<Tr> <Td_colspan="2"> Different areas of the body have receptive fields of different sizes, giving some better resolution in two - point discrimination . The tongue and finger pads have very high resolution, while the back has very low . This is illustrated as the distance where the two points can be felt as separate . </Td> </Tr> <P> Two - point discrimination (2PD) is the ability to discern that two nearby objects touching the skin are truly two distinct points, not one . It is often tested with two sharp points during a neurological examination and is assumed to reflect how finely innervated an area of skin is . In clinical settings, two - point discrimination is a widely used technique for assessing tactile perception . It relies on the ability and / or willingness of the patient to subjectively report what they are feeling and should be completed with the patient's eyes closed . The therapist may use calipers or simply a reshaped paperclip to do the testing . The therapist may alternate randomly between touching the patient with one point or with two points on the area being tested (e.g. finger, arm, leg, toe). The patient is asked to report whether one or two points was felt . The smallest distance between two points that still results in the perception of two distinct stimuli is recorded as the patient's two - point threshold . Performance on the two extremities can be compared for discrepancies . Although the test is still commonly used clinically, it has been roundly criticized by many researchers as providing an invalid measure of tactile spatial acuity, and several highly regarded alternative tests have been proposed to replace it . </P> <P> Body areas differ both in tactile receptor density and somatosensory cortical representation . Normally, a person should be able to recognize two points separated by 2 to 8 mm on fingertips . On the lips, it is 2 to 4 mm, and on the palms, it is 8 to 12 mm and 30--40 mm on the shins or back (assuming the points are at the same dermatome). The posterior column - medial lemniscus pathway is responsible for carrying information involving fine, discriminative touch . Therefore, two - point discrimination can be impaired by damage to this pathway or to a peripheral nerve . </P> <P> Although two - point testing is commonly used clinically, evidence accumulated from many research studies indicates that 2PD is a flawed measure of tactile spatial acuity . Research studies have shown that the two - point test may have low sensitivity, failing to detect or underestimating sensory deficits, that it only poorly tracks recovery of function following nerve injury and repair, that it has poor test - retest reliability, and that it fails to correlate with validated measures of tactile spatial acuity such as grating orientation discrimination . Two - point testing has been criticized for yielding highly variable performance both across and within participants, for being reliant on the subjective criterion adopted by the participant for reporting "one" compared to "two," and for resulting in performance that is "too good to be true," as the measured two - point threshold can fall--unrealistically--well below the skin's receptor spacing . </P>

Which area of the body had the largest receptive field