Objective: To demonstrate quantifiable differences in cortical visual processing in patients with Parkinson’s disease with no evidence of dementia, as compared to controls.

Background: Visual symptoms are frequently reported in Parkinson’s disease. Previous reports have focussed on ophthalmic changes such as visual acuity, contrast sensitivity and colour deficits, rather than higher visual perceptual changes. Measurable evidence of cortical visual loss will be important as a behavioural marker of cortical involvement in Parkinson’s disease, and may precede overt cognitive disability.

Methods: Cat and dog stimuli of varying degrees of skewness were presented to patients with idiopathic Parkinson’s disease, defined by QSBB criteria (n=7) and healthy age-matched controls (n=7). Average disease duration was 5 years (± 2.9 years), with a mean L-dopa equivalent dose of 458 mg (± 203 mg). Stimuli were generated using an online database of cats and dogs that were cropped and converted to grayscale. Fourier transforms of each image were computed producing magnitude and phase matrices. For each trial, the phase matrix of a cat or dog image was skewed along the x-axis by a variable amount of skew, combined with a proportion of white noise and then recombined with the average magnitude matrix of the entire series. The resulting images had identical spatial frequency with varying degrees of skew. On each trial participants were asked to decide if the image they had seen was a cat or a dog by pressing one of two keys. Psychophysical curves of relative skew against correctness were generated, to determine the threshold for detection of cat or dog. Neuropsychological, visual and motor assessments were also completed.

Results: Non-demented patients with Parkinson’s disease showed significantly impaired ability to identify skewed images compared with healthy controls. Threshold for identifying skewed images was 57.4 degrees skew (±10.3 degrees) in patients with Parkinson’s disease, compared with 68.2 degrees (± 1.68 degrees) in heathy controls, t(12)=2.724, p=0.018. There was no difference between the groups in visual acuity or in neuropsychological performance.

Conclusions: We present evidence for quantifiable differences in higher visual processing in patients with Parkinson’s disease without dementia. This measure may be used as a marker of cortical disease in Parkinson’s disease.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/quantifiable-changes-in-cortical-visual-processing-in-patients-with-parkinsons-disease-but-no-cognitive-impairment/