Perioral somatosensory but not visual inputs to the flank of the mouse superior colliculus

An electrophysiological and anatomical study identified the sensory inputs to the "flank" of the mouse's superior colliculus, a large, ventrolateral extension of layer IV (stratum griseum intermediate) that has no overlying visual layers (II and III). Electrophysiological recordings with subsequent histological localization showed that the flank receives predominantly somatosensory projections from the perioral region but not visual input. In its caudal parts, the flank also has limb and trunk somatosensory inputs and auditory inputs. The perioral somatosensory projections to the flank are ordered somatotopically. The flank is considered part of the superior colliculus since the perioral inputs are adjacent to inputs from mystacial vibrissae in the more medial parts of the superior colliculus, hence forming a single continuous map. The finding that no visual responses occur above the flank (due to the absence of superficial layers) or within it is in accord with the concept of an intermodality "spatial register". Since flank neurons have somatosensory receptive fields in non-visible parts of the body and they lack visual responses, the flank may be involved more in tactile-dependent rather than in vision-dependent orienting behaviors. Thus, the superior colliculus may, in parallel, carry out sensorimotor transformations related to (1) shift of gaze and (2) tactile-dependent behaviors not involving vision.