Viscosity discrimination: a comparison of an adaptive two-alternative forced-choice and an adjustment procedure.

Differential thresholds for viscosity were measured in ten subjects with the use of an adaptive two-alternative forced-choice procedure. An electromagnetic linear motor was connected to each wrist and the viscosity of the motors was under computer servo control. For each block of 50 trials the viscosity of one motor was fixed at a reference value which ranged from 4 to 512 N s m-1, and the viscosity of the other motor varied according to the subject's responses. On each trial subjects were required to indicate which motor had the greater viscosity and were given feedback of the correct response. By this procedure the Weber fraction for viscosity was calculated to be 19%, which is lower than the Weber fraction of 34% estimated by using the method of adjustment. The criterion used for determining the threshold differs in the two procedures (71% and 84% correct, respectively), and the results from the two studies were found to be consistent. They suggest that the Weber fraction for viscosity remains remarkably stable despite differences in the methods of measurement.