Perceived magnitude of multiple electrocutaneous pulses

Multiplexing intensity and frequency sensations for artificial touch by modulating temporal features of electrical pulse trains

In neural prostheses, intensity modulation of a single channel (i.e., through a single stimulating electrode) has been achieved by increasing the magnitude or width of each stimulation pulse, which risks eliciting pain or paraesthesia; and by changing the stimulation rate, which leads to concurrent changes in perceived frequency. In this study, we sought to render a perception of tactile intensity and frequency independently, by means of temporal pulse train patterns of fixed magnitude, delivered non-invasively. Our psychophysical study exploits a previously discovered frequency coding mechanism, where the perceived frequency of stimulus pulses grouped into periodic bursts depends on the duration of the inter-burst interval, rather than the mean pulse rate or periodicity. When electrical stimulus pulses were organised into bursts, perceived intensity was influenced by the number of pulses within a burst, while perceived frequency was determined by the time between the end of one burst envelope and the start of the next. The perceived amplitude was modulated by 1.6× while perceived frequency was varied independently by 2× within the tested range (20-40 Hz). Thus, the sensation of intensity might be controlled independently from frequency through a single stimulation channel without having to vary the injected electrical current. This can form the basis for improving strategies in delivering more complex and natural sensations for prosthetic hand users.

Auditory clicks elicit equivalent temporal frequency perception to tactile pulses: A cross-modal psychophysical study

Both hearing and touch are sensitive to the frequency of mechanical oscillations—sound waves and tactile vibrations, respectively. The mounting evidence of parallels in temporal frequency processing between the two sensory systems led us to directly address the question of perceptual frequency equivalence between touch and hearing using stimuli of simple and more complex temporal features. In a cross-modal psychophysical paradigm, subjects compared the perceived frequency of pulsatile mechanical vibrations to that elicited by pulsatile acoustic (click) trains, and vice versa. Non-invasive pulsatile stimulation designed to excite a fixed population of afferents was used to induce desired temporal spike trains at frequencies spanning flutter up to vibratory hum (>50 Hz). The cross-modal perceived frequency for regular test pulse trains of either modality was a close match to the presented stimulus physical frequency up to 100 Hz. We then tested whether the recently discovered “burst gap” temporal code for frequency, that is shared by the two senses, renders an equivalent cross-modal frequency perception. When subjects compared trains comprising pairs of pulses (bursts) in one modality against regular trains in the other, the cross-sensory equivalent perceptual frequency best corresponded to the silent interval between the successive bursts in both auditory and tactile test stimuli. These findings suggest that identical acoustic and vibrotactile pulse trains, regardless of pattern, elicit equivalent frequencies, and imply analogous temporal frequency computation strategies in both modalities. This perceptual correspondence raises the possibility of employing a cross-modal comparison as a robust standard to overcome the prevailing methodological limitations in psychophysical investigations and strongly encourages cross-modal approaches for transmitting sensory information such as translating pitch into a similar pattern of vibration on the skin.

The impact of the stimulation frequency on closed-loop control with electrotactile feedback

BACKGROUND

Electrocutaneous stimulation can restore the missing sensory information to prosthetic users. In electrotactile feedback, the information about the prosthesis state is transmitted in the form of pulse trains. The stimulation frequency is an important parameter since it influences the data transmission rate over the feedback channel as well as the form of the elicited tactile sensations.

METHODS

We evaluated the influence of the stimulation frequency on the subject's ability to utilize the feedback information during electrotactile closed-loop control. Ten healthy subjects performed a real-time compensatory tracking (standard test bench) of sinusoids and pseudorandom signals using either visual feedback (benchmark) or electrocutaneous feedback in seven conditions characterized by different combinations of the stimulation frequency (FSTIM) and tracking error sampling rate (FTE). The tracking error was transmitted using two concentric electrodes placed on the forearm. The quality of tracking was assessed using the Squared Pearson Correlation Coefficient (SPCC), the Normalized Root Mean Square Tracking Error (NRMSTE) and the time delay between the reference and generated trajectories (TDIO).

RESULTS

The results demonstrated that FSTIM was more important for the control performance than FTE. The quality of tracking deteriorated with a decrease in the stimulation frequency, SPCC and NRMSTE (mean) were 87.5% and 9.4% in the condition 100/100 (FTE/FSTIM), respectively, and deteriorated to 61.1% and 15.3% in 5/5, respectively, while the TDIO increased from 359.8 ms in 100/100 to 1009 ms in 5/5. However, the performance recovered when the tracking error sampled at a low rate was delivered using a high stimulation frequency (SPCC = 83.6%, NRMSTE = 10.3%, TDIO = 415.6 ms, in 5/100).

CONCLUSIONS

The likely reason for the performance decrease and recovery was that the stimulation frequency critically influenced the tactile perception quality and thereby the effective rate of information transfer through the feedback channel. The outcome of this study can facilitate the selection of optimal system parameters for somatosensory feedback in upper limb prostheses. The results imply that the feedback variables (e.g., grasping force) should be transmitted at relatively high frequencies of stimulation (>25 Hz), but that they can be sampled at much lower rates (e.g., 5 Hz).

Human ability in identification of location and pulse number for electrocutaneous stimulation applied on the forearm

Background

The need of a sensory feedback system that would improve users’ acceptance in prostheses is generally recognized. Feedback of hand opening and position are among the most important concerns of prosthetic users. To address the two concerns, this study investigated the human capability to identify pulse number and location when electrical stimulation applied on the forearm skin. The pulse number may potentially be used to encode the opening of prosthetic hands and stimulation location to encode finger position.

Methods

Ten able-bodied subjects participated in the study. Three electrodes were placed transversely across the ventral forearm spatially encoding three fingers (i.e., thumb, index, and middle finger). Five different pulse numbers (1, 4, 8, 12, and 20) encoded five levels of hand opening. The study consisted of three experiments. In the three experiments, each after a training session, the subjects were required to identify among: (a) five stimulation locations, (b) five pulse numbers, or (c) ten paired combinations of location and pulse number, respectively. The subjects’ performance in the three identification tasks was evaluated.

Results

The main results included: 1) the overall identification rate for stimulation location was 92.2 ± 6.2%, while the success rate in two-site stimulation was lower than one-site stimulation; 2) the overall identification rate for pulse number was 90.8 ± 6.0%, and the subjects showed different performance in identification of the five pulse numbers; 3) the overall identification rate decreased to 80.2 ± 11.7% when the subjects were identifying paired parameters.

Conclusions

The results indicated that the spatial (location) and temporal (pulse number) identification performance are promising in electrocutaneous stimulation on the forearm. The performance degraded when both parameters had to be identified likely due to increased cognitive load resulting from multiple tasks. Utilizing the proposed coding strategy in practical prosthetic hands remains to be investigated for clinical evaluation of its feasibility.

Electric pulse frequency and magnitude of perceived sensation during electrocutaneous forearm stimulation

OBJECTIVES

To investigate the effect that electric pulse frequency has on the perceived magnitude of sensation and to quantify the relationship between electric pulse frequency and perceived magnitude of sensation during low-intensity electrocutaneous stimulation.

DESIGN

A repeated-measures research design was applied to evaluate the effect of electric pulse frequency on the perceived magnitude of electrocutaneous stimulation.

SETTING

Electrocutaneous agents laboratory.

PARTICIPANTS

University students (N=26) with normal hearing and normal sensation were recruited for the study.

INTERVENTIONS

Electrocutaneous stimulation was applied to the forearm at 10 electric pulse frequencies.

MAIN OUTCOME MEASURES

A cross-modality matching procedure was used in which stimulation intensity was matched with the level of loudness. Pairwise comparisons with 2 degrees of freedom at a power of 80% was performed. Statistical significance was set at P equal to .05.

RESULTS

Electric pulse frequency had a significant effect on the perceived magnitude of sensation, with the perceived sensation growing between 0 and 120Hz (F=36.02; P<.001). The relationship between the 2 variables was strong (r(2)=.99; P<.01).

CONCLUSIONS

Increasing the electric pulse frequency of electrocutaneous stimulation increases the perceived magnitude of the resulting sensation. This has implications for the use of electrocutaneous stimulation for both analgesia and muscle stimulation.

Effectiveness of supplemental grasp-force feedback in the presence of vision

Previous studies have shown that supplemental grasp-force feedback can improve control for users of a hand prosthesis or neuroprosthesis under conditions where vision provides little force information. Visual cues of force are widely available in everyday use, however, and may obviate the utility of supplemental force information. The purpose of the present study was to use a video-based hand neuroprosthesis simulator to determine whether grasp-force feedback can improve control in the presence of realistic visual information. Seven able-bodied subjects used the simulator to complete a simple grasp-and-hold task while controlling and viewing pre-recorded, digitised video clips of a neuroprosthesis user's hand squeezing a compliant object. The task was performed with and without supplemental force feedback presented via electrocutaneous stimulation. Subjects had to achieve and maintain the (simulated) grasp force within a target window of variable size (+/- 10-40% of full scale). Force feedback improved the success rate significantly for all target window sizes (8-16%, on average), and improved the success rate at all window sizes for six of the seven subjects. Overall, the improvement was equivalent functionally to a 35% increase in the window size. Feedback also allowed subjects to identify the direction of grasp errors more accurately, on average by 10-15%. In some cases, feedback improved the failure identification rate even if success rates were unchanged. It is thus concluded that supplemental grasp-force feedback can improve grasp control even with access to rich visual information from the hand and object.

Individual differences in perceived pinch force and bite force

Six subjects made cross-modal matches of pinch force and bite force to an electrocutaneous stimulus. The electrocutaneous stimulus consisted of bursts of pulses, and the intensity of the stimulus was varied by changing the number of pulses per burst. All of the individual matching functions were fit well by power functions. The scaling constants and exponents of the power functions covaried systematically with the maximum pinch force for 5 of the 6 subjects. The relationship was consistent with the hypothesis that subjects perceive their physical maxima equally, in agreement with Borg's theory of relative perceived exertion. For both pinch force and bite force functions, the scale factors could be described by a single linear function of the exponents, suggesting that all of the matching functions converged at a single point with extreme values.

Maximal dynamic range electrotactile stimulation waveforms

A new method to measure the dynamic range of electrotactile (electrocutaneous) stimulation uses both steepest ascent (gradient) and one-variable-at-a-time methods to determine the waveform variables that maximize the subjective magnitude (intensity) of the electrotactile percept at the maximal current without discomfort for balanced-biphasic pulse bursts presented at a 15-Hz rate. The magnitude at the maximal current without discomfort is maximized by the following waveform (range tested in parentheses): number of pulses/burst = 6 (1-20), pulse repetition rate within a burst = 350 Hz (200-1500), and phase width = 150 microseconds (40-350). The interphase interval (separation between positive and negative phases in a biphasic pulse) does not affect dynamic range from 0-500 microseconds. The number of pulses/burst has a large effect on the perceived dynamic range when this is measured using a subjective-magnitude-based algorithm, whereas it has little effect on the traditional dynamic range measure, i.e., (maximal current without discomfort)/(sensation threshold current). The perceived stimulus magnitude at the maximal current without discomfort is approximately twice as strong with 6 pulses/burst as it is with 1 pulse/burst (a frequently-used waveform).

Magnitude estimates for electrical pulses: evidence for two neural mechanisms

The hypothesis that there are two neural mechanisms for electrocutaneous stimulation--one that is sensitive to low current and is adaptive to repeated stimulation and another that is responsive to high current and is less adaptive--was tested in a control and four main experiments. In the main experiments, magnitude estimates obtained for single electrical pulses (of 2-msec duration) were described by a simple power function for each combination of high- and low-current levels and 10 trial blocks. The results were: (1) The slope of the power function for low current was steeper than was that for high current; (2) for low current, the intercept of the power function decreased with increasing block, whereas for high current, it remained constant over blocks; (3) this decrease of the intercept for low current disappeared when judgmental blocks were separated by a rest period of 8 min; (4) the modulus did not affect the slope; (5) for a large modulus combined with low current, the intercept decreased rapidly over trial blocks, whereas for a small modulus combined with high current, the intercept increased over trial blocks. The first four findings support the two-mechanism hypothesis, but the last one may also be interpretable in terms of the regression to absolute scale values.

Itch evoked by electrical stimulation of the skin

Psychophysical experiments were done to test the possibility that a single receptor population signals both itch and pain by generating different patterns of activity for each type of stimulus. Electrical stimulation of hairy skin evoked pruritus in 92% of the subjects tested, and for the majority the pruritus elicited by electrical stimulation felt the same as that provoked by cowhage. The intensity of pruritus increased with the frequency of stimulation with no change in the quality of the sensation from itch to pain. Electrical stimulation of human skin with response patterns obtained from individual cat polymodal nociceptive neurons to pain- and itch-producing stimuli caused no differences in the quality of the evoked pruritic sensations. These results do not support the idea that the same population of primary sensory neurons can produce both itch and pain by changing their pattern of discharge.