Visually Driven Innate Defensive Behaviours in Mice: A study of detection, habituation and processing

All animals need to react appropriately to their environment in order to survive. To accomplish that, they make use of their senses to detect relevant stimuli to avoid potential danger in their surroundings. The brain gathers this information and in keeping with its prior experiences, generates an appropriate response, some of which are innately present in animals. Innate defensive responses such as freezing or escape are essential for animal survival. Mice show defensive behaviour to stimuli sweeping overhead, like a bird cruising the sky. In chapter 2 of this thesis, we tested this in young male mice and found that mice reduced their defensive freezing after sessions with a stimulus passing overhead repeatedly. This habituation is stimulus-specific, as mice freeze again to a novel shape. We found no evidence for head-centred stimulus location-specific habituation. The mice generalized over a range of sizes and shapes, but distinguished objects when they differed in both size and shape. Innate visual defensive responses are thus strongly influenced by previous experience as mice learn to ignore specific stimuli. In chapter 4, we inhibited the Periaqueductal Grey (PAG) area of the mouse brain using chemogenetics during exposure to overhead stimuli and the process of habituation. We found that mice did not freeze in response to overhead stimuli during inhibition of the PAG. To check whether inhibition of PAG affects mice’s ability to habituate, we followed the habituating sessions by a session without inhibition of the PAG. In this case, these mice froze less in response to over-head stimuli than control mice exposed for the first time, and not more than control mice after habituation. This suggests that the behavioural habituation to these overhead stimuli occurred normally and happens in a brain area upstream of the PAG. Before an animal can react to its environment, it needs to detect the presence of relevant stimuli through its senses. We detect objects more readily if they differ from their surroundings in motion, colour, or texture. This increased saliency is thought to be related to increased responses in the visual cortex. The superior colliculus is another brain area involved in vision and especially in directing gaze and attention. In chapter 3, we show that differences in texture orientation also increase responses in the superficial layers of the superior colliculus that receive retinal and cortical input. We found that gratings evoke more neural response when surrounded by orthogonal gratings than when surrounded by parallel gratings, particularly in the awake mouse. This pop-out is not originating from the visual cortex, and silencing visual cortex increased the relative difference in response. A model shows that this can result from retinotopically matched excitation from visual cortex to the superior colliculus. We suggest that the perceptual saliency of a stimulus differing from its surround in a low-level feature like grating orientation could depend on visual processing in the superior colliculus.