NROC64H3 Lecture Notes - Lecture 2: Homonymous Hemianopsia, Dalmatian (Dog), Optic Chiasm
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NROC64 – Lecture 2
• How vision works in its bottom up processing and top down processing. If you look at
the image, what do you see? There is a Dalmatian dog. You see the butt of the dog. The
dog is sniffing on the ground. Knowing that this is a Dalmatian dog makes it easier to
perceive that it’s a Dalmatian dog. This shows that the top down processes are important
• V1 is in the cerebral cortex.
CHIASM AND LGN
• Last week we talked about how light from the visual field is projected through the lens to
an eye. Eyes are fixated towards the middle between the green and red bar. The visual
field is projected into each of the two eyes. Some people have the notion that the left eye
sees the right visual filed and the right eye sees the left visual field. This is not true
because the two visual fields overlap. This is what helps us with stereo vision. Most of
the visual fields goes into the left and right eye. The right visual field on the right eye
becomes the nasal portion of the retina. It is stimulating cells on the nasal portion of the
• The other part of retina is the temporal retina because it is closer to your temporal.
• In retina, the light info gets passed on ( the neuro signals) to the ganglion cells. The
ganglion cells have axons are very long which go all the way to the thalamus. Ganglion
cells of the nasal portion of the eye project from the optic nerve and then cross over into
the opposite part of the brain, from left eye to the right hemisphere, and nasal portion
from the right eye crosses over to the left brain. Nasal portions from the ganglia, axons
project across the optic chiasm. The crossing over only happens with the nasal portions of
the retina. The temporal potions stay on the same side, only the nasal portions cross over.
The left eye the temporal portion would be information from the right visual field that
doesn't’t cross over as color coded in the diagram, it stays at the end and information gets
sorted in terms of not through the eyes but the visual fields. The right visual field
connects to right brain processes and left visual filed connects to left brain processes.
• In the figure on the right the red area in figure shows deficits/ damage. If you have
damage to different parts of the system, you get different types of visual field deficits. It
is indicated by the circles where black means you cant see and white means you can see,
so you see light or you don’t.
• Unilateral anopia → Lesions in the optic nerve on the right side you are blind on the right
• Homonymous hemianopia → lesion comes after the optic chiasm like in the optic tract, or
thalamus or cerebral cortex, then you have visual filed defects that is the same in both
eyes aka homonymous.
• The optic chiasm can be effected because certain tumors can push down on chiasm and
makes it dysfunctional. The tumors grow where you completely blind. The deficits that
they cause first are heteronymous hemianopia → In the two eyes the visual field defect
is different. The defect is on the left side for the left eye and the right side for the right
eye. Its like partial tunnel vision. You still have right visual field in the left eye and the
left visual field in the right eye but tis only partial. When you close one eye you can see
there is a visual field defect for sure. In hemianopia the entire half field is damaged.
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• LGN is part of the thalamus.
• LGN consist of 6 layers. The layers get input from one or the other eye.
• First layer is always from the contralateral eye, if this is the left LGN then contra means
right eye. The LGN in the right hemisphere then contra means left eye. Layer 2 is the
ipsilateral eye. The first two layers make up the magnocellular layer. These layers get
input from the M ganglion cells.
• Layers 3,4,5,6 they belong to the parvocellular system. They also get info from the the
two eyes starting from the layer 3 input from the ipsilateral eye then the contralateral eye
then ipsilateral then contralateral again. These 4 layers get input from the P ganglion
cells. Each of these layers are a entire net of half of the visual field. If it’s the left LGN it
would be the right half and the LGN of the right hemisphere would be the visual field on
the left eye. But it’s the entire visual field. If you find neurons in the receptive field
anywhere in the respective visual field.
• Ganglion cells in the retina that we talked about last week are P-type ganglion cells in the
contralateral or ipsilateral eye. They project to the layers 3-6. This is the parvocellular
system. Layers 2 and 1 are the magnocellular system. Finally there is a third system that
was last to be discovered between the 6 layers there are additional layers called the
• Dots are bigger in green then orange layer. These neurons have bigger body cell lines. In
between are pink coloured layers that’s the koniocellular system that comes between each
• Magnoceullar system has to do with motion, larger receptor field. The konicellular
system is for color perception. It projects directly into the socalled CO blobs in the
• Inside the LGN the layers do not talk to one and other too much. If input from each layer
is mostly coming from one or the other eye and there is not much cross talk between the
layers this means the neurons there are mostly only for one eye.
• So in the LGN there is atleast input from three different systems. The LGN then projects
somewhere else. The magnocellular projects to the subcanteous nucleus.
• In this lecture we are talking about how LGN projects to the cerebral cortex. The first
system targeted by LGN is area V1.
• V1 is one of many visual areas in the brain.
• V1 is near the medial portion of the two hemispheres.
• Information gets from LGN to V1 and gets passed on to other areas.
• V1 the striate cortex has its name because its has prominent layers especially layer 4.
Layer 4 gets so much input form LGN the thalamus, the major target of the thalamus
which is the major input. In total, there are 6 layers.
• Layer 4 gets lots of input from the thalamus.
• Across the layers there are different types of cells.
• Pyramidal cells in layers 5,3 and 6. Pyramidla cells have large axons going into other
cells. This is the big output and gossip neurons in the brain.
• Input happens for different layers of the brain. LGN is mostly going into layer 4.
Layer 4 can be broken up into several sublayers. Layer 4 is split up into layer a, b.
and c. The layer C is broken up into lower case a and b. The Magnocellular system
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