Vision problem and Virtual Reality

P C Knox pck at castle.ed.ac.uk
Mon Nov 23 04:54:26 EST 1992

spotter at sanger.bio.uci.edu (Steve Potter) writes:

>With all due respect to the ol' professor, Dr. Dubin, dont believe anyone
>who says something can't be done.  Especially when it comes to issues
>of neural plasticity.  Monkeys and cats are different than humans in the
>very important respect that we can follow instructions and intentionally
>carry out excercises that can re-wire out visual system.  For instance
>I have recently successfully trained my eyes to merge images that are
>further apart than my interocular distance.  This requires the eyes to
>diverge, something they never needed to do in our evolutionary history.

>Also, in order to cross-fuse images without using lenses, it is necessary
>to focus at a distance that is different than the point of convergence,
>another behavior that Mother Nature never intended us to do.  This
>can be learned by most people with practice, even full-grown ADULTS
>(but maybe not cats and monkeys).  

But there are surely two separate issues here. Learning "new" eye
movements can clearly be done; indeed Roger Carpenter mentions this in
his excellent book "Movements of the Eyes" (Pion, London). And new eye
movement may allow you to exploit the connections in th visual system.
But new patterns of wiring, particularly the binocular convergence
patterns, do not develop in the adult visual cortex. However, there is
some int4eresting work being done in Frankfurt that suggests that
reintroduction of immature glial cells into an adult cortex does restore
developmental plasticity (published within the last year or two in
"Nature"). If menory serves, NMDA receptors are involved.
	I've found this an interesting thread for another reason. In the
UK strabismus surgery is very common. The objective is to retore
allignment of the eyes to allow the normal cortical wiring to develop.
But in the process, the surgery on the extra-ocular muscles frequently
disturbs the proprioceptive feedback signals the provide. This is enough
to leave deficits in spacial localisation even if stereopsis develops.
The clinical investigations are done by Steinbach in Canada. I'm
interested to see that some of the contributers to this thread talk
about "clumsiness". This could be due to disruptions to spatial visual
processing causeing mislocation of objects and even of limbs  with
respect to external objects. 
	We're about to start some human pilot experiments to invetigate
the role of EOM afferent feedback signals in the control of eye
movement. If anyone's interested let me know and I'll keep you posted.

Paul C Knox

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