And so if one looks at an animal soon after it has open its eyes, one can
indeed find that their is a system of orientation columns that are organized
into a map of orientation preference. So, this has been known since really, the
early 90s, when it was first possible to apply these methods to the developing
visual cortex. And, what was discovered is that these
columns do indeed self-organize. They appear pretty early in postnatal
life, and those animals that are born with their eyes closed, they seem to
emerge prior to the time of eye opening but they rapidly develop in the days to
weeks that follow the onset of normal visual experience.
So we thought an obvious experiment to test whether vision had an important role
to play in the maturation of this system, was to raise animals in total darkness.
And, thus removing visual experience from the life history of the animals.
And when we did that experiment, what we found was that sure enough.
The circuits for orientation selectivity and preference do self-organize.
It's possible to recognize. It's possible to recognize columns and
the response maps, and in the selectivity maps that we generate from these animals.
And the signals are sufficiently robust. That we can assign color values and
create an orientation preference map. With the proper analysis applied to these
maps, indeed, these maps from animals that never saw any photons of light until
the experiment, do indeed self organize and develop.
Pinwheels with a density of Pi. However, I think, perhaps, even your eye
can pick this out. the contrast of the images that we
recorded from these animals that lacked vision seemed to be sig, significantly
less than what we observed from animals that had normal vision.
Throughout the period of brain maturation that we study.
So, while these cortical networks do indeed self-organize without the benefit
of vision, vision does seem to contribute to the refinement of these maps, and an
increase in the selective responses. Of the neurons, whose signals we're
recording with this method. Well, just because these circuits
self-organize, and are present in animals that have never experienced vision does
not mean that this process. Cannot be influenced by experiment.
So we thought it was important to do a complementary experiment.
Rather than just taking away vision we thought we should make vision abnormal.
And a fairly straightforward way to do that is to raise animals keeping their
eyelids shut. And that's very much like what your eye
might do if we were to close our eyes and then look around.
The visual scene, if I look up towards the florecent lights in the ceiling I can
tell that I'm looking up. And seeing an incease in illumination
compared to if I were to look away. But I see no form.
I see no structure. I see no shape to what.
Eye, eye view, simply an increase or a decrement in light intensity.
So this is the nature of visual experience that we imposed on the set of
animals, and when we did this, what we discovered is that there were some pretty
significant impacts on the development of the circuits that compute orientation
preference. Now this manipulation did not produce
cortical blindness. And we know that because when we looked
at the response maps in the visual cortex, we saw robust activation of the
visual cortex. The problem however, is that we failed to
see the robust development of columnar structure It's as if the very same
circuits in the brain responded to the presentation of a horizontal stimulus and
the presentation of a vertical stimulus. So when these images were subtracted,
what we found was virtually no evidence of columnar structure that was organized
into a map of orientation preference. So, I want to notice the implication of,
this finding here. What this shows us is that when we made
vision abnormal, what we found was actually more significant imparement of
the visual cortex then when we simply deprived the animal of vision.
Now, this reminds me, at least, of a famous, aphorism.
That was suggested by, one of our, seminal neurosurgeons of the 20th
century. Dr.
Wilder Penfield. Who said that no brain is better than bad
brain. Well, that justified some neurosurgical
procedures aimed at the removal of bad brain.
In this case, I would suggest that bad experience is worse.
More detrimental to the development of visual cortical circuits, than is no
experience. So what have we learned from these
studies? I think we can make some provisional
conclusions based on these experiments looking at the development of orientation
preference.`` I think what we've learned is that normally circuits in the visual
cortex self-organize. And they operate synergistically with
normal sensorimotor experience. And this synergy promotes the full
maturation of these circuits. However, when experience is rendered to
be abnormal, then this synergy is broken. And self-organization goes awry.
[INAUDIBLE]. The neural circuits that develop as a
consequence are functionally impaired. Now, these neural circuits, they
self-organize to adapt to the quality of the incoming sensory signals.
So, by self-organization going awry, I mean relative to the synergy that our
brain has evolved to anticipate From the world in which we live.
But when that synergy is aggregated by some kind of ocular impediment, in our
case, keeping the eyelids closed or perhaps in clinical populations
congenital cataract, for example. would be one human condition that we
would model through this method of keeping the eyelids shut.
well, such conditions. Keeping the eyelids shut or congenital
cataract. Are those that, would provide for the
adaptive influence over the circuity of the visual cortex that would then
develop. So consequently, not only do these neural
circuits under these conditions fail to benefit from normal visual experience.
And as a result, they're actually, developing along a trajectory that
instantiates functional impairment. So now I'd like to turn our attention to
the property of direction selectivity and direction preference.
So we can think of this as a further differentiation of the circuitry for
orientation selectivity and orientation preference Where those circuits begin to
specialize for the representation of motion in just one direction or other,
with the preferred direction of motion being orthogonal to the preferred
orientation. That is to say, circuits might develop a
preference, let's say, for vertical stimulus.