First Hour - How does perception depend on sensory processing?

Sensory Processes

Transduction of sensory information

The visual system as the “prototype”• The duplex theory of vision

– Rods and cones and visual acuity• The visual cycle • The stabilized image procedure• Dark adaptation

An exploration of the 6th sense – A test for ESP

Vision Light waves Light-sensitive cells (rods and cones) 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

Scotopic vision: rods•Not very accurate•Highly sensitive•Monochromatic vision

•120,000,000 rods

Photopic vision: cones•Responsible for visual acuity•Not as sensitive as rods•Colour vision•7,000,000 cones

Duplex Theory of Vision:Two types of receptors

Retina (p 185 Gleitman)

Lightentering

To brainvia opticnerveRod Cone

Ganglioncell

Bipolarcell

Photoreceptorlayer

Amacrinecell

Horizontalcell

lightfovea

10º

10º

20º

20º

40º

40º

30º

30º

blindspot

(See Gleitman p. 185)

Geographical Distribution of Retinal Cells

Rods vs. Cones

FoveaBlind spotlight

fovea

blindspot

periphery

periphery

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

-60 -40 -20 0 20 40 60

Distance from fovea (degrees)

Nu

mb

er o

f ce

lls /

sq. m

m. Rods

Cones

(cones)

(rods)

(rods)

What the eye sees:

By daylight, only the central fovea sees clearly and in colour.

On a dark night, only the periphery sees, but only in black and white and with poor resolution. The fovea is “blind.”

The Visual Cycle• Iodopsin• Rhodopsin

The stabilized image procedure

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 in

ten

sity

of

ligh

t re

qu

ired

to s

ee

Cone adaptationRod adaptation

“Rod-cone break” at about 7 minutes

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