Sensory, Attentional and Perceptual Processes

Written and reviewed by Subhra Gupta, Humanities faculty at UniDrill (M.A. Sociology, University of Delhi · 7 years teaching Humanities). Based on NCERT Psychology Class XI (Introduction to Psychology), Chapter 4.

Why do you hear your name in a noisy canteen?

The canteen is loud. Plates clatter, a dozen conversations overlap, and you hear none of them clearly. Then, from the far corner, someone says your name, and your head turns at once. Your ears received all the sounds. Your brain let almost all of them pass. One got through.

That small event contains the whole chapter. First, your sense organs collect stimuli. Second, attention selects a few of them. Third, perception gives them meaning. These three processes are so closely tied that they are often treated as parts of one process, cognition.

The senses: how information gets in

We have seven sense organs, often called sensory receptors, because they receive information. Five collect information from the outside world and two are deep senses that report on the body itself.

Each sense organ is specialised for one kind of information, which is why each is called a sense modality. Sensation is the initial experience of a stimulus. It is the process by which we detect and encode physical stimuli, and it gives basic experiences such as hard, warm, loud and blue.

Our senses also have limits. Eyes cannot see very dim or very bright things, and ears cannot hear very faint or very loud sounds. A stimulus must have an optimal intensity to be noticed. The study of the relationship between stimuli and the sensations they produce is called psychophysics.

Thresholds: how much is enough to notice?

Both thresholds are measured as the point where the person reports the sensation on 50 per cent of trials. They are not fixed numbers. They vary across people and situations with their organic condition and motivation, so many trials are needed.

Worked example: sugar and water. Suppose a participant tastes water with 2, 4, 6 and 8 granules of sugar, many times each. She says "sweet" on 10, 30, 50 and 80 per cent of trials. The AL of sweetness is 6 granules, the amount that gets "sweet" 50 per cent of the time. Now keep 6 granules as the standard and suppose she says the water tastes sweeter than the standard on 50 per cent of trials only when 9 granules are used. The DL is 9 − 6 = 3 granules. A common wrong step is to pick the lowest amount that was ever called sweet. That is not the AL, because the threshold is defined by the 50 per cent point.

A sensation also needs a working pathway. The sense organ encodes the stimulus as an electrical impulse, and the impulse must reach the higher brain centres. A defect in the receptor, its neural pathway or the brain area can cause partial or complete loss of sensation.

Attention: choosing what to notice

Many stimuli strike our sense organs at once, but only a selected few are noticed. Attention is the process by which certain stimuli are selected from a group of others. It has three further properties.

Attention means "effort allocation". It also has a focus (the centre of awareness) and a fringe (things you are only vaguely aware of).

What grabs your attention (selective attention)

Three theories of selective attention

Pause and predict: in the noisy canteen, why did your own name get through when other conversations did not? Which theory fits best?

Check your answer: filter attenuation theory. Unattended speech is weakened, not blocked, and a personally relevant word like your name is strong enough, even when weakened, to reach higher levels of processing. Under filter theory alone, nothing from the unattended conversations should have got through.

Divided attention and automatic processing. You can do two things at once only when one of them is highly practised, because practised activities become nearly automatic and need less attention. Even then, the driver gives more effort to driving than to the conversation. Automatic processing has three features: it occurs without intention, it takes place unconsciously, and it involves little or no thought. Reading words and tying shoelaces are examples. This is also why a learner driver cannot chat easily while driving, but an experienced driver can.

Sustained attention (vigilance). Errors in detecting signals can be fatal for air traffic controllers and radar readers, so vigilance matters. Four factors affect it.

Span of attention is the number of objects you can attend to in a brief exposure, a fraction of a second. It is measured with an instrument called the tachistoscope. Miller reported that it is about seven plus or minus two, the "magic number". That is why number plates have only four digits with a few letters, so that the traffic police can read them in one glance.

Attention deficit hyperactivity disorder (ADHD) is a common behavioural disorder of primary school age children, marked by impulsivity, excessive motor activity and an inability to attend. It is more common among boys than girls. Difficulty in sustaining attention is the central feature. These children are highly distractible and do poorly at school despite no deficit in intelligence. It is understood to have multiple causes. Treatment includes drugs such as Ritalin, which reduce over-activity and raise attention but do not cure the problem and can have side effects, and behavioural programmes using positive reinforcement. A cognitive behavioural method teaches the child to use self instructions: stop, think, and then do.

Two views. For the exam: the chapter states that studies generally do not provide evidence for a biological basis of ADHD, and that social and psychological factors such as home environment have been found to account for it more reliably. Another reading: current clinical and research opinion, including twin studies, holds that ADHD has a strong genetic and neurobiological component alongside environmental influences. Both agree on the point that matters for the exam, which is that the causes are multiple.

Perception: making sense of sensation

Sensation tells you that there is a flash, a sound or a smell. It does not tell you what it is or where it came from. Perception is the process of recognising, interpreting and giving meaning to the information from the sense organs, using learning, memory, motivation and emotions. It is a construction, not a photocopy.

In real perception both processes work together.

The perceiver matters

You can test expectation yourself. Show a friend the sequence 12, 13, 14, 15 for five seconds, then A 13 C D. Most people read the middle item as B, because the neighbouring letters set an expectation.

Gestalt principles: how we see wholes

We see a bicycle, not a heap of lines, and form perception is the process of organising the visual field into meaningful wholes. The Gestalt psychologists (Köhler, Koffka and Wertheimer) answered how. "Gestalt" means a regular figure or form, and the central claim is that we perceive organised wholes that are different from the sum of their parts. A flower pot with flowers is one configuration and without them another. The brain is oriented toward a "good figure", called pragnanz.

Figure and ground. The most primitive organisation is figure-ground segregation. The words on a page, the painting on a wall and the birds in the sky are figures, and the page, wall and sky are the ground. In Rubin's vase, you see either a white vase or two black faces, but never both at once.

Seeing depth

The image on the retina is flat and two dimensional, yet we see in three dimensions. This is depth perception, and it helps you judge the distance of an approaching vehicle. It depends on two sets of cues.

Monocular cues work with one eye and are also called pictorial cues, because artists use them to create depth on a flat canvas.

Binocular cues need both eyes.

Try it: hold a pencil in front of you and close one eye and then the other. The pencil jumps from side to side. That jump is retinal disparity.

Beyond the textbook. For the exam: relative height is described as larger objects seeming closer and smaller ones farther, which closely resembles relative size. In reality: in most perception texts, relative height means that objects higher in the visual field, nearer the horizon, appear farther away. A tree whose base is higher up in a photograph is read as more distant. Why it matters: if a question asks for relative height in a picture, use the textbook description. If it asks how height in the picture gives depth, use the horizon rule.

Pause and predict: you close one eye and still see a road narrowing into the distance. Which kind of cue are you using, and which one?

Check your answer: a monocular cue, specifically linear perspective. The parallel edges of the road converge toward a vanishing point, and the more they converge, the farther away the road appears.

Constancies: why the world stays steady

The retinal image changes every time you move, but objects appear stable. This is perceptual constancy.

Illusions: when perception goes wrong

An illusion is a misperception caused by misinterpreting the information from the sense organs. Illusions come from the external stimulus situation, produce the same experience in everyone, and are therefore called "primitive organisations". Universal (permanent) illusions do not change with experience, such as rail tracks appearing to converge. Personal illusions vary from person to person.

Why illusions matter. They show that we construct the world rather than copy it, sometimes from the features of stimuli and sometimes from our experience.

Culture and perception

Two views. For the exam: perception is a joint product of the stimulus (bottom up) and the perceiver (top down), and culture shapes the perceiver. Another reading: some researchers stress that basic perceptual mechanisms such as depth cues and constancies are shared across humans. Culture then tunes habits and interpretation, but does not create perception from scratch. The Müller-Lyer illusion is experienced even by children, and some studies suggest by animals too, which hints at a shared biological base.

Exam traps

60-second revision

Where this chapter connects

• Previous: Human Development shows how infants' senses, such as vision and hearing, develop from birth.
• Next: Learning builds on attention and perception, because conditioning and observational learning depend on noticing the right stimuli.
• Same book: What is Psychology? introduced Gestalt psychology as the reaction to structuralism, which this chapter develops fully, and Human Memory continues the information processing story.
• Class XII link: Attitude and Social Cognition extends perception from objects to people.