New concept may explain vision problems in Parkinson's disease
(Medical Xpress)—Are patients with Parkinson's disease "blind to blindsight?" That's not a trick question, but the focus of an inquiry by neuroscientists from Rush University Medical Center as well as the Centre Hospitalier and University of Luxembourg.
The "blind to blindness" concept is described in the June issue of the journal Brain by Dr. Nico J. Diederich, from Centre Hospitalier and University of Luxembourg, who is a visiting scholar at Rush University. He was joined by the Rush researchers Glenn Stebbins, PhD, and Dr. Christopher G. Goetz, and neuropsychologist Christine Schiltz, PhD, from the University of Luxembourg.
The scientists developed the concept of "blind to blindsight" to integrate data on visual impairments that contribute to the disability and diminished quality of life in patients with Parkinson's disease.
Blindsight is observed in people who are blind as a result of a lesion in the visual cortex of their brain. Although these individuals are blind, they maintain the ability to sense accurately a light source or a rapid movement without being aware of it. Strangely, blindsighted patients even can respond appropriately to emotional facial expressions, especially those expressing fear or danger. It is believed that these visual stimuli can be turned directly into actions (e.g., movement of the eyes) by passing through lower areas of the brain. Thus, these retained visual functions operate as unconscious responses to visual stimulation even when there is extensive damage to the visual cortex. .
Conversely, patients with Parkinson's disease, who do not have a problem with their general vision, are unable to do these tasks: they display slowness and reduced accuracy of pursuit eye movements. They often have difficulties grasping a moving object, and show decreased sensitivity to low contrast and impaired ability to read "right away" other people's facial expressions.
Taken together, these Parkinson's disease symptoms represent major impairments in blindsight – hence, "blind to blindsight." Based on this new concept, the researchers could now propose a new concept how to comprehensively understand within one visual system—blindsight—numerous visual signs and symptoms of patients with Parkinson's disease. Impairment of the evolutionary old networks in the brain operating within the blindsight visual system form the basis of the visual problems in Parkinson's disease.
Parkinson's disease is a chronic, neurodegenerative disorder that affects predominantly people over age 60. About 1 million people in the United States, and over 5 million worldwide, have Parkinson's disease.
Interview studies of patients with Parkinson's disease show that approximately one third of patients with Parkinson's disease experience visual hallucinations at a given time, but up to three fourths of all affected patients might develop visual hallucinations during a 20-year period of extant Parkinson's disease. Hallucinations have substantial psychosocial effects and are the main reason for admitting patients with Parkinson's disease into nursing homes.
The researchers propose that the impaired brain networks involved in "blind to blindsight" syndrome also may give rise to the highly distinctive "passage" hallucinations of Parkinson's disease. These hallucinations involve the false "guess" of movements in the peripheral visual field, such as a cat appearing in a doorway or seen in the corner of one's eye.
The slow or inadequate reaction of patients with Parkinson's disease to movements may also endanger their driving capacities, although these patients are known for mostly driving slowly and cautiously.
The researchers emphasized, however, that not all visual deficits in patients with Parkinson's disease can be classified within the concept of "blind to blindsight," but they hope their findings will prompt neuropathological and neuroimaging studies to better understand the networks underpinning this syndrome.