Auditory and Visual Space Maps in the Cholinergic Nucleus Isthmi Pars Parvocellularis of the Barn Owl

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Abstract
The nucleus isthmi pars parvocellularis (Ipc) is a midbrain cholinergic nucleus that shares reciprocal, topographic connections with the optic tectum (OT). Ipc neurons project to spatially restricted columns in the OT, contacting essentially all OT layers in a given column. Previous research characterizes the Ipc as a visual processor. We found that, in the barn owl, the Ipc responds to auditory as well as to visual stimuli. Auditory responses were tuned broadly for frequency, but sharply for spatial cues. We measured the tuning of Ipc units to binaural sound localization cues, including interaural timing differences (ITDs) and interaural level differences (ILDs). Units in the Ipc were tuned to specific values of both ITD and ILD and were organized systematically according to their ITD and ILD tuning, forming a map of space. The auditory space map aligned with the visual space map in the Ipc. These results demonstrate that the Ipc encodes the spatial location of objects, independent of stimulus modality. These findings, combined with the precise pattern of projections from the Ipc to the OT, suggest that the role of the Ipc is to regulate the sensitivity of OT neurons in a space-specific manner.