus (TRN), a cluster of GABAergic cells, modulates sensory attention and perception through its inhibitory projections to thalamic nuclei. Cortical and thalamic topographic projections to the auditory TRN are thought to compose tonotopic organizations for modulation of thalamic auditory processing. The present study determined tonotopies in the TRN and examined interactions between probe and masker sounds to obtain insights into temporal processing associated with tonotopies. Experiments were performed on anesthetized rats, using juxta-cellular recording and labeling techniques. Following determination of tonotopies, effects of sub-threshold masker sound stimuli on onset and late responses evoked by a probe sound were examined. The main findings are as follows. Tonotopic organizations were recognized in cell location and axonal projection. Tonotopic gradients and their clarities were diverse, depending on sound intensity, response type and the tiers of the TRN. Robust alterations in response magnitude, latency and/or burst spiking took place following masker sounds in either a broad or narrow range of frequencies that were close or far away from the probe sound frequency. The majority of alterations were suppression recognizable up to 600 ms in the interval between masker and probe sounds, and directions of alteration differed depending on the interval. Finally, masker sound effects were associated with tonotopic organizations. These findings suggest that the auditory TRN is comprised of sound intensity-dependent multiple tonotopic organizations, which could configure temporal interactions of auditory information across sound frequencies and impose complex but spatiotemporally structured influences on thalamic auditory processing.

Subcortical circuits mediate communication between primary sensory cortical areas in mice

Integration of information across the senses is critical for perception and is a common property of neurons in the cerebral cortex, where it is thought to arise primarily from corticocortical connections. Much less is known about the role of subcortical circuits in shaping the multisensory properties of cortical neurons. We show that stimulation of the whiskers causes widespread suppression of sound-evoked activity in mouse primary auditory cortex (A1). This suppression depends on the primary somatosensory cortex (S1), and is implemented through a descending circuit that links S1, via the auditory midbrain, with thalamic neurons that project to A1. Furthermore, a direct pathway from S1 has a facilitatory effect on auditory responses in higher-order thalamic nuclei that project to other brain areas. Crossmodal corticofugal projections to the auditory midbrain and thalamus therefore play a pivotal role in integrating multisensory signals and in enabling communication between different sensory cortical areas.

Two dynamically distinct circuits drive inhibition in the sensory thalamus

Most sensory information destined for the neocortex is relayed through the thalamus, where considerable transformation occurs^1,2. One means of transformation involves interactions between excitatory thalamocortical neurons that carry data to the cortex and inhibitory neurons of the thalamic reticular nucleus (TRN) that regulate the flow of those data^3-6. Although the importance of the TRN has long been recognised^7-9, understanding of its cell types, their organization and their functional properties has lagged behind that of the thalamocortical systems they control. Here we address this by investigating the somatosensory and visual circuits of the TRN in mice. In the somatosensory TRN we observed two groups of genetically defined neurons that are topographically segregated and physiologically distinct, and that connect reciprocally with independent thalamocortical nuclei through dynamically divergent synapses. Calbindin-expressing cells-located in the central core-connect with the ventral posterior nucleus, the primary somatosensory thalamocortical relay. By contrast, somatostatin-expressing cells-which reside along the surrounding edges of the TRN-synapse with the posterior medial thalamic nucleus, a higher-order structure that carries both top-down and bottom-up information^10-12. The two TRN cell groups process their inputs in pathway-specific ways. Synapses from the ventral posterior nucleus to central TRN cells transmit rapid excitatory currents that depress deeply during repetitive activity, driving phasic spike output. Synapses from the posterior medial thalamic nucleus to edge TRN cells evoke slower, less depressing excitatory currents that drive more persistent spiking. Differences in the intrinsic physiology of TRN cell types, including state-dependent bursting, contribute to these output dynamics. The processing specializations of these two somatosensory TRN subcircuits therefore appear to be tuned to the signals they carry-a primary central subcircuit tuned to discrete sensory events, and a higher-order edge subcircuit tuned to temporally distributed signals integrated from multiple sources. The structure and function of visual TRN subcircuits closely resemble those of the somatosensory TRN. These results provide insights into how subnetworks of TRN neurons may differentially process distinct classes of thalamic information.

Cross-modal modulation of cell activity by sound in first-order visual thalamic nucleus

Cross-modal auditory influence on cell activity in the primary visual cortex emerging at short latencies raises the possibility that the first-order visual thalamic nucleus, which is considered dedicated to unimodal visual processing, could contribute to cross-modal sensory processing, as has been indicated in the auditory and somatosensory systems. To test this hypothesis, the effects of sound stimulation on visual cell activity in the dorsal lateral geniculate nucleus were examined in anesthetized rats, using juxta-cellular recording and labeling techniques. Visual responses evoked by light (white LED) were modulated by sound (noise burst) given simultaneously or 50-400 ms after the light, even though sound stimuli alone did not evoke cell activity. Alterations of visual response were observed in 71% of cells (57/80) with regard to response magnitude, latency, and/or burst spiking. Suppression predominated in response magnitude modulation, but de novo responses were also induced by combined stimulation. Sound affected not only onset responses but also late responses. Late responses were modulated by sound given before or after onset responses. Further, visual responses evoked by the second light stimulation of a double flash with a 150-700 ms interval were also modulated by sound given together with the first light stimulation. In morphological analysis of labeled cells projection cells comparable to X-, Y-, and W-like cells and interneurons were all susceptible to auditory influence. These findings suggest that the first-order visual thalamic nucleus incorporates auditory influence into parallel and complex thalamic visual processing for cross-modal modulation of visual attention and perception.