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Investigating habituation to a stimulus

Aim: To investigate habituation of snails.

Problem statement: Do snails become habituated to a stimulus?

Hypothesis: Time taken for tentacles to re-extend decreases with the increase in number of times, stimulation been given to the snail.

VARIABLES: 

Independent variable = Number of times the snail has been stimulated(number of pokes )

Dependent variable = retraction time

Other variables:using same type of snails, approx same size and age

Equal handling history

Drying out

Apparatus: 1 giant African land snail

1 dampened cotton wool bud

Clean firm surface

Stop watch

Method:1. Collect one giant African land snail, and place it on a clean, firm surface. 2. Allow the snail to get used to its new surroundings for a few minutes until it has fully

emerged from its shell.3. Dampen a cotton wool

bud with water.4. Firmly touch the snail

between the eye stalks with the dampened cotton wool bud and immediately start the stopwatch.

5. Measure the length of time between the touch

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and the snail being fully emerged from its shell once again, with its eye stalks fully extended.

6. Repeat the procedure in step 3 for a total of 10 touches, timing how long the snail takes to re-emerge each time.

7. Record your results in a suitable table. 8. Present your results in an appropriate graph

Results:

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DISCUSSION:Data evaluation. The results are tabulated as in Table 1. A total of ten stimulations had been given to the snail during the experiment. The time taken for its tentacle to re-extend after each stimulation is recorded and after a constant time-interval, stimulation is re-applied. From Table 1, data shows the longest time it took to re-extend its tentacle is after the first stimulation which 15.65 seconds. Meanwhile, the shortest time recorded is 2.31 seconds which is after the last (tenth) stimulation given to snail. The difference between the first and last reading is as much as 13.34 seconds. Graph 1 shows a line graph. However, generally, the trend can be seen to be declining from first stimulation to tenth stimulation. The overall result follows the principal of habituation hence positive result of experiment.Habituation is defined as decreased response to a stimulus that is repeated without reinforcement (reward or punishment). Habituation is important for animals to learn and differentiate between dangerous stimuli and harmless ones around them. It takes place as memories formed and may be relatively short term but can become long term so that a response is lost permanently. However, in this experiment, the

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habituation is a short term memory. This is based on further test on snail after a spontaneous recovery. The habituation response can re-occur after a period of time where stimulus was not presented to the snail. Sources of error and modifications

Degree of response of snail which is the length of tentacle retracted during the stimulation may vary with degree of force used when touching its head.

Therefore, stimulation must be given by the same person throughout the experiment and by poking with the same force each time.

Besides that, as different spots of the snail may have different sensitivity, it should be ensured that stimulation is applied to precisely the same area between snail’s eye stalks.

  Limitations: The condition during the experiment is different when compared to

environment of snail’s habitat. Therefore, its reaction may be different from its usual in the wild habitat. As snail’s response can be unpredictable at times. Even though degree of force is constant for every stimulation, how far the body

retracts for each stimulation may still vary. Other than that, the age of snail used in this experiment can be a factor related to

habituation result. If an aged snail is used, the result will show a significant decrease as older snails

habituate faster.

Ethical issues In this experiment too, since the experimental data are obtained from

experiments with living organism which is giant African snail, ethical issue arise. Stimulus should cause no harm to the snails. Reduce any possibilities of snail suffering during this experiment. In addition, the snails used in this experiment should be released instantly to its

original habitat after the practical.

Further exploration. Experiment can be repeated with snails of different age. It can be investigated whether age of snails is a factor in habituation.. Force or stimulus is applied to different part of snail body.

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 Rs= -0.939

Interpretation of Rs: There is a strong negative correlation.  

Risk assessment: 

Hands should be washed thoroughly for disinfection after touching the snails once all the equipment has been put ready.

Disinfect all glassware etc. used.

CONCLUSION: The time taken for snail’s tentacle to re-extend decreases with the increase of number of stimulation given to it.

Outcome: spearman’s rank statistical test is used to look for correlation in data. There is a negative correlation – as the number of stimuli increase the time taken for the snail to re-emerge decreases. Students should make a reference to the data. With repeated stimulation, Ca2+ channels in the pre synaptic membrane become less

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responsive. Less Ca2+ crosses the

membrane into the pre-synaptic (sensory) neurone. As a result less neurotransmitter is released into the synaptic cleft. This means that an action potential across the postsynaptic membrane is less likely. Fewer action potentials are produced in the postsynaptic motor neurone so less of a response is observed .

Possible evaluation issues: Snails already handled before the experiment may not react in the same way

Determining when a snail has fully emerged

Lack of moisture may encourage snail to stay more in its shell

Measuring eye stalk length instead

Questions:

1. Write a hypothesis which this experiment will test.

2. Using your graph, state if you think there is a positive, negative, or no, correlation between the number of stimulations and the time for eye stalk withdrawal.

3. Explain any patterns or trends in your data, supporting your ideas with evidence from the data and your biological knowledge of habituation. Relate your findings to your hypothesis.

4. Suggest a reason why snails may become habituated to a prodding stimulus in the wild.

5. Evaluate the procedure used for this experiment.6. This experiment has been shown to be less successful if the snails are handled

regularly prior to the experiment. Suggest why handling prior to the experiment could affect the results of the experiment.

7. Write a null hypothesis that this experiment will test.8. Complete a Spearman’s rank rs correlation test to determine if there is a statistically

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significant correlation between the variables. A table with the headings below will help.

9. Use a table of critical values to accept or reject your null hypothesis. If your calculated Spearman rank value (rs) is greater than the critical value, then the null hypothesis is rejected. If your calculated rs value is less than the critical value, then the null hypothesis is accepted.

10. Write a statistical conclusion for your experimental data. Make sure you include: Your ca l cu l a t ed va lue o f r s

t he number o f pa i r s o f da t a t h e s i g n i f i c a n c e l e v e l t h e c r i t i c a l v a l u e whether the null hypothesis is being accepted or rejected.

Past questions:

2012 june Q.2 A student decided to investigate habituation in Giant African Land snails (Achatinafulica). The student selected a snail (A) and placed it on a glass plate across which it was able to move. The student tapped the plate with a glass rod, next to the snail, causing the snail to stop moving and withdraw into its shell. The student recorded the time, in seconds, it took for the snail to re-emerge from its shell and start moving again.The student tapped the plate at the start of the investigation and then

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every twominutes for a total of 12 minutes.The investigation was repeated with two other snails (B and C).

A copy of the raw results for each snail is shown below.A 90, 40, 30, 10, 0, 0, 2B 108, 60, 40, 15, 5, 0, 0C 80, 48, 80, 20, 0, 0, 0(a) Write a suitable null hypothesis for this investigation.(1)

there will be no significant {difference / correlation} in the time it takes for the snail to {re-emerge / start moving / eq} and the {number of taps received /eq ;(b) Calculate suitable means from the raw results and prepare a table to display these and the raw data for this investigation.(4)

1. suitable table format e.g. tap time or tap number in 1st column ;2. correct column headings with units ;3. means correctly calculated ;4. all raw data and means included ;

c) On the graph paper below, draw a suitable graph to show the effect of repeatedtapping on the time taken for a snail to start moving again.(3)

(d) State an anomalous result in the data.(2)

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Give one reason for your answer1. identified anomalous result:snail C at {4 min / 3rd tap /80} /ORmsnail A at {12 min / 7th tap / 2} /OR(mean result at) {4 min / 12 min /3rd tap / 7th tap/ 50 / 0.7} eq ;2. time to respond increased from 2nd to third tap/ eqORwould expect all at 12 min to be zero / eqORappropriate reference to width of the range bar;(2) (e) The student used a statistical test to investigate the significance of the correlation between the mean time for the snails to start moving and the repeated tapping.His calculation gave a negative correlation value of 0.93.The table below shows significance levels and correlation values for this statisticalTest.

What conclusions can be drawn from this investigation?Use the information provided in the table above and on the graph you havedrawn, together with your knowledge and understanding of habituation, toexplain your answer.(5)

1. reference to critical value is 0.79 ;2. the calculated value is greater than the {critical value / 0.79} at the 95% confidence level /eq ;3. therefore there is a (significant negative) correlation between the number of taps received by the snail and the time it takes the snail to start moving again / eq ;4. repeated stimulation results in loss of response/eq ;5. lack of {reinforcement / reward /punishment/eq} ;

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6. idea that Ca2+ ion channels become less responsive /eq ;7. fewer {action potentials / waves of depolarisation / impulses /eq} / eq