Which secondary receptive field size is better for 2-point discrimination?

• A. 10 mm field
• B. 40 mm field
• C. 20 mm field
• D. 5 mm field

Answer: (A)

Two-point discrimination depends on how finely the somatosensory system can separate inputs from adjacent skin areas, which is governed by the size of secondary receptive fields in the pathway. Smaller receptive fields provide higher spatial resolution because two nearby stimuli are more likely to activate distinct neurons, leading to two perceived points. If the secondary receptive field is too large, nearby points fall within the same field and feel like one point; if it’s too tiny, the system may struggle to generate a robust, reliable signal across the population, making discrimination less practical in everyday contexts. A middle-ground size around 10 mm offers a good balance: small enough to distinguish two nearby points, but large enough to ensure consistent signaling across neurons. Larger fields (20–40 mm) blur two points into one sensation, while an extremely small field (5 mm) can be too demanding for reliable discrimination given natural variability.