Ali Israr

Detection Threshold and Mechanical Impedance of the Hand in a Pen-Hold Posture

We report position and force detection thresholds for sinusoidal waveforms in the frequency range 10-500 Hz delivered through a stylus. The participants were required to hold the stylus in a way similar to that of holding the stylus of a force-feedback device. A minishaker moved the stylus along its length so that the majority of vibrations were presented tangentially to the skin of the hand. The measured position thresholds decreased initially with an increasing stimulus frequency and formed a U-shaped curve in the high frequency region. The thresholds of high frequency vibrations were lower than those reported previously for vibrations that were perpendicular to the skin, but were similar to the thresholds reported earlier using vibrations that were tangential to the skin. A similar force threshold curve was obtained using a force sensor attached to one end of the stylus. Mechanical impedance of the skin derived from velocity estimates and force measurements indicated that the skin and tissues in the hand holding the stylus can be modeled with mass-, damper- and spring-like elements. A comparison of the mechanical impedance from the present study with those reported previously showed similar results for vibrations delivered in the tangential and normal directions to the skin

Frequency and amplitude discrimination along the kinesthetic-cutaneous continuum in the presence of masking stimulia)

Frequency and amplitude discrimination thresholds along the kinesthetic to cutaneous continuum were evaluated on the left index fingerpad using a multifinger tactual display. Target stimuli were presented either in isolation (no-masker condition) or in the presence of masking stimuli (one- or two-masker conditions). Six reference target signals in the frequency range 2-300 Hz (two each from low-, medium-, and high-frequency regions) and at an amplitude of either 20 or 35 dB sensation levels (SL) were used. In the no-masker condition, the range of frequency Weber fraction was 0.13-0.38 and 0.14-0.28, and the range of amplitude discrimination threshold was 1.82-2.98 dB and 1.65-2.71 dB, at 20 and 35 dB SL, respectively. In the masking conditions, average frequency Weber fractions rose to 0.60 and 0.46, and average amplitude thresholds rose to 3.63 and 3.72 dB, at 20 and 35 dB SL, respectively. In general, thresholds were largest in the two-masker condition and lowest in the no-masker condition. Although the frequency and amplitude thresholds generally increased in the presence of masking stimuli, there was some indication of channel independence for low- and high-frequency target stimuli. The implications of the results for tactual communication of speech are discussed.

Expertise-based performance measures in a virtual training environment

This paper introduces and validates quantitative performance measures for a rhythmic target-hitting task. These performance measures are derived from a detailed analysis of human performance during a month-long training experiment where participants learned to operate a 2-DOF haptic interface in a virtual environment to execute a manual control task. The motivation for the analysis presented in this paper is to determine measures of participant performance that capture the key skills of the task. This analysis of performance indicates that two quantitative measurestrajectory error and input frequencycapture the key skills of the target-hitting task, as the results show a strong correlation between the performance measures and the task objective of maximizing target hits. The performance trends were further explored by grouping the participants based on expertise and examining trends during training in terms of these measures. In future work, these measures will be used as inputs to a haptic guidance scheme that adjusts its control gains based on a real-time assessment of human performance of the task. Such guidance schemes will be incorporated into virtual training environments for humans to develop manual skills for domains such as surgery, physical therapy, and sports.

Mechanical Impedance of the Hand Holding a Spherical Tool at Threshold and Suprathreshold Stimulation Levels

We report mechanical impedance of the hand for sinusoidal stimulation at the threshold and suprathreshold levels in the frequency range of 10-500 Hz delivered through a ball-shaped interface. The participants held the ball mounted on a minishaker in a way similar to that of holding a ball interface of a force-feedback device. A minishaker excited the ball in the vertical direction, resulting in vibrations on the skin of the hand in mostly the tangential direction. The position detection threshold curve was similar to that measured earlier using a stylus in the pen-hold posture, but the force detection threshold curve and the mechanical impedance was shifted upwards in the high frequency region. The mechanical impedance at the threshold and suprathreshold levels were essentially the same, indicating that skin characteristics do not change in the dynamic range of tactual perception for the same tool-holding posture

Effects of magnitude and phase cues on human motor adaptation

Recent findings have shown that humans can adapt their internal control model to account for the changing dynamics of systems they manipulate. In this paper, we explore the effects of magnitude and phase cues on human motor adaptation. In our experiments, participants excite virtual second-order systems at resonance via a two-degree of freedom haptic interface, with visual and visual plus haptic feedback conditions. Then, we change the virtual system parameters and observe the resulting motor adaptation in catch trials. Through four experimental conditions we demonstrate the effects of magnitude and phase cues on human motor adaptation. First, we show that humans adapt to a nominal virtual system resonant frequency. Second, humans shift to higher and lower natural frequencies during catch trials regardless of feedback modality and force cues. Third, participants can detect changes in natural frequency when gain, magnitude, and phase cues are manipulated independently. Fourth, participants are able to detect changes in natural frequency when the feedback (visual or visual plus haptic) is delayed such that the phase shift between the nominal system and catch trial system is zero. The persistent ability of participants to perform system identification of the dynamic systems which they control, regardless of the cue that is conveyed, demonstrates the humans versatility with regard to manual control situations. We intend to further investigate human motor adaptation and the time for adaptation in order to improve the efficacy of shared control methodologies for training and rehabilitation in haptic virtual environments.

Passive and Active Discrimination of Natural Frequency of Virtual Dynamic Systems

It has been shown that humans use combined feedforward and feedback control strategies when manipulating external dynamic systems, and when exciting virtual dynamic systems at resonance, that they can tune their control parameters in response to changing natural frequencies. We present a study to determine the discrimination thresholds for the natural frequency of such resonant dynamic systems. Weber fractions (WFs in %) are reported for the discrimination of 1, 2, 4, and 8 Hz natural frequencies. Participants were instructed either to passively perceive or actively excite the virtual system via a one degree-of-freedom haptic interface with visual and/or haptic feedback. The average WF for natural frequency ranged from 4% to 8.5% for 1,2, and 4 Hz reference natural frequencies, while the WF was approximately 20% for systems with a reference natural frequency of 8 Hz. Results indicate that sensory feedback modality has a significant effect on WF during passive perception, but no significant effect in the active perception case. The data also suggest that discrimination sensitivity is not significantly affected by excitation mode. Finally, results for systems with equivalent natural frequencies but different spring stiffness indicate that participants do not discriminate natural frequency based on the maximum force magnitude perceived.

A Psychophysical Model of Motorcycle Handlebar Vibrations

In this study, we developed a perception-based quantitative model to relate broadband vibrations transmitted through a motorcycle handlebar to a rider’s hands. The test apparatus consisted of the handlebar of a motorcycle rig assembly driven by a computer-controlled actuator. Participants were instructed to hold the handlebar and maintain a sitting posture as they would while riding a motorcycle. In Exp. 1, psychophysical detection thresholds for 10 participants were estimated at ten test frequencies between 20-300 Hz using a two-interval one-up twodown adaptive procedure. The interpolated threshold vs. frequency function specified the minimum acceleration required before a user could perceive the vibration at a particular frequency. In Exp. 2, participants were asked to rate 15 representative handlebar vibrations using a magnitude estimation procedure. The vibration patterns were measured on an actual motorcycle handlebar while the motorcycle traveled at speeds ranging from 25 to 75 mph. Several weighting functions, including the ISO-5349 standards, were applied to the broadband vibration signal in the frequency domain to estimate the total vibration energy by summing up all weighted components. The best weighting function, in the sense that the estimated total energy correlated linearly with the subjective magnitude ratings obtained in Exp. 2, were based on the detection threshold data obtained in Exp. 1. Specifically, the strength of each vibration component was calculated relative to the human detection threshold at the same frequency, thereby taking into account human sensitivity to vibration signals at different frequencies. The resulting weighting function can be applied to other recorded vibration signals to predict user rating of perceived vibration intensities.

A Two DOF Controller for a Multi-Finger Tactual Display Using a Loop-Shaping Technique

The design and evaluation of a new controller for a multi-finger tactual (kinesthetic and cutaneous) display, the TACTUATOR, is discussed. A crucial performance requirement is that the relative amplitude of spectral components be preserved in terms of perceived intensity as judged by human users. In this article, we present a two degree-of-freedom controller consisting of a feedback controller and a pre-filter, and its digital implementation. The overall system was evaluated with frequency-response function measurements and with human psychophysical experiments. The measurement results confirm that the steady-state frequency response closely follows the design specifications. The psychophysical results indicate a deviation in the model of the human detection threshold curve at frequencies below 30 Hz. Future work will compensate for this deviation by reshaping the pre-filter. Our work demonstrates the validity of designing controllers that take into account not only the electromechanical properties of the hardware, but the sensory characteristics of the human user.Copyright

Controller design and consonantal contrast coding using a multi-finger tactual display.

This paper presents the design and evaluation of a new controller for a multi-finger tactual display in speech communication. A two-degree-of-freedom controller consisting of a feedback controller and a prefilter and its application in a consonant contrasting experiment are presented. The feedback controller provides stable, fast, and robust response of the fingerpad interface and the prefilter shapes the frequency-response of the closed-loop system to match with the human detection-threshold function. The controller is subsequently used in a speech communication system that extracts spectral features from recorded speech signals and presents them as vibrational-motional waveforms to three digits on a receivers left hand. Performance from a consonantal contrast test suggests that participants are able to identify tactual cues necessary for discriminating consonants in the initial position of consonant-vowel-consonant (CVC) segments. The average sensitivity indices for contrasting voicing, place, and manner features are 3.5, 2.7, and 3.4, respectively. The results show that the consonantal features can be successfully transmitted by utilizing a broad range of the kinesthetic-cutaneous sensory system. The present study also demonstrates the validity of designing controllers that take into account not only the electromechanical properties of the hardware, but the sensory characteristics of the human user.

Discrimination of consonant articulation location by tactile stimulation of the forearm

In this paper, we evaluate the ability of four able-bodied participants to discriminate the articulation location for spoken consonants, using tactile cues presented on the dorsal side of their forearm. Additionally, we determine the processing capability of the dorsal forearms skin with a tactile sleeve worn by ten participants using two psychophysical studies. Our first study shows that 2–3 tactors arranged along the length of the forearm can be reliably identified by human users, when only the location of vibration is varied. Our second study indicates that the physical placement of localized vibrations map linearly to the perceived physical arrangement. Based on these findings, the subsequent speech experiment uses six tactors placed equidistant from each other and maps location of the constriction inside the mouth as directional cues on the forearm. Results of the speech study show that participants are able to indicate which of two randomly presented tactile cues (derived from consonant-vowel-consonant (CVC) non-sense syllables) has the preceding consonant closer to the lips. The discrimination performance is better (i) with fricatives than plosives, (ii) when the consonants are produced further apart inside the mouth, and (iii) when both place and manner of articulation feature is varied. The study also shows that discrimination performance with cues applied to the forearm is inferior to that with the fingerpads utilized in a previous study.