Visually evoked activity and variable modulation of auditory responses in the macaque inferior colliculus.

How multisensory cues affect processing in early sensory brain areas is not well understood. The inferior colliculus (IC) is an early auditory structure that is visually responsive (Porter KK, Metzger RR, Groh JM. Proc Natl Acad Sci USA 104: 17855-17860, 2007; Bulkin DA, Groh JM. Front Neural Circuits 6: 61, 2012; Bulkin DA, Groh JM. J Neurophysiol 107: 785-795, 2012), but little is known about how visual signals affect the IC's auditory representation. We explored how visual cues affect both spiking and local field potential (LFP) activity in the IC of two monkeys performing a task involving saccades to auditory, visual, or combined audiovisual stimuli. We confirm that LFPs are sensitive to the onset of fixation lights and the onset of visual targets presented during steady fixation. The LFP waveforms evoked by combined audiovisual stimuli differed from those evoked by sounds alone. In single-unit spiking activity, responses were weak when visual stimuli were presented alone, but visual stimuli could modulate the activity evoked by sounds in a stronger way. Such modulations could involve either increases or decreases in activity, and whether increases or decreases were observed was variable and not obviously correlated with the responses evoked by visual or auditory stimuli alone. These findings indicate that visual stimuli shape the IC's auditory representation in flexible ways that differ from those observed previously in multisensory areas.NEW & NOTEWORTHY We find that the inferior colliculus, a primarily auditory brain area, displays distinct population-level responses to visual stimuli. We also find that visual cues can influence the auditory responses of individual neurons. Together, the results provide insight into how relatively early sensory areas may play a role in combining multiple sensory modalities to refine the perception of complex environments.