Detection of vibration transmitted through an object grasped in the hand

A tool or probe often functions as an extension of the hand, transmitting vibrations to the hand to produce a percept of the object contacting the tool or probe. This paper reports the psychophysical results of a combined psychophysical and neurophysiological study of the perception of vibration transmitted through a cylinder grasped in the hand. In the first part of the psychophysical study, 19 subjects grasped a cylinder, 32 mm diam, with an embedded motor that caused vibration parallel to the axis of the cylinder. The relationship between threshold and frequency was the traditional U-shaped function with a minimum between 150 and 200 Hz. Except a study by Békésy in which subjects grasped a rod that vibrated parallel to the skin surface, thresholds above 20 Hz were lower and the slopes were steeper than any reported previously. Thresholds were <0.01 microm in some subjects. Data from both the psychophysical and the neurophysiological studies suggest that detection performance at frequencies >20 Hz was based on activity in Pacinian afferents. The extreme sensitivity compared with previous reports may have resulted from differences in contact area, direction of vibration, contact force, and the shape of the stimulus probe. The effects of each of these variables were studied. At 40 and 300 Hz (frequencies near the lower and upper end of the Pacinian range) thresholds were 9.8 and 18.5 dB (68 and 88%) lower, respectively, when subjects grasped the cylinder than when a 1-mm-diam probe vibrated perpendicular to the skin. These differences were accounted for as follows: 1) thresholds at a single fingerpad obtained with the large cylindrical surface were, on average, 20 and 60% lower, respectively, than thresholds with the punctate probe; 2) thresholds at the palm were, on average, 15 and 40% lower, respectively, than at the fingerpads; 3) thresholds obtained when the subjects grasped the cylinder averaged 40 and 20% less, respectively, than when the cylinder contacted only the palm; 4) thresholds with the cylinder contacting two fingers were 10 and 30% lower, respectively, than thresholds with the cylinder contacting a single finger; and 5) thresholds with vibration parallel to the skin surface were, on average, 10 and 30% lower, respectively, than thresholds with vibration perpendicular to the skin. Contact force, which was varied from 0.05 to 1.0 N, had no effect.