PERCEPTION

•The feel of cotton wool (TOUCH)

•The smell of a cake cooking (SMELL)

•The taste of chocolate (TASTE)

•The sound of sad song (HEARING)

•Knowing that we are sitting up (KINAESTHESIA)

•And seeing a beautiful flower (VISION)

Visual Perception

Sensory Processes: are involved with the detection, reception, conversion and transmission of raw data.

Visual Perception consists of two main processes: sensory processes and cognitive processes.

Cognitive Processes: are where the brain processes, analyses, organises and interprets information so that it becomes meaningful.

(Adapted from Grivas, et al. 2004)

Bottom up Top down

Blind Spot

(and Fovea)

The visible light spectrum is just a tiny ‘slice’ of the total electromagnetic energy (radiation) spectrum.

www.visionweb.com

Thresholds: absolute and differential

• How dim can a light be and still be visible? Is there a specific light intensity that represents the limit which a person is able or unable to perceive a visual stimulus? How much change in light intensity is required for our perceptual system to notice the difference?

• When examining sensory processes, psychologists use the term threshold to refer to our ability to detect a stimulus or changes in a stimulus. There are two main types of thresholds for vision: absolute and differential.

(Grivas, et al, 2004)

Absolute Threshold

• The absolute threshold for vision refers to the minimum amount of light energy that is necessary in order for a visual stimulus to be perceived. (Grivas, et al, 2004).

• Examples of what the absolute thresholds for our senses would be are as follows:

Vision: a candle flame seen from a distance of 50 kilometres on clear, dark night.

Hearing: the ticking of a watch under quiet conditions at a distance of six metres.

Touch: the wing of a bee falling on your cheek from a height of one centimetre.

Smell: one drop of perfume diffused throughout a three-room apartment.

Taste: one teaspoon of sugar dissolved in eight litres of water.

A Snellen Chart

THE NATURE OF PROCESSES IN VISUAL PERCEPTION

There are six key processes that occur in visual perception. These are:

1. The capture of visual information (reception): like taking a photo, the eye captures a visual image and focuses it on the photoreceptors in the retina.

2. Conversion of the information into a form that can be processed (transduction): photoreceptors in the retina convert light energy into neural impulses.

3. Sending the information from the eye to the brain (transmission): once converted, the information is transmitted from neuron to neuron to the visual cortex in the brain.

4. Identifying specific features of a visual stimulus (selection): involves differentiating between the various features that make up a visual stimulus such as colours, and orientation and angles of lines.

5. Arranging features of the visual stimulus into a form (organisation): in this process we reassemble the key elements of a visual stimulus in a meaningful way. We use ‘rules’ to do this.

6. Making sense of the visual information (interpretation): this involves cognitive processes and will vary between individuals.

(Grivas, et al, 2006)

The following slide provides and example of how we can interpret information differently depending on which features we focus on more.

ORGANISATION IN VISUAL PERCEPTION

There are two broad categories of cognitive processes that we use when we assign meaning to incoming visual information. One category comprises the principles or ‘rules’ of visual perception that are believed to be innate or learned very early in childhood.

The other category comprises psychological factors, which are predominantly learned.

(Grivas, et al, 2006).

The simultaneous use of these two cognitive processes is illustrated in our interpretation of the following picture…….

In order to perceive a Dalmation (dog) walking along and sniffing the ground from an apparent series of meaningless splotches your visual perception system would probably have first established that there are a number of black splotches against a white background. In doing so, you applied one principle of perceptual organisation (called figure-ground).

Furthermore, to assist your interpretation of the various splotches, you probably used another perceptual principle (called closure). This involved completing the figure by mentally filling in the missing bits to help organise the splotches into a recognisable form).

Another perceptual principle (called proximity) may have been used to support your interpretation of the dog’s legs and head. These forms were interpreted by organising them on the basis of how close the splotches were to each other, the assumption being that they were somehow related because they were close together.

Still another factor is our individual experiences and psychological factors often unique to the individual. These can affect how readily we recognise different forms, for instance. Without past experience, which involves the use of information in memory, the interpretation of the dog would not be possible.

(Adapted from Grivas, et al, 2006)

VISUAL PERCEPTION PRINCIPLES

Visual perception principles can be classified into three broad categories: Gestalt principles, depth principles and perceptual constancies.

Gestalt principles

Gestalt principles of visual perception refer to the numerous ways in which we organise the elements in our visual field by grouping them into the perception of a whole, complete form, usually in the simplest possible way. For example, if part of what we are observing is hidden or obscured, then in order to make sense of what is being observed, we simply construct a whole or complete form by mentally filling in the parts that are not visible.

The two following paintings by Giuseppe Archimboldo (1527-93) provide an example of how the ‘parts’ (fruit and vegetables) are organised into the ‘whole’ form of a person.

Do we process the parts or the whole first??? Do we act globally or locally??

Do you recognise the big letter before the smaller letters?

H H HHHH H H HH H H HHHHHH HHHH H HH H H HH H HHH

David Navon (1977) found that the local letter (the smaller one) had no impact on the speed of the recognition of the global letter (the bigger one), however, identification of the smaller, local letter was much slower when the global and local letters did not match.

Navon concluded that we mentally process the whole before we analyse the parts. (Grivas, et al, 2006).

Types of gestalt principles

1. Figure-ground

When we use figure-ground, we organise visual information by perceptually separating important aspects of the visual field into the ‘figure’, which stands out from the ‘ground’.

You are doing this while reading this sentence.

By making the object the centre of our focus it becomes the figure, while all other visual information becomes the ground, or background.

This figure-ground organisation is achieved by the observer separating the figure from the ground using an existing (actual) or perceived (inferred) line. This line of separation between figure and ground is known as a contour.

When this becomes difficult, whether intentionally or not, this creates camouflage which restricts our ability to mentally attach contours to figures to separate them from the background.

(Grivas, et al, 2006).

These signs are designed to make the figure stand our clearly from the background.

Artists have cleverly used figure-ground principles to create ambiguous contours.

Can you find 13 faces hidden in this artwork?

Types of gestalt principles

2. ClosureClosure refers to our perceptual tendency to mentally ‘close up’, fill in or ignore gaps in a visual stimulus and to perceive objects as complete.

This technique is common in advertising. With the Telstra logo, we mentally impose a missing vertical stroke to the letter ‘T’. And, with the IBM logo we fill in the gaps between each of the horizontal lines to mentally form solid letters and/or ignore the gaps that prevent solid letters.

(Grivals, et al, 2006).

Types of gestalt principles

3. SimilarityThe tendency to perceive stimuli or parts of a visual stimulus that have similar features – such as size, shape, texture or colour – as belonging together in a unit, group or ‘whole’.

Impressionist artists using a technique called ‘pointillism’ relies on our use of of the perceptual principle of similarity.

(Grivas, et al, 2006)

The Ishihara tests are used to diagnose colour blindness. When tested, people are required to visually perceive the number in the stimulus figure. What two Gestalt principles would a person who did not suffer from colour blindness apply here?

Types of gestalt principles

4. Proximity

Also called ‘nearness’, is the tendency to perceive parts of a visual stimulus that are positioned close together as belonging together in a group.

This can be seen with letters of the alphabet. Several series of letters located in close proximity to each other will be perceptually groups and interpreted as words. For example:

What’s that on the road ahead?

What’s that on the road a head?

The whole can be interpreted differently when there is a subtle difference in the proximity of just one of its parts.

Also consider the spots on the dalmation from the earlier slide. (Grivas, et al, 2006)

Depth perception

In addition to the principles that influence how we perceive objects, there is another group of principles that helps us to determine where objects are perceptually located. Depth cues are sources of information from the environment (external cues) or from within our body (internal cues) that help us to perceive how far away objects are (a perception of depth and/or distance).

Furthermore, our retina actually holds two-dimensional image from the world around us, but we visually perceive the world in three-dimensional forms. Depth cues allow us to do this. Depth cues can be binocular (requiring the use of both eyes) or monocular (requiring the use of only one eye).

(Grivas, et al, 2006)

Retinal Disparity

Because our eyes are about 6 or 7 centimetres apart, each retina receives a slightly different visual image due to the different angles of view. Retinal Disparity refers to the very slight difference in the location of the visual images on the retinas which enables us to make judgements about the distance of an object from the viewer.

Convergence

Convergence involves the brain detecting and interpreting depth or distance (up to 6 metres) from changes in tension in the eye muscles.

Monocular depth cues

Require only the use of one eye to provide information to the brain about depth and distance, but they also operate with both eyes.

Accommodation

Involves the automatic focusing mechanism of the lens in the eye to adjust the shape of the lens in response to differing distances of view from the object. The lens bulges to focus on nearby objects and elongates to focus on distant objects.

From Grivas, et al, 2006

Accommodation

The brain monitors the movement of the ciliary muscles that control the shape of the lens. Information about how much the lens bulges or elongates is used by the brain to determine the depth and distance of the object in focus.

Accommodation enables the continuous focusing of moving objects – even those travelling at great speed.

From Grivas, et al, 2006

Pictorial cues

Artists use these extensively to portray depth and distance on a two-dimensional surface. Pictorial cues include linear perspective, interposition, texture gradients, relative size, and height of the visual field.

Linear perspective

Is the apparent convergence of parallel lines as they recede into the distance.

Pictorial Cues

linear perspective

texture gradient

The Flight Into Egypt(1306) by Giotto.

The Annunciation by Crivelli (1430-95)

Texture gradients

Texture gradients refer to the gradual diminishing of detail that occurs in surfaces as they recede into the distance, compared with objects in the vidual field that are close and seen in fine detail. Thus, our judgement about depth and distance is influenced by the extent to which we can detect fine detail.

(Grivas, et al, 2006)

Interposition

Interposition, or overlap as it is also called, occurs when one object partially obscures another, and the partially obscured object is perceived as further away than the object that obscures it (and vice versa).

Relative size

Refers to the tendency to visually perceive the object that produces the largest image on the retina as being closer, and the object that produces the smallest image on the retina as being further away.

Height in the visual field

Refers to the location of objects in our visual field, whereby objects that are located closer to the horizon are perceived as being more distant than objects located further from the horizon. An object on the ground and below the horizon gets higher in the visual field (and therefore smaller) as it moves away towards the horizon. However, an object in the sky and above the horizon gets lower in the visual field (and therefore smaller) as it moves away towards the horizon.

(Grivas, et al, 2006)

Pictorial Cues

a) Occlusion/Interposition (overlap)

b) Relative Size in the field of view

c) height in the field of view

shading

Pictorial Cues

Visual constancy

Visual constancy refers to the tendency to perceive a visual stimulus as remaining stable and unchanging despite any changes that may occur to the image despite any changes that may occur to the image cast on the retina. Size, shape, brightness and orientation are all aspects of visual constancy.

Size constancy

Size constancy involves recognising that an object’s actual size remains the same, even though the size of the image that is cast on the retina changes.

Shape constancy

Shape constancy is the tendency to perceive an object as maintaining its shape despite any change in the shape of the image cast on the retina.

Brightness constancy

Brightness constancy is the tendency to perceive an object as maintaining its level of brightness relative to its surroundings, despite changes in the amound of light being reflected from the object onto the retina.

Orientation constancy

Orientation constancy is the tendency to visually perceive the true position of an object in the environment even though the retinal image of the object may be at a different orientation.

Perceptual set

Look around you and write a brief description of your surroundings………………………

It is unlikely that anyone described their surroundings in exactly the same way. This is because there are psychological factors which influence how we interpret incoming sensory stimuli. These psychological factors are learned and sometimes cause us to perceive visual stimuli in a subjective (personal) manner. That is, we have a predisposition ( a readiness in advance) about how we will visually perceive objects and events. Psychologists refer to this predisposition as a perceptual set.

A perceptual set is the readiness to perceive stimuli in accordance with certain expectations that often cause us to select certain aspects of the visual stimulus to be organised and interpreted, yet ignore other aspects.

Two key factors that contribute to a perceptual set are context and past experience. These factors often work together in influencing our interpretations of a visual stimulus.

(Grivas, et al, 2006)

Context

Context refers to the environment or setting in which a perceived event occurs or an object exists.

The context provides a framework for the observer to consider on object or event. In doing this, this framework leads the observer to eliminate any other context in which the object or event may have been considered. Context therefore plays a focusing role for the observer.

Imagine the different interpretations you might make of a fast-moving bright light in the sky that has a tail streaking behind it. How would you interpret this visual stimulus if you observed it in the sky over:

(1) Outback Australia

(2) A war zone

(3) Cape Canaveral, Florida

(4) Hobart Wharf on New Year’s Eve

(5) A ship at sea

(Grivas, et al, 2006)

Titchener Cirlces

Bruner and Minturn (1955) B/13

Context experiment

Learning activity 18 p 229 Grivas.

Past experience

Past experience refers to personal experiences that have happened throughout one’s life. These experiences are subjective, in that a given experience may be interpreted in a very ‘personalised’ way by different people. Every individual has a unique combination of past experiences, and it is the individual nature of these experiences that leads to many individual differences in visual perception. Such experiences set or predispose the individual to perceive information in a particular way. (Grivas, et al, 2006)

Our past experiences can help us to make quick judgements, however, they can also cause us to make perceptual errors.

Bruner and Postman aces here.

Past experiences experiment

Cultural factors

Culture refers to the way of life of a particular community that sets it apart from other communities. Culture includes things such as the customs, language, traditions, beliefs, values, attitudes, values, rules about what is right and wrong, food and music.

Experience in a particular culture can influence the way we process and interpret visual information.

Malawi people and photographs.

Bottom-up and top-down processing

Bottum-up processing involves processing information by starting with the individual elements of a visual stimulus and gradually building up a final representation and interpretation. This involves all the sensory processes and those parts of brain processing over which we exert little conscious control; for example, the application of perception principles.

Top-down processing involves using psychological factors such as pre-existing knowledge (past experience) and the current setting (context) to interpret and assign meaning to a visual stimulus.

Visual perception can be enhanced by perceptual set when we correctly anticipate a visual stimulus and thus interpret it more quickly than we would otherwise do. For example, as your retina receives raw sensory information that your teacher writes on the board (bottom-up processing), your perceptual set may enable you to interpret a misspelt work such as ‘rember’ as a meaningful word (‘remember’) in order for you to make sense of what is being written. Your past experience with language and the current context of the sentence would both have played a top-down processing role in allowing you to quickly perceive the mispelt and apparently meaningless word in a meaningful way.

(Grivas, et al, 2006)

Effects of motivation and emotional states on perceptual set

Motivation:

Motivation refers to the process within us that activate, direct and sustain our behaviour towards achieving a particular goal. Motives can be influenced by psychological factors (such as interests, ambitions and desires) or physiological factors (such as bodily responses associated with hunger or thirst). Visual perception can be influenced by our motives when, for example, we see want we want to see, rather than what is actually there (e.g. eye-witness testimonies).

Emotional state:

Our emotional state – how we are feeling – can also influence the way in which we perceive visual information. Different emotions can ‘set’ us to perceive information in a particular way that is consistent with how we are feeling.

(Grivas, et al, 2006)

Context

Motivation

Emotional state

Past experience

Cultural factors

Perceptual set

(Expectancy)Interpretation of a visual stimulus

influences

AWARD REQUIREMENTS

The final award will be determined by the Tasmanian Qualifications Authority from the 15 ratings (10 ratingsfrom the internal assessment and 5 ratings from the external assessment).The minimum requirements for an award in this syllabus are as follows:

EXCEPTIONAL ACHIEVEMENT (EA)13 ‘A’, 2 ‘B’ ratings (4 ‘A’, 1 ‘B’ from external assessment).

HIGH ACHIEVEMENT (HA)6 ‘A’, 7 ‘B’, 2 ‘C’ ratings (2 ‘A’, 2 ‘B’, 1 ‘C’ from external assessment).

COMMENDABLE ACHIEVEMENT (CA)9 ‘B’, 5 ‘C’ ratings (2 ‘B’, 2 ‘C’ from external assessment).

SATISFACTORY ACHIEVEMENT (SA)12 ‘C’ ratings (3 ‘C’ from external assessment).

PRELIMINARY ACHIEVEMENT (PA)7 ‘C’ ratings.

A student who otherwise achieves the ratings for a CA (Commendable Achievement) or SA (SatisfactoryAchievement) award but who fails to show any evidence of achievement in one or more criteria (‘z’ notation) willbe issued with a PA (Preliminary Achievement) award.

Distortions of visual perception: visual illusions

Distortions, or mistakes in visual perception, sometimes occur. Visual illusions demonstrate casees in which reality is misperceived.

For example, when viewing the pattern below it is impossible to perceive the horizontal lines as parallel, yet they are.

A visual illusion is a misinterpretation (distortion or mistake) of real sensory stimuli. It is an experience in which there is a mismatch between our perception and what we understand as physical reality.

Generally, the illusory effects are unavoidable. Even with the knowledge that we are looking at an illusion and an understanding of why the illusion occurs, we continue to see the illusion as powerfully as when we first saw it.

(Grivas, et al, 2006)

The following optical illusions are a sample of over 200 visual illusions identified by psychologists……

Fraser Spiral: Although we perceive a spiral, this is actually a picture of a series of concentric circles.

Poggendorf Illusion: The diagonal line running from bottom right to top left appears to exit the vertical bar too high. A ruler placed on the line can confirm that the line is perfectly straight.

Zollner Illusion: The diagonal lines are all parallel but do not appear so because of the changing direction of the small diagonal lines crossing them.

Horizontal-vertical illusion: (a) although the two lines are equal in length, the vertical line appears to be longer. (b) this illusion is also apparent with a real structure – The Gateway Arch (in St. Louis, Missouri, USA).

Hering Illusion: the parallel lines appear to bow around the point of convergence of the diagonals and vice versa.

Orbison Illusion: the red square appears to be misshapen when placed over the series of concentric circles.

Perceptual Illusions

Which monster appears larger?

The Ponzo Illusion

Perceptual IllusionsThe Ponzo Illusion

Illusion, delusion or hallucination?

An illusion is a perceptual misrepresentation of reality. It is different from a delusion, which is a false belief that is firmly held despite obvious proof that the belief is incorrect; for example, believing that one is Napolean Bonaparte and has been sent to save us all!

An illusion is also different from a hallucination, which is a perception that occurs without external stimulation of the relevant sensory organ and which seems to be real but is not real; for example seeing Napolean Bonaparte walking across the water towards you.

(Grivas, et al, 2006)

The muller-lyer illusion

Are the horizontal lines the same length?

Muller-Lyer Illusion

The Muller-Lyer illusion is a visual illusion in which one of the two lines of equal length each of which has opposite ends, is incorrectly perceived as being longer than the other.

Psychologists has proposed various explanations of the Muller-Lyer illusion as well as other illusions shown previously:One approach is in terms of biological factors: it may be the case that we have a biological tendency to misperceive simple geometric patterns that appear in a two-dimensional form.

Another approach to explaining this type of illusion is in terms of cognitive factors. This explanation suggests that the perceptual error may be due to using inappropriate mental strategies when making perceptual interpretations.

Perceptual compromise: an explanation for the Muller-Lyer illusion

One of the most widely accepted explanations for this illusion is because certain features of the figure provide visual cues that contradict each other and, therefore, give us conflicting information. We deal with this by making a perceptual compromise. A perceptual compromise occurs when two or more visual cues conflict with each other when we are interpreting visual information, and we make an interpretation that is not due solely to one visual cue or the other. In order to deal with conflicting information we tend to compromise by making a middle ground interpretation.

(Grivas, et al, 2006)

Conflicting cues

1. The length of the horizontal lines of each figure - the central horizontal line in (a) is equal in length to the central horizontal line in (b).

2. The length of each whole figure - the entire length of (a) is longer than the entire length of (b).

3. the length between the tips at each end of each figure - see next slide.

We use the Gestalt principle of ‘closure’ to unconsciously draw these red lines in

Muller-Lyer Illusion

Ames room illusion

Ames Room

• The Ames room is designed so that the monocular depth cues give the illusion that the two people are equally far away

Accommodation is evident in the Ames Room because the lens is adjusting to accommodate short and long distances – people in the room are moving towards the subject, not along the straight wall.Room is two illusions:Room appears cubic when viewed monocularly through the peep hole. Room is actually trapezoidal.Floor is inclined/walls slanted but appear perpendicularConstanciesSize – person walks along slanted wall, getting closer and closer to observer therefore gets larger in relation to the room. Shape – regularity of familiar objects (clocks) – all distorted – illusionSecondary depth cuesRelative size – of the people compared to the room – people grow and shrink Conflict between relative size and size constancy contributes to the illusion in the Ames RoomHypothesis testing – check with schema already present then acted on by top down processesTop Down Culture – western culture familiar with vertical and horizontal linesMotivation – highly motivated to make the room make sensePast experienceMostly Bottom Up but some top Down – Analysis by Synthesis.

MOON ILLUSION

Naturally occurring illusionThe moon illusion occurs when the moon appears to be considerably larger on the horizon than when it is high up in the sky, even though the retinal image is equal in both situations, and the moon does not actually change its size as it moves across the sky (Grivas).Apparent distance theory (Kaufman & Rock 1962)Proposed that viewing the moon over a visible stretch of terrain makes it appear further away. They found that when subjects viewed the moon at the horizon over a visible surface (eg, trees) it appeared on average 1.3 times larger than the moon seen at its highest point. However, when terrain kept out of view (observed through hole in sheet of paper) no difference in size between horizon and zenith moons.Criticisms:1.Coren & Aks (1990) – have found that people sometimes perceive the moon as closer not more distant2.Reed & Krupinski (1992) – illusory effect cannot be created when the stimulus is a star rather than the moon. Summary – no one completely satisfactory explanation (Kassin, 1990)