The Two-Point Threshold
USD Internet Sensation & Perception Laboratory


Touch Acuity Experiment

Background

Although humans rely heavily on the sense of vision to detect and recognize objects, the sense of touch is also very important. Touch can provide information about an object, such as surface texture, that is not easily detectable by vision. Touch experiences are triggered by mechanical disturbance of the skin produced by physical contact with an object. The human skin contains mechanoreceptors, or receptors that are sensitive to mechanical pressure or deformation of the skin. However, the concentration of mechanoreceptors within the skin is not uniform. Rather, the highly sensitive areas of skin, such as the lips and fingertips, contain densely packed mechanoreceptors, while insensitive areas, such as the stomach and back, contain lower concentrations of mechanoreceptors. More sensitive areas of the skin also project to a larger proportion of the somatosensory cortex than less sensitive areas. Thus, the area of the brain which receives touch sensations (for example, from the fingertip) is proportional to the actual sensitivity of the skin area.

Experiment

Touch acuity is conventionally measured using the two-point threshold test. The basic question is this: How far apart do two separate points need to be before they are perceived as two points rather than one? In this experiment we will test the sensitivity of five separate areas of the skin: the pad of the middle finger, the dorsal (or backside) of the middle finger, the back of the hand, the forearm, and upper arm. Begin the experiment by forming a 2 person team.  Next, decide who will do the testing and who will be the participant. The person chosen to be the experimenter will begin by calibrating the test apparatus (an adjustable 2-point caliper) to a inter-point gap size of 1 mm (using a ruler).

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Adjustable 2-point caliper

Start with the fingertip and then continue through the five testing areas using a gap size of 1mm. After testing each area record whether the participant perceives one or two points on the skin surface. Once you have tested all five areas at 1mm of separation (and record the responses) proceed to test all five areas in the same sequence at the following increments: 2mm, 3mm, 4mm, 5mm, 10mm, 15mm, 20mm, 25mm, and 30mm. Be sure to record the amount of separation and whether the participant can perceive two separate points. Once the participant has perceived two points on two consecutive trials feel free to skip that testing area, and use the 1st two-point increment as the threshold value. For example, if on the back of the hand the participant feels two points at 10mm and 15mm, use 10mm as the threshold. Proceed through the remaining increments until the participant can feel two separate points on each of the five testing areas.

 

 


 

 

Data Sheet

  1mm 2mm 3mm 4mm 5mm 10mm 15mm 20mm 25mm 30mm
Pad of Middle Finger                    
Back of Middle Finger                    
Back of Hand                    
Forearm                    
Upper Arm                    

Lab Report

1. What is the independent variable?
2. What is the dependent variable?
3. What was the psychophysical method used in this experiment?
4. Plot your threshold as a function of the tested area.
5. What is the relationship between the two-point threshold and the tested area? Why is this so?


Different regions of the somatosensory cortex process tactile information from different parts of the body, with these regions forming a "map" of the body surface on the cortical surface.  In general, the more cortex dedicated to a body surface,  the greater the sensitivity of that region (Blake & Sekuler, 2006). As seen in the Sensory Homunculus, a disproportionately large volume of cortical tissue is devoted to certain parts of the body such as the hands and mouth (Blake & Sekuler, 2006).

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