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An intrinsic function of the reticular activating system (RAS) is its participation in fight vs. flight responses such that alerting stimuli simultaneously activate thalamocortical systems, as well as postural and locomotor systems, in order to enable an appropriate response. The P50 midlatency auditory-evoked potential appears to be an ascending manifestation of the cholinergic arm of the RAS in eliciting changes in arousal state. Abnormalities in the manifestation of the P50 potential are present in disorders which include: (1) dysregulation of sleep-wake cycles; (2) abnormalities in reflex/postural, especially, startle, responses; and (3) malfunctions in flight vs. flight responses. In general, the P50 potential appears to be upregulated (increased amplitude and/or decreased sensory gating) in disorders which are marked by upregulation of RAS outputs (hypervigilance), and downregulated in disorders characterized by decreased RAS outputs (hypovigilance). Many of the disorders discussed have a developmental etiology and a postpubertal age of onset.

Much of the controversy surrounding the induction of locomotion following stimulation of mesopontine sites, including the pedunculopontine nucleus (PPN), appears based on procedural differences, including stimulus onset, delay preceding stepping, and frequency of stimuli. The results reviewed in this chapter address these issues and provide novel information suggesting that descending projections from the PPN may exert a frequency-dependent effect. Stimulation at approximately 60 Hz (which induces prolonged tonic firing) may exercise a "push" towards locomotion (activation of pontine interneurons) as well as a "pull" away from decreased muscle tone (inhibiting giant pontine reticulospinal cells). Higher frequencies of stimulation (> 100 Hz, which induces phasic burst-like activity) may "push" towards decreases in muscle tone, including the atonia of rapid eye movement sleep (activating giant pontine reticulospinal cells).

The P13 mid-latency auditory evoked potential in Rat is a sleep state-dependent response thought to be equivalent to the human P50 potential, a measure of the output of the reticular activating system. The amplitude of these potentials can be considered a measure of level of arousal, while, using a paired stimulus paradigm, the degree of habituation of the responses also can be assessed. Different durations of rotation were found to reduce the amplitude of the P13 potential, which recovered in a duration-dependent manner. Different durations of rotation led to decreases in habituation of the P13 potential again in a duration-dependent manner. These results suggest that rotation may affect the level of arousal as well as habituation to repetitive sensory inputs. Such effects could be interpreted to imply the presence, following rotation of sufficient duration, of a deficit in sensory gating, or distractibility, and are relevant for the study of the effects of space motion sickness.