Assessing skin sensitivity – touch discrimination
Class practical
This procedure has been developed with the help of the Physiological Society.
The underlying assumption of this procedure is that one touch is felt when one receptor, or two adjacent receptors are stimulated. We feel two points of touch when there is an unstimulated sensory unit between the two points touched. The results depend on reliable responses from the person being tested. It is therefore important that the person tested cannot see (and does not know) whether they are being tested with one point or two. It is also important to allow a margin of error – a percentage of incorrect descriptions of the touch as it is difficult to discriminate accurately.
Lesson organisation
Students could work in threes – one being touched, one touching, and one recording the results. It is important that the person whose skin is being assessed has volunteered to be tested, that they are comfortable with the idea of being touched by their colleague, and that the areas of skin tested are appropriate – for example, fingers, palm, back of the hand, upper arm, back of the neck, earlobe, cheek or brow.
Input the class data to a final results table so that individual student results remain anonymous.
This investigation takes time and care to obtain reliable results and students can find analysis and interpretation of the results quite difficult. It will add to the teaching and learning value of the procedure to spend a significant effort on evaluation of the procedure and discussion of both the practical and analytical methods used.
Apparatus and Chemicals
For each group of students:
Ball of plasticine/ blu-tack with 2 cocktail sticks, 1
Ruler, 1
Die, 6-sided, 1
Record sheet
Access to Excel to analyse results – optional
Health & Safety and Technical notes
The hazard in this practical is that students are touching one another’s skin with a sharp implement that could cause damage. This is especially the case if students test the skin near one another's eyes. Students behaving in a calm, methodical, purposeful way should not cause one another any harm.
Ethical issues
Students may be embarrassed to be touched by their colleagues in this way. Students from some cultural backgrounds may need to work with colleagues of the same gender, or touch and be touched on more limited parts of the body. Students are effectively test subjects for this practical and so should sign the attached form to state that they understand the procedure, agree to be involved, and have the right to stop their participation and remove their results at any time.
Procedure
SAFETY: Students should touch one another carefully and without causing damage.
Preparation
a Decide on the roles of the students in each working group
b Ensure that the student whose skin sensitivity is to be assessed has understood the procedure and signed the 
student briefing and consent form.
Investigation
c Choose three areas of the body where a volunteering student is happy to be touched. This could be a finger, palm, back of the hand, upper arm, back of the neck, earlobe, cheek or brow.
d Make sure that the student is sitting comfortably and not looking at the part of their body being tested.
e Set the points of the cocktail sticks a known distance apart and record the distance.
f Throw the die. If the result is odd, touch with only one point of the dividers. If the result is even, touch with both points. Ask the student whether they felt one or two touches. Record what was done and the answer they gave.
g Repeat this step around 10 times.
h If the student’s answers were mostly correct, reduce the distance between the points of the dividers. If their answers to questions about a double touch were mostly incorrect, increase the distance between the points of the dividers.
i Repeat steps f to h.
j When you think you have found the minimum distance at which the student being tested can reliably discriminate between two points, repeat the procedure for another part of the body.
Teaching notes
It is essential that students being tested do not perceive a pattern in the testing – otherwise they may give results based on what they know the answer should be rather than what they really feel. This is the reason for using dice to decide on one or two touches.
Analyse the pattern of percentages for each part of the body at each distance:
- correctly identified single touch
- correctly identified double touch
- incorrectly identified single touch
- incorrectly identified double touch
The first page of this spreadsheet provides a recording structure for this practical. The second page gives a structure for analysing accumulated data. Once you have added your data in columns H to K, you can ‘fill’ columns D-G with the formula from Row 4.
In this procedure, it may make sense to make qualitative comparisons of the skin sensitivity of a particular body part for several students, but calculating averages for each part of the body is not likely to be meaningful.
Health & Safety checked, March 2009
Downloads
Download the student briefing and consent sheet Assessing skin sensitivity - touch discrimination (60KB)
Download this 
Assessing skin sensitivity spreadsheet
Web links
www.childrensuniversity.manchester.ac.uk/interactives/science/brainandsenses/sensitivity.asp
This is the Children’s University of Manchester site in the Brain and Senses section. There is an interactive sequence that allows you to mimic this protocol with an animated drawing. The close of the sequence introduces a distorted drawing of the body showing each body part in proportion to its sensitivity – this is similar to the ‘sensory homunculus’ described below.
http://piclib.nhm.ac.uk/piclib/www/image.php?img=87495&frm=ser&search=homunculus&first=1
The Natural History Museum’s picture library includes a picture of a sensory homunculus (and a motor homunculus) – models around a metre high that are on display in the Museum’s Human biology exhibition. Type ‘homunculus’ into the picture library search if this link fails.
(Websites accessed October 2011)
Related experiments
Other practicals on the site explore different aspects of the sensitivity of the skin.