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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

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Question 1

• What do these devices have in common?

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These devices make use of electromagnetic waves

Capture electromagnetic waves and transform them into various forms.

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What does the eye do?

Transducing light energy into electrical energy

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Transduction Light enters the eye A photon hits a receptor changes the shape of pigment molecules triggers massive chemical reactions generate electrical signals

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• Solar cells (photovoltaics) produce electricity in a similar way as our eyes do.

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Rods and cones

• Morphology

• Distribution on the retina

• Dark adaptation

• Spectral sensitivity

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Photo receptors: Rods and cones

• Rods have bigger outer segments than cones.• Why?

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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.

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• How can we see a book?

• How can we see a desk?

• Why don’t we see light?

How can we see objects?

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Reflection of light• What we see is a reflection of light.

• Different objects reflect different wavelengths, different objects show different colors

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• Photo receptors in the eye are geared to capture different wavelengths

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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

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Retina:

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• 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

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Some messages: how to improve your vision

• Massage your eye muscles• Eat carrots• Massage the back of your head.

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Rods and cones

• Morphology

• Distribution on the retina

• Dark adaptation

• Spectral sensitivity

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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.

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Demonstration

• Blind spot

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Rods and cones are different

• In their dark adaptation rates

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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.

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• In terms of the activity of neurons,

what is the difference between

A and B ?

Any guess?

A. B.

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Measuring the electrical activity of a neuron directly by inserting a thin needle into animal brains.

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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.

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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.

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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.

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Ganglion cell

• Ganglion cell

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• Convergence:

• The ratio of connections with two groups of neurons.

• Rods vs. Ganglion cells– 120:1

• Cones vs. Ganglion cells– 6:1

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Why does this matter?

• How is this related to the higher sensitivity of rods?

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The cones result in better detail vision than the rods

• Visual acuity– How far apart are two dots?

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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.

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Fig. 2.11, p.53

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• 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.

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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.

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Visual Cortex

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The cones result in better detail vision than the rods

• Visual acuity– How far apart are two dots?

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Neurons

• How do you detect there are two separate dots (lights)?

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• How do you detect there are two separate dots (lights)?

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• Rods are bigger than cones• Convergence:

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Lateral Inhibition & Mach bands

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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

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Questions: What happens to B?

0 t

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Questions: What happens to B?

Excitatory Inhibitory

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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

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Receptive field of a ganglion cell

Measuring the frequency of action potentials elicited by this ganglion cell.

ConesGanglion cell

B

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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

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Questions: What happens to B?

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Measuring the receptive field of a ganglion cell

Change the size of the stimulus and see the way a ganglion cell respond

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Cones Ganglion cell

B

12 3 4 5 6 7

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Excitatory Inhibitory

Excitatory-center-inhibitory-surround receptive field

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Questions: What happens to B?

Excitatory Inhibitory

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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.

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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.

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How does this happen?

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=sum(B)

=sum(B)

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Fig. 3-6, p. 50

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Fig. 3-7, p. 51

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Why is this important?

help you to detect the edge of a figure

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Light intensity

location

Perceived Light intensity

location

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Physical stimuli Your perception

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Lateral inhibition

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+ +-- --+ +-- --

100 20

+ +-- --+ +-- --

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White’s illusion

• Can you explain this by lateral inhibition?

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Application: Machine vision

• Implementing the mechanism of lateral inhibition to a computer program.

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Image

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• Edge detection algorithm– Zero-crossing