Learning and habituationEdexcel A-Level Biology A: Revision notes
Section 1
What is learning?
Learning is a change in behaviour that results from experience. It allows animals, including humans, to adjust their behaviour to a changing environment, which improves their chances of survival and reproduction.
Habituation is the simplest form of learning: a reduced response to a repeated stimulus that is neither harmful nor rewarding. The animal learns to ignore it.
Habituation is not tiredness or damage. The animal still responds to other, new stimuli, and the response recovers after a rest.
Section 2
Examples and advantages
Examples: humans stop noticing the feel of clothes or a ticking clock; birds ignore a scarecrow; a snail stops withdrawing its tentacle when it is touched repeatedly; the sea slug Aplysia withdraws its gill less each time its siphon is touched.
Advantages:
- Saves energy that would be wasted on unnecessary responses (such as fleeing).
- Allows the animal to keep feeding, resting or reproducing.
- Lets the nervous system focus on new or important stimuli, such as a real predator.
If a different stimulus is given, the response returns (dishabituation). The response also recovers after a rest without the stimulus.
Section 3
The mechanism at the synapse
Habituation takes place in the synapse between the sensory neurone and the next neurone in the pathway. With repeated stimulation:
- The sensory neurone still carries action potentials as before.
- Fewer voltage-gated calcium channels open in the presynaptic knob, so less Ca²⁺ enters.
- Fewer vesicles fuse with the presynaptic membrane, so less neurotransmitter is released.
- The postsynaptic membrane is depolarised less, so it is less likely to reach threshold.
- Fewer action potentials pass on, so the response (e.g. gill withdrawal, escape) is reduced or lost.
After a rest the calcium channels recover and the response returns.
Link the chain in order: fewer Ca²⁺ in, less transmitter released, smaller depolarisation, no threshold, no response.
Section 4
Core practical 18: investigating habituation
Aim: investigate habituation to a stimulus, e.g. in a garden snail, woodlouse or Daphnia.
Method (snail): touch the same tentacle gently with a clean cotton bud at a fixed interval (e.g. 30 s) and count the touches until the snail fails to withdraw on three consecutive touches. Repeat with several snails.
Control variables: same stimulus and pressure, same interval, temperature, humidity, species and size of animal, and rest before testing.
Checks for habituation: touch a different part (or give a different stimulus) to show the response is still possible; rest and test again to show recovery.
Safety and ethics: handle animals gently, use a mild stimulus, keep tests short and return animals to their habitat. Wash hands afterwards.
Do not just say 'repeat the experiment'. Name the repeats (several snails) and what is kept the same.
Section 5
Analysing results
Record the number of stimuli needed before the response stops, or the response time or size against stimulus number. A graph of response against trial number falls and levels off.
Calculate a mean from several animals and compare conditions, e.g. with a different stimulus interval. Use a statistical test (such as a t-test) to decide whether differences are significant.
Limitations: judging a 'withdrawal' is subjective; animals vary; few animals reduce reliability.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Learning and habituation
- A student moves into a flat beside a railway line. During the first week she is woken several times each night by passing trains. By the fourth week the trains no longer wake her, although she can still hear her alarm clock and wakes at once when her phone rings.Suggest how the fact that she still wakes to her alarm clock shows that the loss of response to the trains is habituation and not damage to her hearing.2 marks
- Researchers repeatedly touched the siphon of a sea slug (Aplysia) with a soft brush at one-minute intervals. At first the slug withdrew its gill sharply each time. After 30 touches the withdrawal was very weak. Recordings showed that the sensory neurone still produced action potentials of the same size and frequency, but the depolarisation produced in the motor neurone had become smaller. After a ten-minute rest, a touch again produced a strong withdrawal.Explain how the change at the synapse leads to the weaker withdrawal of the gill.2 marks
- A student investigated habituation in garden snails. She touched one tentacle of each snail gently with a clean cotton bud every 30 seconds and counted the touches until the snail failed to withdraw its tentacle on three consecutive touches. She tested ten snails. She then touched the other tentacle of each snail, and after five minutes of rest touched the first tentacle again. All ten snails withdrew the other tentacle at once. Eight of the ten snails withdrew the first tentacle again after the rest.Describe three variables that the student should keep constant to make the results valid.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).