Although kinesthetic illusions are attractive to occupational therapy professionals because they can be applied to new therapies, there are currently insufficient quantitative data for them. Toward applications of such illusions, using psychophysical experiments this paper reveals two basic properties of kinesthetic illusions: minimum stimuli for creating the illusions and the appearance conditions of vivid illusions. First, we estimated the stimulus thresholds that are required to induce illusions under several frequency, acceleration and contact force combinations. Five subjects participated in a series of psychophysical experiments in which we determined the stimulus threshold using the staircase method. When the vibration frequency condition was 70 [Hz], the vibration stimuli evoked the strongest illusion, and when the acceleration condition was 40 [m/s^2], the vibration stimuli induced the kinesthetic illusions. Second, we investigated differences in the illusion intensity (vividness) at different frequencies, accelerations and contact forces. We applied 40 kinds of different vibration stimuli to each subject and, using magnitude estimation, investigated the illusion intensity (vividness) induced by each stimulus. Our result showed that when the contact force was 0.3 [N] between vibration stimuli of 50 [Hz] and 90 [Hz], increased acceleration made the illusion intensity about two points more vivid. However, when the contact force was 1.5 [N] among all the frequency conditions, this effect was not observed.
