Increasing Contrast Improves Object Perception in Parkinson's Disease with Visual Hallucinations

Abstract

Background: Deficits in basic vision are associated with visual hallucinations in Parkinson's disease. Of particular interest is contrast sensitivity loss in this disorder and its effect on object identification.

Objectives: Evaluate whether increased contrast improves object perception in persons with Parkinson's disease and visual hallucinations, without dementia.

Methods: We assessed 26 individuals with mild to moderate idiopathic Parkinson's disease, half of whom reported one or more episodes of hallucinations/unusual perceptual experiences in the past month, with a letter-identification task that determined the contrast level required to achieve 80% accuracy. Contrast sensitivity was further assessed with a chart that presented stimuli at multiple spatial frequencies. The groups were closely matched for demographic and clinical characteristics except for experience of hallucinations.

Results: Relative to participants without visual hallucinations, those with hallucinations had poorer spatial frequency contrast sensitivity and required significantly greater contrast to correctly identify the letters on the identification task. Specifically, participants with hallucinations required a mean contrast of 52.8%, whereas participants without hallucinations required 35.0%. When given sufficient contrast, the groups with and without hallucinations were equally accurate in letter identification.

Conclusions: Compared to those without hallucinations, individuals with Parkinson's disease and hallucinations without dementia showed poorer contrast sensitivity. Once contrast was individually enhanced, the groups were equally accurate at object identification. These findings suggest the potential of visual perception tests to predict, and perception-based interventions to reduce, hallucinations in Parkinson's disease.

Keywords: Parkinson's disease, hallucinations, contrast sensitivity, object perception, vision.

Conflict of interest statement

This research was supported by a Research Supplement to Promote Diversity Award from the National Institute of Neurological Disorders and Stroke (NINDS) (R01 NS050446‐03S1), a Ruth L. Kirschstein National Research Service Award (F31NS074682), and a Clara Mayo Research Fellowship, Boston University, to MDS; a conference travel award from the Undergraduate Research Opportunities Program of Boston University to ZAM, and grants from NINDS (R01 NS067128, R01 NS050446) to ACG. The authors report no conflicts of interest.

© 2020 International Parkinson and Movement Disorder Society.

Figures

FIG 1

Letter identification task. The letter identification task used an interleaving staircase procedure that identified the target contrast threshold that the participant needed to correctly identify the letter at an 80% accuracy rate. Each of the four letters H, O, T, and X was displayed within a box measuring 256 × 256 pixels, which functioned as a background and was held at a constant gray level. Stimuli were presented one at a time on the screen for 12 ms followed by a constant interstimulus interval of 59 ms, and followed by a visual mask for 506 ms. The visual mask consisted of the multiple overlapping letters H, O, T, and X positioned randomly in the display field to create an unstructured pattern (more unstructured than represented in this schematic for clarity). The participants' task was to orally report the perceived letter. See text for details.

FIG 2

Mean contrast thresholds on the letter detection task at the criterion accuracy of 80%. Error bars represent standard error of the mean. The entire group (left column) required an overall mean contrast of 44.3% (SD = 19.3). Relative to participants with PD without visual hallucinations (PD‐NVH, n = 12), those with hallucinations (PD‐VH, n = 13) required significantly more contrast to correctly identify the target letters, *P < 0.05. At the adjusted contrast levels, the VH group performed at the same level of accuracy (i.e., comparable number of errors) as the NVH group. See text for details.