Escaping from imminent danger is an instinctive behaviour that is fundamental for survival, and requires the classification of sensory stimuli as harmless or threatening. The absence of threat enables animals to forage for essential resources, but as the level of threat and potential for harm increases, they have to decide whether or not to seek safety . Despite previous work on instinctive defensive behaviours in rodents, little is known about how the brain computes the threat level for initiating  escape. Here we show that the probability and vigour of escape in mice scale with the saliency of innate threats, and are well described by a model that computes the distance between the threat level and an escape threshold. Calcium imaging and optogenetics in the midbrain of freely behaving mice show that the activity of excitatory neurons in the deep layers of the medial superior colliculus (mSC) represents the saliency of the threat stimulus and is predictive of escape, whereas glutamatergic neurons of the dorsal periaqueductal grey (dPAG) encode exclusively the choice to escape and control escape vigour. We demonstrate a feed-forward monosynaptic excitatory connection from mSC to dPAG neurons, which is weak and unreliable-yet required for escape behaviour-and provides a synaptic threshold for dPAG activation and the initiation of escape. This threshold can be overcome by high mSC network activity because of short-term synaptic facilitation and recurrent excitation within the mSC, which amplifies and sustains synaptic drive to the dPAG. Therefore, dPAG glutamatergic neurons compute escape decisions and escape vigour using a synaptic mechanism to  threshold threat information received from the mSC, and provide a biophysical model of how the brain performs a critical behavioural computation.

8Escaping from imminent danger is an instinctive behaviour fundamental for survival that requires classifying 9 sensory stimuli as harmless or threatening. The absence of threat allows animals to forage for essential resources, 10 but as the level of threat and potential for harm increases, they have to decide whether or not to seek safety 1 . 11Despite previous work on instinctive defensive behaviours in rodents 2-13 , little is known about how the brain 12 computes the threat level for initiating escape. Here we show that the probability and vigour of escape in mice 13 scale with the intensity of innate threats, and are well described by a theoretical model that computes the distance 14 between threat level and an escape threshold. Calcium imaging and optogenetics in the midbrain of freely behaving 15 mice show that the activity of excitatory VGluT2 + neurons in the deep layers of the medial superior colliculus 16 (mSC) represents the threat stimulus intensity and is predictive of escape, whereas dorsal periaqueductal gray 17 (dPAG) VGluT2 + neurons encode exclusively the escape choice and control escape vigour. We demonstrate a feed-18 forward monosynaptic excitatory connection from mSC to dPAG neurons that is weak and unreliable, yet 19 necessary for escape behaviour, and which we suggest provides a synaptic threshold for dPAG activation and the 20 initiation of escape. This threshold can be overcome by high mSC network activity because of short-term synaptic 21 facilitation and recurrent excitation within the mSC, which amplifies and sustains synaptic drive to the dPAG. 22

GABAergic projection neurons in the cerebellar nuclei (CN) innervate the inferior olive (IO) that in turn is the source of climbing fibers targeting Purkinje neurons in the cerebellar cortex. Anatomical evidence suggests that CN synapses modulate electrical coupling between IO neurons. In vivo studies indicate that they are also involved in controlling synchrony and rhythmicity of IO neurons. Here, we demonstrate using virally targeted channelrhodopsin in the cerebellar nucleo-olivary neurons that synaptic input can indeed modulate both the strength and symmetry of electrical coupling between IO neurons and alter network activity. Similar synaptic modifications of electrical coupling are likely to occur in other brain regions, where rapid modification of the spatiotemporal features of the coupled networks is needed to adequately respond to behavioral demands.

Escape behavior is a defensive action deployed by animals in response to imminent threats. In mammalian species, a variety of different brain circuits are known to participate in this critical survival behavior. One of these circuits is the periaqueductal gray, a midbrain structure that can command a variety of instinctive behaviors. Recent experiments using modern systems neuroscience techniques have begun to elucidate the specific role of the periaqueductal gray in controlling escape. These have shown that periaqueductal gray neurons are critical units for gating and commanding the initiation of escape, specifically activated in situations of imminent, escapable threat. In addition, it is becoming clear that the periaqueductal gray integrates brainwide information that can modulate escape initiation to generate flexible defensive behaviors.

The generation of temporal patterns is one of the most fascinating functions of the brain. Unlike the response to external stimuli temporal patterns are generated within the system and recalled for a specific use. To generate temporal patterns one needs a timing machine, a “master clock” that determines the temporal framework within which temporal patterns can be generated and implemented. Here we present the concept that in this putative “master clock” phase and frequency interact to generate temporal patterns. We define the requirements for a neuronal “master clock” to be both reliable and versatile. We introduce this concept within the inferior olive nucleus which at least by some scientists is regarded as the source of timing for cerebellar function. We review the basic properties of the subthreshold oscillation recorded from olivary neurons, analyze the phase relationships between neurons and demonstrate that the phase and onset of oscillation is tightly controlled by synaptic input. These properties endowed the olivary nucleus with the ability to act as a “master clock.”