Segregated fronto‐cortical and midbrain connections in the mouse and their relation to approach and avoidance orienting behaviors
The superior colliculus (SC) is a multimodal sensory‐motor midbrain structure, involved in visual, auditory, and somatosensory triggered orienting (Meredith, Wallace, & Stein, 1992; Stein, 1981; Thiele, Rübsamen, & Hoffmann, 1996; Wallace, Meredith, & Stein, 1993; Westby, Keay, Redgrave, Dean, & Bannister, 1990). In most species, the spatial representation of sensory inputs is aligned to the retinotopic organization of the superficial layers where the central or frontal field/space is represented in the anterior SC, the upper visual hemi‐field in the medial SC, and the lower visual hemi‐field in the lateral SC (Drager & Hubel, 1976; Goldberg & Wurtz, 1972; Meredith & Stein, 1990; Thiele, Vogelsang, & Hoffmann, 1991). Multimodal sensory processing occurs in the intermediate and lower layers where sensory neurons are intermixed with sensory‐motor responses coding for eye (Wurtz & Albano, 1980), head (Harris, 1980), pinnae (Stein & Clamann, 1981), and whisker movements (Bezdudnaya & Castro‐Alamancos, 2014). In primates, electrical microstimulation in intermediate and deep layers of the SC results in defined saccadic eye‐movements, with endpoints in the visual receptive field locations of the stimulation sites (Stryker & Schiller, 1975). This suggests that sensorimotor integration in the SC invariably triggers orienting responses toward the object of interest. However, in rats, stimulation of the SC can elicit orienting responses toward the visual field representation at the stimulation site, and it can result in defensive behaviors such as freezing, or orienting movements away from the visual field region (Dean, Mitchell, & Redgrave, 1988; Dean, Redgrave, & Westby, 1989). These different types of behavior are, at least to some extent, mediated by two separate output pathways from the intermediate and deep layers of the SC. The crossed descending tecto‐reticulo‐spinal projection, which preferentially arises from the lateral SC (Redgrave, Odekunle, & Dean, 1986), is speculated to be involved in approach movements toward novel stimuli. Whereas the uncrossed ipsilateral pathway, of which certain parts arise in the medial SC, is likely involved in avoidance and escape‐like behavior (Westby et al., 1990). This view is in accord with the ecological niches which rodents occupy, where predators most likely appear in the upper visual field, represented medially in the SC, while prey most likely appear in the lower visual field where they can also be detected by the whisker system (Furigo et al., 2010; Westby et al., 1990), which is represented preferentially in the lateral SC (Favaro et al., 2011). In line with this, medial and the lateral parts of the SC in the rat show an anatomical segregation of inputs from subcortical and from cortical sources, which may feed into the avoidance and approach related pathways (Comoli et al., 2012). It is currently unknown whether this distinction holds for the mouse SC, although a recent study has dissected a pathway originating in the intermediate layers of the medial SC. This is involved in defensive behavior, and provides a short latency route through the lateral posterior thalamus to the lateral amygdala (Wei et al., 2015). Beyond the level of the SC, the larger scale cortical and subcortical anatomical networks involved in approach and avoidance behavior in rodents have not been delineated in great detail. In pursuit of this goal, we injected retrograde tracers into the medial or lateral parts of the murine SC (SCm, SCl) to determine their specific input connections. We found that SCl and SCm receive inputs from shared, but also largely distinct sources.