Purchase this article with an account.
S. Madill, G.B. Arden, D.H. Ffytche; Changes in Colour Thresholds Associated With Colour Hallucination Susceptibility in the Charles Bonnet Syndrome . Invest. Ophthalmol. Vis. Sci. 2005;46(13):624.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Purpose: Hallucinatory percepts in the Charles Bonnet Syndrome (CBS) can be of a single vivid colour. A previous study using functional MRI scanning has demonstrated transient, spontaneous increases in activity in the human colour centre, V4 coincident with hallucinations of colour. The finding is suggestive of a cortical hyperexcitability within V4 in patients with colour hallucinations. Here we use a simple test of colour threshold to establish whether this change in cortical excitability has a psychophysical correlate and, if so, whether the change is specific to the colour hallucinated. Methods: 10 patients with AMD and CBS with continuing (most recent episode within a month of testing) hallucinations of, principally, one colour were recruited. Because central vision was impaired in all patients, the ChromaTest© was used to test the extra–macular retinae at four positions (superior and inferior nasal and temporal quadrants at 12.5° eccentricity). The test involves the presentation of a protan or tritan colour–contrast defined stimulus against a background of luminance noise in a staircase procedure, the location of the stimulus being randomised between the four locations. Patients were tested monocularly with the best eye and test order counterbalanced across patients. Results: For each patient, the threshold value for the hallucinated colour axis was significantly reduced (Fisher's exact test, p<0.05) compared to the non–hallucinated colour axis, the relative increase in sensitivity lowering the hallucinated–axis threshold for some patients below that found for normal sighted, predominantly younger, control subjects. Conclusions: Cortical hyperexcitability in CBS has a correlate in psychophysical threshold. In susceptible patients, de–afferentation of the central visual field changes the sensitivity of cortical colour representations outside the de–afferentated zone. This change in sensitivity seems to relate specifically to hallucinated colour representations rather than generalising across all colours. This is the first study to demonstrate a correlation between visual hallucination susceptibility and psychophysically–defined visual sensitivity, raising the possibility of a predictive test for this common, often distressing, symptom.
This PDF is available to Subscribers Only