The Visual Pathways: Roadmaps and impacts following brain injury

The Visual Pathways:
Roadmaps and impacts following brain injury

brain pathwayThe visual process is so tremendously complex.  SO much neurology is involved.  And therefore, so much is exposed, following brain injury.

It is not possible to anticipate all of the problems which may result, following a brain insult (such as a stroke or a trauma).  However, the array of possible problems may come to light by studying a quick overview of the Visual Pathways.

This summary was presented, following a question which arose from a Medical Resident studying to be an Internist, who received his first exposure to Behavioral Optometry watching my interview.  He asks:

I watched part of your interview and I was really impressed at your  concept of holistic eye care. This sounds very novel in the field of eye care.  

Do you also work with post-stroke patients who have issues with sight due to involvement of their optic track?

 

My response includes an outline of the visual pathways…

Thank you for taking the time to watch my television interview!
I am pleased you find the concept of holistic vision care to be of interest.  

Oculomotor impacts of brain injury

I do, indeed, support a great number of patients with Acquired Brain Injury (ABI), including stroke (cerebral vascular accident, or CVA) and traumatic brain injury (TBI).
Often, their issues are not limited to “sight.”  The most common visual impacts, post-ABI, include effects on saccadic eye movements, smooth pursuit, and convergence (eye-teaming).  These are all managed by neuro-muscular pathways originating in the brainstem, which send signals to the extraocular muscles via Cranial Nerves III, IV, and VI.  
  • These signals vary in quality (for example, providing a steady movement, for “smooth pursuit,” or providing a rapid movement or eye-jump, for “saccades”).  
  • They also vary in the muscle pairings:  For example, to look from left to right with both eyes as a team, we require paired constriction of the left medial rectus with the right lateral rectus.  But to look from far to near, or to converge the eyes, requires pairing of the left medial rectus and the right medial rectus.

 

fnint-08-00071-g0021

 

When an injury affects the brainstem, eye-movement control is therefore frequently “un-hinged” post-injury.  This is particularly common in TBI or whiplash injuries. This is something that we treat in a Vision Rehabilitation program.

“Eyesight” & steady fixation

eye chart retinaEye “sight” is merely a matter of “resolution” at the central part of the vision.  Eye-movement skills do play some role in that: If the eyes cannot maintain steady fixation, it is more difficult to sustain an image on the center of the eye (fovea) while viewing detail.

 

Visual information processing post-brain injury

Visual discrimination/ recognition, the ability to “make sense” of things, is largely impacted post-ABI, as this entails visual information PROCESSING.  This is activity taking place in the cerebral cortex.  It does not only take place in the occipital lobe of the cortex, which is a common misconception among lay-people.  True, the visual data which reaches the cortex goes to the striate cortex (IVth layer) of the occipital lobe first.  But next the data branches upwards and downwards for gross and fine processing of the “receptive fields.”  (Receptive fields are areas over which data is gathered from the retina.)  scientificamericanmind1208-20-I31Next, associative cortices support the processing of the data, as we make sense of the world:

  • Data are carried towards the temporal lobe for spatial processing, “WHERE is it?”
  • Data are carried towards the parietal lobe for identification, “WHAT is it?”  
  • The frontal lobe also plays a role in planning, such as visual-motor planning- more like, “HOW do I get there, and what should I manage first, etc…”

Visual Pathways

In your question, you mentioned the “optic track.”  It would be more appropriate to look at all of the visual pathwayS that get affected:
optic nerve tract
Once light strikes the retina, 90% of the data it is carried via the OPTIC NERVE and RE-SORTED along the OPTIC TRACT on its way to the THALAMUS (Lateral Geniculate Nucleus of the thalamus).  Within the thalamus, the neurons synapse before the upper neurons carry the data to the VISUAL CORTEX.  

 

Top-down neurological connections >> bottom-up!

HOWEVER, for every ONE neural pathway carrying data UP to the cortex, there are NINE pathways FROM the CORTEX, TO the THALAMUS, which FILTER the data.  Basically, we have 9 times as many “traffic-cops” which limit and filter the data available to the visual system.  THEREFORE, when there is damage to the cerebral cortex, some of those Top-down FILTERS are disrupted, and the individual has a great deal of trouble making sense of the overwhelming amount of data available via the visual process. 

This is also something that we treat in a Vision Rehabilitation program.

top-down management of vision

a) 90% of data from the retina goes through the LGN of the thalamus, and onto the visual cortex, then onto associated cortices. 10% bypasses the cortex and goes to Superior colliculus in the brainstem.
b) Diagram shows inter-cortical associations as well as two-way communications between the higher centers of the cortex and the integration centers in the thalamus (pulvinar nucleus), as well as those between the cortex and amygdala (emotional centers), also feeding back to the thalamus. There are many more top-down than bottom-up pathways in the Visual Process.

 

Sub-cortical visual pathway and Blindsight

I mentioned only 90% of the data coming through the retina goes up to the cortex.  The other 10% of neurons peel off and travel directly to the Superior Colliculus (SC) within the mid-brain.  These are used for light-orientation and general spatial awareness.  I have had a patient, for example, with an arterio-venous (A-V) malformation in the brain which left him with right hemianopsia (loss of visual field on his right side, after a life-saving operation). In the course of our Vision Therapy program, he developed BLINDSIGHT, with amazingly precise ability to localize in his BLIND field, even though he could not CONSCIOUSLY see the targets.  He simply knew where they were.  I have an example of that on video. I believe the data which bypasses the cortex and goes to the SC is responsible for this phenomenon.
slide_13 visual pathway

Other links of interest

I have written a little blog piece on the eye/brain connection, which you might wish to read.  The follow-up piece has some fascinating links on the use of vision in non-sighted individuals as well.

I hope you continue to keep your eyes and ears open, as your patients stand to benefit greatly when their physician listens openly and offers whole-person solutions for any sort of problem with which they present.  Being able to refer to the right support is a tremendous impact and service to those in your care.
Posted in Education, Public Health, Vision & Brain injury

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