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Optical Imaging of a Tactile Illusion in Area 3b of the Primary Somatosensory Cortex

Purpose: Tactile displays convey various aspects of touch through mechanical or electrical stimuli. Recent research focuses on combining these two types of stimulation (hybrid stimuli) to achieve naturalness, comfort, and reduction in the threshold, only by experiment. However, there are no computational models to study the Pacinian corpuscle’s behavior under hybrid-stimuli.

Computational Model of a Pacinian Corpuscle for Hybrid-Stimuli: Spike-Rate and Threshold Characteristics

There are use cases where presenting spatial information via the tactile sense is useful (e.g., situations where visual and audio senses are not available). Conventional methods that directly attach a vibrotactile array to a user's body present spatial information such as direction by having users localize the vibration source from among the sources in the array. These methods suffer from problems such as heat generation of the actuator or the installation cost of the actuators in a limited space. A promising method of coping with these problems is to place the vibrotactile array at a distance from the body, instead of directly attaching it to the body, with the aim of presenting spatial information in the same way as the conventional method. The present study investigates the method's effectiveness by means of a psychophysical experiment. Specifically, we presented users with sinusoidal vibrations from remote vibrotactile arrays in the space around the hand and asked them to localize the source of the vibration. We conducted an experiment to investigate the localization ability by using two vibration frequencies (30 Hz as a low frequency and 230 Hz as a high frequency). We chose these two frequencies since they effectively activate two distinctive vibrotactile channels: the rapidly adapting afferent channel and the Pacinian channel. The experimental results showed that humans can recognize the direction of the vibration source, but not the distance, regardless of the source frequency. The accuracy of the direction recognition varied slightly according to the vibration source direction, and also according to the vibration frequency. This suggests that the calibration of stimulus direction is required in the case of both high and low frequencies for presenting direction accurately as intended. In addition, the accuracy variance of direction recognition increased as the source became farther away, and the degree of increase was especially large with the low-frequency source. This suggests that a high frequency is recommended for presenting accurate direction with low variance.

Sinusoidal Vibration Source Localization in Two-Dimensional Space Around the Hand

Electrotactile displays reproduce tactile properties of an object such as texture and shape to provide feedback in human-machine interactions. Electrotactile perception threshold (EPT) is a critical psychophysical parameter in designing a safe display as prolonged usage causes skin irritation. This study's main objective was to reduce EPT with background thermal stimulation (BTS) and analyze the qualitative and quantitative aspects of EPT with and without BTS. A psychophysical experiment was conducted to measure EPT with and without BTS (+7 °C)) over a wide range of stimulus frequencies of EPT from 20 Hz to 640 Hz. Findings from these experiments showed a reduction in the EPT (∆EPT), which was 13% to 17% (with an average of 15%). Reduced EPT may provide safer and comfortable electrotactile displays for prolonged usage.

Reduction of Electrotactile Perception Threshold Using Background Thermal Stimulation.

High-fidelity localized feedback has the potential of providing new and unique levels of interaction with a given device. Achieving this in a cost-effective reproducible manner has been a challenge in modern technology. Past experiments have shown that by using the principles of constructive wave interference introduced by time offsets it is possible to achieve a position of increased vibration displacement at any given location. As new interface form factors increasingly incorporate curved surfaces, we now show that these same techniques can successfully be applied and mechanically coupled with a universal actuation plate.

Haptic Actuation Plate for Multi-Layered In-Vehicle Control Panel

‘‘Out of body” tactile illusions can serve as a tool to exploit the limitations of human perception to augment the information displayed in the mobile haptic devices. The objective of this study was to establish a psychophysics based mathematical relationship between the intensity of physical and phantom stimulus rendered “out of the body.” In order to achieve this objective, three experiments were conducted, the first experiment was to compare three existing algorithms (linear, logarithmic, and square) defined for “on the body” when extended to “out of the body,” the second experiment was to test the pertinence of the best performing algorithm from first experiment, and the third experiment was designed to identify an ideal mathematical relation using psychophysical studies. Fifteen subjects participated in the experiments and the physical stimuli were provided to the index fingers of both the hands such that the phantom stimulus was perceived in the mid-air between them. Results of the first experiment depicted that the “square algorithm” was the best among the three. Results of the second experiment proved “square algorithm” to be better than other two algorithms but not ideal to define mathematical relation for “out of body” tactile funneling illusion. Results of the third experiment established a novel and better performing algorithm using “power relationship” between the intensity of physical and phantom stimulus leading to “out of body” tactile funneling. Although this paper defines the power relationship between the phantom and physical stimulus intensity, the relationship between location of phantom stimulus and the physical stimulus intensity remains the future scope of this study.

Power Law Based “Out of Body” Tactile Funneling for Mobile Haptics

Understanding the response of Pacinian Corpuscle (PC) for an electrical stimulus through a computational model can give better insight into the physiology. Although there are simpler models available in the literature, models simulating spike-rate and threshold characterizations are still missing. These characterizations may lead to the development of tactile displays combining both electrical and mechanical stimuli, especially high-frequency vibrations. We developed a PC model with equivalent circuits of the electrode-skin interface, PC’s neurite, and the first Ranvier node. The input electrical stimulus is a current pulse with varying amplitude (0 to 2 mA) and varying frequency (5 Hz to 1600 Hz). The model is characterized initially for the frequency response, and then the spike-rate and threshold characteristics were simulated. The spike-rate traces for electrical stimuli show the phase-locking phenomenon similar to the mechanical stimuli responses of PC, however the plateau lengths are larger for the spike-rate traces with electrical stimuli compared to that of the mechanical stimuli. This is reflected as a large difference in the threshold characteristics for one and two impulses-per-cycle. Moreover, threshold characteristics are little influenced by the neural noise. This model can be extended to study the combination of electrical and mechanical stimuli.

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Computational Model of a Pacinian Corpuscle for an Electrical Stimulus: Spike-Rate and Threshold Characteristics.

Electrotactile displays provide tactile feedback using electrical currents. However, prolonged usage of them causes skin irritation and burns. This study presents a novel method to use the electrotactile display at a lower threshold by applying two subthreshold electrotactile stimuli placed nearby. This method is tested on fourteen subjects over a frequency range from 20 to 1280 Hz in the tuning curve. Two Psychophysical experiments have been conducted to measure their thresholds for individual stimuli and presented together. In the first experiment, the frequencies of both the stimuli were the same, and it showed 36% to 18% reduction in EPT of the second stimulus when the first stimulus was at 10% to 50% of its threshold. In the second experiment, Meissner and Pacinian were selectively stimulated by applying stimuli at their resonant frequencies (30 and 250 Hz, respectively), results showed slightly more reduction in the case of 250 Hz compared to 30 Hz.

Rahul Kumar Ray

Papers

Power Law Based “Out of Body” Tactile Funneling for Mobile Haptics

Computational Model of a Pacinian Corpuscle for an Electrical Stimulus: Spike-Rate and Threshold Characteristics.

Spatial Summation of Electro-Tactile Displays at Subthreshold

Reduction of Electrotactile Perception Threshold Using Background Thermal Stimulation.

Reduction of electrotactile perception threshold using subthreshold vibrotactile stimuli