Stimulus-driven competition in a cholinergic midbrain nucleus.

The mechanisms by which the brain selects a particular stimulus as the next target for gaze are poorly understood. A cholinergic nucleus in the owl's midbrain exhibits functional properties that suggest its role in bottom-up stimulus selection. Neurons in the nucleus isthmi pars parvocellularis (Ipc) responded to wide ranges of visual and auditory features, but they were not tuned to particular values of those features. Instead, they encoded the relative strengths of stimuli across the entirety of space. Many neurons exhibited switch-like properties, abruptly increasing their responses to a stimulus in their receptive field when it became the strongest stimulus. This information propagates directly to the optic tectum, a structure involved in gaze control and stimulus selection, as periodic (25-50 Hz) bursts of cholinergic activity. The functional properties of Ipc neurons resembled those of a salience map, a core component in computational models for spatial attention and gaze control.