Chapter 4 – The Visual System Beyond the Eye – the LGN
G8 Ch 4 Outline
Following the Signals from Retina to Cortex
The Visual System
Processing in the Lateral Geniculate Nucleus
Receptive Fields of LGN Neurons
Information Flow in the Lateral Geniculate Nucleus
Organization by Left and Right Eyes
Organization as a Spatial Map
Receptive Fields of Neurons in the Striate Cortex
Do Feature Detectors Play a Role in Perception?
Selective Adaptation and Feature Detectors
Grating Stimuli and the Contrast Threshold
Selective Rearing and Feature Detectors
Maps and Columns in the Striate Cortex
Maps in the Striate Cortex
Columns in the Striate Cortex
Location Columns
Orientation Columns
Ocular Dominance Columns
Hypercolumns
How is an Object Represented in the Striate Cortex
Streams: pathways for What, Where, and How
Streams for Information About What and Where
Streams for Information about What and How
The Behavior of Patient D.F.
The Behavior of People Without Brain Damage
Modularity: Structures for Faces, Places, and Bodies
Face Neurons in the Monkey’s IT Cortex
Areas for Faces, Places, and Bodies in the Human Brain
Something to Consider: How Do Neurons Become Specialized
How Neurons Can Be Shaped by Experience
Visual pathways from the retina
Looking down on the pathway from the retina . . .
Overview of the Visual Pathways
Play and do VL 4.1 here
Relationship between Retinal Images and Projections in LGN
Key concepts associated with the LGN
1. Why have separate layers in the LGN? – functional specialization.
It is believed that neurons in different layers carry different types of information.
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.
2. Retinotopic maps within each layer.
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.
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.
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 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.
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.
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.
In contrast, individual neurons in the visual cortex receive input from both eyes. So they can compare activity from the two eyes.
5. The “separation” of the retina – each LGN represents only half of the visual field.
There is probably some overlap between halves
Functions of the LGN
1. To separate neurons based on function. 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 – a neural volume control.
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
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 odulation 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.
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.
The LGN - 110/2/2018