2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002
1
The Brain
from retina to extrastriate cortex
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 2
Neural processing responsible for vision• photoreceptors• retina
– bipolar and horizontal cells– ganglion cells (optic nerve)
• optic nerves• optic chiasma (X)• lateral geniculate body• striate cortex• extrastriate cortex
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 3
Photoreceptors
Ganglion cells
Light
Lateral inhibition• Edge detection and contrast enhancement• Bipolar, Horizontal and Ganglion cells
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 4
1000 0
100
Lateral inhibition• If no activity in neighboring photoreceptors,
output = output of photoreceptor
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 5
100100 100
0
Lateral inhibition• if activity in neighboring photoreceptors,
– output is decreased, possibly absent
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 6
100100 100
0
(-.5) (-.5)
-50 -50
+
(1.0)
100
200200 200
0
(-.5) (-.5)
-100 -100
+
(1.0)
200
Lateral inhibition via addition and negative weights
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 7
2000 200
100
(-.5) (-.5)
0 -100
+
(1.0)
200
Another example
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 8
100100 100
0
(-.5) (-.5)
-50 -50
+
(1.0)
100
100100 100
?
(-.1) (-.1)
? ?
+
(1.0)
?
Different kinds of ganglion cells == different sets of weights
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 9
cornea
crystallinelens
retina: photoreceptors = rods + cones
opticnerve
Optic nerve• axons of the ganglion cells
– 1 million optic nerves– 120 million photoreceptors
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 11
From light to vision
Lateral Geniculate Nucleus (LGN)
StriateCortexGeniculo-Striate Pathway
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 12
(LGN)
StriateCortex
Striate cortex(primary visual centre)• Neurons are edge detectors
fires when an edge of a particular orientation is present
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 13
(LGN)
StriateCortex
Striate cortex(primary visual centre)• Neurons are edge detectors
fires when an edge of a particular orientation is present
frequent output
vertical bar
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 14
(LGN)
StriateCortex
Striate cortex(primary visual centre)• Neurons are edge detectors
fires when an edge of a particular orientation is present
infrequent output
diagonal bar
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 15
Edge detection• each cell “tuned” to particular orientation
– vertical– horizontal– diagonal
• cats: only horizontal and vertical• humans: 10 degree steps• edges at particular orientations and
positions
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 16
Extrastriate cortex(beyond the striate cortex)
V1
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 17
Extrastriate cortex• Each area handles separate aspect of
visual analysis– “V1-V2 complex”: Map for edges– V3: Map for form and local movement– V4: Map for colour– V5: Map for global motion
• Each is a visual module– connects to other areas– operates largely independently
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 18
Finally• What was the purpose of this
presentation?
• Which question remains unanswered?
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002
19
Slides after this point review neuron function and vocabulary. We did not cover them in lecture, but you may find them useful.
Gregory covers this pp 68-74Coren, Ward, Enns in an appendix
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 20
The brain - processor of information
Eyes are actually a part of the brain
Ganglion cells are a special type of neuron:
Output
Ganglion cell
Inputs from photoreceptors
Output (to other neurons)
Neural cell (neuron)
Inputs from other neurons
Neural cell (neuron)
Inputs from other neurons
Neural cell (neuron)
Inputs from other neurons
D e n d r i t e s
Neural cell (neuron)
Inputs from other neurons
D e n d r i t e s
Cell body
Neural cell (neuron)
Inputs from other neurons
D e n d r i t e s
Cell body
A x o n
Output (to other neurons)
Neural cell (neuron)
Inputs from other neurons
D e n d r i t e s
Cell body
A x o n
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 21
The brain - processor of information
Eyes are actually a part of the brain
Ganglion cells are a special type of neuron:
Output
Ganglion cell
Inputs from photoreceptors
Output (to other neurons)
Neural cell (neuron)
Inputs from other neurons
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 22
The brain - processor of information
Eyes are actually a part of the brain
Ganglion cells are a special type of neuron:
Output
Ganglion cell
Inputs from photoreceptors
Output (to other neurons)
Neural cell (neuron)
Inputs from other neurons
Neural cell (neuron)
Inputs from other neurons
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 23
The brain - processor of information
Eyes are actually a part of the brain
Ganglion cells are a special type of neuron:
Output
Ganglion cell
Inputs from photoreceptors
Output (to other neurons)
Neural cell (neuron)
Inputs from other neurons
Neural cell (neuron)
Inputs from other neurons
Neural cell (neuron)
Inputs from other neurons
D e n d r i t e s
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 24
The brain - processor of information
Eyes are actually a part of the brain
Ganglion cells are a special type of neuron:
Output
Ganglion cell
Inputs from photoreceptors
Output (to other neurons)
Neural cell (neuron)
Inputs from other neurons
Neural cell (neuron)
Inputs from other neurons
Neural cell (neuron)
Inputs from other neurons
D e n d r i t e s
Neural cell (neuron)
Inputs from other neurons
D e n d r i t e s
Cell body
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 25
The brain - processor of information
Eyes are actually a part of the brain
Ganglion cells are a special type of neuron:
Output
Ganglion cell
Inputs from photoreceptors
Output (to other neurons)
Neural cell (neuron)
Inputs from other neurons
Neural cell (neuron)
Inputs from other neurons
Neural cell (neuron)
Inputs from other neurons
D e n d r i t e s
Neural cell (neuron)
Inputs from other neurons
D e n d r i t e s
Cell body
Neural cell (neuron)
Inputs from other neurons
D e n d r i t e s
Cell body
A x o n
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 26
The brain - processor of information
Eyes are actually a part of the brain
Ganglion cells are a special type of neuron:
Output
Ganglion cell
Inputs from photoreceptors
Output (to other neurons)
Neural cell (neuron)
Inputs from other neurons
Neural cell (neuron)
Inputs from other neurons
Neural cell (neuron)
Inputs from other neurons
D e n d r i t e s
Neural cell (neuron)
Inputs from other neurons
D e n d r i t e s
Cell body
Neural cell (neuron)
Inputs from other neurons
D e n d r i t e s
Cell body
A x o n
Output (to other neurons)
Neural cell (neuron)
Inputs from other neurons
D e n d r i t e s
Cell body
A x o n
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 27
Input signals on dendrites affect voltage on cell body
-action potentials (spikes) sent along axon, towards terminals
-speed ≈1 m/sec (narrow) to 100 m/sec (wide)
axon axonterminals
axon axonterminals
axon axonterminals
-when voltage change is sufficiently high,
cell begins to fire
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 28
Input signals on dendrites affect voltage on cell body
-action potentials (spikes) sent along axon, towards terminals
-speed ≈1 m/sec (narrow) to 100 m/sec (wide)
axon axonterminals
-when voltage change is sufficiently high, cell begins to fire
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 29
Input signals on dendrites affect voltage on cell body
-action potentials (spikes) sent along axon, towards terminals
-speed ≈1 m/sec (narrow) to 100 m/sec (wide)
axon axonterminals
axon axonterminals
-when voltage change is sufficiently high, cell begins to fire
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 30
Input signals on dendrites affect voltage on cell body
-action potentials (spikes) sent along axon, towards terminals
-speed ≈1 m/sec (narrow) to 100 m/sec (wide)
axon axonterminals
axon axonterminals
axon axonterminals
-when voltage change is sufficiently high, cell begins to fire
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 31
Output strength represented by rate of firing along axon of neuron
axon axonterminals
Typical rates: 200-1000 spikes per second
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 32
Output strength represented by rate of firing along axon of neuron
axon axonterminals
Typical rates: 200-1000 spikes per second
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 33
Output strength represented by rate of firing along axon of neuron
axon axonterminals
axonspikes travel down axon
axonterminals
Typical rates: 200-1000 spikes per second
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 34
Output strength represented by rate of firing along axon of neuron
axon axonterminals
axonspikes travel down axon
axonterminals
axonspikes travel down axon
axonterminals
low output strength = low frequency of spikes
Typical rates: 200-1000 spikes per second
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 35
Output strength represented by rate of firing along axon of neuron
axon axonterminals
axonspikes travel down axon
axonterminals
axonspikes travel down axon
axonterminals
low output strength = low frequency of spikes
axonspikes travel down axon
axonterminals
low output strength = low frequency of spikes
axon axonterminals
high output strength = high frequency of spikes
Typical rates: 200-1000 spikes per second
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 36
Information processed by neurons activating each other in sequence
-output of one neuron = input of next-connection = synapse
connections(s y n a p s e s )
Activity in one cell increases activity in connected cell -> excitation
2002/01/21 PSCY202-005, Term 2, Copyright Jason Harrison, 2002 37
But excitation is not the only way that neurons interact…
Activity in one cell decreases activity in connected cell -> inhibition
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