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A Theory to Account for Vision Changes in Parkinsonís Disease
- Jun 03 2014
Changes in vision are among the many symptoms of Parkinson’s disease (PD). According to a new hypothesis published in the April 24 online edition of Brain, funded in part by the Parkinson’s Disease Foundation (PDF), changes in vision that accompany PD can be understood as the loss of an unconscious aspect of vision known as “blindsight.”
People who live with advanced PD often experience vision challenges. For example, they may have trouble accurately perceiving objects in their peripheral vision or may be unable to make the small, rapid eye movements called saccades, which enable people to visually track moving objects. In addition, people with PD have been shown to experience difficulty in recognizing emotions in the faces of others. Yet the mechanisms behind these visual challenges are not well understood.
The authors of the new paper, funded in part by the PDF Center Grant to Rush University Medical Center, propose that people with PD have trouble with the vision characteristics enabled by blindsight – the aspect of vision that the brain processes subconsciously.
Blindsight refers to aspects of vision of which individuals are not aware. Scientists only know about blindsight because of studying people with brain injuries that have caused people to lose part or all of their vision yet their blindsight still works. Even though these people may be “blind,” they still seem to have “sight” as they can locate objects moving in their blind spot and their eyes will track those objects normally though they cannot be consciously seen.
Blindsight also enables people to recognize facial expressions that convey danger or anger — and respond viscerally to them — even though they do not consciously see the faces. This phenomenon occurs normally in everyone, whether they have vision loss or not, because the brain processes some visual information in several different areas of the brain.
- The authors reviewed existing evidence that demonstrates that people with PD may lack subconscious visual abilities found in blindsight such as perceiving motion or responding to facial expressions.
- They propose that the visual difficulties in PD occur because people with PD have impaired blindsight.
- They suggest their theory may unify existing research and provide a new way of investigating visual problems associated with PD.
What Does It Mean?
This paper is unique in that it provides a potential framework for understanding vision perception difficulties in advanced PD.
This idea of losing the function called blindsight may help explain why people with PD experience abnormal peripheral vision and have difficulty following moving objects (which in turn can make it difficult to drive in advanced PD). The theory that the brain’s subconscious visual centers are impaired would help to explain the visual hallucinations that some people with PD experience, such as seeing a cat or small animal out of the corner of the eye.
In addition, it might explain why people with PD often make fewer facial expressions. The study authors suggest that this “masked” face may reflect, in part, an inability to visually perceive and react to emotions expressed on the faces of other people.
Having a conceptual framework can help scientists to design research to better understand how PD affects visual perception, and potentially to address the ways in which vision difficulties affect the quality of life of people with PD.
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Reference: Diederich NJ, Stebbins G, Schiltz C, Goetz CG (2014) Are patients with Parkinson's disease blind to blindsight? Brain. DOI: 10.1093/brain/awu094 http://dx.doi.org/10.1093/brain/awu094
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Source Date: Jun 03 2014