(A) High amplitude, slow waves # SLEEP PATTERNS:> There are two different kinds of sleep: rapid eye movement (REM) sleep and non-REM (NREM), or slow wave, sleep.> NREM sleep is divided into four stages. A person falling asleep first enters stage 1, which is characterized by low-amplitude, high-frequency EEG activity> Stage 2 is marked by appearance of sleep spindles.> These are bursts of alpha-like, 10-14-Hz, 50-pV waves.> In stage 3, the pattern is one of lower frequency and increased amplitude of the EEG waves.> Maximum slowing with large waves is seen in stage 4.> Thus, the characteristic of deep sleep is a pattern of rhythmic slow waves, indicating marked synchronization.# REM Sleep:> The high-amplitude slow waves seen in the EEG during sleep are sometimes replaced by rapid, low- voltage EEG activity, which in primates, including humans, resembles that seen in stage 1 sleep.> However, sleep is not interrupted; indeed, the threshold for arousal by sensory stimuli and by stimulation of the reticular formation is elevated.> This condition is sometimes called paradoxical sleep, since the EEG activity is rapid.> Rapid, roving movements of the eyes occur during paradoxical sleep, and it is for this reason that it is also called REM sleep.> No such movements are present in slow-wave sleep, which consequently is often called NREM sleep.> Another characteristic of REM sleep is the occurrence of large phasic potentials, in groups of three to five, that originate in the pons and pass rapidly to the lateral geniculate body and from there to the occipital cortex.> For this reason, they are called pontogeniculo-occipital (PGO) spikes.