Previous research has shown that subjects appear unable to restrict processing to a single finger and ignore a stimulus presented to an adjacent finger. Furthermore, the evidence suggests that, at least for moving stimuli, an adjacent nontarget is fully processed to the level of incipient response activation, The present study replicated and expanded upon these original findings. The results of Experiment 1 showed that an equally large response-competition effect occurred when the nontarget was presented to adjacent and nonadjacent fingers (on the same hand). The results of Experiment 2 showed that the effects observed in Experiment 1 (and in previous studies) were also obtained with stationary stimuli. Although small, there was some indication in the results of Experiment 2 that interference may dissipate more rapidly with distance with stationary stimuli. An additional finding was that interference effects were observed in both experiments with temporal separations between the target and nontarget of up to 100 msec. In Experiment 3, target and nontarget stimuli were presented to opposite hands. Although reduced, interference was still evident with target and nontarget stimuli presented to opposite hands. Varying the physical distance between hands did not produce any change in the amount of interference. The results suggest that the focus of attention on the skin extends nearly undiminished across the fingers of one hand and is not dependent upon the physical distance between sites of stimulation. 571A recent study by Evans and Craig (1991) indicated that subjects have difficulty focusing their attention on one finger and ignoring stimulation at an adjacent site. In this study, subjects were instructed to focus their attention on the left index fmger and to identify the direction of movement of the stimulus presented to that finger. A second stimulus, also moving, was presented to the middle finger, adjacent to the index finger. The stimuli moved in the same direction or in opposite directions. We reasoned that if subjects could restrict their attention to the target location, the presence of the nontarget would produce little, if any, interference. If, however, subjects are unable to restrict their attention to a single finger on one hand, then the direction of movement of the adjacent nontarget might interfere with target performance in a systematic fashion. Specifically, target performance might be interfered with when the direction of movement of the nontarget was opposite to that of the target. The results showed that when the target and nontarget moved in opposite directions, accuracy was reduced and response latencies (correct trials) This research was supported by Grant DC-00095 from the National Institutesof Health and was conducted while the first author was a visiting scholar in the Institute for the Study of Human Capabilities at Indiana University. The authors thank Roger P. Rhodes for his assistance in conducting these experiments. Reprint requests should be addressed to P. M. Evans, Department o...

Previous studies have shown that the perception of spatial patterns, such as letters, presented to the hand is affected by the spatial orientation of the hand. The present study investigated how the perception of direction of motion across the fingerpads changes with the position of the hand in space. The moving stimuli were generated on two displays. In one condition, the displays were placed horizontally in front of the subject, with the subject's thumb (target site) and index finger (nontarget site) placed flat on the displays. In a second condition, the displays were vertically oriented and gripped between the thumb and index finger. Using a selective-attention paradigm in which subjects are instructed to respond only to the direction of motion at the target site, performance was still affected by the direction of motion at the nontarget site. Changing the orientation of the displays changed the effectiveness of the nontarget in interfering with the identification of the target movement. Nontarget stimuli that produced no interference in the horizontal orientation did so in the vertical, and vice versa. It appears that subjects are not using the local direction of movement across the fingerpads to judge the relative direction of movement at the two sites; rather, they are using the external direction of movement.

As part of a project to examine the ability of the hand to receive speech information, the present study examined subjects' ability to discriminate finger movements along the dimensions of amplitude and period (movement duration). The movements consisted of single-cycle, sinewave movements and single-cycle, cosine movements presented to the index finger. Difference thresholds were collected using an adaptive, two-interval, temporal forced-choice procedure. Amplitudes from 6 to 19 mm were examined, and the difference thresholds ranged from 10% to 18%. The thresholds were unaffected by the period of the movement. Periods from 3000 to 111 ms (0.33-9 Hz) were examined, and thresholds ranged from 6% to 16%. The thresholds were unaffected by the amplitude of the movement. Further measurements in which period was varied in the amplitude discrimination task and amplitude was varied in the period discrimination task indicated that subjects were not using peak velocity as the basis for discrimination. These measurements were collected using a display specifically designed for the examination of haptic stimulation and capable of presenting controlled patterns of movement and vibration to the fingers.

The present study describes experiments comparing human observers’ ability to resolve the orientation of grating stimuli presented on a 3-DOF haptic interface (PHANToM) and a 2-DOF haptic interface (Immersion IE2000 joystick). Although the force renderings for the two devices are of necessity quite distinct, both yield surprisingly credible percepts of surface texture. Accordingly, resolution for sinusoidal gratings (3-DOF device) and “square-wave” gratings (2-DOF device) was measured as the minimum spatial period required to determine which of two grating orientations had been presented. For the 3-DOF device, this ability was tested for a range of grating amplitudes. For the 2-DOF device, a range of damping constants was employed. Pilot results showed better performance for the 2-DOF device, most likely attributable to the sharp delineation of “edges” for the gratings presented, as compared to the “edgeless” sinusoidal gratings presented on the 3-DOF device. In addition, observers reported a number of interesting perceptual phenomena when stimuli were close to threshold.

Differences in the force profile delivered by different types of end-effectors suggest that the choice of end-effector for a haptic interface can have a considerable effect on the perception of the human user. In the present study two different end-effector types were evaluated for two different haptic interfaces. Conventional probe-stylus end-effectors were tested on the PHANToM 3-degree-of-freedom (DOF) force feedback haptic interface and for the Immersion IE2000 2-DOF force feedback joystick. These were compared to thimble-gimbal end-effectors into which the index fingertip is inserted (standard for the PHANToM and specially constructed for the IE2000). In a task in which subjects were asked to judge the orientation of virtual sinusoidal gratings, no significant differences in performance were observed. Results are discussed in terms of tasks in which the differential cues delivered by different end-effectors might influence performance.