Page 4: Freakscience - The nature of hallucinations

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The nature of hallucinations
 
The general theory of hallucinations here delineated rests upon two fundamental assumptions.
One assumption states that life experiences influence the brain in such a way as to leave, in the brain, enduring physical changes that have variously been called neural traces, templates, or engrams. Ideas and images are held to derive from the incorporation and activation of these engrams in complex circuits involving nerve cells. Such circuits in the cortex (outer layers) of the brain appear to subserve the neurophysiology of memory, thought, imagination, and fantasy.
 
The emotions associated with these intellectual and perceptual functions seem to be mediated through cortex connections with the deeper parts of the brain (the limbic system or “visceral brain,” for example), thus permitting a dynamic interplay between perception and emotion through transactions that appear to take place largely at unconscious levels.
 
Conscious awareness is found to be mediated by the ascending midbrain reticular activating system (a network of nerve cells in the brainstem). Analyses of hallucinations reported by sufferers of neurological disorders and by neurosurgical patients in whom the brain is stimulated electrically have shown the importance of the temporal lobes (at the sides of the brain) to auditory hallucinations, for example, and of other functionally relevant parts of the brain in this process.
 
A second assumption states that the total human personality is best understood in terms of the constant interplay of forces that continually emanate from inside (as internal physiological activity) and from outside the individual (as sensory stimuli).
Such transactions between the environment and the individual may be said to exert an integrating and organizing influence upon memory traces stored in the nervous system and to affect the patterns in which sensory engrams are activated to produce experiences called images, fantasies, dreams, or hallucinations, as well as the emotions associated with these patterns. If such a constantly shifting balance exists between internal and external environmental forces, physiological considerations (e.g., brain function) as well as cultural and experiential factors emerge as major determinants of the content and meaning of hallucinations.
 
The brain is bombarded constantly by sensory impulses, but most of these are excluded from consciousness in a dynamically shifting, selective fashion. The exclusion seems to be accomplished through the exercise of integrative inner mechanisms that focus one’s awareness on selected parts of potential experience. (The sound of a ticking clock, for example, fades in and out of awareness.) Functioning simultaneously, these mechanisms survey information that is stored within the brain, select tiny samples needed to give adaptive significance to the incoming flow of information, and bring forth only a few items for actual recall from the brain’s extensive “memory banks.”
From Encyclopedia Britannica (www.britannica.com)

What Are Visual Hallucinations?

Hallucinations, defined as the perception of an object or event (in any of the 5 senses) in the absence of an external stimulus, are experienced by patients with conditions that span several fields (e.g., psychiatry, neurology, and ophthalmology). When noted by non-psychiatrists, visual hallucinations, one type of sensory misperception, often trigger requests for psychiatric consultation, although visual hallucinations are not pathognomonic of a primary psychiatric illness.
Visual hallucinations have numerous etiologies. Here, we discuss possible mechanisms and offer a differential diagnosis of visual hallucinations, with an emphasis placed on conditions that arise in the context of medical and surgical illness. Treatment typically rests on the underlying etiology, so timely recognition and an understanding of causative mechanisms are crucial.
What Causes Visual Hallucinations?
Numerous hypotheses have been suggested to explain the genesis of visual hallucinations. These have been summarized and categorized: psycho physiologic (i.e., as a disturbance of brain structure), psycho biochemical (as a disturbance of neurotransmitters), and psychodynamic (as an emergence of the unconscious into consciousness). Visual hallucinations can be the result of all 3 processes, given the interplay among disturbances of brain anatomy, brain chemistry, prior experiences, and psychodynamic meaning.
To date, no single neural mechanism has explained all types of visual hallucinations; however, the similarity of visual hallucinations that are associated with seemingly diverse conditions suggests a final common pathway. There are three pathophysiologic mechanisms thought to account for complex visual hallucinations.

The first mechanism involves irritation (e.g., seizure activity) of cortical centers responsible for visual processing. Irritation of the primary visual cortex causes simple elementary visual hallucinations, while irritation of the visual association cortices causes more complex visual hallucinations. These data are supported by both electroencephalographic (EEG) recordings and direct stimulation experiments.

Lesions that cause differentiation of the visual system may lead to cortical release phenomenon, including visual hallucinations. Normal inputs are thought to be under the control of inhibitory processes that are effectively removed by differentiation. It has been further suggested that differenced neurons undergo specific biochemical and molecular changes that lead to an overall increase in excitability.

A multitude of lesions can cause this loss of input and inhibit other cognitive functions. Of note, visual hallucinations may be induced by prolonged visual deprivation. One study reported visual hallucinations in 10 of 13 healthy subjects blindfolded for a period of 5 days; this finding lends strong support to the idea that the simple loss of normal visual input is sufficient to cause visual hallucinations.
Finally, due to its role in the maintenance of arousal, the reticular activating system has been implicated in the genesis of visual hallucinations. Lesions of the brainstem have led to visual hallucinations (as in peduncular hallucinosis). Further, visual hallucinations are common in those with certain sleep disorders, and occur more frequently in those who are drowsy. The observation that visual hallucinations occur more frequently in those who are drowsy (even in the absence of frank sleep pathology) suggests that the reticular activating system plays a role in visual hallucinations, although the precise mechanism has not yet been established.
Read 1191 times Last modified on Wednesday, 11 June 2014 17:01

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