View
222
Download
0
Tags:
Embed Size (px)
Citation preview
ch 3 1
Sensation & Perception
• Ch. 3: Vision
© Takashi Yamauchi (Dept. of Psychology, Texas A&M University)
• Main topics– convergence– Inhibition, lateral inhibition and lightness perception– Interactions between neurons– Feature detectors
ch 3 2
Question 1
• What do these devices have in common?
ch 3 3
These devices make use of electromagnetic waves
Capture electromagnetic waves and transform them into various forms.
ch 3 4
What does the eye do?
Transducing light energy into electrical energy
ch 3 5
Transduction Light enters the eye A photon hits a receptor changes the shape of pigment molecules triggers massive chemical reactions generate electrical signals
ch 3 6
ch 3 7
• Solar cells (photovoltaics) produce electricity in a similar way as our eyes do.
ch 3 8
Rods and cones
• Morphology
• Distribution on the retina
• Dark adaptation
• Spectral sensitivity
ch 3 9
Photo receptors: Rods and cones
• Rods have bigger outer segments than cones.• Why?
ch 3 10
Outer segments capture photons
• Bigger outer segments can capture more light.
• Rods have bigger outer segments.
– Rods allow us to see in the dark.
– Cones are mainly for day vision.
– Cones are for color perception.
ch 3 11
• How can we see a book?
• How can we see a desk?
• Why don’t we see light?
How can we see objects?
ch 3 12
Reflection of light• What we see is a reflection of light.
• Different objects reflect different wavelengths, different objects show different colors
ch 3 13
• Photo receptors in the eye are geared to capture different wavelengths
ch 3 14
Lens: focuses light rays.
Iris: control the size of the pupil regulating the amount of light reaching the retina
Retina: a layer of receptor cells
Receptor cells rods and cones
ch 3 15
Retina:
ch 3 16
• Photo receptors are facing away from the light source.• The optic nerve carries neural information to this spot. • What happens?
– No receptors, no vision blind spot
ch 3 17
Some messages: how to improve your vision
• Massage your eye muscles• Eat carrots• Massage the back of your head.
ch 3 18
Rods and cones
• Morphology
• Distribution on the retina
• Dark adaptation
• Spectral sensitivity
ch 3 19
The distribution of cones and rods on the retina
• Cones are concentrated mainly on the fovea.
• There are no rods on the fovea.• We move eyes to capture images
on the fovea.
ch 3 20
Demonstration
• Blind spot
ch 3 21
Rods and cones are different
• In their dark adaptation rates
ch 3 22
Dark adaptation rates of rods and cones
• When you enter a dark room from outside, you can’t see well at first. But gradually, your eyes are adjusted to the dark, and see better.
ch 3 23
• In terms of the activity of neurons,
what is the difference between
A and B ?
Any guess?
A. B.
ch 3 24
Measuring the electrical activity of a neuron directly by inserting a thin needle into animal brains.
ch 3 25
Time0 t
The frequency of action potential
Time0 t
Time0 t
The number of action potential emitted by a neuron is correlated with the intensity of the stimulus.
ch 3 26
Neural Processing by Convergence
• Why are rods more sensitive to light than cones?
• Because rods are bigger than cones.
• Because rods and cones are connected to ganglion cells in different manners.
ch 3 27
Activities of neurons can be schematically shown as
B
a1 a2 a3 a4The firing rate of neuron B is determined by the activation sent by neurons a1-a4.
ch 3 28
Ganglion cell
• Ganglion cell
ch 3 29
• Convergence:
• The ratio of connections with two groups of neurons.
• Rods vs. Ganglion cells– 120:1
• Cones vs. Ganglion cells– 6:1
ch 3 30
ch 3 31
Why does this matter?
• How is this related to the higher sensitivity of rods?
ch 3 32
The cones result in better detail vision than the rods
• Visual acuity– How far apart are two dots?
ch 3 33
Time0 t
The frequency of action potential
Time0 t
Time0 t
The number of action potential emitted by a neuron is correlated with the intensity of the stimulus.
ch 3 34
ch 3 35
ch 3 36
Fig. 2.11, p.53
ch 3 37
ch 3 38
ch 3 39
ch 3 40
• Demonstration:
• On a scratch paper, draw two vertical lines of about 2 inches (1/2 inch apart).
• Close your left eye, and focus your right eye on your index figure, and move the figure.
• At some point, you can’t distinguish the two vertical lines.
ch 3 41
The distribution of cones and rods on the retina
• Cones are concentrated mainly on the fovea.
• There are no rods on the fovea.
• We move eyes to capture images on the fovea.
ch 3 42
Visual Cortex
ch 3 43
ch 3 44
ch 3 45
The cones result in better detail vision than the rods
• Visual acuity– How far apart are two dots?
ch 3 46
Neurons
• How do you detect there are two separate dots (lights)?
ch 3 47
ch 3 48
• How do you detect there are two separate dots (lights)?
ch 3 49
• Rods are bigger than cones• Convergence:
ch 3 50
Lateral Inhibition & Mach bands
ch 3 51Herman grid
ch 3 52
ch 3 53
ch 3 54
Time0 t
The frequency of action potential
Time0 t
The number of action potential emitted by a neuron is correlated with the intensity of the stimulus.
Time0 t
ch 3 55
Questions: What happens to B?
0 t
ch 3 56
Questions: What happens to B?
Excitatory Inhibitory
ch 3 57
Receptive field• The receptive field of a neuron in the visual
system is the area on the retina that influences the firing rate (action potential) of the neuron.
• Measuring the receptive field of a ganglion cell
ch 3 58
Receptive field of a ganglion cell
Measuring the frequency of action potentials elicited by this ganglion cell.
ConesGanglion cell
B
ch 3 59
Receptive field of a ganglion cell
ConesGanglion cell
B12 3 4 5 6 7
Firing rate of B
4 3-5 2-6 2-7
ch 3 60
Questions: What happens to B?
ch 3 61
Measuring the receptive field of a ganglion cell
Change the size of the stimulus and see the way a ganglion cell respond
ch 3 62
Cones Ganglion cell
B
12 3 4 5 6 7
ch 3 63
Excitatory Inhibitory
Excitatory-center-inhibitory-surround receptive field
ch 3 64
Questions: What happens to B?
Excitatory Inhibitory
ch 3 65
Excitatory and inhibitory connections
• What neurons transmit is electricity.
• Some neurons send positive (excitatory) signals (+) increase the firing rate of the target neuron.
• some neurons send negative (inhibitory) signals (-) depress the firing rate of the target neuron.
ch 3 66
Spatial Summation
c1 c2 c3 c4
B
a1 a2 a3 a4
+ +
= 4
+ +
B
a1 a2 a3 a4
+ +
= 0
+ + - - - -The firing rate of neuron B can be expressed by the overall summation of the signals that B receives.
ch 3 67
ch 3 68
ch 3 69
How does this happen?
ch 3 70
=sum(B)
=sum(B)
ch 3 71
Fig. 3-6, p. 50
ch 3 72
Fig. 3-7, p. 51
ch 3 73
Why is this important?
help you to detect the edge of a figure
ch 3 74
ch 3 75
Light intensity
location
Perceived Light intensity
location
ch 3 76
Physical stimuli Your perception
ch 3 77
Lateral inhibition
ch 3 78
+ +-- --+ +-- --
100 20
+ +-- --+ +-- --
ch 3 79
ch 3 80
ch 3 81
White’s illusion
• Can you explain this by lateral inhibition?
ch 3 82
ch 3 83
ch 3 84
ch 3 85
ch 3 86
ch 3 87
ch 3 88
ch 3 89
Application: Machine vision
• Implementing the mechanism of lateral inhibition to a computer program.
ch 3 90
Image
ch 3 91
ch 3 92
ch 3 93
ch 3 94
• Edge detection algorithm– Zero-crossing