Amanda D. Francoeur, firstname.lastname@example.org
No, you’re not seeing things. Researchers can trigger visual hallucinations using intensely flashing LEDs. Employing photic stimulation to promote brain responses could bring scientists closer to understanding the phenomena and, in turn, could reveal more about the psychiatric, neurological and eye disorders associated with them. Unlike other vision disorders, hallucinations are especially difficult to study because of their irregularity and unpredictability. However, Dr. Dominic H. ffytche at the Institute of Psychiatry in London devised a way to manipulate the same anomalous activity within the same regions of the brain and to monitor the activity by using EEG and functional MRI scanning.
“In the past, the study of visual hallucinations has had to rely on the chance occurrence of an hallucination while a patient was having a brain scan or EEG recording. Being able to induce visual hallucinations on demand makes it easier to study the phenomena,” ffytche said.
Hallucinations create false vivid or distorted perceptions of objects, colors and sounds by affecting all sensory modalities, from vision, hearing, smell, taste and touch to even balance, physiological pain and body temperature fluctuations. Activity occurs within specialized cortical regions of the brain (topological) as well as changes in connections between these regions (hodological). In his study, ffytche focused on visual hallucinations and the hodotopic framework to help assimilate neurobiological data into neuropsychiatric symptoms.
Ffytche mimicked a model created by physiologist Jan Purkinje called Purkinje patterns. It is an experiment that exposes normal subjects to recurring changes of hallucinatory and nonhallucinatory states by using light sources. The lights execute repetitive flashes that generate activity in various regions of the brain, whether or not hallucinations occur. Frequencies below 5 Hz or above 30 Hz do not induce hallucinations.
Visual stimulation was created by functional MRI-compatible goggles comprising 30 high-intensity white LEDs. Subjects underwent hallucination-inducing stimuli, which included a sequence of low hallucination-inducing frequencies (8 to 11 Hz) and high hallucination-inducing frequencies (19 to 25 Hz). The experiment also combined two control stimuli – frequency and luminance – that did not provoke hallucinations.
During frequency control stimulation, the frequency of flashes stayed the same as the hallucination-inducing stimuli, but light intensity was reduced by 15 to 30 percent. With luminance control stimulation, subjects underwent the same light intensity as hallucination-inducing stimuli, but the frequency changed as flashes were irregularly spaced. Flash sequences were in pseudorandom order, so subjects did not know whether they would have hallucinations or not. Each flash sequence lasted 6 s, followed by 10 s of rest, and a total of 10 flash sequences were given within a period of 8 min.
Ffytche discovered that both control stimuli increase activity throughout the bilateral and ventral cortical regions of the brain. Activity of the lateral geniculate nucleus (LGN), where nerve signals from the eye are passed through to the brain, increased during frequency-control stimuli. Hallucination-inducing stimuli produced a decrease in LGN activity but resulted in an increase in activity of the occipital lobe. Luminance control caused only a small decrease in LGN activity, and activity of the occipital lobe was lower than with hallucination-induced stimuli.
The occipital lobe of the brain reveals activity during visual hallucinations after an experiment that involved sequences of intensely flashing LEDs. Being able to induce hallucinations and observe brain responses could help researchers better understand the phenomena.
As a result of the hallucination-inducing stimuli, subjects saw a vivid array of geometrical shapes, simple lines, colors, checkerboards and grids. Topological changes revealed that specific areas of the visual cortex, when activated, produced their intended hallucination – activity in the color region triggered color hallucinations. Hodological changes revealed that each sensory modality hallucination was based on the collaboration of the appropriate networks – visual hallucinations were derived from communication within the visual cortical areas of the brain. Therefore, by inducing visual hallucinations, ffytche could observe the correlation between changes of increased activity and connectivity of specific brain regions.
A disease that has appeared to be most directly related to Purkinje patterns is Charles Bonnet syndrome (CBS), which commonly affects individuals with significant vision loss. During CBS, an individual can experience what ffytche calls a “transient form of ‘blindness’ ” – when the transfer of visual signals to the brain changes because of a shift in neural firing patterns known as burst mode. This impairs vision and temporarily blinds the person to the external world. Although Purkinje hallucinations occur in individuals who do not have severe vision loss, both types of patients can experience this kind of dissociation. Identifying their relationship could bring researchers one step closer to understanding the cause of CBS.
An advantage of applying the Purkinje model within the hodotopic framework is that it allows for specific regions of the brain and their activity to be viewed simultaneously. A disadvantage is that the framework is oversimplified because neither the basal ganglia, brain stem or limbic system, nor changes associated with specific neurotransmitters, are incorporated into the study.
According to ffytche, hallucinations are still extraordinary and complex occurrences that scientists must study further to comprehend their onset.
“We are planning to look at how brain changes in patients at risk of hallucinations relate to changes that occur during the hallucinations themselves to better understand why hallucinations occur,” he said.
Cortex, September 2008, pp. 637-648.