smyth at zorro
Wed Aug 16 04:48:46 EST 1995
Stephen Maidment writes:
> The problem is that the visual pathways have to have been
> active from birth in order to establish the correct connections
> and disconnect unwanted ones. This process is usually complete
> by around the age of 5 years, after which no further development
> can take place. Any such procedure, to be successful would have
> to be performed on an infant and would be unlikely to succeed in an
> eight year old. Even if she can see with her existing eye, this
> will transmit visual information predominantly to one side of the
> brain only. The other side of the brain, even if served by an
> intact optic nerve will have never received any input from this
> nerve and will therefore have been unable to develop into functional
> visual cortex.
First I must also state that I am not a neurosurgeon and so I can
only make suggestions from some basic knowledge. Three points:
1) I heard of some patient that had a cataract removed the eyes
after years of blindness, and then had serious problems with
sight because the visual system was so unused to getting real
stimuli. The patient initially found it more confusing than
anything else to try and do any sort of object recognition with
his eyes. He tended to revert to touch for much better results.
Does anyone know of this case and how has the patient developed
since then? Has his visual system reorganized in any way that may
suggest adaptation is possible in the adult? I don't know if he
was blind from birth or had some initial critical period to allow
2) Following from that, there are some results from experiments on
*adult* cats that suggest that visual cortex (V1, I think) can
reorganize when damaged. In particular, Das & Gilbert (1995) seem
to show that after a retinal lesion, those cells that get no new
stimuli in their receptive fields (RFs) seem to shift their RFs
to outside the area of the retinal lesion. But of importance is
that they preserve the line orientation they used to respond to.
My interpretation of their results was that the horizontal projections
between columns in neighbouring hypercolumns that respond to similar
orientations were guiding the development of adapting RFs. If thats
the case then maybe such connections could guide the organization
of the visual cortex responding to the *new* eye referred to above.
Just a point to Steve Maidment, I think a particular side of
the brain does not perdominantly respond to a single eye, rather
to the opposite side of the visual field: which will be inputs from
both eyes. So the cortex should be organized on both sides but only
responding to one eye.
3) Much work has been done on modelling the development of
orientation columns, ocular dominance and the retinotopic map.
These models generally take the self-organization paradigm of
``winner-take-all'' as in Kohonen learning. But I actually don't
know of any biologically plausible and implementible model for
this self-organization process (any takers?). I have no references
for this so I apologise, but my understanding now is that we are finding
more and more information in the genome for controlling such things
like these chemical markers for guiding the development of the
retinotopic map. I think it may be likely that in 1,5 or 10 yrs time
we may find the genes that specify the hypercolumn/column/ocular
layout of V1/V2. These may not necessarily be completely activity
derived. They do respond to activity of course and adapt to it, but
maybe the necessary underlying architecture is already coded in the
genes. This may offer some hope for reorganization after damage or
after surgical intervention to repair damaged eyes etc. Any more
exact views from people in genetics on this one?
Das,A. & Gilbert,C. (1995) ``Long-range horizontal connections and
their role in cortical reorganization revealed by optical
recording of cat primary visual cortex.'' Nature 375:780-4.
Darragh Smyth smyth at informatik.uni-leipzig.de
Institut fur Informatik, http://www.informatik.uni-leipzig.de/~smyth/
Universitat Leipzig, voice : +49-341-97-32242
Pf. 920, Leipzig 04009, fax : +49-341-97-32209
``Each of us is sometimes a cretin, a fool, a moron or a lunatic.
A normal person is just a reasonable mix of the these components.''
(Umberto Eco, Foucault's Pendulum)
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