Re: stereovision question

[P3D Josh Rubin <jnr@xxxxxxxxx>]

Jim Crowell wrote:  

>   here's something I yanked from the archive--someone's question & my reply.
>   
>   >The other interesting part of the tape involved the development
>   >in infants of stereo vision. Julesz said that there is a small window
>   >(about 5 or 6 weeks long) somewhere around 6 months of age (I can't
>   >remember the details) in which an infant's brain learns stereo vision.
>   >If the infant doesn't have this experience (such as if it has severe
>   >cross-eyedness during this time period), he/she will always be stereo-
>   >blind for the rest of their lives. 
>   
>   In babies, these neurons take inputs from a
>   range of locations in both eyes; there is a sensitive period in which
>   they are tuned by correlated patterns of inputs in the two eyes. This
>   requires that the input regions for a given cell in the two eyes point
>   to a common region of space. If the baby's cross-eyed enough, this
>   doesn't happen, & the neurons end up taking input from one eye only.
>   The connections from the other eye are lost, so correcting the
>   cross-eyedness later doesn't help.

Amazing.  This reminds me of some studies of, I think, decerebrated
cats, in which (as I recall) it was shown that if certain brain 
structures (were they visual pathways?) did not receive normal
stimulation during a crucial developmental phase, they would
actually physically degenerate permanently.  You could readily see 
this from histological photographic studies.   

I would guess that the flip side is that someone born monocular
would perform better than the binocular masses on tests of
monocular depth perception.  If so, what would account for 
the difference?  I guess that the commonsense answer is "practice."
But what these studies seem to show is that "practice" has
an anatomical or physical dimension.  Could the brain
adapt to use the stereo processing algorithms to
enhance depth perception from motion parallax cues?  

Somehow, the Pulfrich effect and the fact that you can
experience a compelling sense of "stereoscopic" depth from
an appropriate monocular display imply to me that
at least some material part of the stereoscopic wetware
can be conscripted to enhance depth perception from monocular 
cues.  I must admit that I haven't thought it out.  How much
of stereo processing or motion parallax processing is
cortical?  Does (can) the visual 
cortex adapt like the language centers do, in which the
right hemisphere will assume the tasks of the damaged left
hemisphere if the damage is early?  Is visual processing
power at all fungible?  There's probably some MRI research
that could answer this.

Josh
w
Joshua N. Rubin (jnr@xxxxxxxxx) 


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