Visual pathways from the retina Not in G9
Retina
Looking down on the pathway from the retina . . .
Nose
Cortex
The LGN - 1
3/10/2016
Overview of the Visual Pathways
Play and do VL 4.1 here
The LGN - 2
3/10/2016
Optic nerve
Optic tract
This figure means that everything you see
to the right of your nose is processed in
your left hemisphere – whether it came
from your left eye or your right eye.
Optic
radiation
The LGN - 3
3/10/2016
Functions of the LGN
1. To group neurons performing the same function from the two eyes together. It’s probably the case
that each layer is a subsystem, devoted to processing only one kind of visual information. This separation
by function occurs first in the LGN.
This may be analogous to doing a jig saw puzzle, in which you first gather all the blue pieces together, then
all the green pieces, all the pieces that are obviously edges, etc.
It’s also analogous to splitting the functions of a business – creating a sales division, an engineering
division, an advertising division. People with the same function are put together so they can communicate
things that a common to them more easily than if they were distributed throughout the company.
2. To provide synapses for modulation of visual input by signals from the Reticular Activating
System and higher level brain centers – a neural volume control.
For example, the LGN cells also receive input from the reticular activation system (RAS). The RAS
becomes active during startle situations. It is quiescent during sleep and during drowsiness.
Here’s a possible circuit
LGN
Area VI
Retina
RAS activation
RAS inhibition
When activated by the RAS, visual information will be more likely to be passed to the visual projection
Area I of the cortex.
When inhibited by the RAS (during quiescence) visual information will be less likely to be passed to higher
levels.
Note the need for synapses for the inhibition that provides modulation.
3. To provide synapses modulation of visual input by the higher cortical area – a neural remote control.
“Dude – only look for red round things.”
“Keep focused on the snake in the center, but keep on the lookout for snakes over on the right.”
Higher order neurons in various parts of the cortex send axons to the LGN. These may serve the same kind
of modulating function that the cells from the RAS serve – to change the likelihood of certain kinds of
visual information getting passed on to the visual cortex – to modulate the level of visual input.
LGN as Memphis
A synapse in the central nervous system is kind of like a city – a place to “mix is up”. Of what value is
Memphis? It’s a gateway – a place for persons from different parts of the Southeast to come together.
Every synapse serves the same function. Note that many synapses connect 100s, perhaps 1000s of neurons
– just as a city like Memphis connects multiple different roads and highways.
The LGN - 4
3/10/2016
Consequences of damage to various parts of the visual pathway.
Location of damage is in the lettered rectangles in the figure. Consequences are on the right.
Red represents what is visible.
Shaded areas are not visible.
Visual field
Both the left and right
visual fields are visible,
but only through the right
eye.
Only the nasal part of the
visual field is visible - the
right visual field through
the left eye and the left
visual field through the
right eye.
Retina
The left visual field is
visible through both eyes.
The right visual field is
not visible.
Optic Nerve
D. Fahgettaboutit
Optic Chiasm
The left visual field is
visible through both eyes.
The right visual field is
not visible.
Optic Tract
Succinct descriptions.
A. Loss of all vision from the
left eye.
B. Loss of peripheral vision.
Optic Radiation
C. Loss of ability to see the
right side of the visual field.
D.
E. Loss of ability to see the
right side of the visual field.
The LGN - 5
3/10/2016
Key concepts associated with the LGN
Skipped
in Spring
2016
1. There are 6 separate
layers
in the LGN.
Question: Why 6?
Layers 1 & 2 probably carry information about movement, location.
Layers 3, 4, 5, & 6 probably carry information about form and color.
Buy why 4 layers for form and color. My guess is that there’s a finer “breakdown” of the layers into 3&4
and 5&6. It is not yet known precisely what the differences between these two pairs are.
Why group them? Think of any large organization. It’s usually more efficient to put the people doing the
same type of work together – so they can communicate with each other more efficiently.
This is called functional specialization.
2. The neurons within each layer form Retinotopic Maps.
The position of activation within each layers is in 1-to-1 correspondence with position of activation in the
retina. The locations of neurons in the LGN form a map of the retina.
C
B
A
Scene in right side of visual scene
Lens+Cornea
A B
C
Activity in left half of retina
A
B
C
Activity over all of left LGN
Note that since each LGN is responsible for only ½ of the visual field, point C, which is near the center of
the left part of the retina, is at the “end” of the LGN layer.
Why a retinotopic map? Why not? What would be gained by having a scrambled representation? Nothing.
As we’ll see, responses of neurons “seeing” adjacent stimuli often need to be combined. It’s easier to
combine those response if the neurons are adjacent to each other.
The LGN - 6
3/10/2016
3. Registration of layers – the individual layer maps are point-for-point one-on-top-of-the-other.
The retinotopic map within each layer lies directly on top (or underneath) the map on adjacent layers. The
Skipped in Spring 2016
maps are in registration with each other, like separate transparent overlays of the same scene. Example
showing layers 3 and 4 of the left LGN.
C
A
A
B
A
B
B
Left
Retinal
activity
C
C
C
B
Right
Retinal
activity
A
C
A
B
Left LGN
Layer 3
Left LGN
Layer 4
Note that layer 3 receives input from the left eye and layer 4 receives input from the right eye.
Registration refers to the fact that the projections of activity in layers 3 and 4 are at the same place in their
respective layers, even though the stimulation is from different eyes.
That is, the activity generated by stimulus A is at the same end of both LGN layers. The activity generated
by stimulus B is right next to A in to layers. The activity generated by stimulus C is at the opposite end in
both layers. The layers are in registration with each other.
Registration carries across all layers. This means, for example, that the information about movement and
location of an object, presumably in layers 1 and 2 is in close physical proximity in the LGN to information
about form and color of the same object in layers 3-6.
Why is registration important? Ever tried to conduct business long distance? It’s much easier to coordinate
activities of different functional units of a business if they’re in close physical proximity to each other.
The LGN - 7
3/10/2016
4. Monocularity.
Each LGN cell receives input from only one eye. So no single cell in the LGN can compare input from the
two eyes.
Skipped in Srping 2016
In contrast, individual neurons in the visual cortex receive input from both eyes. So they can compare
activity from the two eyes.
5. The “splitting” of the retina – each LGN represents only half of the visual field.
The LGN - 8
3/10/2016