Science 7–10 · Year 9
Two-point discrimination: mapping touch receptor density on the skin
Science understanding: Biological sciences (AC9S9U01 is about body systems coordinating a response to a stimulus and negative feedback, and the NSW Stage 5 Disease focus area covers homeostasis, feedback loops and nervous and endocrine coordination; neither deals with sensory receptor density, and no other ACARA v9 content description or NSW content point does, so both sides are coded to science inquiry and Working scientifically only)
The idea
Skin that carries more touch receptors per square centimetre can tell two nearby points apart, so the smallest gap felt as two points measures receptor density.
What you need
- A pair of blunt-ended dividers or two toothpicks taped to a ruler at an adjustable gap, with a millimetre scale
- A blindfold or a request to look away
- A results table listing sites: fingertip, palm, forearm, upper arm, back of the neck
How to do it
- The subject looks away. The tester touches the skin lightly with both points at once, set 60 mm apart so the first gap is wider than the 35 to 40 mm the arm needs, and asks whether one or two points are felt. If only one point is felt at the first touch, widen the gap in 10 mm steps until two are felt, then work down.
- Reduce the gap in 5 mm steps (then 1 mm steps below 10 mm) until the subject reports one point; occasionally touch with a single point as a check on guessing.
- Record the smallest gap reliably felt as two points at each site, testing each site twice.
- Swap roles and repeat.
- Rank the sites from most to least sensitive and pool the class means.
What you should see
Fingertips distinguish two points 2 to 3 mm apart, while the arm needs about 35 to 40 mm, the values reported in the University of Washington lesson; the other sites on the sheet are the learner's own measurements, and the class ranking should place the fingertip first. The learner knows it worked when the two repeats at a site agree to within a few millimetres and the single-point checks were reported correctly.
What changes
- What you change
- Skin site
- What you measure
- Two-point threshold (mm)
- What you keep the same
- Pressure of touch
- Subject looking away
- Step sizes
- Same tester for one subject
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- All skin feels touch equally well.
- Bigger body parts have more touch receptors; the small fingertip resolves a gap more than ten times narrower than the arm can.
- The brain receives a picture of the skin rather than signals from separate receptors.
Safety card
Hazards
- Sharp points on dividers
Controls
- Blunt tips only; light touch; test only on arms, hands and neck; the subject may stop at any time
Note
No hazardous chemicals; record a RiskAssess risk assessment for the activity as school procedure requires.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
Sources
The pages the author read to write this activity.