Neuronal circuitry for stimulus selection in the visual system

Visual objects naturally compete for the brain’s attention, and selecting just one of them for a behavioural response is often crucial for the animal’s survival 1 . The neural correlate of such stimulus prioritisation might take the form of a saliency map by which responses to one target are enhanced relative to distractors in other parts of the visual field 2 . Single-cell responses consistent with this type of computation have been observed in the tectum of primates, birds, turtles and lamprey 2–7 . However, the exact circuit implementation has remained unclear. Here we investigated the underlying neuronal mechanism presenting larval zebrafish with two simultaneous looming stimuli, each of which was able to trigger directed escapes on their own. Behaviour tracking revealed that the fish respond to these competing stimuli predominantly with a winner-take-all strategy. Using brain-wide functional recordings, we discovered neurons in the tectum whose responses to the target stimulus were non-linearly modulated by the saliency of the distractor. When the two stimuli were presented monocularly in different positions of the visual field, stimulus selection was already apparent in the activity of retinal ganglion cell axons, a likely consequence of antagonistic mechanisms operating outside the classical receptive field 8,9 . When the two stimuli were presented binocularly, i.e., on opposite sides of the fish, our analysis indicates that a loop involving excitatory and inhibitory neurons in the nucleus isthmi (NI) and the tectum weighed stimulus saliencies across hemispheres. Consistent with focal enhancement and global suppression, glutamatergic NI cells branch locally in the tectum, whereas GABAergic NI cells project broadly across both tectal hemispheres. Moreover, holographic optogenetic stimulation confirmed that glutamatergic NI neurons can modulate visual responses in the tectum. Together, our study shows, for the first time, context-dependent contributions of retinotectal and isthmotectal circuits to the computation of the visual saliency map, a prerequisite for stimulus-driven, bottom-up attention.