Multiplexing of visual-auditory signals in a predominantly auditory brain region.

A recently proposed theory of neural representation postulates that when more than one stimulus is to be encoded, neurons in that representation may fluctuate between encoding each item. To date, such work has considered only cases in which both stimuli are of the same sensory modality, such as how two sounds are encoded in an auditory brain region. Here, we consider the influence of sensory modality on this process, asking whether stimuli of a non-dominant sensory modality can evoke or influence the fluctuating activity patterns observed for combinations of stimuli of a dominant sensory modality. Specifically, we investigated the impact of combining visual and auditory stimuli on single-unit activity in the inferior colliculus, a predominantly auditory structure, in monkeys performing a behavioral task involving reporting all stimulus locations in a given trial. We found that fluctuating activity is evident when a single sound and a single visual stimulus are paired at the same spatial location, indicating that fluctuating activity can be induced when one item is not from a brain area's preferred modality, and even when only a single spatial location is involved and spatial attention is directed to only one place. Fluctuating activity was also identified when visual stimuli were paired with sounds presented at two spatial locations. The IC appeared to maintain a representation of all stimuli, regardless of the sequence in which the monkey localized the targets (although in one monkey there was a bias toward greater representation of the second reported location). Together, these results indicate that fluctuating activity occurs in response to multimodal stimuli in a predominantly unimodal brain structure and is geared toward preservation of all sensory information rather than attentional filtering. Impact Statement: We found that when visual stimuli are paired with sounds, they can induce fluctuating activity in neurons that are primarily sensitive to sound alone. Such activity fluctuations may permit the representation of both the visual and auditory stimuli in an overlapping population of neurons.