hypnagogia shares with other stages of sleep is amnesia. But this is a selective forgetfulness, affecting the hippocampal memory system, which is responsible for episodic or autobiographical memory, rather than the neocortical memory system, responsible for semantic memory. It has been suggested that hypnagogia and REM sleep help in the consolidation of semantic memory, but the evidence for this has been disputed. For example, suppression of REM sleep due to antidepressants and lesions to the brainstem has not been found to produce detrimental effects on cognition. Hypnagogic phenomena may be interpreted as visions, prophecies, premonitions, apparitions and inspiration (artistic or divine), depending on the experiencers' beliefs and those of their culture. Physiology Physiological studies have tended to concentrate on hypnagogia in the strict sense of spontaneous sleep onset experiences. Such experiences are associated especially with stage 1 of NREM sleep, but may also occur with pre-sleep alpha waves. Davis et al. found short flashes of dreamlike imagery at the onset of sleep to correlate with drop-offs in alpha EEG activity. Hori et al. regard sleep onset hypnagogia as a state distinct from both wakefulness and sleep with unique electrophysiological, behavioral and subjective characteristics, while Germaine et al. have demonstrated a resemblance between the EEG power spectra of spontaneously occurring hypnagogic images, on the one hand, and those of both REM sleep and relaxed wakefulness, on the other. To identify more precisely the nature of the EEG state which accompanies imagery in the transition from wakefulness to sleep, Hori et al. proposed a scheme of 9 EEG stages defined by varying proportions of alpha (stages 1–3), suppressed waves of less than 20μV (stage 4), theta ripples (stage 5), proportions of sawtooth waves (stages 6–7), and presence of spindles (stages 8–9). Germaine and Nielsen found spontaneous hypnagogic imagery to occur mainly during Hori sleep onset stages 4 (EEG flattening) and 5 (theta ripples). The "covert-rapid-eye-movement" hypothesis proposes that hidden elements of REM sleep emerge during the wakefulness-sleep transition stage. Support for this comes from Bódicz et al., who notes a greater similarity between WST (wakefulness-sleep transition) EEG and REM sleep EEG than between the former and stage 2 sleep. Respiratory pattern changes have also been noted in the hypnagogic state, in addition to a lowered rate of frontalis muscle activity. Daydreaming and waking reveries Microsleep (short episodes of immediate sleep onset) may intrude into wakefulness at any time in the wakefulness-sleep cycle,