How does perception depend on sensory processing?

• Transduction of sensory information• The visual system as the “prototype”

– Rods and cones and visual acuity– The visual cycle– Visual adaptation– Introduction to colour vision

Vision Light waves Light-sensitive cells in retina

Sound waves Pressure-sensitive hair cells in cochlea of inner ear

Audition

Touch

Pain

Taste

Smell

Pressure on skin Sensitive ends of “touch” neurons in skin

Variety of potentially or actually harmful stimuli

Sensitive ends of “pain” neurons in skin and other tissue

Molecules dissolved in fluid on tongue

Chemical-sensitive cells on tongue

Molecules dissolved in fluid on mucous membranes in nose

Sensitive ends of olfactory neurons in mucous membrane

SenseStimulus

Receptors

Prism

whitelight

GammaRays

X-rays Ultra-Violetrays

Infraredrays

Radar Broadcastbands

ACcircuits

Wavelength in nanometers (billionths of a meter)

10-5 1017105

Shorter wavelength(or higher frequency):

Longer wavelength(or lower frequency):

Greater amplitude(brighter colours):

Smaller amplitude(duller colours):

lightfovea

10º

10º

20º

20º

40º

40º

30º

30º

blindspot

(See Gleitman p. 185)

Retina

Lightentering

To brainvia opticnerveRod Cone

Ganglioncell

Bipolarcell

Photoreceptorlayer

Amacrinecell

Horizontalcell

(See Gleitman, p. 185)

• Scotopic vision: rods– 120,000,000 rods

• Photopic vision: cones– 7,000,000 cones

• Purkinje “shift”• Rhodopsin and 3 kinds of

iodopsin• Stabilized image

Geographical Distribution of Retinal Cells

Rods vs. Cones

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

-60 -40 -20 0 20 40 60

Distance from fovea (degrees)

Nu

mb

er

of

cell

s /

sq

. m

m. Rods

Cones

Threshold to light at different wave lengths

(Compare with Gleitman, p. 186, where the Y axis is measuring “sensitivity” rather than “energy”)

400 450 500 550 600 650 700

Min

imum

log e

nerg

y t

o g

enera

te a

resp

onse

Wavelength in nanometers

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

Cones

Rods

Dark-adaptation curves

10

9

8

7

6

5

4

3

2

1

00 5 10 15 20 25 30

Minutes in the dark

Min

imu

m d

ete

ctab

le lig

ht

inte

nsi

ty(l

og

un

its)

Cone adaptationRod adaptation

The nature of color vision:Gleitman pp. 192-196

Trichromatic Theory(at the level of the retina)

Opponent-process Theory(further up the system)

The Negative Afterimage Phenomenon(Opponent-process theory in action)