Biological basis for visual hallucinations identified

By Eleanor McDermid, Senior medwireNews Reporter

Visual hallucinations in patients with schizophrenia may be caused by hyperconnectivity in a specific area of the brain, say researchers.

“While [visual hallucinations] are generally neglected in clinical assessments in this population, functional connectivity data provide biological support for their distressing reality”, says the team, led by Judith Ford (San Francisco VA Medical Center, California, USA).

Ford and colleagues studied 180 schizophrenia patients, and found that the 45 who had experienced visual hallucinations in the month prior to brain imaging had hyperconnectivity between the amygdala and the visual cortex, relative to 178 mentally healthy controls matched for age, gender, handedness and ethnicity.

“This hyperconnectivity may facilitate retrieval and reactivation of visual memories, arguably the raw material of [visual hallucinations]”, the team writes in Schizophrenia Bulletin.

Patients with visual hallucinations also had significantly increased connectivity relative to 50 patients with auditory hallucinations and 67 without any visual or auditory hallucinations. The connectivity was strongest for Brodmann area (BA)18, and tended to be stronger for right than left BA18 connectivity.

Of note, connectivity between the amygdala and BA18 was significantly stronger in patients with visual hallucinations than auditory hallucinations for both the right and left hemispheres.

“Hyperconnectivity between the amygdala and visual cortex may reflect the ability of these areas to communicate too efficiently with each other, perhaps resulting in excessive conscious experiences or [visual hallucinations] of negatively charged, threatening images,” say Ford et al.

However, contrary to their expectations, patients with visual hallucinations did not have increased connectivity between the hippocampus and the visual cortex, despite the reported role of the hippocampus in visual hallucinations and memory retrieval.

The lack of hyperconnectivity involving the hippocampus may suggest that visual hallucinations in the schizophrenia patients in their study “relied less on retrieval of memories than on intrusive negative visual images,” say the researchers, although they note that they had no information on the content of the patients’ visual hallucinations.

Forty-two of the patients reported both visual and auditory hallucinations. Although the researchers did not know whether these were experienced simultaneously (eg, “talking heads”) or separately, they did find a trend towards hyperconnectivity between the visual and auditory cortices in these patients, relative to those without hallucinations.

They suggest: “Perhaps the wiring is in place for the simultaneous experience of auditory and visual percepts in this sample, and, perhaps, this relationship would be significant in people who indeed had simultaneous [auditory hallucinations] and [visual hallucinations].”

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