So what we have, then, is an inversion of the representation of the world in the
brain, such that the opposite side of the visual field is represented in one
hemisphere, and the upper bank of the calcarine sulcus is representing the
contralateral inferior quadrant of space. Whereas the, lower bank of calcarine
sulcus is representing the contra lateral, upper field.
So with that background in mind let's turn our attention now to the analysis of
injury or disease that afflicts some local region along this visual pathway.
And before we actually look at representation of such injury, I want to
get some terms out in front of you that will be useful.
as we describe these lesions, and, these terms, seem a li-, a little bit
burdensome, but actually they're quite descriptive.
And I think if you can understand the meaning of the terms, then their proper
clinical use will quickly become second nature.
So let's begin with the term Anopsia. Anopsia refers to a large area of
blindness in one eye. So hemianopsia means blindness in one
hemifield, that is half field that is seen just by one eye.
Quadrantanopsia means blindness in one quadrant within a hemifield that's seen
by an eye. Now we modify these words with a term
homonymous and a complimentary term heteronomous.
So homonymous means same side so this refers to the same visual hemifield that
is seen in each eye. For example, one can have a lesion in the
central visual pathway that produces blindness on the left side of the visual
field for one eye and the left side, or the same side, of the visual field for
the other eye. That's an example of a homonymous
hemianopsia. So if the regions of blindness are not on
the same side of the mid line for the views of each eye, then we call that a
Heteronymous hemianopsia. So Heteronymous means different side,
this refers to opposite vision hemifields being impacted in the field of view of
each eye. And then, finally, the word, scotoma
refers to a much smaller lesion. a small area of blindness somewhere
within a visual field. The blind spot that we each bear in our
visual hemispheres. Due to the fact that the optic nerve head
has no photoreceptors overlying it. Is one example of a natural[SOUND]
scotoma. But, of course, scotomas can arise
because of focal[SOUND] injury to their visual pathways.
Or to the cortex itself. [SOUND] Now here's an important figure
that I want you to spend some time studying.
This figure relates a series of locations[SOUND] along the visual
pathways. That potentially could be the location of
injury or disease to a reprensentaion of the visaul fields of each eye.
And the convention in this representaion is to use black shading to indicate
blindness. So let's walk through each of these.
locations and visual field deficits in turn.
First of all, let's orient ourselves to the brain.
We are looking down through the brain from above.
So notice that we have the right eye on the right side of this illustration.
And the left eye on the left side of the illustration.
That seems quite fundamental. But it will be critical when you're
studying and you're learning to be certain that you know which side of the
brain you're considering. So in this figure, right is right and
left is left. OK, so let's consider what happens if
there were to be an injury. In the right eye or the right optic
nerve. Well that should be pretty
straightforward. The right eye would be blind so this
would be simply blindness in the right eye.
Now, let's imagine that we had a lesion. Not in the optic nerve, but right in the
middle of the optic chiasm. this can happen sometimes when there is a
traumatic injury that induces a sharing force along the midline of the
hemispheres. Or perhaps with a tumor that's growing
out of the pituitary gland. Gland, which attaches right below the
optic chiasm. Well, a lesion of the optic chiasm, would
cut the axons of the nasal retina that cross in the midline.
And that results in blindness, in the peripheral parts of our visual world.
Specifically. The line is in what the nasal retina's C
and the nasal retina of the left eye sees the lateral or the temporal part of the
visual world. The nasal portion of the right eye sees
it's lateral retina. We have preservation of central visual
space but blindness to the left and right sides of our visual fields.
And that's an example of bitemporal hemianopsia, or specifically a
heteronymous hemianopsia. So, let's see how we use those terms.
Hemianopsia means half visual field blindness for what an eye sees.
And then the further modifiers talk about whether it's concordant for the field of
view of each eye. And in this case, it's not.
It's the left visual field that is lost for the view of the left eye.
And the right visual field for the view of the right eye.
That's a bilateral, or bitemporal heteronymous hemianopsia.
Now let's consider the remaining lesions here and let's continue to work our way
back. So, lesion C is a lesion of the right
optic tract. So, at this location we have sorted out
the. Principal of contralateral representation
such that in the right optic tract, we have the axons from the two eyes that see
the left side of the midline. So the image to the right optic tract
produces blindness in the left side of the visual fields that are seen by each
eye. So here is also a hemianopsia but now its
a homonymous hemianopsia. Specifically a left sided homonymous
hemianopsia. The left side of mid-line is lost for the
field of view of each eye. The same would essentially be the case if
we jump all the way back now, and look at what happens with damage to the right
occipital cortex. If we imagined that this was a widespread
lesion in the right occipital cortex, we would likewise produce damage to the
field of view that's seen on the left. Now notice there's a little bit of a
carved region of visual preservation just around the central part of the visual
field there on the left side of the midline.
This is called macular sparing. And there are a couple of ideas as to why
this comes about. One idea is simply the fact that we have
such a large representation of the macular region in the visual cortex.
A brain injury following, let's say, a stroke or a trauma very well may damage a
wide region of the visual cortex, but potentially there is preservation of some
small region that very well may be representing the macula and what it sees.
Just because this is the part of the visual field that is given the most
cortical circuitry. Well should that be the case then we
might expect there to be some sparing of that region of the visual field.