Discrimination of real and virtual surfaces with sinusoidal and triangular gratings using the fingertip and stylus.

Two-interval two-alternative forced-choice discrimination experiments were conducted separately for sinusoidal and triangular textured surface gratings from which amplitude (i.e., height) discrimination thresholds were estimated. Participants (group sizes: n = 4 to 7) explored one of these texture types either by fingertip on real gratings (Finger real), by stylus on real gratings (Stylus real), or by stylus on virtual gratings (Stylus virtual). The real gratings were fabricated from stainless steel by an electrical discharge machining process while the virtual gratings were rendered via a programmable force-feedback device. All gratings had a 2.5-mm spatial period. On each trial, participants compared test gratings with 55, 60, 65, or 70 µm amplitudes against a 50-µm reference. The results indicate that discrimination thresholds did not differ significantly between sinusoidal and triangular gratings. With sinusoidal and triangular data combined, the average (mean + standard error) for the Stylus-real threshold (2.5 ± 0.2 µm) was significantly smaller (p <; 0.01) than that for the Stylus-virtual condition (4.9 ± 0.2 µm). Differences between the Finger-real threshold (3.8 ± 0.2 µm) and those from the other two conditions were not statistically significant. Further studies are needed to better understand the differences in perceptual cues resulting from interactions with real and virtual gratings.

A Frequency-Domain Analysis of Haptic Gratings.

The detectability and discriminability of virtual haptic gratings were analyzed in the frequency domain. Detection (Exp. 1) and discrimination (Exp. 2) thresholds for virtual haptic gratings were estimated using a force-feedback device that simulated sinusoidal and square-wave gratings with spatial periods from 0.2 to 38.4 mm. The detection threshold results indicated that for spatial periods up to 6.4 mm (i.e., spatial frequencies >0.156 cycle/mm), the detectability of square-wave gratings could be predicted quantitatively from the detection thresholds of their corresponding fundamental components. The discrimination experiment confirmed that at higher spatial frequencies, the square-wave gratings were initially indistinguishable from the corresponding fundamental components until the third harmonics were detectable. At lower spatial frequencies, the third harmonic components of square-wave gratings had lower detection thresholds than the corresponding fundamental components. Therefore, the square-wave gratings were detectable as soon as the third harmonic components were detectable. Results from a third experiment where gratings consisting of two superimposed sinusoidal components were compared (Exp. 3) showed that people were insensitive to the relative phase between the two components. Our results have important implications for engineering applications, where complex haptic signals are transmitted at high update rates over networks with limited bandwidths.

Localization of a time-delayed, monocular virtual object superimposed on a real environment.

Observers adjusted a pointer to match the depicted distance of a monocular virtual object viewed in a see-through, head-mounted display. Distance information was available through motion parallax produced as the observers rocked side to side. The apparent stability of the virtual object was impaired by a time delay between the observers' head motions and the corresponding change in the object position on the display. Localizations were made for four time delays (31 ms, 64 ms, 131 ms, and 197 ms) and three depicted distances (75 cm, 95 cm, and 113 cm). The errors in localizations increased systematically with time delay and depicted distance. A model of the results shows that the judgment error and lateral projected position of the virtual object are each linearly related to time delay.

Rotation and direction judgment from visual images head-slaved in two and three degrees-of-freedom.

The contribution to spatial awareness of adding a roll degree-of-freedom (DOF) to telepresence camera platform yaw and pitch was examined in an experiment where subjects judged direction and rotation of stationary target markers in a remote scene. Subjects viewed the scene via head-slaved camera images in a head-mounted display. Elimination of the roll DOF affected rotation judgment, but only at extreme yaw and pitch combinations, and did not affect azimuth and elevation judgement. Systematic azimuth overshoot occurred regardless of roll condition. Observed rotation misjudgments are explained by kinematic models for eye-head direction of gaze.

Dynamic response of electromagnetic spatial displacement trackers.

Overall system latency--the elapsed time from input human motion until the immediate consequences of that input are available in the display--is one of the most frequently cited shortcoming of current virtual environment (VE) technology. Given that spatial displacement trackers are employed to monitor head and hand position and orientation in many VE applications, the dynamic response intrinsic to these devices is an unavoidable contributor to overall system latency. In this paper, we describe a testbed and method for measurement of tracker dynamic response that use a motorized rotary swing arm to sinusoidally displace the VE sensor at a number of frequencies spanning the bandwidth of volitional human movement. During the tests, actual swing arm angle and VE sensor reports are collected and time stamped. By calibrating the time stamping technique, the tracker's internal transduction and processing time are separated from data transfer and host computer software execution latencies. We have used this test-bed to examine several VE sensors--most recently to compare latency, gain, and noise characteristics of two commercially available electromagnetic trackers: Ascension Technology Corp.'s Flock of Birds(TM) and Polhemus Inc.'s Fastrak(TM).

Peripheral mechanical loading and the mechanism of the tremor of chronic alcoholism.

The tremor of chronic alcoholism, although clinically similar to essential tremor, has been considered a distinct syndrome. Its underlying mechanism was analyzed in five patients (none in the acute stages of alcohol withdrawal) hospitalized in an alcohol detoxification program. All five patients performed tracking tasks in which they pursued a linearly moving "target" light with a response light that they controlled by flexion-extension activity of the wrist. Stationary and dynamic targets were used with both isometric and unconstrained wrist mechanical interfaces. Frequency, torque, and displacement tremor characteristics were examined under varying inertial loading or isometric voluntary torque conditions. Two simultaneous tremor components were present in all patients: a prominent 4- to 7-Hz low-frequency peak and a smaller-amplitude 9.4- to 9.6-Hz high-frequency peak. As the inertia of the hand was augmented during unconstrained tasks, the low-frequency peak decreased, while the high-frequency peak was unaffected. As required voluntary effort was increased during isometric testing, the amplitude of the low-frequency peak increased. These findings suggest that the low-frequency peak represents the significant pathologic component of the tremor of chronic alcoholism and that it has a biomechanical reflex mechanism similar to that of the lower-amplitude normal physiologic tremor.

Controlled trial of isoniazid therapy for severe postural cerebellar tremor in multiple sclerosis.

Six patients with severe postural cerebellar tremor were studied in a double-blind, placebo-controlled, crossover trial with isoniazid. Each phase of the study lasted 4 weeks, and in the isoniazid phase patients received an increasing dose up to 1200 mg/d. Patients were studied with self-rating scales, quantitative tremor recording, and blinded ratings of videotapes. All patients improved by at least one method, three improved by all methods, and four improved sufficiently to want to continue with the drug after the trial was finished. There is a definite, but limited, therapeutic role for isoniazid.