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A study of the way in which we perceive the world around us through the sense of vision is a wonderful example of the way the brain and body work together, depend on each other and help each other as we go about our everyday activities.

One of the key objectives of this chapter is to help you to understand how the physical reality of light from an object in the environment (referred to as the distal stimulus) is cast as an image on the retina (called the proximal stimulus), and nerve impulses travel to the brain where higher mental processes enable us to organise and interpret what we see. This process is referred to as visual perception.

figure 6.1 The eye

The actual image on our retina is: - upside-down - back-to-front - blurred - crisscrossed by a network of veins - patched by holes.

Yet when this image has been sent to the brain, it is processed so that we see a crystal-clear picture! The process itself—from receiving an image to ultimately interpreting what we see—is complex and has been studied extensively from a number of psychological perspectives, as discussed in Chapter 5.

The processes involved in sensation and perception are also thought to be adaptive. From an evolutionary perspective, the ability to see, hear, touch, smell and taste has developed over thousands of years and through millions of changes—leaving our senses perfectly suited to our environment and helping us survive and reproduce (Tooby & Cosmides 1992, cited in Westen et al. 2009). Just like frogs, which have an inbuilt ‘bug-detecting’ function in their visual system designed to activate when a tasty insect is in view, humans have specialised areas in the brain that allow the perception of faces and facial expression. This can be seen in infants, who have an innate or inborn tendency to show greater interest in objects that look like a human face (Adophs et al. 1996).

The eye is a fantastic organ—it is very complex in construction, but we only need to know about a few of its structures.

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From the time we receive an image to when we can identify what we see, six stages have been identified, some of which occur at about the same time. Essentially these six stages progress, in sequence, from being reflexive physical functions of the eye and nervous system to being psychological functions of the brain, involving memory and thought processes.

Sensation and perceptionSensation - 1 Reception: Light enters the eye through the cornea, a tough transparent tissue

covering the front of the eye. It then passes through the pupil—the hole in the middle of the coloured part of the eye (the iris). The lens focuses the light on the retina, which contains the photoreceptors—light-sensitive cells called rods and cones.

- 2 Transduction: The electromagnetic energy that we know as light energy is converted by the rods and cones into electrochemical nerve impulses. This allows the visual information to travel along the fibres of the optic nerve to the brain.

- 3 Transmission: The next task for the rods and cones is to send the nerve impulses along the optic nerve to the primary visual cortex in the occipital lobes, at the very back of the brain where specialised receptor cells respond as the process of visual perception continues.

figure 6.2 The movement of light entering the eye

figure 6.3 The brain makes sense of what we see.

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Wavelength in nanometres10-12 10-10 10-8 10-6 10-4 10-2 102 104 1061

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figure 6.5 Electromagnetic spectrum, including the visible spectrum

the processes of visual perceptionreception and light energyThe process of light entering the eye is an important part of our ability to receive and interpret a visual stimulus. However, before the eye can receive the visual stimulus, there are a couple of elements that must be in place.

First, the light energy must be within the visible part of the electromagnetic spectrum. Wavelengths of between 360 and 760 nanometres form the visible spectrum (1 nm = 1 billionth of a metre). The energy that enables us to see is what we call light energy, the visible part of the electromagnetic spectrum.

reception and absolute thresholdThe second important element to enable the eye to detect the light stimulus is that the light energy that falls within the visible light spectrum must be intense enough for the human eye to see. In other words, it must reach absolute threshold. The absolute threshold is the minimum amount of light energy needed for an observer to perceive a stimulus, in ideal conditions, 50 per cent of the time.

One method psychologists use to measure absolute threshold is to present light stimuli at different intensities to see what level of intensity is needed for a person to detect the light. If that person detects it during the experiment about 50 per cent of the time at a particular intensity (the point at which they actually perceive it), then absolute threshold has been reached.

Absolute threshold for the senses are: - hearing: the ticking of a watch 6 m away - smell: one drop of perfume in a large house - taste: one teaspoon of sugar dissolved in 10 l of water. - touch: the wing of a fly falling on the cheek from a height of 1 cm - vision: the flame of a candle 50 km away on a dark, clear night.

These are based on sound scientific research, but they may not be the same for everyone and can vary depending on a range of environmental factors (noise, amount

figure 6.6 Your eyes can detect a candle flame up to 50 km away on a clear dark night.

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of light) and psychological factors (fatigue, motivation, stress, expectations). For example, if a person has had someone break into their house, they will be more highly attuned to sounds at night, and this may affect their usual absolute threshold for sound.

the role of the eye in visual perceptionAs already mentioned, light enters the eye through the cornea, a tough transparent tissue covering the front of the eye. It then passes through the pupil, the hole in the middle of the iris. The lens focuses the light onto the retina, which contains photoreceptors (light-sensitive cells).

The retina is nerve tissue that covers more than 50 per cent of the inner surface of the back of the eye. It contains two types of photoreceptors: rods and cones. - Rods: There are 125 000 000 in each eye.

- They are responsible for vision in low light (that is, they are very sensitive to light). - They are responsible for peripheral vision (out of the corner of the eye). - They are concentrated at the edges of the retina. - They have low visual acuity (they can’t register detail). - They can register only in black and white. - They are most sensitive to light of approximately 500 nm wavelength.

- Cones: There are 6 500 000 in each eye. - They are concentrated in the middle of the retina. - They are responsible for vision of detail. - They are responsible for colour vision (and black-and-white vision in daylight). - They require high levels of light to enable them to respond.

figure 6.7 Your tastebuds can detect one teaspoon of sugar in 10 l of water.

figure 6.8 Structure of the retina

↙ Did you know?Psychologists have learned a great deal about our vision through experimentation on animals. They discovered how receptive fields in ganglion cells respond to an image after it has been transduced (converted to nerve impulses) by inserting a tiny electrode into the brain or retina of an animal. By holding the animal’s head still and flashing light to different sections of the visual field, they were able to identify and map the receptive fields of ganglion cells of the retina (Westen et al. 2009).

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Try this yourself: - Hold a pen at arm’s length and look past it at the other side of the room. - Close one eye and then the other and watch how far the pen ‘ jumps’ from side to side. - Now bring the pen closer—about 40 cm from your eyes—and repeat the process.

What do you notice?

The process of retinal disparity is artificially recreated in ‘magic eye’ pictures from two flat, 2-D patterns viewed from about 20 cm. Each eye observes a slightly different view of the same scene and the brain fuses the two images together in the same way it would when observing a real (3-D) scene.

If you can look ‘through’ the picture in Figure 6.13, you will see a ‘3-D’ star in the middle of it.

figure 6.13 A magic eye picture—what can you see?

convergenceConvergence is also a binocular depth cue. As an object comes closer to us, our eyes turn inwards to keep the object centred on the retina. Again, this cue operates for objects within about 7 m. The brain reads the amount of turning from the tension of the muscles that move the eyes and uses this to make judgments of distance. The more the turning, the closer the object is to the viewer.

Try this yourself: - Hold your pen vertically at arm’s length and slowly bring it closer to your nose,

watching it with both eyes all the time.

- As the pen gets close to your nose, you can feel your eyes turning and soon you go ‘cross-eyed’.

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monoCular depth CuesTwo types of monocular depth cues enable us to judge depth and distance using one eye: accommodation and pictorial cues.

AccommodationAccommodation involves the lens of the eye (located behind the iris) changing shape so that it can focus light rays onto the retina. Small muscles called ciliary muscles control whether the lens bulges (for closer objects) or flattens (for more distant objects). At the same time, the tension in the ciliary muscles is received by the brain to confirm the location of the object being viewed. The greater the tension, the closer the object.

Try this yourself: - Take a pen and close one eye. - Move the pen as close to you as you can while maintaining focus. You should be able

to focus on an object between 8 and 10 cm away (depending on your eyesight).

- Keep focusing on the pen until you feel the tension within your eye. That’s your ciliary muscles at work, keeping your brain informed.

Pictorial depth cuesPictorial depth cues are so called because they are used by artists to create a 3-D perception of something that exists on a 2-D surface. - Linear perspective (first described by Leonardo da Vinci) is one of the most basic

skills an artist uses to create apparent depth. Parallel lines are made to converge as they extend along the page to an imaginary point (where in theory they meet) at the horizon, as shown in Figure 6.14.

- Interposition (overlap) is based on the partial blocking or obscuring of one object by another. The obscured object appears to be further away than the object obscuring (overlapping) it. This is an effective cue for determining which objects are closer than others, but it is not as effective for actually judging distance.

- Texture gradient is used to make surfaces in a picture appear to recede into the distance. Artists draw less and less detail as a surface is more and more distant, the same way we see it in real life. This is illustrated by Figure 6.15, which shows the boardwalk at Rhyll, Phillip Island. In the foreground we can see every detail of the wood and the mesh, but as the boardwalk gets further away it becomes much less detailed.

- Relative size is based on our tendency to perceive the object producing the largest retinal image as being the nearest, and the object producing the smallest retinal image as being the farthest. For this cue, it is necessary to know the real size of the objects so that accurate comparisons can be made. Think about watching a game of football from behind your team’s goal—you realise that the players at the opposite goal are far away; you don’t think they are tiny!

- Height in the visual field shows depth by portraying objects further away as being closer to the horizon. In a picture, objects in the sky—such as aeroplanes, clouds and birds—will be perceived as further away as they become lower in the visual field (closer to the horizon). On the other hand, objects on the ground—such as

figure 6.14 The linear perspective shows parallel lines converging in the distance.

figure 6.15 Rhyll boardwalk—how many different depth cues can you see in this picture?

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distortions of perceptionVisual illusions are mistaken perceptions of visual stimuli. We may misjudge length, curvature, position, speed or direction in a visual illusion, but it is the brain that is tricked, not the eye!

A visual illusion occurs when perception consistently differs from objective reality.

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SynaesthesiaSynaesthesia is an intriguing phenomenon that concerns connections between senses. It is a cross-modal experience—the presentation of a stimulus from one sensory modality (system) automatically triggers a perception in a second sensory modality or cognitive process, such as seeing a certain colour in response to the sound of a certain word, or experiencing particular smells when hearing a particular sound. It is as though the senses have their wires crossed!

The experience only occurs in one direction—the sound of a specific word might elicit a certain taste, but the taste does not elicit the sound of the word. For example, in the case study about Sean and his synaesthesia, music may instantly and automatically trigger Sean’s perception of a vivid colour, but seeing a splash of colour does not cause him to hear music.

It is difficult to say how common synaesthesia is—estimates vary from less than 1 in 10 000 to 4 in 100. Most studies suggest that synaesthesia is more common in females than males, but recent random sampling found no difference between the sexes (Simner et al. 2006).

A person who experiences synaesthesia is called a synaesthete.

2 Show either the ‘faces’ group of cards or the ‘animals’ group of cards, one at a time, to volunteer participants.

3 Show the ambiguous rat-man stimulus (Figure 6.19) to each volunteer and record their response.

4 Compare the responses of the two groups. Did the ‘faces’ group identify an old man more than the ‘animals’ group?

5 Repeat the experiment with other volunteers, but this time show them a mixture of three ‘faces’ and three ‘animals’ before showing the rat-man stimulus.Step 5 is an important step in the research. Why should this be done?

figure 6.20 Letters of the alphabet may trigger vivid colours in synaesthetes.

Did you know?Visual illusions are perceptual distortions; that is, they are caused by psychological factors. There are other distortions of perception that are caused by biological factors.

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figure 6.23 Which room has the higher walls?

produces an equal length image on the retina, we interpret the line with feather-tails to be longer.

Evidence to support this theory came when the illusion was shown to country-dwelling Zulu people who live in round huts with rounded doors and windows. They did not perceive the lines to be different lengths as they had never learned to judge distance from corners and angles. Zulu people living in cities in South Africa were fooled by the illusion like everyone else.

the perCeptual Compromise theoryRoss Day, an Australian psychologist, proposes the perceptual compromise theory.1 Both parallel lines cast identical-sized images on the retina.

2 The arrowhead or feather-tail lines at the ends of the figures create ‘open’ figures that cause us to apply the Gestalt principle of closure.

3 This creates a more ‘solid’ figure as shown by the blue lines in Figure 6.24.

4 Because of the perceptual compromise made, we perceive each figure to be the length of the average between the internal (black) line and the external (blue) lines (see Figure 6.25).

5 As a result of this, Figure A is perceived to be much shorter than Figure B, each being perceived to be as long as the distance between the green lines shown in Figure 6.25.

Evidence in support of this theory is that the illusion occurs even when the ends of the lines are ‘U’-shaped or completed circles, as shown in Figure 6.26.

Most illusions are psychological distortions of perception, but there is one type of illusion that has a physiological basis.

figure 6.25 Our mind averages out the black and blue lines, and we perceive the length to be as shown by the distance between the green lines.

figure 6.24 The blue lines show how our mind applies the Gestalt principle of closure so that Figure B appears larger.

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figure 6.26 If the arrowheads and feather-tails are replaced with ‘U’ shapes and circles, the illusion still occurs.

motion after-effect illusionsIn 1834, Robert Addams was watching the Falls of Foyers, a waterfall in Scotland, and noticed an odd effect (Addams 1834). After staring at the waterfall for about 30 seconds, he noticed that the rocks behind the falls appeared to drift upwards. He coined the term Waterfall Illusion to describe this phenomenon.

The Waterfall Illusion is an example of a motion after-effect (MAE) illusion. The MAE refers to the apparent motion of a stationary stimulus (object) following the extended viewing of a continuously moving stimulus. The stationary stimulus appears to move in the opposite direction. The MAE is an illusion—a normal and relatively consistent phenomenon in which perceptions are different from the physical stimuli themselves.

The spiral MAE, reported in 1849 by the inventor of the stroboscope, Belgian physicist Joseph Plateau, is another common MAE. If you watch a rotating spiral for 30 seconds and then gaze at a stationary object, such as your hand, the object should appear to rotate in the opposite direction.

MAEs are not limited to visual perception. For example, tactile MAEs have been studied, where a stationary stimulus feels as though it is moving (Planetta & Servos 2010). The sense of touch—including pain, pressure and temperature—provides us with vital clues about our immediate environment. Touch allows us to navigate the world safely, and tracking and predicting the motion of a moving tactile stimulus is part of touch perception. Interestingly, tactile MAEs are twice as likely to be reported on the hands as on the cheeks or forearms (Planetta & Servos 2010). figure 6.27 The Falls of Foyers

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