Prof. Greg Francis

1PSY 310: Sensory and Perceptual Processes

Purdue University

Light and the eye

PSY 310

Greg Francis

Lecture 03

Why does my daughter look like a demon?Purdue University

The perceptual process

Attended stimulus

Environmental stimulus

Action

Recognition

Perception

Processing

Transduction

Stimulus onreceptors

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Brain structure

Neuralcircuits are

specialized

to processcertain

types of

information

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Brain structure

Blowing up a partof the imagehighlights a oddfeature about howthe brain works

Some of theinformation fromthe retina crosseson its way to thebrain

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Contralateral processing

Neural fibers from

eye cross on wayto cortex

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Brain structure

The left hemisphere of the brain takes in input from:

the left part of the left eye

The left part of the right eye

==> objects to the right of where you are looking

The right hemisphere of the brain takes in input from:

the right part of the left eye

The right part of the right eye

==> objects to the left of where you are looking

An fMRI brain scan demonstrates that different hemispheresrespond to contralateral stimuli

Movie: moving wedge

Prof. Greg Francis

2PSY 310: Sensory and Perceptual Processes

Purdue University

Brain structure

The two hemispheres are pretty similar

for most perceptual processing

But they differ for cognitive processing

The right hemisphere plays a bigger role

than the left in judging faces

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CogLab: Brain asymmetry

On each trial you saw two oddlyconstructed faces And had to judge which face appeared younger

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CogLab: Brain asymmetry

If you look at the middle of an image, theleft side goes to the right brain, and theright side goes to the left brain

This face might lookyoung, because thejudgment is basedmore on the left sideof the image.

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CogLab: Brain asymmetry

If you look at the middle of an image, theleft side goes to the right brain, and theright side goes to the left brain

This face might lookold, because thejudgment is basedmore on the left sideof the image.

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CogLab: Brain asymmetry

The faces are actually mirror images ofeach other

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CogLab: Brain asymmetry The hemispheres of left-handed people are not

as different as for right-handed people Experiment measures the percentage of choices

with younger half face on left

67Right

57Left

PercentageHandedness

Prof. Greg Francis

3PSY 310: Sensory and Perceptual Processes

Purdue University

Light Visual perception involves the detection of light We interpret patterns of light that vary in space and time in

a way to learn something about objects in the world This wasn’t always obvious

People used to think that perception involved some kind of“ray” going out of the eyes to “touch” objects

The fact that photographs are convincing demonstrates thatthis idea is not correct

Light is electro-magnetic energy It can be described as a wave

The distance between two neighboring peaks of a wave is thewavelength

Usually given in nanometers

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Light It can be described as a wave

The distance between two neighboring peaks of a wave is thewavelength

Can also fix a distance and talk about the number of peaks in thewave

frequency

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Light

People arevisuallysensitive toonly a smallfraction of thewavelengthsof light

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Light

Different species are sensitive to differentwavelengths of light

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Light The wavelength of light often corresponds to a particular color

E.g. blue skies are blue because they scatter low wavelength light more thanhigh wavelength light

The non-scattered light reaches our eyes directly (the sun looks yellow, orange,red)

The scattered light is what we see when we look to other parts of the sky

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Color But be careful! Color is a

perceptualexperience, not aproperty of light

The magenta andorange have

exactly the samewavelength!

Prof. Greg Francis

4PSY 310: Sensory and Perceptual Processes

Purdue University

Illuminant and reflectance We need light to

see things, butwe are not ofteninterested insources of light. A source of

light is called

an illuminant

Light energy

falling onto a

surface isilluminance

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Illuminant and reflectance We are really

interested inobjects

Light from an

illuminantreflects off of

objects

Light energy

reflected of anobject or

surface isluminance

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Measuring light intensity Most illuminants produce light at lots of different wavelengths

We only sense a small part of the wavelengths

So how do we measure the strength of a source of light?

Cannot just measure total electro-magnetic energy

Who cares about the

radio waves or gammawaves? They do not

contribute to visual perception.

There are specialcalculations that weightsdifferent frequencies toinclude only thoseimportant for visual perception

Candelas per square meter

Foot-lamberts Purdue University

The eye Light reflects off objects and some of it enters the eye

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The eye Cross-section

shows moredetail

The eye ishollow, butfilled withclear fluid

The cornea isa clear lens

So isthe”lens”

They helpfocus light

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The eye Cross-section

shows moredetail

Light entersthe pupil

The cornea isa clear lens

So isthe”lens”

They helpfocus light

Prof. Greg Francis

5PSY 310: Sensory and Perceptual Processes

Purdue University

The eye

The interior ofthe eye ishollow, but isfilled with aclear liquid

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The eye Light passes through the cornea, lens, and fluid and projects

on to the back of the eye

The retina

The image is upside down because the lens inverts theimage

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The eye You change the shape of the lens to make certain the image

is sharp on your retina

Accommodation (automatic)

As you age, the lens loses flexibility and cannot be shaped sowell

Then you need glasses

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The eye Why is the pupil black?

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The eye Much of the visible light that goes into the eye is

absorbed by the detection process (next time)

What is reflected back comes out of the right back tothe source

Unless your eye is right where the source of light is

located you do not see the light coming out of the eye

Thus it looks black

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The eye Why is the pupil red in photographs?

With a camera’s flash, the camera lens is very close

to the source of light (the flash)

So it does record some of the light coming out of the eye

Thus we get red-eye

Prof. Greg Francis

6PSY 310: Sensory and Perceptual Processes

Purdue University

Conclusions Light

Wavelength

Luminance

Color is a percept

Eye

Gross anatomy

Lens

pupil

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Next time

More detail on the properties of the retina

Detection of light energy

Fovea

Rods

Cones

Visual acuity