Directional sensitivity to a tactile point stimulus moving across the fingerpad

Abstract

The ability of subjects to discriminate between directions of a point contact moving across the fingerpad was examined. Subjects were required to report, using an adaptive two-interval, twoalternative forced-choice procedure, whether in two sequential stimuli the direction of motion changed in a clockwise or counterclockwise direction. The overall mean orientation-change threshold across eight stimulus orientations was approximately 14°, with the lowest threshold for the point motion toward the wrist. This observed lower threshold in the distal-to-proximal direction is thought to be due to stretching of the skin at the tip of the fingernail, to which one may be particularly sensitive. For all orientations, thresholds were generally more uniform and higher than those reported on vibrotactile linear contactor arrays for horizontal and vertical orientations.Although little research has been reported on the thresholds for tactile angle discrimination of a moving point across the skin, several studies have examined tactile and haptic perception oforientation for static stimuli. In particular, the degree to which deviations can be detected from referent or standard vertical, horizontal, and diagonal orientations has been considered. However, a clear understanding of directional sensitivity for a single point contactor, as opposed to sensitivity for a static linear contactor array, is lacking. Furthermore, it is not clear how directional discrimination at opposite ends of the principal meridians may vary, given skin stretch by a single point. Thus the present study has two major objectives: (I) To estimate the degree to which subjects can discriminate the angle of a point contact moving across the right index finger; and (2) to determine how threshold levels vary with stimulus orientations in relation to a fixed finger position.Studies of nonmoving tactile and haptic line orientation perception suggest that an oblique effect (Appelle, 1972) occurs in the haptic and tactile modalities, whereby the discrimination of stimuli presented along or straight across the finger axis (i.e., in a vertical or a horizontal orientation, respectively) is superior to performance with stimuli in oblique orientations. Lechelt, Eliuk, and Tanne (1976) and Lechelt and Verenka (1980) first reported similar spatial asymmetries in the haptic modality as well as in visuallhaptic cross-modal judgments of stimulus orientations. Lechelt (1988) examined the tactile discrimination threshold for stimulus-orientation discrepancy from standard or referent vertical, horizontal, and diagonal reference orientations. The stimuli were vibrating linear displays presented to the pad of the right index finger via an Optacon (optical-to-tactile converter). The Optacon converted optical line images into a 6 X 24 tactile array using piezoelectric bimorph reeds vibrating at 230 Hz. Results indicated that deviations of2,5O and 5°, respectively, could be discriminated at the 75%-correct level from horizontal and vertical standards, whereas 15°d eviations were requir...

Haptic shape perception from force and position signals varies with exploratory movement direction and the exploring finger

“…That is, its sensing probably involves muscle receptor subsystems that signal rate of change in muscle length (which corresponds to velocity/ acceleration) and cutaneous receptors that are particularly sensitive to shear force. It seems unlikely that the larger estimates for tangential than for radial exploration relate to cutaneous sensing via shear force: Shear forces are perceived more accurately when the skin is stretched longitudinally, as compared with transversely (Keyson & Houtsma, 1995). In contrast, here, the finger crossed the bump transversely with tangential exploration (from left to right side, or vice versa), and longitudinally with radial exploration.…”

HapTip: Displaying Haptic Shear Forces at the Fingertips for Multi-Finger Interaction in Virtual Environments

“…Some previous works have focused first on measuring the just noticeable difference between the directions of shear forces generated at two different angles by a fingertip tactile device. Keyson and Houtsma (1995) obtained a threshold angle between 11°and 17°depending on the tested direction. Drewing et al (2005) found a threshold angle between 23°and 35°.…”

First evaluation of a novel tactile display exerting shear force via lateral displacement

“…A single stimulus consisted of a forward-backward movement traversing twice a straight path of 3.25 mm with a velocity of 41 mm/s. For these stimuli, Keyson and Houtsma [1995] report discrimination thresholds for movement direction of about 14…”

“…Our device is designed to maximize skin stretch, whereas in the Keyson and Houtsma device skin stretch was minimized by minimizing skin friction. Moreover, our device is designed as a multipin display and by the requirement to achive high pin density it is constrained to smaller displacements than investigated by Keyson and Houtsma [1995].…”

“…However, to our knowledge, there is only a single study on human resolution of movement direction, which is an important parameter for display design. Keyson and Houtsma [1995] displayed movements on the distal phalanx of the finger via a 0.8 mm diameter Braille point that was fixed to a trackball. The trackball could be moved in any horizontal direction.…”