Question

The distance by which two touch stimuli must be separated to be perceived as two separate stimuli is greatest at?

• A. The lips
• B. The palm of the hand
• C. The back over the scapula
• D. The dorsum of the hand

Hint:

Two-point threshold is largest where touch receptors are sparsest, like the back.

Answer 1

The Correct Option is C

Step 1: This is the two-point discrimination threshold, the smallest distance between two simultaneous touch stimuli at which they are still felt as two separate points rather than one. A larger threshold means the area has poorer spatial resolution.

Step 2: The threshold is smallest where touch receptors are most densely packed and where the cortical representation is large, such as the lips and fingertips. On the fingertips two stimuli are told apart when separated by as little as 2 mm.

Step 3: The threshold is largest where receptor density is low and receptive fields are wide. On the back, over the scapula, points must be separated by at least about 65 mm before they are recognised as two. Therefore the greatest distance is required over the back, option (c).

Step 4: The lips, palm, and dorsum of the hand all have far higher receptor density than the back, so their thresholds are much smaller, making them incorrect.