Recommended additional practical: habituation to a stimulus

What habituation is

Habituation is a reduced response to a stimulus that is repeated many times but does no harm. The animal saves energy and time by not responding to stimuli that are not a threat.

• In the sea slug Aplysia, repeated stimulation leads to fewer calcium ion channels opening in the presynaptic membrane of the sensory neurone, so less neurotransmitter is released and the motor neurone is less likely to fire.
• Habituation is not fatigue: if a new stimulus (for example a touch in a different place) brings back the full response, the muscles and sensory receptors still work, so the reduced response was habituation.

Method: snail tentacle withdrawal

1. Use one species of land snail (for example garden snails) of similar size; keep them in the same conditions beforehand.

2. Place a snail on a damp, flat surface and leave it for a few minutes to acclimatise until it moves with tentacles fully extended.

3. Touch it firmly between the tentacles with a moistened cotton bud. To standardise the stimulus, use the same place, the same pressure (for example a cotton bud on a pivoted arm dropped from a fixed height) and the same person.

4. Start a stopwatch at the touch and stop it when the tentacles are fully extended again.

5. Repeat the touch at a fixed interval (for example 1 minute after re-extension) for 10 stimuli.

6. Repeat with at least five snails; calculate the mean time for each stimulus number.

Controlled variables: temperature, light intensity, humidity, time between stimuli, position and force of touch, species and size of snail.

Example data, mean of five snails

Ethics, analysis and limitations

• Ethics: snails are invertebrates; handle them gently, keep them damp, use the smallest number that gives valid results, and return them unharmed to where they were found.
• Null hypothesis: there is no significant correlation between stimulus number and the time taken for the tentacles to re-extend.
• Test: Spearman's rank correlation of stimulus number against mean time, with a scatter graph.
• Limitations: the force of a hand-held touch varies; judging 'fully extended' is subjective; snails differ in age and activity; other disturbances (vibration, shadows) also act as stimuli.
• Extension: test dishabituation with a novel stimulus, or compare the effect of different intervals between stimuli.

Worked example

Use the example data to calculate Spearman's rank correlation coefficient between stimulus number and mean time to re-extend, and decide whether the correlation is significant at p = 0.05.

1. Rank stimulus number 1 to 10, and rank the times from smallest (10 s = 1) to largest (38 s = 10).
2. Σd² = 328
3. rₛ = 1 - (6 x 328) / (10 x 99) = -0.988
4. Critical value for n = 10 at p = 0.05 is 0.648; 0.988 > 0.648

Answer: rₛ = -0.988: reject H₀; there is a significant negative correlation: the response decreases as the stimulus is repeated, showing habituation.

Check yourself

1. What is habituation?
   • A reduced response to a stimulus that is repeated but causes no harm
   • An increased response to a repeated stimulus
   • Muscle fatigue after repeated contraction
   • A reflex that cannot change
   Show answer

   A reduced response to a stimulus that is repeated but causes no harm. The animal learns to ignore a harmless repeated stimulus.

2. After habituation, a snail is touched on a different part of its body and withdraws its tentacles fully. What does this show?
   • The snail has become more sensitive to all stimuli
   • The first stimulus was too weak
   • The reduced response was habituation, not fatigue of the muscles or receptors
   • The experiment must be repeated
   Show answer

   The reduced response was habituation, not fatigue of the muscles or receptors. The effectors still work, so the reduced response to the first stimulus was habituation.

3. Which is the best way to standardise the stimulus?
   • Touch the same place with a cotton bud dropped on a pivoted arm from a fixed height, at a fixed interval
   • Touch the snail whenever it moves
   • Use a different person for each touch
   • Touch harder each time so it still responds
   Show answer

   Touch the same place with a cotton bud dropped on a pivoted arm from a fixed height, at a fixed interval. Same place, force and interval mean only the number of repeats changes.

4. Which is an ethical consideration in a snail habituation investigation?
   • Use as many snails as possible
   • Keep snails in a warm dry container between trials
   • Use vertebrates because they respond more clearly
   • Keep snails damp, handle them gently and return them unharmed to their habitat
   Show answer

   Keep snails damp, handle them gently and return them unharmed to their habitat. Minimise harm and numbers, and return the animals.

5. The mean time to re-extend fell from 38 s at the first stimulus to 10 s at the tenth. Calculate the percentage decrease, to 2 significant figures.
   Show answer

   74 %. (38 - 10) / 38 x 100 = 73.7%, which is 74% to 2 s.f.

6. At the first stimulus five snails took 36, 41, 35, 40, 38 s. Calculate the standard deviation to 3 significant figures.
   Show answer

   2.55 s. Mean 38; squared differences 4, 9, 9, 4, 0 = 26; ÷ 4 = 6.5; s = 2.55 s.

7. For 10 stimuli, Σd² = 328 when the times are ranked from smallest. Calculate rₛ to 3 decimal places (include the sign).
   Show answer

   -0.988. 1 - (6 x 328) / 990 = -0.988.

8. rₛ = -0.988 for 10 pairs (critical value 0.648 at p = 0.05). What is the conclusion?
   • Reject H₀: a significant negative correlation between stimulus number and response time
   • Accept H₀: no significant correlation
   • A significant positive correlation
   • The snails were fatigued
   Show answer

   Reject H₀: a significant negative correlation between stimulus number and response time. 0.988 > 0.648; the negative sign shows the response falls as stimuli are repeated.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018): Unit 3, Unit 6 and Appendix 6 mathematical skills

Last updated 8 November 2026. Spotted a mistake? Tell us.