English: From Tong et al (2013) doi: 10.3389/fnhum.2013.00579. (A) Performance on a the traditional two-point discrimination (2PD) task. (B) Performance on the two-point orientation discrimination task (2POD). In both cases, the tests were done with a two-interval forced-choice testing procedure. Note that performance on 2PD is well above chance even at zero mm separation, indicating that 2PD is not a valid measure of tactile spatial acuity. By contrast, performance on 2POD falls to chance level (50% correct) as the point separation approaches zero mm, as expected for a valid measure of spatial acuity.