New Research Explains How Some Blind People Can Have Visual Hallucinations

Priscilla Lynch 

January 11, 2021

Visual hallucinations in people who have lost their sight can stem from spontaneous activity in the brain’s visual centres, according to a study co-led by UK researchers.

The study, published in  Brain,  investigated why some people who have lost their eyesight experience vivid hallucinations, i.e., Charles Bonnet syndrome (CBS).

To determine if and how spontaneous brain activity underlies deprivation-related visual hallucinations, the team recruited five late-onset blind individuals with CBS, 11 late-onset blind control participants who did not experience hallucinations and 13 sighted control participants for a functional magnetic resonance imaging study. CBS participants verbally/manually reported their hallucinations while being scanned. A visual simulation composed of their hallucinatory streams was later presented to sighted control participants. In addition, all participants completed a visual imagery scan.

The same brain visual areas were active in all three groups – those that hallucinated, those that watched the films and those creating imagery in their minds’ eye.

However, the researchers noted a difference in the timing of the neural activity between these groups. In both the sighted participants and those in the imagery group, the activity took place in response either to visual input or to the instructions set in the task. But in the group with CBS, a gradually increasing wave of neural activity was noted, reminiscent of the slow spontaneous fluctuations that emerged just before the onset of the hallucinations.

These results suggest that, in the absence of external visual input, a build-up of spontaneous fluctuations in early visual cortex may activate the visual hierarchy, thereby triggering the experience of vision.

Lead author Dr Avital Hahamy said: “Our findings have clinical significance, in helping us to understand Charles Bonnet syndrome, but they also teach us that spontaneous brain activity has a functional role in evoking unprompted perceptual behaviour – for example, the same spontaneous fluctuations in the brain’s visual centres may underlie dreams.”

Hahamy A, Wilf M, Rosin B, Behrmann M, Malach R. How do the blind 'see'? The role of spontaneous brain activity in self-generated perception. Brain. 2020 Dec 26 [Epub ahead of print]. doi: 10.1093/brain/awaa384. PMID: 33367630.  View full text

This article originally appeared on Univadis, part of the Medscape Professional Network.

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