Behavioral functions of the reticular formation.
Studies of the behavioral correlates of activity in reticular formation cells, usually performed in restrained animals, have found units whose discharge relates to sensory stimuli, pain and escape behavior, conditioning and habituation, arousal, complex motivational states, REM sleep, eye movements, respiration and locomotion. Units with these different behavioral correlates were found in the same anatomical areas. Most studies report that a large proportion of encountered cells related to the behavior being studied. If one adds up the reported percentages, the total far exceeds 100%. Therefore it appears that many investigators are looking at the same cells and reaching very different conclusions about their behavioral roles. On the basis of observations in unrestrained cats, it is hypothesized that discharge in most RF cells is primarily related to the excitation of small groups of muscles. This hypothesis can parsimoniously explain many previous observations on the behavioral correlates of these cells, and is consistent with anatomical, physiological and phylogenetic studies of the reticular formation. The hypothesized simplicity of reticular formation unit function is contrasted with the complexity of the behavioral functions mediated by the RF, and the implications of this contrast discussed.