A synaptic corollary discharge signal suppresses midbrain visual processing during saccade-like locomotion.

In motor control, the brain not only sends motor commands to the periphery, but also generates concurrent internal signals known as corollary discharge (CD) that influence sensory information processing around the time of movement. CD signals are important for identifying sensory input arising from self-motion and to compensate for it, but the underlying mechanisms remain unclear. Using whole-cell patch clamp recordings from neurons in the zebrafish optic tectum, we discovered an inhibitory synaptic signal, temporally locked to spontaneous and visually driven locomotion. This motor-related inhibition was appropriately timed to counteract visually driven excitatory input arising from the fish's own motion, and transiently suppressed tectal spiking activity. High-resolution calcium imaging revealed localized motor-related signals in the tectal neuropil and the upstream torus longitudinalis, suggesting that CD enters the tectum via this pathway. Together, our results show how visual processing is suppressed during self-motion by motor-related phasic inhibition. This may help explain perceptual saccadic suppression observed in many species.

Classification of object size in retinotectal microcircuits.

BACKGROUND: A principal task of the visual system is to detect and classify moving objects in the visual environment. Information about the size of an object is critical for selecting appropriate behavioral responses. Object size is encoded in retinal ganglion cell (RGC) activity. Little is known, however, about how inputs from the multitude of RGC subtypes are distributed to higher visual centers and how information is combined from these feature-selective inputs. RESULTS: Here we show that in the zebrafish optic tectum, prey- or predator-like moving targets evoke activity in distinct groups of RGC fibers dependent on target size, demonstrating a retinal origin of tectal size classification. Small-size-selective retinal inputs are relatively more frequent in the most superficial layer of the tectal neuropil, whereas large-size-selective inputs predominate in deeper layers. Monostratified superficial interneurons (SINs) process large-size- and small-size-selective signals dependent on their dendritic target layer, consistent with the retinal input organization. Further downstream, small- and large-sized objects are encoded in population activity of separate sets of tectal neurons. CONCLUSIONS: Ethologically relevant size classes are preferentially processed in different layers of the tectal neuropil. The tectum categorizes visual targets on the basis of retinally computed size information, suggesting a critical role in visually guided response selection.