Organisation of the
Human Visual System
Higher levels
V5
Velocities fed back to V1
Motion processing
V3
Processing dynamic form
Orientation selective
M pathway
V4
Project to temporal visual cortex
Colour distinction
Both M and P pathways
V2
Increased sensitivity to retinal disparity
Input from V1
The Primary Visual Cortex
Receptive fields are specialised
Chromiance
Luminance
Complex neurons
Nonlinear
Position is unimportant
Orientation and size selectivity
Simple neurons
Receptive field
End stopped cells
Width
Orientation
Length
2 elongated antagonistic regions
Antisymmetric
Central elongate region
with antagonistic regions
Symmetric
Layers 4 & 6 (LGN input)
Projection to other layers
M-pathway
P-pathway
P-I (lines, bars, edges)
P-B (colour or brightness)
LGN input
Layer 6
Layer 4
Sublayer 4C has receptive fields similar to retinal ganglion
axons from LGN project to this layer
Selective towards direction of movement
Orientation selective
Sublayers 4A, 4B, 4C alpha, 4C beta
Striated (6 layers & several sublayers)
The Lateral Geniculate Nucleus
Modulation
Spinal cord
Spatial attention
Saccadic eye motion
Feedback from V1
Attentional mechanisms
May mediate response to certain stimuli
Responses are similar to the retinal ganglion cells
6 Layers
Parvocellular (P)pathway
Recieves input from P cells
Layers 3,4,5,6
Magnocellular (M) layers
Receives input from the M cells
Larger cells
Layers 1 & 2
Spatial topology is preserved
Each layer receives
input form one eye
Interlaced to combine inputs
Depth perception
2,3,5: ipsilateral
1,4,6: contralateral
The Eye
Amacrine Cells
Inner synaptic layer
Lateral connections between retinal ganglion cells
Bipolar cells
Define receptive field
Their effect based on position
Which photoreceptors
Connect a number of photoreceptors
to one retinal ganglion cell
Retinal ganglion cells
Form the optic nerve
Pathway
P
Midget cells
Respond to high spatial frequencies
High wavelength selectivity
M
Parasol cells
Wavelength insensitive
Sensitive to low spatial frequencies
Isotropic receptive field
Centre-surround organisation
On centre receptive field
Off centre receptive field
Horizontal cells
Modulates response sent to bipolar cells
Outer synaptic layer
Lateral connections between photoreceptors
Photoreceptors
Brightness adaptation
Sets bandwidth for perceptual quality
High dynamic range (10^10)
Densely packed at the fovea
Cones
Color vision
Photopic vision
Rods
Scotopic vision
Temporal response
Used to infer velocity