How Stroke Affects Vision

By David Wasielewski

 

      

To understand how the human visual system is affected by stroke one needs to first understand the amazing capabilities the human brain has developed to process light stimulus and create what we know as vision.

 

It is logical to assume that vision problems resulting from a stroke means that the stroke has damaged one or both of the eyes. This is most often not the case. Only by understanding how the brain receives and processes light stimuli can the survivor begin to make some sense of why a stroke has affected his/her ability to see. This brief article attempts to outline the basics of how the brain creates vision.

 

In some rare cases a stroke will result in damage to the vision in just one eye. This indicates that the stroke occurred in that small area between the eyeball and the optic chiasm damaging the eye or optic nerve (see diagram below). This is rare because the optic nerves are short and fairly isolated from the rest of the brain at this point.

 

Much more common is a stroke that affects a portion of the patient’s field of vision. The condition is referred to as a ‘field cut’. In this situation an area of the brain that processes light stimulus received from the eyes is damaged. This problem usually indicates damage to the Thalamus in the center of the brain or to the visual cortex in the lower rear of the brain The area of the field cut helps the neurologist to assess where in the patient’s brain a stroke may have occurred and use that information to devise a treatment plan.

 

As the optic nerves pass thru the optic chiasm they are segregated according to their left and right visual fields. Note from the diagram that because each eye is bowl shaped (concave) each receives light stimulus from the same direction or visual field. The left eye and the right eye both receive stimulus from the left visual field and right visual field.

 

In the optic chiasm, where the optic nerves intersect, the right and left visual fields are sorted and segregated. In this way light stimulus from the right visual field is sent to and processed by the left side of the brain and light stimulus from left visual field is processed by the right side of the brain.

 

Remember that the brain is set up so the right side controls the left and the left side controls the right. Amazingly, visual evolution has resulted in a processing mechanism that allows us to make sense of this very complex set of stimuli and combine the two overlapping visual fields into one seamless 120 degree field of vision.

 

As the optic nerves enter the Lateral Geniculate Nucleus they are layered in such a way that they can be compared to form an impression of our distance from the object we are looking at. This allows for accurate depth perception. A disruption in this area will affect one’s ability to judge distances and navigate through the environment.

 

Once the data is processed by the Thalamus it is transmitted to the right and left optical cortex at the lower rear of the brain. Since this is well after the visual fields have been integrated it is logical that damage to these areas will affect the ‘field’ of vision, not the sight in one eye or the other since the visual field is made up of stimulus received from both eyes.

 

Someone who does not understand brain and eye anatomy and how they process light stimuli to create vision might assume that the inability to see the right visual field is a result of damage to the right eye. A neurologist who diagnoses the same condition will understand that the inability to see the right visual field (a right side field cut) is due to damage to the visual cortex on the left side of the brain – not the eye. The neurologist will investigate the visual cortex, not the optic nerve for signs of a stroke.

 

 

  

Copyright © January 2011

The Stroke Network, Inc.

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