Behavioural sciences chapters
Sensation, perception, attention and concentration
How sensation differs from perception, the principles that organise what we perceive, thresholds and adaptation, disorders of perception (illusions and hallucinations), and the types of attention and concentration with their clinical relevance.
- Sensation is the detection of a stimulus by a sense organ; perception is the brain's interpretation of it, shaped by past experience, expectation, emotion and culture.
- An illusion is a misinterpretation of a real stimulus; a hallucination is a perception without any external stimulus.
- Attention selects some stimuli for processing (selective, sustained, divided); concentration is the ability to maintain attention on one task.
On this page
Overview
We do not simply record the world; we interpret it. Two people can see the same thing and perceive it differently. Understanding how sensation, perception and attention work helps doctors understand patients' experiences, recognise disorders such as hallucinations, and communicate better.
Sensation
Sensation is the process by which sense organs detect stimuli (light, sound, pressure, chemicals) and convert them into nerve signals (transduction).
The senses: vision, hearing, smell, taste, touch (and pressure, temperature, pain), balance (vestibular) and body position (proprioception).
Thresholds
- Absolute threshold: the weakest stimulus that can be detected 50% of the time.
- Difference threshold (just noticeable difference, JND): the smallest change in a stimulus that can be detected.
- Weber's law: the JND is a constant proportion of the original stimulus. Example: if you can just notice 200 g added to a 10 kg load, you would need about 2 kg added to a 100 kg load.
For lifted weights the Weber fraction is about 2% (1/50)
Holding 10 kg → JND = 10 × 0.02 = 0.2 kg (200 g)
Holding 50 kg → JND = 50 × 0.02 = 1 kg
The heavier the load, the bigger the change needed to notice it
Sensory adaptation
When a stimulus stays constant, the response decreases over time (you stop noticing a watch on your wrist or the smell of a ward). This frees attention for new information.
Perception
Perception is the process by which the brain organises and interprets sensations to give them meaning.
Sensation = detecting the stimulus. Perception = interpreting it.
Gestalt principles of organisation
"The whole is greater than the sum of its parts." We tend to see organised patterns:
- Figure-ground: we separate an object (figure) from its background.
- Proximity: things close together are seen as a group.
- Similarity: similar things are grouped.
- Closure: we fill gaps to see complete shapes.
- Continuity: we see smooth, continuous lines.
Perceptual constancy
We perceive objects as stable even when the image on the retina changes: size, shape, colour and brightness constancy.
Depth perception
Uses binocular cues (two eyes' slightly different views) and monocular cues (relative size, overlap, perspective, texture).
Factors that affect perception
- Past experience and learning.
- Expectations (perceptual set): we see what we expect.
- Needs, motivation and emotions: a hungry person notices food; an anxious patient notices threatening signs.
- Culture and beliefs.
- Attention.
- Physical state: fatigue, drugs, illness.
This is why patients may perceive the same symptom (for example chest pain) in very different ways.
Disorders of perception
| Disorder | Meaning | Example |
|---|---|---|
| Illusion | Misinterpretation of a real external stimulus | A frightened patient in a dim ward sees a coat on a hook as a person |
| Hallucination | A perception without any external stimulus, experienced as real and in outer space | Hearing voices when no one is speaking |
| Pseudohallucination | A perception without a stimulus, but experienced inside the mind, and the person may know it is not real | Hearing a voice "inside my head" |
Illusion = a real stimulus misread. Hallucination = no stimulus at all.
Types of hallucination and their meaning
- Auditory: the commonest in schizophrenia (voices commenting or talking about the patient); also in severe depression and mania.
- Visual: suggest an organic cause: delirium, alcohol withdrawal (delirium tremens), drugs, dementia with Lewy bodies.
- Olfactory and gustatory: temporal lobe epilepsy.
- Tactile: cocaine use ("insects crawling under the skin"), alcohol withdrawal.
- Hypnagogic (falling asleep) and hypnopompic (waking): can be normal.
Visual hallucinations point first to an organic brain problem, not to schizophrenia.
Attention
Attention is the process of selecting some stimuli for further processing while ignoring others.
| Type | Meaning | Example |
|---|---|---|
| Selective attention | Focusing on one stimulus and ignoring others | Hearing your name across a noisy room ("cocktail party effect") |
| Sustained attention (vigilance) | Maintaining attention over time | Monitoring an ICU screen for hours |
| Divided attention | Attending to two tasks at once | Taking a history while writing notes |
| Alternating attention | Switching between tasks | Moving between patients in a busy emergency room |
Factors that attract attention: intensity, novelty, movement, contrast, repetition, and personal relevance (needs, interests).
Concentration
Concentration is the ability to keep attention focused on a task over time.
It is reduced by: fatigue, sleep loss, stress, anxiety, depression, pain, hunger, noise, multitasking, drugs and alcohol. In ADHD, delirium and dementia, attention and concentration are impaired.
Tests: serial sevens (subtracting 7 from 100 repeatedly), spelling "WORLD" backwards, months of the year backwards.
Poor attention and concentration are prominent in delirium, which also has a fluctuating level of consciousness.
Expect: sensation vs perception, absolute and difference thresholds and Weber's law, sensory adaptation, Gestalt principles, perceptual constancy, factors that affect perception, illusion vs hallucination vs pseudohallucination, what visual vs auditory hallucinations suggest, types of attention, and tests of concentration.
Quick revision
- Sensation: detection. Perception: interpretation.
- Absolute threshold: detected 50% of the time. JND: smallest detectable change. Weber: JND is a constant proportion.
- Gestalt: figure-ground, proximity, similarity, closure, continuity.
- Illusion: real stimulus misread. Hallucination: no stimulus.
- Auditory hallucinations: schizophrenia. Visual: organic (delirium, drugs).
- Attention: selective, sustained, divided, alternating. Concentration: serial sevens.
Written by The House Job from the Handbook of Behavioural Sciences by Mowadat H Rana (named in the UHS PGET syllabus).
FAQ
What is the difference between sensation and perception?
Sensation is the detection of a stimulus by a sense organ; perception is the brain's interpretation of that sensation.
What is the difference between an illusion and a hallucination?
An illusion is a misinterpretation of a real stimulus; a hallucination is a perception that occurs without any external stimulus.
What does Weber's law state?
The smallest noticeable change in a stimulus is a constant proportion of the original stimulus.
What is selective attention?
Focusing on one stimulus while ignoring others, such as hearing your own name in a noisy room.