Multisensory Integration Enhances Temporal Coding in Ventral Cochlear Nucleus Bushy Cells.

Temporal coding of auditory stimuli is critical for understanding communication signals. The bushy cell, a major output neuron of the ventral cochlear nucleus, can "phase-lock" precisely to pure tones and the envelopes of complex stimuli. Bushy cells are also putative recipients of brainstem somatosensory projections and could therefore play a role in perception of communication signals because multisensory integration is required for such complex sound processing. Here, we examine the role of multisensory integration in temporal coding in bushy cells by activating the spinal trigeminal nucleus (Sp5) while recording responses from bushy cells. In normal-hearing guinea pigs of either sex, bushy cell single unit responses to amplitude-modulated (AM) broadband noise were compared with those in the presence of preceding Sp5 electrical stimulation (i.e., bimodal stimuli). Responses to the AM stimuli were also compared with those obtained 45 min after the bimodal stimulation. Bimodal auditory-Sp5 stimulation resulted in enhanced envelope coding for low modulation frequencies, which persisted for up to 45 min. AM detection thresholds were significantly improved 45 min after bimodal auditory-Sp5 stimulation, but not during bimodal auditory-Sp5 stimulation. Anterograde labeling of Sp5 projections was found within the dendritic fields of bushy cells and their inhibitory interneurons, D-stellate cells. Therefore, enhanced AM responses and improved AM sensitivity of bushy cells were likely facilitated by Sp5 neurons through monosynaptic excitatory projections and indirect inhibitory projections. These somatosensory projections may be involved in the improved perception of communication stimuli with multisensory stimulation, consistent with psychophysical studies in humans. Multisensory integration is crucial for sensory coding because it improves sensitivity to unimodal stimuli and enhances responses to external stimuli. Although multisensory integration has typically been described in the cerebral cortex, the cochlear nucleus in the brainstem is also innervated by multiple sensory systems, including the somatosensory and auditory systems. Here, we showed that convergence of these two sensory systems in the cochlear nucleus results in improved temporal coding in bushy cells, principal output neurons that send projections to higher auditory structures. The improved temporal coding instilled by bimodal auditory-Sp5 stimulation may be important in priming the neurons for coding biologically relevant sounds such as communication signals.