Visual Perception

Simon Bartlett

This area of study focuses on perception and the general characteristics of the visual

sensory system.

What are the stages?

• Reception – eye senses a stimulus• Transduction – changes it so brain can

understand it• Transmission – sends it to the visual cortex• Selection – aspects selected of stimulus• Organisation – grouping of elements to form a

whole• Interpretation – given meaning with the aid of

psychological factors

Reception

• Electromagnetic energy (light) sensed from the environment

• RODS (black and white) and CONES (C for colour) are photoreceptors that pick up the electromagnetic energy

• This energy must be CHANGED for the brain to be able to interpret it…

Transduction

• The electromagnetic energy is converted, changed or ‘transduced’ into electrochemical energy.

• This is the energy all neurons use.

• This means that information sensed by the rods and cones can be sent as neural impulses to the visual cortex.

Transmission

• Neural impulses once triggered are sent to visual cortex of the occipital lobe

• Like any neuron, the “all or none” law applies – there is no such thing as a big or small impulse. The stimulus must be ENOUGH for the threshold to be reached to make the neuron fire.

• They are sent via the optic nerve.

Selection• This is where discrimination and identification of

the FEATURES of the stimulus takes place.

• Feature detectors are cells that are in the retina, optic nerve and visual cortex, that respond to patterns, lines, edges and angles

• Selection is also happening at a basic level in reception with rods and cones responding to different electromagnetic energy wavelengths.

Organisation

• This can only happen once the brain has received the neural impulses.

• Single elements are grouped to form a whole, using perceptual principles that work like ‘rules’ of organisation.

• These principles are called Gestalt principles.

Interpretation• The ‘whole’ that has been created is given

meaning.

• Memory is used to compare incoming information with what is already known.

• This creates a perceptual hypothesis.

• Interpretation involves the brain using psychological factors in order to make sense of what it is considering.

Absolute Threshold

• The minimum amount of energy needed to detect a stimulus under ideal conditions 50% of the time.

• VISION: A candle flame 50km away on a clear, cloudless night with NO light pollution (moon, other lights, cities)

Differential Threshold or JND• The smallest difference in the intensity of a

given stimulus that a specific sense can detect 50 % of the time.

• This is about DIFFERENCE in the intensity of a stimulus; there must be comparison.

• If there is enough change, there will be enough stimulus for receptor cells to fire and send a message to the brain to be interpreted.

• VISION: a dimmer switch does not turn a light off but changes the intensity of the light.

Physical Structure – the Eye

Let there be light!

• Receptor cells respond to 380 nm (violet) to 760 nm (red) of the electromagnetic radiation spectrum.

• This is the visible light spectrum to which humans are able to respond.

• To remember the pattern of light in order, think ROYGBIV (red, orange, yellow, green, blue, indigo, violet).

Eye structure and function• CORNEA – light enters through the cornea and

is bent onto the lens.

• PUPIL – light passes through the SPACE called the pupil.

• IRIS – a band of muscle that contracts/expands to manage the amount of light entering the pupil and hitting the lens.

• LENS – the lens bends the light more and focuses them on the retina – in particular, on the FOVEA. The lens can bulge or stretch to help the light reach the fovea.

Eye structure and function…

• RETINA – registers the electromagnetic energy, processes the incoming information and transduces the energy to a form the brain can interpret.

• BLIND SPOT – where the optic nerve attaches to the retina – no photoreceptors.

• PHOTORECEPTORS – 2 types:Rods – 125 MillionCones – 6.5 Million

Rods VS ConesRODS

• Black and white vision• Operate well in low level light

– night vision• Sensitive to brightness,

darkness and movement• Mainly located in outer part

of retina• Poor acuity (low sharpness

and focus)• Excellent for peripheral

vision rather than direct• 19x the number of cones

CONES• Daytime and colour vision• Excellent visual acuity –

sharp and clean images• Concentrated in the fovea• Not useful at night – can’t

discriminate colours• Outnumbered by rods 19:1

GESTALT Principles• Theory that the whole is greater than the sum of

its parts – Gestalt means ‘form’ or ‘shape’.• We group individual elements of a visual

stimulus into a complete form.• This allows us to perceive objects in the most

simple way possible.

• BOTTOM-UP = building an image from the parts into a whole

• TOP-DOWN = working to move from the whole image to the parts it is assembled from

What are the 4 Gestalt Principles?

• Figure Ground

• Closure

• Similarity

• Proximity

Figure-Ground• We tend to separate the

important aspects of the figure from the surroundings (background). We focus or give our attention to the figure.

• Real or imagined contour lines separate the figure and ground

• Camouflage depends on difficulties with establishing contour lines

• Reversible figures change the ownership of the contour lines

Love and Death?

Proximity

Proximity is the grouping of elements that are close to each other to form an overall figure or pattern. Also known as ‘nearness’.

Similarity

When the elements of a stimulus or pattern have similar features (size, shape, colour) we tend to group them together. Similarity is stronger than proximity if the two are together in a stimulus.

ClosureWhat do we see?

We perceive an object as a whole, despite it being actually incomplete. We group the individual elements to make ‘one’ by filling in the missing contour lines so that it makes sense.

Fido?

Depth Perception

• Depth cues which are internal and involve the eye and the brain and are known as primary cues.

• Depth cues which are external to the body and part of the environment or pictures are known as secondary cues.

Binocular Cues

• Binocular cues are primary cues and involve the eye and its functioning.

• Binocular cues ALWAYS involve BOTH eyes

• Convergence

• Retinal Disparity

What do I need to know about convergence?

• We use this for objects up to 6m away from our eyes.

• When we view objects close to us, our eyes turn inwards or ‘converge’ so that a single image is formed on both retinas.

• Changes in muscle tension are detected and interpreted by the brain and used to determine the depth and distance of an object.

• Our brain detects that the closer the object, the greater the convergence.

How does retinal disparity work?• Because our eyes are 6-7cm apart, the retinas

receive slightly different images. An object must be under 10 metres away for us to use retinal disparity.

• The retinal images are combined together and compared by the brain.

• Any disparity or difference between the 2 images provides us with information about the depth of the object and its distance from us.

• You can check this by only using one eye, in turn, to view the same object on your desk.

Monocular Depth CuesAccommodation:

This is a primary monocular depth cue that involves the lens of the eye changing shape. Information about how much the lens bulges or flattens is used by the brain to determine depth and distance of an object.

Object close? Lens bulges to direct the image clearly onto retina.

Object distant? Lens flattens or elongates to direct the image clearly onto retina.

Secondary Monocular Depth Cues

• Linear Perspective

• Interposition

• Texture Gradient

• Relative size

• Height in the visual field

Pictorial Cues are secondary cues, as they occur in the environment – they are NOT a function of the eyes!

Secondary Monocular Depth Cues

Linear Perspective• Parallel lines appear

to converge towards the horizon and an imaginary’ vanishing point, creating a sense of distance.

Interposition• The object that is

closer obscures part of another, more distant object.

• It is useful to indicate which objects are closer than others but not actual distance.

Secondary Monocular Depth Cues

Texture gradient

• The amount of detail in a scene decreases as the distance increases or moves away from the viewer.

• The foreground is highly detailed and the background is less detailed and less focussed.

Relative Size

• If separate objects are predicted to be the same size, then the one that appears larger, is closer.

• The retinal images of the objects are different and we can only use relative size if we KNOW the size of an object to be able to compare.

Secondary Monocular Depth Cues

Height in the

Visual field

• NEVER just call this ‘height’!

• If land objects appear smaller and closer to the horizon, they appear to be further away.

• If air objects appear smaller and closer to the horizon, they appear to be further away.

• HOWEVER – land based objects will be LOW in the visual field if close and air objects will be HIGH in the visual if close.

Henri Cartier-Bresson

Concert crowd - Bristol, UK

Perceptual Constancies• These are learned visual perception

principles that let us perceive or make sense of stimuli.

• This means when objects change in shape, size or brightness, we still see them as being what they are – a ‘constant’ and familiar thing.

• Our perception stays the same even though the retinal images may have altered; we are familiar with the objects.

Size Constancy

• We maintain an object’s perceived size even though the size of the retinal image alters due to the object’s distance from us.

• If we look at a truck outside our homes, we KNOW that the truck isn’t smaller if we stand and look at it as it drives down the street. We don’t think it ‘shrinks’ as it is driving away from us.

Brightness Constancy• We maintain the perceived brightness of

an object, despite actual changes in the amount of light being received by the retina.

• Because we look at objects in their immediate environment, we perceive that everything else has altered by the same amount of brightness too and so know that the brightness has remained constant.

Shape Constancy

• We maintain an object’s perceived shape even though the image cast on the retina changes if the object is observed from a different angle.

• If we see a closed door, a half open door and a fully open door, we still understand that the door has a rectangular shape, even if it appears different when closed, half open and fully open.

Orientation Constancy• This is our tendency to maintain an

object’s location in ‘space’ as constant – and perceive the world as being upright and vertical.

• If we hang upside down on monkey bars, we don’t believe that the world is no longer vertical and that the trees are upside down and sideways! Again, we compare and use our body’s signals to help us work out what’s going on.

Psychological factors affecting Visual Perception

Perceptual Set or ‘Expectancy’

• We interpret stimuli by comparing it to what we already know and have experienced = prior experience.

• We tend to see what we EXPECT to see – this includes ignoring some stimuli and giving attention to others.

• Factors like emotions, motivation, context or setting, cultural background and past experience all affect our perceptual set.

Physiological factors affecting Visual Perception

Alcohol… Slows us down, changes coordination, reduces attention, concentration and ability to make judgements.

Old Age…cataracts, long-sightedness, age-related macular degeneration, retinal detachment, diabetic retinopathy, glaucoma. Our eyes age as we do and become more susceptible to disease and damage.

OPTICAL ILLUSIONS

OR

HOW EASY IT IS TO TRICK HUMAN BEINGS….

How do Optical Illusions work?

• A visual stimulus misleads our perception (or meaning-making) of that stimulus.

• This happens because we APPLY perceptual constancies to what we are seeing – they are our RULES.

• We make a false judgement because we misjudge length, position, speed, direction or curvature.

MULLER LYER Illusion

There are 2 theories to explain this illusion!

The Muller Lyer illusion consists of 2 lines that are equal in length to each other.

•One has ‘arrow-heads’ attached.

•One has fish-tails’ attached.

Theory of Perceptual Compromise

1. The length of the solid horizontal lines of each figure are actually equal in length.

2. BUT the length of each WHOLE figure is not the same.

3. The lengths between the added tips (heads or tails) is very different. We use closure to estimate the length and compromise somewhere between the actual length and the perceived length.

Diagram of Perceptual Compromise

The Apparent Distance Theory• We use our knowledge from depth cues and

size constancy.• We liken the line with ‘fish-tails’ as being the

inside corner of a room in a building.• We liken the arrow-heads as the closest

outside corner of a building .• BOTH lines cast the same size image on the

retina.• As the fish-tails are perceived to be further away

(inside the building), we apply size constancy and interpret that the line must be longer.

Diagram of Apparent Distance Theory

The Ames Room• We must only use MONOCULAR depth cues in

order to perceive the Ames room as an illusion.

• It is a trapezium - a shaped room that is much higher and longer on one side than the other, however, it LOOKS like a rectangle to the viewer.

• The back corner on the left hand side is double the distance away from the viewer, compared to the back right hand side of the room.

The Ames Room

• The roof is also higher on the left hand side of the room, compared to the right hand side of the room.

• All of the decorations, windows and furnishings are also not ‘square’ – there are no right angles to be seen. Even clocks are not circles but odd ovals.

Ames Room construction

The Ames Room• A person standing in the back left hand

corner of the room is going to appear very small.

• A person standing in the back right hand corner of the room looks like a giant.

• If a person crosses the room, they appear to shrink or grow in size.

• This creates a conflict for the viewer!!! The retinal images of the room’s corners (etc) are the same shape and size.

The Ames Room

EXPLANATIONS?SHAPE CONSTANCY:

If the viewer maintains shape constancy, they continue to perceive the room as SQUARE or RECTANGULAR – this means they see the person shrink and grow. Most people do this.

EXPLANATIONS?

SIZE CONSTANCY:

It is difficult to maintain size constancy in the Ames room as it means that the brain must remain totally confused! The retinal information (the same sized retinal image of the room’s corners) cannot be corrected due to the lack of accurate depth information – even if we know what we are ‘seeing’ is not possible.

Perception Deception

Is this picture REALLY moving?