BRAIN STRUCTURES involved in light sleep include the raphe
system, which, by producing the monoamine serotonin, serves to counteract
the alerting effects of the brain's reticular formation ("a,"
color at left). The author suggests that other nearby structures act to
modulate the fast wave pattern of the alert cortex into the slower pattern
typical of light sleep. Such slow activity, however, is known to depend
on higher as well as lower brain structures (b); when a cat is deprived
of its cerebral cortex and thalamus, the wave pattern characteristic of
light sleep disappears. The reason for this is not yet understood.