September 4, 2008 A hallucination is a perception in a conscious awake state, in the absence of external stimuli, that has qualities of real perception in that they are vivid, substantial, and located in external objective space. For most of us, hallucinations are a normal, fleeting, brain glitch; yet for a few they are persistent, distressing and associated with a range of psychiatric, neurological and eye conditions. In the September Issue of Cortex, Dominic H. ffytche at the Institute of Psychiatry in London reviews what we do know and moves the field forward, by introducing a new experimental approach to studying hallucinations as they occur.
Surprisingly little is known about brain changes that occur during hallucinations because of their brief, unpredictable nature. One cannot anticipate when a hallucination will occur, so the chances of capturing one during a brain scanning experiment are small. It has long been recognized that flashes of light at particular frequencies produce hallucinations of intricate patterns and vivid colours. Using a combination of brain imaging methods in normal subjects, the author harnesses the technique to examine localized changes in brain activity and changes in brain connections during hallucinations.
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“We observed increases in activity in visual brain regions”, says ffytche, “Increases in visual connection strength and an alteration in relationship between visual relay and receiving stations, together suggesting that hallucinations were caused by a transient form of 'blindness'”.
The work highlights the need to consider the hallucinating brain from a wider perspective than previously thought. Changes in both localized brain activity and in connections between brain areas occur during hallucinations, raising further questions as to how these changes interact with pre-existing abnormalities in patients susceptible to hallucinations.
The article “The Hodology of Hallucinations” is authored by Dominic H. ffytche, and it appears in Cortex, Volume 44, Issue 8 (September 2008), pp 637-648, published by Elsevier in Italy.
The article is published in a Special Issue of Cortex entirely dedicated to the novel MRI methods to study human brain connections. In the same issue an article by Thiebaut de Schotten et al shows that disorders of spatial awareness are frequently associated with lesions of the fibres connecting the frontal and parietal lobes. Similarly for the left hemisphere Catani and Mesulam revisit the anatomy of the language pathways and show that beyond the classical language network connecting Wernicke's and Broca's area there are additional circuits involved in complex language functions. This hodological approach begins to reveal important details on the architectural backbone of human cognition and could explain the anatomical basis of symptoms commonly observed in a wide range of disorders including schizophrenia, autism, Alzheimer's disease and stroke.