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This component of the Lecture will have these learning outcomes: Introduce the concept of vision ~5 mins. Disseminate a few (ir)relevant facts about the eye ~5 mins. Understand the working of the eye ~10 mins. Introduction to Illusions ~15 mins. Optical Illusions ~20 mins.

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Vision: Lecture Objectives

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Vision lecture objectives

  • This component of the Lecture will have these learning outcomes:

    • Introduce the concept of vision~5 mins.

    • Disseminate a few (ir)relevant facts about the eye~5 mins.

    • Understand the working of the eye~10 mins.

    • Introduction to Illusions~15 mins.

    • Optical Illusions~20 mins.

    • Depth Perception and Movement Detection~15 mins.

Vision: Lecture Objectives

Damian Schofield


Vision lecture objectives1

  • This component of the Lecture will have these learning outcomes:

    • Introduce the concept of vision~5 mins.

    • Disseminate a few (ir)relevant facts about the eye~5 mins.

    • Understand the working of the eye~10 mins.

    • Introduction to Illusions~15 mins.

    • Optical Illusions~20 mins.

    • Depth Perception and Movement Detection~15 mins.

Vision: Lecture Objectives

Damian Schofield


Lecture objectives

Lecture Objectives

Damian Schofield


Seeing is believing

Elkins, J., The Object Stares Back, Simon & Schuster: New York, 1996

… nothing could be easier than seeing.

We just point our eyes where we want them to go, and gather in whatever there is to see. Nothing could be less in need of explanation.

The world is flooded with light, and everything is available to be seen.

We see people, pictures, landscapes, and whatever else we need to see, and with the help of science we can see galaxies and viruses and the insides of our own bodies.

Seeing does not interfere with the world or take anything from it, and it does not hurt or damage anything.

Seeing is detached and efficient and rational.

Unlike the stomach or the heart, eyes are our own to command: they obey every desire and thought.

SO ARE THESE STATEMENTS TRUE ?

Seeing is Believing ?

Damian Schofield


Seeing is believing1

Elkins, J., The Object Stares Back, Simon & Schuster: New York, 1996

Each one of those ideas is completely wrong.

The truth is more difficult: seeing is irrational, inconsistent, and undependable.

It is immensely troubled, cousin to blindness and sexuality, and caught up in the threads of the unconscious.

Our eyes are not ours to command: they roam where they will and tell us they have only been where we have sent them.

No matter how hard we look, we see very little of what we look at….

Seeing is Believing ?

Damian Schofield


Seeing is believing example

Seeing is Believing - Example

Damian Schofield


Seeing is believing example1

Elkins, J., The Object Stares Back, Simon & Schuster: New York, 1996

… Even when I am not thinking of the use of objects, they remind me of use.

And there is a curious thing here that easily passes unnoticed: I do not focus on anything that is not connected in some way with my own desires and actions.

I fail to see the stretches of wall between the lamp and the coffee cup, or the manila paper of the file folders, or the black plastic calendar holder.

My eyes can understand only desire and possession.

Anything else is meaningless and therefore invisible.

Seeing is Believing - Example

Damian Schofield


Biological vision

Biological vision is the process of using light reflected from the surrounding world as a way of modifying behavior.

Generally, with humans, we say that the surrounding environment is interpreted by visual input.

This usually implies some form of conscious understanding of the three-dimensional (3D) world from the two-dimensional (2D) projection that it forms on the retina of the eye.

Biological Vision

Damian Schofield


Biological vision1

Solving the problem of converting light into ideas, of visually understanding features and objects in the world, is a complex task far beyond the abilities of the world's most powerful computers.

Much of our visual computation is carried out unconsciously and often our interpretations can be fallacious.

Vision requires distilling foreground from background, recognising objects presented in a wide range of orientations, and accurately interpreting spatial cues.

Biological Vision

Damian Schofield


Biological vs machine vision

In this part of the course we will briefly overview the human visual system and try to understand the ways in which this system interprets its input.

Although not strictly correct, the analogy between machine vision and biological vision is currently the best model available.

Biological vs. Machine Vision

Damian Schofield


Biological vs machine vision1

We use the human visual system as an existence proof that visual interpretation is even possible in the first place, and its response to optical illusions as a way to guide our development of algorithms that replicate the human system;

We use our understanding of machine vision and our ability to generate ever more complex computer images as a way of modifying, or evolving, our visual system in its efforts to interpret the visual world.

Biological vs. Machine Vision

Damian Schofield


Vision lecture objectives2

  • This component of the Lecture will have these learning outcomes:

    • Introduce the concept of vision~5 mins.

    • Disseminate a few (ir)relevant facts about the eye~5 mins.

    • Understand the working of the eye~10 mins.

    • Introduction to Illusions~15 mins.

    • Optical Illusions~20 mins.

    • Depth Perception and Movement Detection~15 mins.

Vision: Lecture Objectives

Damian Schofield


Eye facts 1

The eye is considered by most neuroscientists as part of the brain.

It consists of a small spherical globe of approximately 2cm in diameter, which is free to rotate under the control of six extrinsic (or extraocular) muscles.

The tough, outermost layer of the eye is called the sclera. It maintains the shape of the eye.

The front sixth of this layer is clear and is called the cornea. All light must first pass through the cornea when it enters the eye.

Eye Facts (1)

Damian Schofield


Eye facts 2

  • The choroid is the second layer of the eye. It contains two structures:

  • The ciliary body - The ciliary body is a muscular area that is attached to the lens. It contracts and relaxes to control the size of the lens for focusing.

  • The iris - The iris is the colored part of the eye. The iris is an adjustable diaphragm around an opening called the pupil.

  • Inside the eyeball there are two fluid-filled sections separated by the lens. The larger, back section contains a clear, gel-like material called vitreous humor. The smaller, front section contains a clear, watery material called aqueous humor.

Eye Facts (2)

Damian Schofield


Eye facts 3

The lens is a clear, bi-convex structure about 10 mm (0.4 inches) in diameter. The lens changes shape because it is attached to muscles in the ciliary body. The lens is used to fine-tune vision.

The eye is unique in that it is able to move in many directions to maximise the field of vision, yet is protected from injury by a bony cavity called the orbital cavity. The eye is embedded in fat, which provides some cushioning.

Eye Facts (3)

Damian Schofield


Eye facts 4

The eyelids protect the eye by blinking. This also keeps the surface of the eye moist by spreading tears over the eyes. Eyelashes and eyebrows protect the eye from particles that may injure it.

Tears are produced in the lacrimal glands, which are located above the outer segment of each eye. The tears eventually drain into the inner corner of the eye, into the lacrimal sac, then through the nasal duct and into the nose.

That is why your nose runs when you cry.

Eye Facts (4)

Damian Schofield


Lastly pupils

Pupil size can change from 2 millimeters to 8 millimeters.

This means that by changing the size of the pupil, the eye can change the amount of light that enters it by 30 times.

Pupil size between 6 and 8 mm may indicate the use of marijuana, cocaine, crack, meth, hallucinogens, crystal, ecstasy or other stimulants.

Pupil size between 1 and 2 mm may indicate the use of heroin, opiates or other depressants.

Lastly, Pupils …..

Damian Schofield


Lastly pupils1

We subconsciously pick up clues from others’ pupil sizes and use them to help us form opinions about people.

Eckhard Hess, a Chicago biopsychologist, performed a study in which men were shown retouched photographs of women.

In half the photographs, the pupils were made to appear larger than normal, and in the other half they were smaller.

The men in the study invariably perceived the women with larger pupils as being more attractive and friendlier than the very same women when shown with smaller pupils.

Lastly, Pupils …..

Damian Schofield


Lastly pupils2

We subconsciously pick up clues from others’ pupil sizes and use them to help us form opinions about people.

Eckhard Hess, a Chicago biopsychologist, performed a study in which men were shown retouched photographs of women.

In half the photographs, the pupils were made to appear larger than normal, and in the other half they were smaller.

The men in the study invariably perceived the women with larger pupils as being more attractive and friendlier than the very same women when shown with smaller pupils.

So, following logically from this,

does the use of drugs make you more

attractive to potential sexual partners ?

Lastly, Pupils …..

Damian Schofield


Vision lecture objectives3

  • This component of the Lecture will have these learning outcomes:

    • Introduce the concept of vision~5 mins.

    • Disseminate a few (ir)relevant facts about the eye~5 mins.

    • Understand the working of the eye~10 mins.

    • Introduction to Illusions~15 mins.

    • Optical Illusions~20 mins.

    • Depth Perception and Movement Detection~15 mins.

Vision: Lecture Objectives

Damian Schofield


The human eye

The Human Eye

Damian Schofield


The retina

  • The retina is complex in itself. This thin membrane at the back of the eye is a vital part of our ability to see.

  • Its main function is to receive and transmit images to the brain, by converting light into neural signals that can be relayed to the cortex through the optic nerve.

  • The retina consists of a team of five types of cells whose role it is to collect light, extract basic information about color, form, and motion, and pass the pre-processed image on to centers in the brain.

  • These cell types are:

  • photoreceptors,

  • bipolarcells,

  • horizontalcells,

  • amacrinecells,

  • ganglioncells.

The Retina

Damian Schofield


The retina1

The Layers of the Retina

Light passes all the way through the retina before reaching the photoreceptor cells at the back.

Photoreceptors convert light signals into neural impulses that are relayed to a variety of other cells types in the retina for processing.

The ganglion cells at the front of the retina are the final relay station in the eye, and they pass signals into the brain via the optic nerve.

The Retina

Damian Schofield


The retina2

The location of the optic nerve on the retina obviously prohibits the existence of photoreceptors at this point.

This point is known as the blind spot and any light that falls upon it is not perceived by the viewer. Most people are unaware of their blind spot, although it is easy to demonstrate that it exists.

The Retina

Damian Schofield


Vision worksheet 01

Why not try decapitating your friends?

Where the optic nerve exits the eyeball is a gap in the light-sensitive retinal cells, creating a "blind spot."

You can locate your own spot by marking two X's side by side on a sheet of paper about three inches apart, then closing your right eye and viewing the X on the right with your left eye.

Start with the paper about a foot and a half from your face, then bring it slowly toward you. At some critical distance (which you may need to "hunt" for), the left X will fall entirely on your blind spot and disappear.

What good is all this? Now you're ready to play the decapitation game: From about 10 feet away, shut your right eye and stare at your friend’s head with your left eye, then slowly move your open eye horizontally toward the right until the head falls onto your blind spot and disappears.

"When King Charles II heard about the blind spot, he took great delight in walking around his court decapitating his ladies in waiting or beheading criminals with his blind spot before they were actually guillotined.”

Vision Worksheet 01

Damian Schofield


Vision worksheet 011

Why not try decapitating your friends?

XX

Vision Worksheet 01

Damian Schofield


The retina3

The Fovea

The fovea is the region of the retina that allows us to see detail.

At the fovea, the top two

layers of the retina thin

out, allowing light to fall

directly onto the

photoreceptor cells.

Foveal photoreceptors

are mostly cone cells,

meaning that the fovea

is particularly good at

seeing color in daylight.

At night, the activity of

color-insensitive rod cells

in the periphery of the

retina dominates vision.

The Retina

Damian Schofield


Rods and cones

An image shining upon the retina traverses these layers to reach the photoreceptor cells, which absorb the incoming light and transform it into electrochemical signals.

Photoreceptors are divided into two subtypes, rods and cones. Generally, the outer segment of rods are long and thin, whereas the outer segment of cones are more cone shaped.

Rods and Cones

Damian Schofield


Rods and cones1

Rod cells are very sensitive to changes in contrast even at low light levels (they can detect a single photon) and can create black and white images without much light, but consequently are imprecise in detecting position (due to light scatter) and insensitive to color.

Rods are generally located in the periphery of the retina and used for night vision..

Rods and Cones

Damian Schofield


Rods and cones2

Once enough light is available (for example, daylight or artificial light in a room), cones give us the ability to see the colour and detail of objects.

These are the high-precision cells that are specialised to detect red, green, or blue light. They are generally located in the center of the retina in a region of high spatial acuity (the fovea).

Cones are responsible high acuity

tasks like reading

(they allow you to read this slide)

they allow us to see at a high resolution.

Rods and Cones

Damian Schofield


Rods and cones3

Rods and Cones

Damian Schofield


Rods and cones4

The photoreceptors are in a continuous turnover, and the outer segment ‘discs’ have a turnover of around 9 to 13 days.

The rod disc shedding is maximal in the morning, and cone shedding maximal at dusk. These are probably mediated through melatonin.

The information received by the rods and cones are then transmitted to the nearly 1 million ganglion cells in the retina.

These ganglion cells interpret the messages from the rods and cones and send the information on to the brain by way of the optic nerve.

Rods and Cones

Damian Schofield


The visual pathway

Signals from the

photoreceptors pass

forward into the

next layer of the

retina containing

horizontal, bipolar,

and amacrine cells.

The Visual Pathway

Damian Schofield


The visual pathway1

Signals from the photoreceptors pass forward into the next layer of the retina containing horizontal, bipolar, and amacrine cells.

These cells form small networks that are able to extract information about form and motion from an image.

The neurons in these two retinal layers exhibit complex receptive fields that enable them to detect contrast changes within an image; these changes might indicate edges or shadows.

That information continues to the front of the retina

where it is received by a layer of ganglion cells.

The Visual Pathway

Damian Schofield


The visual pathway2

The optic nerves within each eye meet in the front part of the head at a point called the optic chiasm, which functions like a cloverleaf on a highway.

All the fibers from the left half of each retina turn towards the right side of the brain, and the fibers from the right half of each retina turn towards the left side of the brain.

The end result of this is that the left half of the brain looks at the right visual world, and the right half of the brain looks at the left visual world.

The Visual Pathway

Damian Schofield


The visual pathway3

First stages of the visual pathway.

A series of splits and

crossing in the first parts of

the visual system may appear

complicated, but the end

result is simple: the left half

of the brain receives

information about the right half

of the world (in this case the

spoon handle), and the right

half of the brain receives

information about the left

half of the world

(here the spoon tip)

The Visual Pathway

Damian Schofield


The visual cortex

A small group of fibers in the optic nerve splits off and travels down to brainstem nuclei, which are groups of cells governing reflex actions.

Those fibers mediate automatic responses, such as adjusting the size of the pupil, blinking, and coordinating the movement of the eyes.

The majority of fibers in the optic nerve take another path that leads to the very back of the brain, to a part of the occipital lobe called primary visual cortex, or V1.

The Visual Cortex

Damian Schofield


The visual cortex1

The primary visual cortex (V1) is part of the cerebral cortex.

The V1, also called striate cortex because of the distinctive stripe it bears, is responsible for creating the basis of a three-dimensional map of visual space, and extracting features about the form and orientation of objects.

Once basic processing has occurred in V1, the visual signal enters the secondary visual cortex, V2, which surrounds V1.

The Visual Cortex

Damian Schofield


The cerebral cortex

The cerebral cortex is the extensive outer layer of gray matter of the cerebral hemispheres, and is involved in higher brain functions, including sensation, voluntary muscle movement, thought, reasoning, and memory.

The grooves on the brain's surface results in much of the cortex being buried; over 60% of the cerebral cortex in primates is buried and not visible from the surface.

Cerebral cortex is typically 2-4mm thick and folded, but if unfolded it has the same area as a 21” television.

The Cerebral Cortex

Damian Schofield


Visual worksheet 02

Illusions That Won’t Go Away.

Stare at some sights long enough, and you'll continue to see an afterimage even after you've looked away. Now vision scientists report that the brain can see afterimages of things that weren't even there in the first place, but were created by optical illusions. The cerebral cortex is responsible for the trick.

Afterimages come in two basic types: those formed on the eye's retina and those conjured up in the brain. For example, a bright light can bleach pigments in the retina and cause the retina's neurons to adapt; these effects linger after the light disappears, creating an afterimage. But most other aftereffects, such as continuing to hear a sound when it has ceased or seeing color-based patterns after looking away, are formed in the brain's cerebral cortex.

Now cognitive scientist Shinsuke Shimojo and his colleagues at the California Institute of Technology in Pasadena have discovered that the cerebral cortex can create afterimages even of illusory surfaces. They worked with a well-known effect called perceptual color spreading, or filling in. If you stare at the illustration long enough, you will start to see a box between the four circles with wedges cut out of them (researchers call them pacmen for their resemblance to the video game heroes).

Shimojo and his colleagues noticed that if you shift your gaze to the blank area at right, you continue to see the box. Illusions, they found, can create afterimages. Then they fiddled with the pacmen to find out whether the newfound perception relies more on the retina-based afterimages created by the pacmen or on a cortex-based process of filling in. The cortex won, they report in the 31 August issue of Science.

The demonstration that illusions can create afterimages is "cute," says vision scientist Mary Hayhoe of the University of Rochester in New York state. "No matter how much we study perception we still find it puzzling when we see something that isn't there," Hayhoe says.

Visual Worksheet 02

Damian Schofield


Visual worksheet 021

Illusions That Won’t Go Away.

Visual Worksheet 02

Damian Schofield


Visual acuity

Visual acuity is a measure of the spatial resolving power of the visual system; it indicates the angular size of the smallest detail that can be resolved.

Visual acuity is measured for various purposes. When determining the appropriate eyeglasses, the corrective lens power that permits the best visual acuity is prescribed.

Visual acuity measurements are also used by some licensing authorities and employers as eligibility criteria for some occupations (e.g., airline pilot, police officer) and activities (e.g. driving). Visual acuity has traditionally been used as the primary indicator of the magnitude of functional impairment due to vision loss.

Visual Acuity

Damian Schofield


Visual acuity1

Visual Acuity

Damian Schofield


Visual acuity2

The notation of visual acuity is written as a fraction, with normal vision being 20/20 (twenty twenty vision).

At a 20 foot distance, (the top number in the fraction, or testing distance), a person with normal vision should be able to read the small 20/20 line on an eye chart. The smallest line that you can read on the chart is your visual acuity.

If larger lines than the 20/20 line are all that can be read, the visual acuity may be 20/30, 20/60, etc. The larger the second number is, the worse is the vision. A person with 20/200 vision would have to come up to 20 feet to see a letter that a person with normal vision could see at 200 feet! If the vision is 20/10, it means that the vision is better than normal.

A person with 20/10 vision can read a letter at 20 feet that a person with normal vision would have to come up to 10 feet to read.

Certain visual acuities have special significance. Some of these are

• 20/20 vision is considered normal vision

• 20/40 vision in at least one eye is the vision required to pass the driving test

• 20/200 vision or worse is the legal definition of blindness

Visual Acuity

Damian Schofield


Visual worksheet 03

Vision Testing Instructions

The blue "standardization bar" must be

measured in centimeters on your

computer screen or printout.

You should then stand in a position back

from your computer screen so that your

eye is this same number of feet from the

screen. i.e. if the blue bar measures 9 1/2

centimeters in length, you should stand

back so that your eye is 9 1/2 feet from

the screen.

Vision testing should be performed on

each eye separately, wearing distance

eyeglasses if required.

There are six lines on the screen, you

should note the last line number in which

you can read most (50-75%) of the letters.

Visual Worksheet 03

Damian Schofield


Visual worksheet 031

Vision Testing Instructions

The middle letter of the top line is equivalent to 20/200.

A person unable to read this letter with best eyeglass correction is considered legally blind.

The "T" and the "B" on the top line are equivalent to 20/100. The second line is 20/50.

Best corrected acuity in the range between 20/50 and 20/100 is considered disabling in occupations which require work with numbers or extensive reading.

The third line is equivalent to 20/40, it is the "driver's test" line. You must be able to read most of the letters on this line in order to obtain an unrestricted drivers license in most US states.

The bottom three lines represent 20/30, 20/20 and 20/10. These are considered to be "normal vision" with the 20/20 line being the traditional normal vision line.

Visual Worksheet 03

Damian Schofield


Dark adaptation

Our eyes have to dark-adapt before we can see in the dark or dim light. Usually, this requires fifteen to twenty minutes (or more) in an environment as dark as the environment you will be viewing.

First of all, the pupil needs to dilate to its maximum aperture in order to collect the most light. Another component of night vision is contained in the biochemistry of the eye. The rhodopsin contained in our rod cells is sensitive to very low light levels.

We need to use our rods, as opposed to the color-vision cone cells, in order to see faint objects in the dark. Any source of bright light will saturate the rod cells, destroy the sensitivity of the rhodopsin, and require twenty minutes of further dark adaptation.

Rhodopsin is less sensitive to red light than to other wavelengths. This is why astronomers read star charts and make log entries with the help of dim, red lights. Our red-absorbing cone cells allow us to read, while we maintain most of our dim-light sensitivity.

Dark Adaptation

Damian Schofield


Dark adaptation1

Dark adaptation is how the visual system adapts when going from a bright environment to a dark one. 

Light adaptation happens when we go from a dark environment to a bright one.

For example, you spend the afternoon in the movies and when you leave the sun is still shining, your eyes may hurt when you get outdoors. 

Or perhaps, on a snowy winters day you go from indoors where the lighting is moderate and you find that it is difficult to see for some seconds, perhaps as long as a minute.

One of the major differences between dark adaptation and light adaptation is their time course.  Whereas dark adaptation takes about 30 minutes to be complete, light adaptation happens very quickly, usually in less than a minute.

Dark Adaptation

Damian Schofield


Dark adaptation2

Another difference between these to type of adaptation is that when you are light adapted and then go into a very dark room for a while you may not see anything at all. 

As you dark adapt more and more things become visible.

When you go from a darker area to a very bright one you usually are not temporarily blind. It is just that your vision, temporarily, is not very good. 

In technical jargon, your contrast sensitivity is poor until you become light adapted. 

By that we mean that you will have difficulty in perceiving areas of low contrast.  It is like every thing is all washed out. 

But as you quickly light adapt the darker areas become darker and the lighter areas become easier to see. 

Dark Adaptation

Damian Schofield


Vision lecture objectives4

  • This component of the Lecture will have these learning outcomes:

    • Introduce the concept of vision~5 mins.

    • Disseminate a few (ir)relevant facts about the eye~5 mins.

    • Understand the working of the eye~10 mins.

    • Introduction to Illusions~15 mins.

    • Optical Illusions~20 mins.

    • Depth Perception and Movement Detection~15 mins.

Vision: Lecture Objectives

Damian Schofield


Introduction to illusion

Amongst the senses, Plato gave primacy to sight. Aristotle ranked sight over hearing: 'Of all the senses, trust only the sense of sight'.

Plato and Aristotle closely associated vision and reason.

This has been a persistent bias in Western culture.

Thinking is associated with visual metaphors: phenomenon (Greek: 'exposing to sight'); definition (from definire, to draw a line around); insight, illuminate, shedding light, enlighten, vision, reflection, clarity, survey, perspective, point of view, overview, farsighted.

Other words associated with thinking also have visual roots: intelligent, idea, theory, contemplate, speculate, bright, brilliant and dull.

And there is no shortage of commonly-used phrases which emphasise the primacy of the visual: Seeing is believing , Let me see, I see, I'll believe when I see with my own eyes, Seeing eye to eye, The mind's eye, See what I mean?

It is likely that the spread of literacy in modern times has helped to privilege sight.

Introduction to Illusion

Damian Schofield


Introduction to illusion1

One thing we do know, is that the human visual system is a mixture of distributed and central processing.

There are at least two levels of vision, autonomous and perceptual.

Shapes are perceived primarily by contours, edges and light level changes.

Vision without contours (edge detection) can cause physical and psychological disorientation.

However, the edges we see may not be real.

Introduction to Illusion

Damian Schofield


Mach banding

We will start with a simple vision problem\illusion, Mach bands. This illusion describes a particular vision property which many optical illusions exploit.

In the image below, Mach bands appear at the boundary between squares of differing gray levels.

Along the boundary the dark side looks darker and the light side looks lighter.

The Mach band effect exaggerates the change in intensity at any boundary where there is a discontinuity in magnitude or slope of intensity.

These bars, known as Mach Bands after their discoverer, the physicist Ernst Mach are illusory.

Mach Banding

Damian Schofield


Mach banding1

The best way to explain this is with an example. In the graph below, the solid black curve represents the amount of light being reflected from the figure on the left.

The dashed red curve represents the brightness of this figure as it is usually perceived. To the left of the point where the figure just starts to get lighter people usually see a dark bar that is slightly darker that the area to the left of it.

At the point where the brightness just stops increasing, people usually perceive a bright bar.

Mach Banding

Damian Schofield


Mach banding2

The receptive fields are represented as a disk (+) and annulus (-) in the figure above. The center disk is an excitatory area and the annulus an inhibitory area.

The receptive fields in the uniformly white and uniformly black areas receive about the same stimulation in their excitatory centers and inhibitory surrounds.

Therefore the center excitations are in balance with the surround inhibitions.

Mach Banding

Damian Schofield


Mach banding3

The receptive field over the bright Mach Band gives a stronger response in the center because part of the surround is in the darker area.

Therefore it receives less inhibition from the surround than did the center at the extreme left and right ends.

The receptive field over the dark band receives more surround inhibition because part of the surround is in the brighter area.

Therefore, the excitatory response is less and results in seeing the area as darker.

Mach Banding

Damian Schofield


Grouping and similarity

It is natural for humans to group together

similar visual objects.

The image on the right is a prime example;

humans are more likely to impose a particular

grouping on what they see.

People tend to refer to five pairs of lines which

are close together with fairly broad gaps between them.

They are less likely to group together the lines which are further apart, perhaps partly because this would leave lonely lines on each side of the image, but also, perhaps, because we seem to have a predisposition to associate things which are close together.

Groupings can be triggered by position or similarity. Shapes are also seen in order to maintain continuity, humans fill in the gaps. The context in which a shape is seen can also determine its perception.

Grouping and Similarity

Damian Schofield


Grouping and similarity1

Grouping and Similarity

Damian Schofield


Grouping and similarity2

Grouping and Similarity

Damian Schofield


Categories of illusion

A picture assembled on the basis of partial information must be expected to occasionally be in error.

The mind will always try to match stimulus and memory to create a picture.

It will make what seems to be the most likely choice, and present that to the consciousness.

An illusion occurs when the choice is incorrect.

If a picture is created solely from memory, without visual stimulus (or with only a minimal visual stimulus) the result is hallucination, with which we shall not be concerned here, since it is a disorder of perception, not a normal or intended part of it.

Things that are not there can also appear in illusion, it must be emphasised, but here it is normal.

Categories of Illusion

Damian Schofield


Categories of illusion1

An illusion can arise in any of the three links of

visual perception.

The mirage is an example of an external illusion,

created by light rays.

When we stare at a brightly illuminated red

disk for a time, then transfer our attention to a

white paper, we see a green disk as a result of

what is called fatigue.

When the full moon is seen at the horizon, it

seems much larger than when riding high in

the sky, though physically it subtends exactly

the same angle at the eye. This familiar

illusion occurs in the third, mental link of

vision, and a satisfying explanation of it is unknown.

Illusions in the third link are most generally recognised as optical illusions.

Categories of Illusion

Damian Schofield


Optical illusions camouflage

Tricking the eye into recognising one thing while

observing another is often very useful to living things.

There are three different ways to do this.

First, one might mimic something dangerous or

nasty-tasting, as does the fly who resembles a wasp,

or a butterfly with large eyes on its wings.

Another way is to merge with the background, as do moths, stick insects, tabby cats etc.

The third way is to look like something else. Cylindrical snakes and lizards are dark on top and light on the bottom, contrary to the normal modelling of a cylinder, so they can resemble flat objects containing no substance.

MOVIE

Optical Illusions - Camouflage

Damian Schofield


Classic illusion

A picture drawn on a flat background is an attempt to trick the eye into perceiving a three-dimensional scene.

This is very effective, since the eye must do something similar in its normal functioning, because the retina is two-dimensional.

The skill of perceiving depth and perspective in a painting is learned, not innate.

In moving pictures, the mind interprets the succession of static frames as continuous motion, again something it must do in its normal functioning.

Let's look at some classic static illusions created by black-and-white figures.

These are mainly third-link illusions resulting from a failure of estimation, or from the faulty comparison of distances or objects.

There is no satisfying explanation for any of these illusions, or even of the reasons why they should exist. They can, however, be recognised and classified, and have some practical application.

Classic Illusion

Damian Schofield


Vision lecture objectives5

  • This component of the Lecture will have these learning outcomes:

    • Introduce the concept of vision~5 mins.

    • Disseminate a few (ir)relevant facts about the eye~5 mins.

    • Understand the working of the eye~10 mins.

    • Introduction to Illusions~15 mins.

    • Optical Illusions~20 mins.

    • Depth Perception and Movement Detection~15 mins.

Vision: Lecture Objectives

Damian Schofield


Classic illusion1

In the bisection illusion, the vertical line is the same length as the horizontal line it bisects, though it seems about 25% longer.

The illusion persists if the figure is rotated 90°, so it is not due to asymmetry of the retina, as one witless psychologist asserted.

Classic Illusion

Damian Schofield


Classic illusion2

In the Müller-Lyer illusion, the line is bisected by the center arrowhead.

The segment with the diverging wings appears longer, but it is not.

Classic Illusion

Damian Schofield


Classic illusion3

In the annulus illusion, the area of the central disk is equal to the area of the annulus surrounding it, though it appears greater.

Classic Illusion

Damian Schofield


Classic illusion4

Distance b-c in the lozenge illusion is equal to distance a-b, though appearing significantly longer.

Classic Illusion

Damian Schofield


Classic illusion5

In the curvature illusion, all three arcs have exactly the same radius of curvature.

Classic Illusion

Damian Schofield


Classic illusion6

Poggendorff's and Zollner’s illusions are very famous.

Classic Illusion

Damian Schofield


Classic illusion colour

Puzzle: Which of the two knights is the white knight (i.e. which is the lighter of the two)?

Is it the one on the middle of the

chessboard, or the one on the top left?

Which square is lighter, the

knight square in the middle or the

knight square at the top?

Classic Illusion - Colour

Damian Schofield


Classic illusion colour1

The incredible answer is: both knights,

and both squares on which they stand,

are exactly the same shade of grey

(RGB = 107, 107, 107) with identical

light shading of the pieces.

We know that this is difficult to believe.

Even enlarging the image and examining

it closely fails to convince you.

Classic Illusion - Colour

Damian Schofield


Classic illusion colour2

Classic Illusion - Colour

Damian Schofield


Vision lecture objectives6

  • This component of the Lecture will have these learning outcomes:

    • Introduce the concept of vision~5 mins.

    • Disseminate a few (ir)relevant facts about the eye~5 mins.

    • Understand the working of the eye~10 mins.

    • Introduction to Illusions~15 mins.

    • Optical Illusions~20 mins.

    • Depth Perception and Movement Detection~15 mins.

Vision: Lecture Objectives

Damian Schofield


Eye movements

There are four basic types of eye movements:

Saccades are rapid, ballistic movements of the eyes that abruptly change the point of fixation. They range in amplitude from the small movements made while reading, for example, to the much larger movements made while gazing around a room.

Smooth pursuit movements are much slower tracking movements of the eyes designed to keep a moving stimulus on the fovea. Such movements are under voluntary control.

Vergence movements align the fovea of each eye with targets located at different distances from the observer. Unlike other types of eye movements in which the two eyes move in the same direction, these movements are disconjugate (disjunctive).

Vestibulo-ocular movementsstabilise the eyes relative to the external world, thus compensating for head movements. These reflex responses prevent visual images from "slipping" on the surface of the retina as head position varies.

Eye Movements

Damian Schofield


Eye movements reading

Reading is not performed through continuous eye movement but through sudden changes of fixation, fixating a given point in a space, encompassing the surrounding letters.

The speed limit of the eye to shift from one fixation to another is determined by the time that brain takes to process the information input.

Faster reading is not achieved by quicker eye movement but an expansion of visual field.

Eye Movements - Reading

Damian Schofield


Depth perception

Depth perception is the ability to see the world in three dimensions and to perceive distance.

Although this ability may seem simple, depth perception is remarkable when you consider that the images projected on each retina are two-dimensional.

From these flat images, we construct a vivid three-dimensional world.

To perceive depth, we depend on two main sources of information: binocular disparity, a depth cue that requires both eyes; and monocular cues, which allow us to perceive depth with just one eye.

Depth Perception

Damian Schofield


Depth perception1

Distance \ depth perception is almost impossible to turn off

Depth Perception

Damian Schofield


Depth perception2

Distance \ depth perception is almost impossible to turn off

Depth Perception

Damian Schofield


Depth perception3

Binocular Disparity

Perhaps the most important perceptual cues of distance and depth depend on so-called binocular disparity.

Because our eyes are spaced apart, the left and right retinas receive slightly different images.

This difference in the left and right images is called binocular disparity.

The brain integrates these two images into a single three-dimensional image, allowing us to perceive depth and distance.

The phenomenon of binocular disparity functions primarily in near space because with objects at considerable distances from the viewer the angular difference between the two retinal images diminishes.

Depth Perception

Damian Schofield


Depth perception4

Binocular Disparity

Depth Perception

Damian Schofield


Depth perception5

  • Monocular Disparity

  • Monocular cues are cues to depth that are effective when viewed with only one eye. Although there are many kinds of monocular cues, the most important are interposition, atmospheric perspective, texture gradient, linear perspective, size cues, height cues, and motion parallax.

    • Interposition:

    • Atmospheric Perspective:

    • Texture Gradient:

    • Linear Perspective:

    • Size Cues:

    • Height Cues:

Depth Perception

Damian Schofield


Depth perception6

Monocular Disparity

Motion Parallax: Motion parallax appears when objects at different distances from you appear to move at different rates when you are in motion. The rate of an object's movement provides a cue to its distance. The more distant objects appear to move in a more slower pace.

Depth Perception

Damian Schofield


Movement detection

Movement detection is the oldest and most important visual process.

All seeing animals can detect movement, though they may vary widely in their other perceptual abilities.

If one regards vision as an "early warning" system, aiding in the detection of predators, prey and other biologically-salient events, then the important thing to detect is environmental change, and movement always accompanies this.

Movement helps in encoding the third dimension (by means of motion parallax) and aids image segmentation and hence object recognition.

Movement Detection

Damian Schofield


Movement detection1

There are various types of apparent (i.e. illusory) movement. As with other illusions, they give us some insight into the processes underlying normal movement perception.

(a) Induced Movement:

(b) Apparent Motion (Stroboscopic Motion):

(c) Autokinetic Movement:

(d) Movement After-Effect (MAE);

Movement Detection

Damian Schofield


Movement detection2

Movement Detection

Damian Schofield


Final quote

To finish the topic of vision a quote taken from “The Object Stares Back” based on the work of a French surrealist writer.

…. There are three things that throw the eyes into such confusion that they may not be able to see at all:

The Sun

Death

and Genitals

Confronted with objects like these, vision goes out of control, and we see where we do not want to, or fail to see where we should, and our eyes no longer obey our conscious wishes.

Final Quote

Georges Bataille, Oevres Completes, ed (Paris, 1970), Volume 1.

Damian Schofield


Vision lecture achievements

  • This component of the Lecture will have explained learning outcomes:

    • Introduce the concept of vision~5 mins.

    • Disseminate a few (ir)relevant facts about the eye~5 mins.

    • Understand the working of the eye~10 mins.

    • Introduction to Illusions~15 mins.

    • Optical Illusions~20 mins.

    • Depth Perception and Movement Detection~15 mins.

Vision: Lecture Achievements

Damian Schofield


Vision lecture objectives

End

Damian Schofield


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