2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison

1

The eyesreceivers of information

Shaping and transforming light into sensory perceptions for nearly 600 million years.

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 2

Why are we talking about eyes today?

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 3

Approaches to perception and cognition• Philosophy (BC 600 - early 1800s)• Early Psychology (early 1800s - early 1900s)• “Classic” Psychology (early 1990s - 1950s)• Modern Psychology (1950s - )

– information in light -> ecological optics– information used to choose best hypothesis

-> cognitive psychology (symbolic manipulation)– information coded, recoded and decoded

(rewoven)-> eg, intelligent eye

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 4

Light - carrier of information1. Travels in straight lines

• Assumption: optical density unchanging

2. Reflects off of surfaces (i = r)

3. Refracts when travelling into new medium

• ni sin (i) = nr sin (r)

4. Has various frequencies (colours)5. Has various amplitudes (intensities)

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 5

Refraction review

n1n

2

n1<n2

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 6

Refraction quizA

BC

1

2 3

n1n

2

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 8

The eye - receiver of information

sun observer

• Light sources: sun, light bulbs, candles, moon• Light reflects off of objects in environment

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 9

The eye - receiver of information

observersun observer

• Light sources: sun, light bulbs, candles, moon• Light reflects off of objects in environment

objects effectively become light sources

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 10

Implications for vision• Does not matter if an object is a source or

a reflector of light• Strength of reflection is a function of:

– color of object– smoothness of object– relative orientation between light rays,

surface normal, and observer

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 11

What do eyes do?• why do most creatures have them?• how do they do their work?• what do they pass on to the brain?

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 12

Photoreceptor“receptor of light [photons]”• photoreceptor cell transforms light into

nerve impulses

incoming light ray

output of cell(signal along axon)

photoreceptor

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 13

Photoreceptor“receptor of light [photons]”• photoreceptor cell transforms light into

nerve impulses

incoming light ray

output of cell(signal along axon)

photoreceptor

[input intensity = 10]

[output signal = 1]

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 14

Photoreceptor“receptor of light [photons]” • photoreceptor cell transforms light into

nerve impulses• more light == higher frequency of impulses

incoming light ray

output of cell(signal along axon)

photoreceptor

[input intensity = 200]

[output signal = 20]

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 15

Photoreceptortransducer of light into neural signal • process: transduction• object: transducer• action: transducing

incoming light ray

output of cell(signal along axon)

photoreceptor

[input intensity = 200]

[output signal = 20]

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 16

What good is a single photoreceptor?• How can the utility of single

photoreceptor be improved?• Assumption: small creature

incoming light ray

output of cell(signal along axon)

photoreceptor

[input intensity = 200]

[output signal = 20]

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 17

Directional sensitivity• if photoreceptor responds to light from

any direction– cannot determine direction of light– can only determine overall amount of light

incoming light ray 1

output of cell(signal along axon)

photoreceptor

[output signal = 1]

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 18

Directional sensitivity• if photoreceptor responds to light from

any direction– cannot determine direction of light– can only determine overall amount of light

incoming light ray 1

output of cell(signal along axon)

photoreceptor

[output signal = 2]

incoming light ray 2

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 19

Directional sensitivity• if photoreceptor responds to light from

any direction– cannot determine direction of light– can only determine overall amount of light

incoming light ray 1

output of cell(signal along axon)

photoreceptor

[output signal = 3]

incoming light ray 2 incoming light ray 3

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 20

Directional sensitivity: eye cups• Need to exclude all light, except rays

which come from a particular direction• pigmented cells behind

eye cups: Cambrian period, 570–500 million years ago

incoming light ray 1

output of cell(signal along axon)

photoreceptor

[output signal = 3]

incoming light ray 2 incoming light ray 3

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 21

From http://www.sfu.ca/biology/faculty/burr/

Directional sensitivity in a parasitic worm• Mermis nigrescens

Photoreceptor

Pigment

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 22

Directional sensitivity: ommatidium• Need to exclude all light, except rays

which come from a particular direction• pigmented cells around:

extend eye cup into a tube

incoming light ray 1

output of cell(signal along axon) [output signal = 1]

incoming light ray 2incoming light ray 3

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 23

Ommatidium• long narrow light conductor (tube)

– selects light traveling in direction along axis.

.

.

ommatidium 3

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 24

Image formation• array of ommatidia: compound eye

From http://ebiomedia.com/gall/eyes/octopus-insect.html

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 25

Compound eyes• insects• trilobites (300 million years ago)

• very near sighted: wide field microscopic vision

• very poor far vision• average human vision

requires array 1m diameter

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 26

Array of ommatidium…

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 27

Array of photoreceptors

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 28

How to form an image?

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 29

Pinhole eye• array of photo receptors along cavity• pinhole selects light traveling in direction

between it and photoreceptor

Pinhole

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 30

Pinhole eye

• result is a complete image on photoreceptor array

• image represented by set of outputs

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 31

Nautilus: squid that lives in a shell

http://www.paleobase.com/gallery/gallery2.htmlhttp://platea.pntic.mec.es/~rmartini/ciencias.htm

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 32

Drawbacks of a pinhole eye…

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 33

Lens eyevertebrates, octopus

• Lens focuses and selects light rays• instead of a single ray through a narrow

pinhole

PinholeOutput = 1

ray of light

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 34

Lens eyevertebrates, octopus

• beam of light focused onto each photoreceptor

Largeopening

Output = 20

Beam of light

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 35

Human eye

Fovea

Optic nervefibers

Retina

Vitreoushumor

Lens

Iris

Pupil

Cornea

Aqueoushumor

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 36

Human Eye

• lens eye• array of photoreceptors: retina

– rods and cones

• focusing: cornea plus crystalline lens• photoreceptors are “backwards”

– axons (nerves) leave through blind spot

cornea

crystallinelens

retina: photoreceptors = rods + cones

opticnerve

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 37

Retina• Retina covered with light-sensitive

receptors:– rods

• primarily for night vision & perceiving movement• sensitive to broad spectrum of light• can’t discriminate between colors• sense intensity or shades of gray

– cones• used to sense color

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 38

Retina• Center of retina has most of the cones

– allows for high acuity of objects focused at center

– cones packed very tightly in fovea “depression”

• Edge of retina is dominated by rods– allows detecting motion of threats in

periphery

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 39

Edge detection• Photoreceptor array “copies” incoming light

– lens forms an image on the retina– photoreceptors fire at a rate “proportional” to

intensity of light

• But, absolute intensity is not that useful– changes when light level changes– for example?

• Better to represent objects via changes of intensity over space: edges

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 40

Intensity

100

0

Change

0

For example, consider pattern of light rays from an object with a patch of black paint on it

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 41

Intensity

100

0

Change

0

200

However, changes (edges) are still in same place.

If incoming light is twice as strong, twice as much will get reflected…

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 42

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 43

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 44

Representing objects by edges• Very efficient compression• 100 million axons from photo receptors

per eye• 1 million axons from ganglion cells per

eye• Reduction by a factor of 100!!

2002/01/14 PSCY202-005, Term 2, Copyright 2002 Jason Harrison 45

Finally• What was the purpose of this

presentation?

• Which question remains unanswered?