What is a stimulus?
A stimulus is any change, outside or inside the body, that a sense organ can pick up. Light, sound, heat, a smell or a pain are all stimuli.
The first contact with a stimulus is called sensation. It is raw: "something bright", "something loud". Making meaning comes later (perception).
Sense modalities
Each kind of sense is called a sense modality. Each one catches only its own kind of stimulus.
- Vision (eye): light
- Hearing (ear): sound waves
- Smell (nose): chemicals in air
- Taste (tongue): chemicals in food
- Touch/skin: pressure, heat, cold, pain
- Kinaesthetic sense: tells where our body parts are
- Vestibular sense (inner ear): balance and head movement
Limits of our senses
Senses do not catch everything.
- Absolute threshold: the smallest amount of a stimulus we notice half the time. A very faint light may be missed.
- Difference threshold: the smallest change between two stimuli we can notice. Adding one candle in a dark room is noticed; in a bright hall it is not.
- Range: we see only a small band of light and hear only about 20 to 20,000 vibrations per second. Dogs and bats hear higher sounds.
- Adaptation: a steady stimulus slowly stops being felt, like the smell of your own room.
Attention and selective attention
Attention is choosing some stimuli and focusing on them. It has three features: selection (what we pick), alertness (how ready we are) and concentration (how focused).
Selective attention
We pick a few stimuli from many. Factors that pull it:
- External: size, loudness, brightness, movement, newness, contrast.
- Internal: needs (a hungry person notices food smells), interests, motives, expectations.
Hearing your name in a noisy crowd is called the cocktail party effect. Some ideas say we filter early (a filter lets one channel pass) and some say we filter late, after meaning.
Divided attention
Doing two things at once works only if one is very practised, like walking while talking.
Sustained attention (vigilance)
Sustained attention is keeping focus on one task for a long time, like an air traffic controller or a radar operator.
- It drops after about 20โ35 minutes (vigilance decrement).
- It stays better when signals are clear, loud or long, when they come often, and when the place is not too warm or too quiet.
- Tiredness, boredom and a monotonous task make it fall faster.
ADHD (attention-deficit hyperactivity disorder) is a condition where a child finds it very hard to keep attention and sit still. It needs expert help, not scolding.
Key formulas and definitions
- Stimulus: a change the senses can pick up
- Sensation: first raw contact with a stimulus
- Absolute threshold: smallest stimulus noticed (50% of the time)
- Difference threshold: smallest change we can notice
- Selective attention: choosing a few stimuli from many
- Sustained attention (vigilance): keeping focus over time
Worked examples
1. Riya studies with the fan on. After ten minutes she no longer hears the fan. Which idea explains this?
Sensory adaptation. A steady, unchanging stimulus slowly stops being felt, so her ears stop reporting the fan sound.
2. A shop puts a big, flashing, moving sign outside. Which factors of attention is it using?
External factors of selective attention: size, brightness, movement and change. They pull our attention without effort.
3. A radar operator catches 95% of blips in the first 15 minutes and 70% after one hour. What is this and how can it be reduced?
This is a fall in sustained attention (vigilance decrement). It can be reduced by short breaks, clearer and stronger signals, and changing duty often.
4. You smell food in a crowded market only when you are hungry. Which factor is at work?
An internal factor: a need (hunger) makes food-related stimuli stand out. This is selective attention guided by motives.
Common mistakes
- Thinking sensation and perception are the same. Sensation is raw input; perception adds meaning.
- Believing we sense everything around us. Many stimuli are below the threshold or outside our range.
- Thinking multitasking is easy. We can divide attention well only when one task is automatic.
- Saying attention is only about outside things. Needs, interests and expectations inside us also steer it.