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Lecture 11

PSL300H1 Lecture Notes - Lecture 11: Parietal Lobe, Cytochrome C Oxidase, Color Vision


Department
Physiology
Course Code
PSL300H1
Professor
Michelle French
Lecture
11

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PSL300H1F L 11; Oct 05, 2011
Optic Chiasm
Each retina ‘sees’ both visual hemifields, but
each half of brain processes only contralateral
field
therefore, half of afferents in each optic
nerve cross in optic chiasm; the rest project to
same side of brain
after chiasm, optic nerve becomes optic tract
Two eyeballs looking at visual field, pretty much
looking at entire field – which is revealed since
left field is on right side of each retina and vice
versa
So to analyzed entire right field in left
hemisphere and left field in right hemisphere –
easiest way of collecting these axons
But means all the info of the right hemifield of
the right eye has to cross to the other side and
vice versa
And all info of left hemifield in left eye must
cross to other side
Other half stays on same side
Joining together on one side all info of
contralateral hemifield
About half of axons crossing
Blind spot at medial part of retina, fovea away
from blind spot
Blind spot near bottom outer corner of eyeballs
in the pic
Always one eye covering the blind spot
1st place in brain where info collected: Lateral
geniculate nucleus in thalamus – layered
structure, sticks out like a bump in back of
thalamus, some of the axons also terminate in
the SC at top of midbrain – rapidly-adapting (M
pathway)
Visual nuclei
Afferents in optic tract have many targets:
1) lateral geniculate nucleus (thalamus)
2) superior colliculus; M stream only (rapidly
adapting info, important for orienting eyes
and head towards stimuli, so motor structure)
3) pretectal nuclei: more autonomic functions
like pupillary light reflex, (right in front of
SC/tectum in midbrain), and accessory optic
system for motion detection (a lot of M
stream input)
2&3 side by simple, very simple system
for detecting motion
4) suprachiasmatic nucleus(in hypothalamus,
essential, master clock): circadian cycle –
govern 24/7 cycles, day/night, input from
optic tract
LGN@back of thalamus, SCN right on top of optic
chiasm, SC&pretectal also receive branches from
axons going to the LGN
Primary visual cortex V1 – 1st station in cerebral
cortex receiving visual info from the thalamus,
right at back of occipital lobe of the brain (as far
as possible from eyeballs)
Lateral geniculate nucleus (LGN)
6 layers: 2 for M stream, 4 for P stream (slowly
adapting)
Similar to cortex but dif organizations
alternate layers receive afferents/input from the
dif retina (the two eyes)
so LGN not attempting to fuse the info, just
collecting the info and trying to organize in a
methodical manner
Retinotopic manner so receptive field of
neurons in all the layers lying along the radial
axis are the same
Note bigger r.f.’s than ganglion cells in retina,
due to convergence since more synapses gone
thru; same centre organization (light in centre
and dark in surround or vice versa)

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3 layers for input from same side
(white,ipsilateral) and 3 layers for opposite side
(grey, contralateral)
Point is dividing up layers btwn the 2 eyes
First 2 layers for magnocellular input
Last 4 for parvocellular input
But wonder why need 2 sets for p? Colour-related
info rather than just black/white!
If take radial axis thru all layers, the neurons in
each layer along it will respond to same spot in
contrlateral visual field
Ex. Right LGN mapping left half of visual
world – all nerons along radial axis respond
to same place in visual field
Organization - Retinotopic mapping, each
neuron has place in layer, lined up across the
layers
Same retinotopic mapping in each layer –
makes easier to gather the info to send to
the same place in the cortex to V1 since all
from same point in visual field
LGN neuron receptive field
‘OFF’ response strongest when dark stimulus
over a discrete set of cones
(retina)
Primary visual cortex (V1)
LGN organizes inputs to visual cortex
V1 lies along calcarine sulcus in medial
wall of occipital lobe
retinotopic organization – maintained, starts in
retina
largest area devoted to fovea and macula, where
photoreceptors are most numerous – also true in
LGN
like in S1 w magnification of areas of fingers
w more sensitivity and mechano receptors
Photoreceptors squished into fovea blown
up out of proportion in V1, since most
receptors in fovea and surrounding macula
periphery (largest surface of retina) not getting
proportional share of V1
Calcarine (curving) sulcus in medial wall of
occipital lobe
V1 line the banks at the bottom of the calcarine
sulcus, extends to back of occipital pole (comes
out at gyrus at back), most in calcarine sulcus
Fovea, central 3 degrees (actually 1.5 since
hemifield) of physical visual field
blown up disproportionately in v1 – over a
third of V1 devoted to this central portion of
visual field
macula has 6.5 degrees per hemifield (13
degrees total)
Mapping upside side like in eyeball, upper field in
lower bank of calcarine sulcus and vice versa
Note: Periphery of retina (mainly rod input to
brain) is way over at end of calcarine sulcus
proportionally biggest surface area of retina
but much smaller representation in terms of
area in v1 – so mapping population of
receptors
Input Layers of V1
6 layers
Layer 4, thalamic input layer, is very wide and
complex in V1
V1 most complicated piece of cortex in brain
because many submodalities and
characteristics extracted about stimuli in
retini and all need to be dealt with w dif pops
of neurons
Many sublayers of layer 4 in V1 so very
complicated and twice density of neurons as
anywhere else in the brain, so the cortex is
quite thick
Each subclass of input has its own target
within layer 4
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