Spatial Dependence of Stimulus Competition in the Avian Nucleus Isthmi Pars Magnocellularis

The nucleus isthmi pars magnocellularis (Imc) is a group of specialized inhibitory neurons in the midbrain tegmentum, thought to be conserved across vertebrate classes. Past anatomical work in reptiles has suggested a role for it in stimulus selection, which has been supported by recent studies in avians. Additionally, focal inactivation of Imc neurons is known to abolish all competitive interactions in the optic tectum (OT; SC in mammals), a midbrain sensorimotor hub that is critical for the control of spatial attention, thereby revealing a key role for Imc in stimulus selection. However, the functional properties of Imc neurons are not well understood. Here, with electrophysiological experiments in the barn owl Imc, we show that Imc neurons themselves exhibit signatures of stimulus competition. Distant competing stimuli outside the spatial receptive field (RF) suppressed powerfully, and divisively, the responses of Imc neurons to stimuli inside the RF, and did so from all tested locations along the elevation as well as azimuth. Notably, this held true even for locations encoded by the opposite side of the brain from the one containing the recording site. This global divisive inhibition operated independently of the sensory modality of the competing stimulus. Thus, the Imc not only supplies inhibition to the OT to support competition there, but may itself be an active site of stimulus competition. These results from experiments in the barn owl shed light on the functional properties of a vital node in the vertebrate midbrain selection network.