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