image:Antonio de Pereda - The Knight's Dream.JPG Dreams are the images, sounds, thoughts and feelings experienced while sleeping, particularly strongly associated with rapid eye movement sleep. The contents and biological purposes of dreams are not fully understood, though they have been a topic of speculation and interest throughout recorded history. The scientific study of dreams is known as oneirology.

The word "dream" may also be used synonymously with "aspiration".

Neurology of sleep and dreams

main: REM sleep

During REM sleep, the release of certain neurotransmitters is completely suppressed. As a result, motor neurons are not stimulated, a condition known as REM atonia. This prevents dreams from resulting in dangerous movements of the body.

Studies show that various species of mammals and birds experience REM during sleep.

Discovery of REM

In 1953 Eugene Aserinsky discovered REM sleep while working in the surgery of his PhD advisor. Aserinsky noticed that the sleepers' eyes fluttered beneath their closed eyelids, later using a polygraph machine to record their brain waves during these periods. In one session he awakened a subject who was wailing and crying out during REM and confirmed his suspicion that dreaming was occurring. In 1953 Aserinsky and his advisor published the ground-breaking study in Science.

Activation-synthesis

In 1976, J. Allan Hobson and Robert McCarley proposed a new theory that changed dream research, challenging the previously held Freudian view of dreams as unconscious wishes to be interpreted. The activation synthesis theory asserts that the sensory experiences are fabricated by the cortex as a means of interpreting chaotic signals from the pons. They propose that in REM sleep, the ascending cholinergic PGO (ponto-geniculo-occipital) waves stimulate higher midbrain and forebrain cortical structures, producing rapid eye movements. The activated forebrain then synthesizes the dream out of this internally generated information. They assume that the same structures that induce REM sleep also generate sensory information.

Hobson and McCarley's 1976 research suggested that the signals interpreted as dreams originated in the brain stem during REM sleep. However, research by Mark Solms suggests that dreams are generated in the forebrain, and that REM sleep and dreaming are not directly related. While working in the neurosurgery department at hospitals in Johannesburg and London, Solms had access to patients with various brain injuries. He began to question patients about their dreams and confirmed that patients with damage to the parietal lobe stopped dreaming; this finding was in line with Hobson's 1977 theory. However, Solms did not encounter cases of loss of dreaming with patients having brain stem damage. This observation forced him to question Hobson's prevailing theory which marked the brain stem as the source of the signals interpreted as dreams. Solms viewed the idea of dreaming as a function of many complex brain structures as validating Freudian dream theory, an idea that drew criticism from Hobson. Unhappy about Hobson's attempts at discrediting him, Solms, along with partner Edward Nadar, undertook a series of traumatic-injury impact studies using several different species of primates, particularly howler monkeys, in order to more fully understand the role brain damage plays in dream pathology. Solms' experiments proved inconclusive, however, as the high mortality rate associated with using an hydraulic impact pin to artificially produce brain damage in test subjects meant that his final candidate pool was too small to satisfy the requirements of the scientific method.