Chapter 2 Perception, Study notes of Psychology

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Paper 1: Cognition and behavior

Chapter 2: Perception

2.1 What are sensation and perception?

Synoptic link

The electrical signals are carried from sensory organs to the brain by special cells called neurons. These take the information to the part of the brain that will decode it, so we can make sense of it (see page 59).

Did you know?

People who are missing one sense often find that their other senses become stronger to compensate. For example, people who are blind often have very acute hearing, and people who are deaf can be very sensitive to subtle touches or slight movements.

Getting started

Sensation is the information that we receive through our senses. Our senses are active all the time, receiving different kinds of sensory information. Take a minute to note down some of the information you are receiving right now.

Think about each of these senses:

 Vision (what can you see?)  Hearing (what can you hear?)  Smell (can you smell anything?)  Touch (are parts of your body in contact with anything?)  Taste (can you taste anything?)  Proprioception (what positions are parts of your body in?)  Kinaesthesia (are you moving at all?)

Building skills 2

Sometimes, our senses can fool us. Make up two parcels, each weighing one kilogram. One parcel should be quite small, and the other quite large. Ask other people to lift each parcel and tell you if they think one is heavier than the other, and, if so, which one is heavier.

We would normally expect a large parcel to weigh quite a lot, because what is inside a large box should be ‘big’. So, when we pick up the large parcel, we are surprised that it is not heavier. As a result, we think that the large parcel is light, while the small one seems heavy.

Perception

We need to make sense of the information we receive from our senses. This is what perception is all about.

If we did not organise the information somehow, we would get overwhelmed and not know what anything meant. When we use our eyes, for example, they receive information about lightness, darkness, and colour, as tiny dots. This is because each sense receptor in our retinas picks up information from just one tiny part of the whole visual field that you can see.

However, we do not see a mass of tiny dots even though this is what our visual receptors receive. Instead, we see patterns, shapes, people, and things. We may see them clearly or faintly. Sometimes, when we see just a suggestion of a shape and not a proper shape, our experience allows us to work out what is there. This is because of the processes of perception.

Visual perception is how we perceive the information received from our eyes. It is possibly the most important sense for human beings, although not for all animals. For this reason, psychologists have studied visual perception in detail, and we know quite a lot about how it works. Our brains use all the information they receive to construct a mental representation for us (the picture we ‘see’) of the outside world.

Exam tip

When distinguishing between two psychology terms, such as sensation and perception, remember that examples of the terms can help you access maximum marks.

Visual cues and constancies:

How do we judge distance?

The image that our eyes receive is two-dimensional, like a picture, spread out in front of us. But we live in a three-dimensional world, so we are to avoid bumping into things or getting run over by a car, we need to know how far away things are.

We do this without thinking. However, it is a complicated process.

We judge distance using depth cues. These cues tell us about the third

dimension of the real world (depth or distance) and they allow us to

work out how far away from us something is.

This is an important survival tool.

One cue you might use is the way that things further away often appear to be higher up. This monocular depth cue is known as height in plane. As you

can see inFigure 2.3, the cars that are further away are also higher up in

the picture.

2. Relative size

Another possible cue is the way that things that are closer often seem to be larger. This monocular depth cue is known as relative size. We

can see inFigure 2.4 that the cars that are further away are not only

higher up in the photograph, they are getting smaller and smaller. However, we do not believe they are actually tiny cars. They are still the size of a real car. It is just that they are further away from us.

3. Occlusion

Occlusion is another monocular depth cue. You can see inFigure 2.

how an object that is covering up another object appears to be closer to us. Parts of the cars that are nearer to us cover or hide parts of the cars that are just behind them. When we are outside, we see lots of examples of occlusion and we know that any object that is partly covered by another object must be behind it and, therefore, further away from us.

4. Linear perspective

The fourth monocular depth cue that you might use in a picture is known as linear perspective. This is the way that straight lines seem to be pointing towards a single point on the horizon.

When you look atFigure 2.6, you know that the sides of the road are

parallel to each other (that they never meet), but they look like they do. This point on the horizon is known as the vanishing point because the lines disappear when they reach it.

The vanishing point is a useful depth cue if we want to show distance in a landscape, and you will find it in a visual illusion called the

Ponzo illusion.

Did you know?

Having two good eyes is essential for people whose activities depend on precise judgements, such as gymnasts and goalkeepers. Even everyday activities, like driving or crossing the road, are more difficult if we only have one eye to see with.

Binocular depth cues

Binocular depth cues are also clues that help us to decide how far away things are. Humans can use binocular depth cues because we have two eyes at the front of our head that are about 7.5 cm apart.

This means that we see two images that are nearly (but not quite) the same. By comparing these two images, our brains can work out how far away things are.

Using binocular cues allows us to be much more accurate in our judgements of depth.

The brain detects the differences in how the muscles are working, and uses them as a cue to distance. Looking at objects that are close makes the muscles work harder as the eyes turn in towards your nose.

2. Retinal disparity

Retinal disparity is a binocular depth cue that uses the slight difference between the images seen by our left and right eyes.

 Because our eyes are about 6 – 7 cm apart, each eye receives a slightly different image of the same object.

 The brain compares these two images and uses the degree of difference (disparity) to judge depth.

 If the two images are very different, the object is close.  If the images are very similar, the object is far away.

Example

 Hold your finger at arm’s length and look at it with one eye closed, then switch ey es. The images don’t shift much — so your brain judges the finger as far away.  Now bring your finger close to your nose and repeat the process. The image shifts a lot — the large disparity tells your brain the object is close. By comparing the images received by our two eyes. If something is close, there is quite a difference between the two eyes’ images. If it is further away, there will be much less difference

between the images (after about ten metres, the difference is hardly noticeable).

By comparing the images received by our two eyes. If something is close, there is quite a difference between the two eyes’ images. If it is further away, there will be much less difference between the images (after about ten metres, the difference is hardly noticeable).

James J. Gibson’s Direct Theory of Perception

The Importance of Nature in Perception

 Direct perception is the idea that we perceive simply by using the messages we receive through our senses.  This gives our brains enough information to understand the world we live in.  James J. Gibson developed the direct theory of perception, and believed that perception involves the information in the optic array when it is used as the key to understanding perception itself.  He argued that people (and animals) do not just receive passive images of the world.  We are active in it, and this activity is part of our perception because it changes the visual images we receive.

Eleanor Gibson’s Contribution

 Eleanor Gibson, who was married to James J. Gibson, was also interested in perception.  She spent much of her life studying depth perception in animals.  She demonstrated that most animals can perceive depth by the time they can walk, and those that can walk at birth (like goats and chicks) can perceive depth at birth too.

The Visual Cliff Experiment

 Eleanor Gibson and researcher Richard Walk demonstrated this awareness of perception using a piece of apparatus called a visual cliff.  This is a countertop where half is solid plastic covered with a red and white chequered pattern, and the other half is transparent plastic with the same red and white chequered pattern continuing down a drop under the plastic and across the floor.  Gibson and Walk placed babies aged 6 – 14 months on the centre of the visual cliff and encouraged them to crawl across the countertop.  Over 90% of the babies refused to crawl across the side with the ‘ drop ’, but all crawled across the ‘ safe ’ side.

Findings:

 Eleanor Gibson’s study shows that at the earliest time babies could be tested (when they could crawl), they had the ability to perceive depth.  Some babies could be seen checking the clear plastic on the ‘drop’ side, but even feeling the surface was not enough to convince them to crawl across.  Later research in 2014 showed that when babies aged 3 months are placed ov er the apparent ‘drop’, their heart rates and breathing rates increase and their eyes open wide, indicating they are having a fear response to the perceived ‘danger’.

 Although babies are able to perceive depth, they might still fall off furniture or down steps. Their motor skills are not developed enough to prevent themselves from falling when they lean forward to explore the ‘drop’ they can see.

Extra Cues for Perception

Key term:

 Inference: a guess or conclusion reached on the basis of information (often past experience) or knowledge.  There are other cues we can use to perceive figure and depth. We do not just see blocks of colours and shapes — different objects have different textures (patterns).  If you look at a field of grass, for example, you will see individual blades close up, but further away, the texture becomes finer and smoother. This is known as a gradient of texture.

 Similarly, things further away can also look paler in colour. This is called a gradient of colour.

 James J. Gibson saw texture gradients and colour gradients as examples of how the real world gives us information about depth and distance, allowing us to perceive in three dimensions.

 Our brain understands that it is us moving, not the objects, giving us a stable picture of the world. This adds ecological validity, as it reflects real-life perception.

3. Separates Sensation and Perception  The theory explains how sensation and perception are distinct.  For example, visual illusions work because they play tricks on our visual cues.  This supports the idea that perception is not simply the same as raw sensory input.

Weaknesses of Gibson’s Direct Theory

1. Role of Past Knowledge Overlooked

 The theory underestimates how much our past knowledge and expectations affect perception.  For example, we often interpret things based on what we expect them to be (schemas), not just the sensory input.  This is better explained by Gregory’s constructivist theory.

2. Illusions Challenge the Theory  Some illusions suggest that perception is not always direct and accurate.  Instead, they show that we sometimes make mistaken hypotheses, which Gregory argued happens when sensory input is unclear or ambiguous.  This ch allenges Gibson’s claim that sensory information alone is sufficient.

Practice exam questions

1. Using an example, explain the difference between perception and sensation. (4 marks)

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2. What are binocular depth cues? (2 marks)

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3. Identify and explain one monocular depth cue. (2 marks)

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4. Explain how ‘convergence’ helps us perceive how far away objects are. (3 marks)

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2.2 Can you believe what you see?

What Are Visual Illusions?

 Visual illusions occur when our perception does not match reality.  Our senses receive information correctly (sensation), but our brain interprets it in a misleading way.  They demonstrate that perception is not always direct and that our brain can be “tricked” by visual cues.

Visual illusions happen when our perc eption is ‘tricked’ into creating a mental representation of the world that is inaccurate.

Reasons for visual illusions:

1. Misinterpreted depth cues: Using depth cues incorrectly (e.g., seeing a 2D image as 3D).
2. Ambiguity: The image could be seen in more than one way (e.g., Rubin’s vase).
3. Fiction: Perceiving something that is not really there (e.g., Kanizsa triangle).
4. Size constancy: Our perception adjusts the size of objects to remain constant despite changes in their distance.

Explaining visual illusions

a) The Ponzo illusion

The Ponzo illusion uses linear perspective to trick our brain into thinking two lines are different lengths when they are actually the same.

It is usually shown as two horizontal lines placed across a background of converging lines (like railway tracks or a road disappearing into the distance).

Uses linear perspective so that the top line appears further away and therefore longer, even though both lines are the same length.

Why It Happens

 Our brain interprets the converging lines as depth cues (linear perspective).

 The upper line appears to be further away in the distance.  Because it appears further away, our brain interprets it as being longer, even though both lines are the same size on the page.

Example

Imagine two identical lines placed across a drawing of railway tracks that converge at the horizon:

 The top line (closer to the vanishing point) looks longer than the bottom line.  But if you measure them, they are exactly the same length.