Question

Suppose you perform an experiment similar to the two-point threshold test, but you use tiny flashlights to test the two-point threshold of the retina. How would the retina's two-point threshold distance to light stimuli compare to the finger's two-point threshold to cutaneous stimuli?

Answer 1

Each sensory neuron receives information from a particular sensory area called its receptive field. The ability to distinguish between two adjacent mechanical stimuli to the skin is known as two-point discrimination.The minimum distance by which two touch stimuli can be perceived as separate stimuli varies from 2–3 mm on the lips and fingertips to over 60 mm on the back of the trunk. The difference in the distance between the two points seems to be related to the density of touch receptors in different parts of the body.

Two-point discrimination is greater on the fingertips and lips where the receptive fields are much smaller and overlap considerably than on the hands and back where the receptive fields are large and widely separated.

Discrimination of two spatially separated targets is termed resolution. The minimum separation between the two points, which can be discriminated as two, is known as minimum resolvable (minimum separable). The distance between the two targets is specified by the angle subtended at the nodal point of the eye. The normal angular threshold of discrimination for resolution measures approximately 30–60 s arc; it is usually called the minimum angle of resolution. The distance on retina separating two images is approximately 4.5 μm.