Annals of BiomedicalEngineering,Vol. 20, pp. 647-665, 1992 Printed in the USA. All rights reserved.
0090-6964/92 $5.00 + .00 Copyright 9 1992 Pergamon Press Ltd.
Optimization of Single Electrode Tactile Codes Andrew Y.J. Szeto and Glenn R. Farrenkopf Department of Electrical and Computer Engineering San Diego State University San Diego, CA
(Received3/27/91; Revised 10/15/91) The effects of a frequency modulated electrocutaneous signal's (code's) characteristics on the interpretability of the signal were investigated using an electrocutaneous tracking approach. The characteristics investigated include the functional relationship (exponential and hybrid) between an informational signal and the stimulation frequency, the range of stimulation (2-50 Hz and 2-100 Hz), and the impact o f pulse width compensation on a code's efficacy. The interpretability of six different single bipolar electrode codes was examined by 30 subjects using a balanced incomplete block experimental design. Codes with exponentially shaped transfer functions resuited in generally lower electrocutaneous tracking errors than codes utilizing hybridshaped transfer functions. Hybrid codes had a transfer function that was linear in the lower frequency range and exponential in the higher frequency range. Codes with a 2-1 O0Hz frequency range were interpreted better than codes with a 2-50 Hz frequency range. The use o f pulse width compensation to maintain a more even level o f stimulation intensity had a slightly negative effect on the subjects" abilities to cutaneously track the information signal
Keywords- Eiectrocutaneous stimulation, Tactile codes, Sensory feedback.
INTRODUCTION Human beings perceive and interact with their surroundings through their senses. Unfortunately, one or more of the primary senses (i.e., vision, audition, tactile, and proprioception) may be missing or significantly impaired at birth or by traumatic injuries. When one of these senses is impaired or lost, compensating for the missing sensory input must be shared among the remaining senses. Because the sense of touch in persons with an amputation, spinal cord injury, blindness, or deafness is often intact, the tactile sense can serve as that alternate supplemental sensory input channel. Using the tactile sense as a supplemental input channel for the communication of important environmental information is attractive because the tactile sense is available and easily accessible. In contrast to vision or hearing, the sense of touch is not usually encumbered by continuous and fast changing environmental stimuli. Acknowledgments-This project was supported in part by intramural research grants from the SDSU Foundation. The assistance of Dr. James Hwang in the statistical aspects of the work is greatly appreciated. Address correspondence to Andrew Y.J. Szeto, Department of Electrical and Computer Engineering, San Diego State University, San Diego, CA 92182-0190. 647
648
A. Y.J. Szeto and G.R. Farrenkopf
By applying mild, well-controlled electrical pulses to the skin surface (i.e., electrocutaneous stimulation), one can reliably generate safe and comfortable tactile sensations. Depending on the frequency and intensity, electrocutaneous stimuli can elicit slow tapping to buzzing sensations. With practice, a person can relate such tactile sensations to changes in his/her environment (i.e., sensory feedback). Applications of electrocutaneous stimulation include prosthetic limb feedback (10,11,14,18,19); sensory substitution for the insensate hand or foot (32); reading and mobility aids for blind persons (4,6); and vocoders (speech and sound analyzing aids) for people with severe hearing loss (12,16). Investigations during the past 20 years have characterized the tactile sense in terms of its psychophysical characteristics. Collins and Bach-y-Rita (5) reported that a significant amount of neuroplasticity underlaid the relatively undeveloped sense of touch in normally sighted persons. Solomonow et ai. (20) and Szeto and Chung (24), respectively, found that training could make the tactile sense more sensitive and enable it to process more information. Szeto and co-workers demonstrated that not all codes (i.e., the functional relationship between an informational signal and the stimulus waveform) are equally effective in terms of information transmission (22,27), and that human discrimination of pulse rate changes does not necessarily follow the familiar Weber's law of psychophysics (AS/So = k) (25), where AS represents the change in a stimulus from its reference value, So. Previous work has also elucidated the stimulation parameters needed to reliably and safely produce comfortable electrotactile sensations (2,17,26). Lastly, a number of investigators (1,9,15,29,30,31) have reported that the channel capacity in terms of bits of information per stimulation site is limited to about 2-5 bits per second whether the stimulation is by surface electrodes, subdermal electrodes, or intraneural electrodes. Because the informational bandwidth (or dynamic range) of the tactile sense is limited, the manner in which sensory information is encoded into stimulation patterns needs to be optimized. Studies described in this article compared various single electrode codes deemed suitable for sensory communications. The investigations attempted to ascertain the impact of three parameters on a code's efficacy-the transfer function used to map the sensory informational signal into the frequency (or pulse rate) of stimulation; the best range of pulse rates to use; and the desirability of changing the pulse width in coordination with pulse rate changes to maintain a more even intensity of stimulation. Preliminary Study
Using a computer controlled stimulation apparatus, Hudson (7) compared three electrotactile pulse rate modulation (PRM) codes having a linear, exponential, or variable exponential transfer function (Fig. 1). His pilot study involved six subjects in a repeated measures, randomized block experimental design that compared these three codes. The study used a variation of the pursuit tracking task by having subjects try to follow changes in electrocutaneous stimulus rate, presented via a bipolar concentric electrode placed over the forearm. The subjects' tracking performances, when using a particular code, would thus reflect the relative clarity and efficacy of that code. The linear PRM code produced stimulation frequencies that were linearly related
Single Electrode Codes
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Target Values 9 Linear Pulse Rate Modulation 9 Exponential Pulse Rate Modulation a Variable Exponential Pulse Rate Modulation FIGURE 1. The relationships between the stimulus frequency and the 5-bit target signal for the three pulse rate modulation codes examined in a pilot study by Hudson (7}.
to a 5-bit informational (or 32-position target) signal. The exponential PRM code mapped the target information signal exponentially into pulse rates so that the percentage change in the stimulus frequency was the same for each interval of change in the target signal. The variable exponential PRM code attempted to compensate for the variations in human sensitivity to different stimulus rates by taking the inverse of the pulse rate differential sensitivity curve reported by Szeto et ai. (25). The variable exponential transfer function produced larger incremental frequency changes in regions of lesser pulse rate sensitivity (larger j n d thresholds) and smaller frequency changes in regions of greater pulse rate sensitivity (smaller j n d thresholds). Hudson's pilot study (7) found that all three codes produced similar overall electrocutaneous tracking errors. However, detailed off-line analysis of the tracking errors coupled with the unsolicited comments from the experienced test subjects indicated that the linear PRM code seemed to be more interpretable at the lower frequencies (2-11 Hz) whereas the two exponential codes gave better tracking performances at the upper half of the stimulus frequencies (25-50 Hz). Furthermore, tracking performances by some of the test subjects indicated that their frequency discrimination was still useful beyond 50 Hz. These results motivated a full scale study (described below) which investigated the useful frequency bandwidth for electrotactile stimuli and the efficacy of codes that combined the best features of the linear and exponential PRM codes.
650
A.Y.J. Szeto and G.R. Farrenkopf METHOD
Selection of Single Electrode Codes Based on the above pilot study results, three hybrid codes having the best features of the linear code and exponential code were designed, so that the transition from a linear relationship to an exponential relationship occurred at about 40% of the target signal's maximum value. The transfer function for one of the hybrid codes (Hyb50) was linear from 2-11 Hz and exponential from 11-50 Hz (Fig. 2). A second hybrid code (Hyb-100) was linear from 2-20 Hz and exponential from 20-100 Hz. The third hybrid code (Hyb-100C) had the same frequency relationship as the Hyb-100 except that Hyb-100C also incorporated pulse width compensation to achieve a more constant level of sensation intensity. In order to test the hypothesis that the hybrid PRM codes would perform better than the corresponding exponential PRM codes, three "pure" exponential codes were also included in this study. Two of the exponential codes (Exp-50 and Exp-50C) had a 50 Hz maximum pulse frequency while the third code had a 100 Hz maximum (Exp-100). The pulse width compensation used in Hyb-100C and Exp-50C (Fig. 3) was mo-
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TARGETVALUES FIGURE 2. The relationships between the stimulus frequency and the target signal for the exponential and hybrid pulse rate modulation codes (2-50 Hz}.
Single Electrode Codes
651
500 450
PW COMPENSATION TRANSFER FUNCTION [
400
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9 300 0 r ~
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200 150 100 5
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F R E Q U E N C Y (Hertz) FIGURE 3. The pulse width compensation curve used to elicit a constant intensity level of electrocutaneous stimulation.
tivated by the desire to decouple the frequency cues from the intensity cues during electrocutaneous stimulation. By decoupling these two cues, two independent channels of information could theoretically be encoded into a single stimulus waveform. An earlier report (23) quantified the amount by which the pulse width of a constantcurrent amplitude pulse train should decrease as its pulse rate increased, in order to produce a constant level of sensation intensity. To check if pulse width compensation would compromise a code's effectiveness, the pulse width (PW) and pulse rate (PR) of the stimulus waveform used in Hyb-100C and Exp-50C were related by the following logarithmic equation: log(PW) = 2.82 - 0.412 x log(PR)
(1)
where PW is in microseconds and PR is from 1 Hz-100 Hz. Codes without pulse width compensation used a constant 200 microsecond pulse width. Table 1 summarizes the key features of the six codes chosen for comparison in this psychophysical study.
A.Y.J. Szeto and G.R. Farrenkopf
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TABLE 1. Key features of electrocutaneous codes tested. Stimulation Variables Frequency Range (Hz) Electrocutaneous Code Exp-50 Hyb-50 Exp- 1O0 Hyb- 1O0 Exp-50C Hyb- 100C
2-50
2-1 O0
x x
Transfer Function Shape
Pulse Width Compensation
exp
yes
hybrid
x
x x x x
x x x
x
x x x
x
x
no
x x
Choice o f Stimulation Parameters
Purely monophasic waveforms can cause minor tissue damage during stimulation due to the accumulation of reaction products and electrode oxidation (2,3). This harmful process can be minimized, however, by ensuring that zero net charge is delivered to the skin. Zero net DC stimulation may be achieved by capacitively coupling the monophasic stimulating waveform to the skin surface. By using a large coupling capacitor, a stimulus current waveform that is primarily monophasic can be produced. Szeto and Mao (26) found that such a capacitively coupled waveform was safe and relatively immune to tactile sensory adaptation. This type of waveform was therefore used in this study. Stimulus Frequency. Kato et aL (9), Szeto et al. (22,27), and Riso et al. (15) reported that frequency modulation methods are more effective than intensity modulation methods for encoding one or two channels of information electrocutaneously. For this reason, only frequency modulated codes were studied. The stimulus pulse frequency range was chosen based on the work of several investigators. Anani et al. (1) studied a group of subjects to determine how they perceived electrical impulses delivered to the hand as the frequency and current amplitude o f the pulses increased. The subjects generally reported perceiving gentle taps at low frequencies (less than 10 Hz) and low current amplitudes. As the current increased, the sensation became firmer taps. Increases in frequency elicited higher rates of tapping until a low frequency vibration was perceived starting at about 30 Hz. Severe sensory adaptation to the electrical stimulus was observed in some of the subjects when stimulus frequency was increased to 120 Hz. Anani et al. found that stimulus frequencies of 10-80 Hz yielded the most accuracy in the recognition of the frequency being transmitted. Solomonow et al. (21) and Szeto et al. (27) reported that stimulus frequencies near 20 Hz were the most easily and accurately discriminated. In view of these findings, the frequency ranges (or pulse rates) of the stimulus waveforms used in this study were 2-50 Hz and 2-100 Hz. Pulse A m p l i t u d e and Width. The perception threshold for electrocutaneous stimu-
lation follows the familiar hyperbolic strength/duration curve. Typical values of current amplitude and pulse width used in previous investigations were 1-10 milliamps
Single Electrode Codes
653
and 0.1-1 milliseconds, respectively (8,9,30). In view of these values, the stimulus waveform used in this study was limited to a maximum of 10 milliamp of peak current and the pulse width varied between 100 and 500 microseconds.
Electrode Type. The bipolar surface electrode used in this study was a concentric silver electrode having an 0.32 cm diameter active center (cathode) surrounded by a rectangular indifferent anodal electrode (2.54 cm by 1.27 cm). A Plexiglas dielectric sleeve separated the anode and cathode. The concentric type of electrode was chosen to limit current spread, and thus localize the tactile sensation generated by the applied stimulus (17). Method o f Comparing Codes: Electrocutaneous Tracking The interpretability of the six selected codes was compared using a single channel electrocutaneous tracking task. Electrocutaneous tracking of one and two target channels have been successfully used in previous studies (15,22,27). During electrocutaneous tracking (28), a test subject attempts to track quickly and accurately one or two informational signals based on skin sensations evoked at the corresponding stimulation sites. Since codes which are easier to interpret should result in lower tracking errors, the evaluation of a code's relative merits could be made by analyzing tracking error data associated with various electrocutaneous codes. The electrocutaneous tracking apparatus used in the study is depicted in Fig. 4. It consisted of an Apple lie computer with a 6522 parallel I/O card, display scope, joystick, tracking performance monitoring device, concentric silver electrodes, software, and the computer-controlled electrocutaneous stimulation interface unit (ESIU). Although the computer-controlled stimulation system could implement a dual channel electrocutaneous tracking regimen, the vertical channel was not used in this study. The operational characteristics for single channel horizontal tracking were as follows: 1. A 5-bit signal (or 32 target positions) represented the sensory informational signal to be tracked. During training, the experimenter entered the desired target values using the computer keyboard. During tracking, the target signal varied randomly among the 32 target positions under real-time control of the computer. 2. The informational or target signal from the computer was then converted into an analog voltage by a D/A converter and displayed as the horizontal position of a target dot on a display screen located in front of a seated subject. The target dot jumped horizontally to a different screen location once every 5 s. 3. During a 120-s visual tracking run, both the target dot and the subject's joystick response dot were visible and the electrocutaneous stimulation circuits were turned off. The visual tracking task required the subject to keep his/her response dot atop the target dot as continuously, quickly, and accurately as possible. 4. To generate the tactile stimulus, the computer encoded the random target signal into a stimulus control signal that the external ESIU hardware then converted into the stimulus waveform delivered at the silver concentric electrode. The stimulus waveform's frequency and pulse width characteristics depended on the frequency range, transfer function shape, and pulse width compensation characteristics of the code being tested. After hydrating the stimulation site, an
654
A.Y.J. Szeto and G.R. Farrenkopf Target Signal
D/A X - Channel Y - Channel
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Tactile Display F I G U R E 4. S y s t e m block diagram of the electrocutaneous stimulation interface u n i t (ESIU). For electrocutaneous tracking of two different codes, both X and Y channels would be used, For t h i s study, the tracking of just one code was i m p l e m e n t e d u s i n g t h e X - c h a n n e l .
5.
6.
7.
8.
elastic cloth bandage held the electrode against the ventral surface of the subject's nondominant forearm. The subject's other hand manipulated the joystick controller. During training, both the target dot and the subject's response dot were displayed so that a subject could learn to associate the tactile sensations with the target dot's different screen positions. The stimulus frequency of the ventral electrode decreased when the target dot moved to the right and increased when it moved to the left. During a 90-s electrocutaneous tracking run, only the subject's response dot was visible on the display screen. Thus the subject had to track the target dot based solely on the perceived frequency of the tactile sensations at the electrode. The subject estimated location of the unseen target dot (i.e., informational signal) by moving a joystick to place his/her response dot at that location. The subject's tracking errors were found by subtracting the target dot's voltage from the subject's response dot's (i.e., joystick's) voltage. The error voltage then passed through a fullwave rectifier and voltage to frequency converter before being summed by a counter to yield the integrated absolute mean error (AME) score for that tracking trial. To permit off-line detailed analysis of the tracking performance, the target signal and subject's response signal were also digitized at 20 Hz and stored on a floppy data disk.
Single Electrode Codes
655 Experimental Design
The study used a repeated measures, balanced incomplete block design involving 30 subjects, each of whom was randomly assigned to test a pair of codes. Each of the six electrocutaneous codes was tested by 10 different subjects, five times during the first test session and five times during the second test session. To mitigate against possible learning effects, each pair of codes tested by one subject was also tested by a second subject in the reverse order. Eighteen males and 12 females, ranging in age from 18 to 33 years, were recruited from students at San Diego State University. Each subject completed two test sessions of 1.5 to 2 hours each and was paid $20 for his/her participation (see Table 2).
Experimental Protocol One electrocutaneous code was used per test session. Before a code was tested, the subject needed to learn the characteristic set of tactile sensations associated with the various horizontal screen positions of the target dot (informational signal). The train-
TABLE 2. Experimental design. Subject
Sex
Age
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
M F F M M F M M M M F M F M M F M M M M M F F F F M F M M F
33 23 18 24 22 24 25 24 26 22 24 26 24 20 20 22 24 22 23 21 20 22 19 19 18 20 22 19 20 20
Sequence of Codes Tested Hyb-lOOC Exp-50 Hyb-50 Exp-50 Hyb-lOOC Hyb-lO0 Hyb-lOOC Hyb-50 Hyb-lO0 Hyb-lOOC Exp-lO0 Exp-50C Exp-100 Hyb-50 Exp- 1O0 Exp-50 Hyb-50 Hyb-100 Exp-50C Exp-lO0 Exp-50 Hyb-lO0 Exp-50C Exp-50 Exp-lO0 Exp-50C Hyb-50 Hyb-lO0 Exp-50C Hyb-lOOC
Hyb-lO0 Exp-lO0 Exp-lO0 Hyb-lO0 Hyb-50 Hyb-50 Exp-lO0 Exp-50 Exp-50C Exp-50C Exp-50C Exp-50 Exp-50 Hyb-100C Hyb- 1O0 Exp-50C Exp-50C Exp-lO0 Hyb-lOOC Hyb-50 Hyb-50 Exp-50 Hyb-lO0 Hyb-lOOC Hyb-lOOC Hyb-50 Hyb-lO0 Hyb-lOOC Exp-lOO Exp-50
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A.Y.J. Szeto and G.R. Farrenkopf
ing period began with explanations of the tracking task and followed by systematic presentations o f the target dot at various basic screen positions and the corresponding tactile sensations. Initially the subject felt the tactile sensations while seeing the target dot's position on the display screen. Later, the target dot was removed from the display screen, and the subject had to position the response dot where he/she believed the unseen target dot to be. During training, the experimenter provided verbal and visual feedback as to the correctness of the subject's estimates. Training was concluded and formal electrocutaneous tracking began when the subject's estimates of the target dot's locations reached the criterion level of task mastery, about 10070 error or less. This training procedure allowed the subjects to reach a similar level of task proficiency at their own rates o f learning. To ensure consistency from the first test session to the second, the same stimulation site was used in both test sessions. A cross modality check o f the stimulation intensity was made by asking all subjects to mark the perceived level of stimulation intensity on a 10 cm long visual analog scale (VAS). The VAS spanned the intensity range from barely perceptible to barely tolerable. The experimenter instructed the subject to set the stimulus current so that the resulting tactile sensations were clear and comfortable. In summary, the following data were collected during every test session: 9 9 9 9 9 9 9 9 9 9 9 9
test session's date and time; subject's name, age, and gender; electrocutaneous code being tested; integrated AME scores for three visual tracking trials of 120 s each; digitized tracking error data for the visual tracking trials; integrated AME error scores for 7-9 electrocutaneous tracking trials o f 90 s each; digitized tracking error data for each of the electrocutaneous tracking trials; training time needed to achieve the criterion mastery o f the electrocutaneous code under test; stimulus currents needed to produce clear and comfortable tactile sensations and the location of the intensity marks on the VAS; experimenter's observations; subject's verbal and written comments; and questionnaire responses. ANALYSIS OF DATA
The data collected during the 60 test sessions were first consolidated into a form suitable for analysis using S P S S / P C + , a statistical software package designed for a personal computer. For each visual and electrocutaneous tracking trial performed during a test session, four tracking performance indices were calculated: zero time shift percent absolute mean error (070AME); zero time shift percent root mean squared (070RMS) error; response latency; and time shifted minimum 070RMS. The calculated indices for individual tracking trials were then averaged together to form the composite tracking performance indices for that particular test session. The percentage of fullscale AME (070AME) for a given 90-s or 120-s trial reflected the absolute difference in the horizontal distance between the target and response dots on the screen during a tracking run. The 070AME score was obtained by normalizing
Single Electrode Codes
657
the error score displayed on the on-line tracking error monitor (Fig. 4) at the end o f a tracking trial. The percentage of full scale RMS tracking error (for a given time shift) for a tracking trial was computed by the Apple computer using the following equation: 070RMS(t) - 100~176x ~ [target(i) - response(i + t)] 2 31 i=0 n
(2)
where n is the number of data samples, t is the time shift between the target and response data, i denotes the particular time sample, and 100~ is a conversion factor. Thus, if a subject's responses were maximally wrong for the entire tracking run (i.e., the average difference between the target and response equaled _+31), then his/her RMS error for that run would be 100070. The third index o f tracking performance was the average response latency for each tracking trial. Because human reaction time depends on the complexity and clarity of the stimulus signal (13), changes in the tactile sensation generated by the target signal should be more slowly detected for electrocutaneous codes that were less clear. Hence response latency should reflect the clarity o f an electrocutaneous code and thus could be useful in discriminating among the various codes. The response latency for a given tracking trial was estimated by shifting the response data (in increments of 0.05 s or one data sample) relative to the target signal. The time shift (or delay) that yielded the lowest 07oRMS was considered to be an unbiased estimate of the average response latency for that tracking trial. The corresponding time shifted minimum 070RMS was also used in the statistical analysis of the data. Table 3 lists the mean and standard deviation for the nonshifted 070AME, the non-
TABLE 3. Summary of tracking data (mean _+ S.D.|.
%AME
%RMS
Min %RMS
Response Latency (s)
Exp-50
15.20 _+1.58
22.54 +2.09
15.56 + 1.57
1.36 +0.29
17.96 _+8.00
Hyb-50
15.65 _+2.49
23.96 +2.93
16.66 _+2.60
1.37 _+0.54
15.40 _+5.38
Exp-1 O0
14.27 _+1.29
21.95 -+ 1.39
14.72 _+1.92
1.30 _+0.46
15.07 _+4,91
Hyb-1 O0
14.63 -+3.70
23.81 _+4.71
17.24 _+4.95
1.34 _+0.68
14.98 _+8.47
Exp-50C
15.52 +_2.23
22.73 _+3.00
15.66 _+2.88
1.44 _+0.40
19.51 +4.00
Hyb- 100C
14.34 _+2.30
21.54 _+2.95
14.92 _+2.55
1.13 _+0.26
16.66 _+4.10
Electrocutaneous Code
Training Time (min)
%AME = percentage of fullscale absolute mean electrocutaneous tracking error. %RMS = percentage of fullscale root mean square electrocutaneous tracking error. Min %RMS = minimum %RMS electrocutaneous tracking error with time shifting between target and response. Response latency is the time shift which resulted in minimum electrocutaneous RMS error.
658
A. ZJ. Szeto and G.R. Farrenkopf
shifted ~ error, the time shifted minimum %RMS, and the response latency (or time shift) for each of the codes tested. The %RMS errors are graphed in Fig. 5 by code and gender of the test subjects. Figure 6 displays the visual and electrocutaneous tracking latencies as a function of gender and test session. STATISTICAL ANALYSES
In analyzing the collected data, the factors or independent variables under consideration were the test session, gender, and electrocutaneous code. The dependent variables were %AME, %RMS, time shifted minimum %RMS, electrocutaneous tracking response latency, and training time. In addition to the dependent variables described above, two other dependent variables were calculated. Each subject's tracking errors (%AME and %RMS) were adjusted to compensate for their individual differences in manual tracking skills. Since visual tracking scores measured each subject's manual tracking skills, the electrocutaneous %AME and %RMS tracking performance scores were adjusted by subtracting out the corresponding %AME and %RMS visual tracking scores. Hence, the resulting adjusted %AME and %RMS error scores were expected to be less depen-
27.0
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ELECTROCUTANEOUS CODE FIGURE 5. Average root mean square tracking error graphed as a function of the tested code and test subjects" gender.
Single Electrode Codes
659
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Electrocutan Tracking FIGURE 6.
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Tracking response latency for visual and electrocutaneous tracking in test sessions l and2.
dent on the reaction times of the subjects and more dependent on the interpretability of the stimulus codes. A number of two-tailed and one-tailed t-test comparisons were made on the experimental data based on common characteristics (such as range of stimulus frequencies, transfer function shape, and the use of pulse width compensation). Tables 4 and 5 list the comparisons made, the data used in the analysis, the degrees of freedom, the tvalue, and the significance level (p-value). In some of the t-tests, the assumption of that the two comparison groups had equal variances could not be made. Hence a tvalue based on a separate variance estimate was used together with the corresponding fractional degrees of freedom. DISCUSSION OF RESULTS The first group of t-tests compared tracking performances between sessions 1 and 2. Despite the relative simplicity of the visual tracking task, the subjects tracked the target dot significantly better (i.e., achieved a smaller tracking error) in the second session. The training time data for the two sessions showed that the subjects also required significantly less time in the second session to reach the criterion level of tac-
A.Y.J. Szeto and G.R. Farrenkopf
660
TABLE 4. Summary of two-tail t-test statistical results. Comparison
Data Used
df
t
p$
1. Tracking performances in sessions 1 vs. 2
VAME VRMS %AME %RMS Adj %AME Adj %RMS Min %RMS Resp lat Training time
58 58 58 58 58 58 53. 12 58 58
2.53 1.45 1.08 0.64 0.15 0.04 0.74 1.01 1.86
** NS NS NS NS NS NS NS *
2. Tracking performances of males vs. females
VAME VRMS %AME %RMS Adj %AME Adj %RMS Min %RMS Resp lat Training time
58 58 38.69 37.79 38.31 58 58 29.23 58
-3.13 -2.46 -2.79 -1.97 -1.96 -1.20 -1.33 -3.55 O. 15
*** ** *** * * NS NS *** NS
3. E x p - l O 0 vs. Hyb-50
VAME VRMS %AME %RMS Adj %AME Adj %RMS Min %RMS Resp lat Training time
18 18 13.51 12.87 18 18 18 18 18
0.56 -0.27 -1.56 -1.97 -2.24 - 1.99 -1.90 -0.33 -0.14
NS NS NS * ** * * NS NS
~t = p-value for a two-tail t-test statistic. * =p
0090-6964/92 $5.00 + .00 Copyright 9 1992 Pergamon Press Ltd.
Optimization of Single Electrode Tactile Codes Andrew Y.J. Szeto and Glenn R. Farrenkopf Department of Electrical and Computer Engineering San Diego State University San Diego, CA
(Received3/27/91; Revised 10/15/91) The effects of a frequency modulated electrocutaneous signal's (code's) characteristics on the interpretability of the signal were investigated using an electrocutaneous tracking approach. The characteristics investigated include the functional relationship (exponential and hybrid) between an informational signal and the stimulation frequency, the range of stimulation (2-50 Hz and 2-100 Hz), and the impact o f pulse width compensation on a code's efficacy. The interpretability of six different single bipolar electrode codes was examined by 30 subjects using a balanced incomplete block experimental design. Codes with exponentially shaped transfer functions resuited in generally lower electrocutaneous tracking errors than codes utilizing hybridshaped transfer functions. Hybrid codes had a transfer function that was linear in the lower frequency range and exponential in the higher frequency range. Codes with a 2-1 O0Hz frequency range were interpreted better than codes with a 2-50 Hz frequency range. The use o f pulse width compensation to maintain a more even level o f stimulation intensity had a slightly negative effect on the subjects" abilities to cutaneously track the information signal
Keywords- Eiectrocutaneous stimulation, Tactile codes, Sensory feedback.
INTRODUCTION Human beings perceive and interact with their surroundings through their senses. Unfortunately, one or more of the primary senses (i.e., vision, audition, tactile, and proprioception) may be missing or significantly impaired at birth or by traumatic injuries. When one of these senses is impaired or lost, compensating for the missing sensory input must be shared among the remaining senses. Because the sense of touch in persons with an amputation, spinal cord injury, blindness, or deafness is often intact, the tactile sense can serve as that alternate supplemental sensory input channel. Using the tactile sense as a supplemental input channel for the communication of important environmental information is attractive because the tactile sense is available and easily accessible. In contrast to vision or hearing, the sense of touch is not usually encumbered by continuous and fast changing environmental stimuli. Acknowledgments-This project was supported in part by intramural research grants from the SDSU Foundation. The assistance of Dr. James Hwang in the statistical aspects of the work is greatly appreciated. Address correspondence to Andrew Y.J. Szeto, Department of Electrical and Computer Engineering, San Diego State University, San Diego, CA 92182-0190. 647
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A. Y.J. Szeto and G.R. Farrenkopf
By applying mild, well-controlled electrical pulses to the skin surface (i.e., electrocutaneous stimulation), one can reliably generate safe and comfortable tactile sensations. Depending on the frequency and intensity, electrocutaneous stimuli can elicit slow tapping to buzzing sensations. With practice, a person can relate such tactile sensations to changes in his/her environment (i.e., sensory feedback). Applications of electrocutaneous stimulation include prosthetic limb feedback (10,11,14,18,19); sensory substitution for the insensate hand or foot (32); reading and mobility aids for blind persons (4,6); and vocoders (speech and sound analyzing aids) for people with severe hearing loss (12,16). Investigations during the past 20 years have characterized the tactile sense in terms of its psychophysical characteristics. Collins and Bach-y-Rita (5) reported that a significant amount of neuroplasticity underlaid the relatively undeveloped sense of touch in normally sighted persons. Solomonow et ai. (20) and Szeto and Chung (24), respectively, found that training could make the tactile sense more sensitive and enable it to process more information. Szeto and co-workers demonstrated that not all codes (i.e., the functional relationship between an informational signal and the stimulus waveform) are equally effective in terms of information transmission (22,27), and that human discrimination of pulse rate changes does not necessarily follow the familiar Weber's law of psychophysics (AS/So = k) (25), where AS represents the change in a stimulus from its reference value, So. Previous work has also elucidated the stimulation parameters needed to reliably and safely produce comfortable electrotactile sensations (2,17,26). Lastly, a number of investigators (1,9,15,29,30,31) have reported that the channel capacity in terms of bits of information per stimulation site is limited to about 2-5 bits per second whether the stimulation is by surface electrodes, subdermal electrodes, or intraneural electrodes. Because the informational bandwidth (or dynamic range) of the tactile sense is limited, the manner in which sensory information is encoded into stimulation patterns needs to be optimized. Studies described in this article compared various single electrode codes deemed suitable for sensory communications. The investigations attempted to ascertain the impact of three parameters on a code's efficacy-the transfer function used to map the sensory informational signal into the frequency (or pulse rate) of stimulation; the best range of pulse rates to use; and the desirability of changing the pulse width in coordination with pulse rate changes to maintain a more even intensity of stimulation. Preliminary Study
Using a computer controlled stimulation apparatus, Hudson (7) compared three electrotactile pulse rate modulation (PRM) codes having a linear, exponential, or variable exponential transfer function (Fig. 1). His pilot study involved six subjects in a repeated measures, randomized block experimental design that compared these three codes. The study used a variation of the pursuit tracking task by having subjects try to follow changes in electrocutaneous stimulus rate, presented via a bipolar concentric electrode placed over the forearm. The subjects' tracking performances, when using a particular code, would thus reflect the relative clarity and efficacy of that code. The linear PRM code produced stimulation frequencies that were linearly related
Single Electrode Codes
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Target Values 9 Linear Pulse Rate Modulation 9 Exponential Pulse Rate Modulation a Variable Exponential Pulse Rate Modulation FIGURE 1. The relationships between the stimulus frequency and the 5-bit target signal for the three pulse rate modulation codes examined in a pilot study by Hudson (7}.
to a 5-bit informational (or 32-position target) signal. The exponential PRM code mapped the target information signal exponentially into pulse rates so that the percentage change in the stimulus frequency was the same for each interval of change in the target signal. The variable exponential PRM code attempted to compensate for the variations in human sensitivity to different stimulus rates by taking the inverse of the pulse rate differential sensitivity curve reported by Szeto et ai. (25). The variable exponential transfer function produced larger incremental frequency changes in regions of lesser pulse rate sensitivity (larger j n d thresholds) and smaller frequency changes in regions of greater pulse rate sensitivity (smaller j n d thresholds). Hudson's pilot study (7) found that all three codes produced similar overall electrocutaneous tracking errors. However, detailed off-line analysis of the tracking errors coupled with the unsolicited comments from the experienced test subjects indicated that the linear PRM code seemed to be more interpretable at the lower frequencies (2-11 Hz) whereas the two exponential codes gave better tracking performances at the upper half of the stimulus frequencies (25-50 Hz). Furthermore, tracking performances by some of the test subjects indicated that their frequency discrimination was still useful beyond 50 Hz. These results motivated a full scale study (described below) which investigated the useful frequency bandwidth for electrotactile stimuli and the efficacy of codes that combined the best features of the linear and exponential PRM codes.
650
A.Y.J. Szeto and G.R. Farrenkopf METHOD
Selection of Single Electrode Codes Based on the above pilot study results, three hybrid codes having the best features of the linear code and exponential code were designed, so that the transition from a linear relationship to an exponential relationship occurred at about 40% of the target signal's maximum value. The transfer function for one of the hybrid codes (Hyb50) was linear from 2-11 Hz and exponential from 11-50 Hz (Fig. 2). A second hybrid code (Hyb-100) was linear from 2-20 Hz and exponential from 20-100 Hz. The third hybrid code (Hyb-100C) had the same frequency relationship as the Hyb-100 except that Hyb-100C also incorporated pulse width compensation to achieve a more constant level of sensation intensity. In order to test the hypothesis that the hybrid PRM codes would perform better than the corresponding exponential PRM codes, three "pure" exponential codes were also included in this study. Two of the exponential codes (Exp-50 and Exp-50C) had a 50 Hz maximum pulse frequency while the third code had a 100 Hz maximum (Exp-100). The pulse width compensation used in Hyb-100C and Exp-50C (Fig. 3) was mo-
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TARGETVALUES FIGURE 2. The relationships between the stimulus frequency and the target signal for the exponential and hybrid pulse rate modulation codes (2-50 Hz}.
Single Electrode Codes
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PW COMPENSATION TRANSFER FUNCTION [
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tivated by the desire to decouple the frequency cues from the intensity cues during electrocutaneous stimulation. By decoupling these two cues, two independent channels of information could theoretically be encoded into a single stimulus waveform. An earlier report (23) quantified the amount by which the pulse width of a constantcurrent amplitude pulse train should decrease as its pulse rate increased, in order to produce a constant level of sensation intensity. To check if pulse width compensation would compromise a code's effectiveness, the pulse width (PW) and pulse rate (PR) of the stimulus waveform used in Hyb-100C and Exp-50C were related by the following logarithmic equation: log(PW) = 2.82 - 0.412 x log(PR)
(1)
where PW is in microseconds and PR is from 1 Hz-100 Hz. Codes without pulse width compensation used a constant 200 microsecond pulse width. Table 1 summarizes the key features of the six codes chosen for comparison in this psychophysical study.
A.Y.J. Szeto and G.R. Farrenkopf
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TABLE 1. Key features of electrocutaneous codes tested. Stimulation Variables Frequency Range (Hz) Electrocutaneous Code Exp-50 Hyb-50 Exp- 1O0 Hyb- 1O0 Exp-50C Hyb- 100C
2-50
2-1 O0
x x
Transfer Function Shape
Pulse Width Compensation
exp
yes
hybrid
x
x x x x
x x x
x
x x x
x
x
no
x x
Choice o f Stimulation Parameters
Purely monophasic waveforms can cause minor tissue damage during stimulation due to the accumulation of reaction products and electrode oxidation (2,3). This harmful process can be minimized, however, by ensuring that zero net charge is delivered to the skin. Zero net DC stimulation may be achieved by capacitively coupling the monophasic stimulating waveform to the skin surface. By using a large coupling capacitor, a stimulus current waveform that is primarily monophasic can be produced. Szeto and Mao (26) found that such a capacitively coupled waveform was safe and relatively immune to tactile sensory adaptation. This type of waveform was therefore used in this study. Stimulus Frequency. Kato et aL (9), Szeto et al. (22,27), and Riso et al. (15) reported that frequency modulation methods are more effective than intensity modulation methods for encoding one or two channels of information electrocutaneously. For this reason, only frequency modulated codes were studied. The stimulus pulse frequency range was chosen based on the work of several investigators. Anani et al. (1) studied a group of subjects to determine how they perceived electrical impulses delivered to the hand as the frequency and current amplitude o f the pulses increased. The subjects generally reported perceiving gentle taps at low frequencies (less than 10 Hz) and low current amplitudes. As the current increased, the sensation became firmer taps. Increases in frequency elicited higher rates of tapping until a low frequency vibration was perceived starting at about 30 Hz. Severe sensory adaptation to the electrical stimulus was observed in some of the subjects when stimulus frequency was increased to 120 Hz. Anani et al. found that stimulus frequencies of 10-80 Hz yielded the most accuracy in the recognition of the frequency being transmitted. Solomonow et al. (21) and Szeto et al. (27) reported that stimulus frequencies near 20 Hz were the most easily and accurately discriminated. In view of these findings, the frequency ranges (or pulse rates) of the stimulus waveforms used in this study were 2-50 Hz and 2-100 Hz. Pulse A m p l i t u d e and Width. The perception threshold for electrocutaneous stimu-
lation follows the familiar hyperbolic strength/duration curve. Typical values of current amplitude and pulse width used in previous investigations were 1-10 milliamps
Single Electrode Codes
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and 0.1-1 milliseconds, respectively (8,9,30). In view of these values, the stimulus waveform used in this study was limited to a maximum of 10 milliamp of peak current and the pulse width varied between 100 and 500 microseconds.
Electrode Type. The bipolar surface electrode used in this study was a concentric silver electrode having an 0.32 cm diameter active center (cathode) surrounded by a rectangular indifferent anodal electrode (2.54 cm by 1.27 cm). A Plexiglas dielectric sleeve separated the anode and cathode. The concentric type of electrode was chosen to limit current spread, and thus localize the tactile sensation generated by the applied stimulus (17). Method o f Comparing Codes: Electrocutaneous Tracking The interpretability of the six selected codes was compared using a single channel electrocutaneous tracking task. Electrocutaneous tracking of one and two target channels have been successfully used in previous studies (15,22,27). During electrocutaneous tracking (28), a test subject attempts to track quickly and accurately one or two informational signals based on skin sensations evoked at the corresponding stimulation sites. Since codes which are easier to interpret should result in lower tracking errors, the evaluation of a code's relative merits could be made by analyzing tracking error data associated with various electrocutaneous codes. The electrocutaneous tracking apparatus used in the study is depicted in Fig. 4. It consisted of an Apple lie computer with a 6522 parallel I/O card, display scope, joystick, tracking performance monitoring device, concentric silver electrodes, software, and the computer-controlled electrocutaneous stimulation interface unit (ESIU). Although the computer-controlled stimulation system could implement a dual channel electrocutaneous tracking regimen, the vertical channel was not used in this study. The operational characteristics for single channel horizontal tracking were as follows: 1. A 5-bit signal (or 32 target positions) represented the sensory informational signal to be tracked. During training, the experimenter entered the desired target values using the computer keyboard. During tracking, the target signal varied randomly among the 32 target positions under real-time control of the computer. 2. The informational or target signal from the computer was then converted into an analog voltage by a D/A converter and displayed as the horizontal position of a target dot on a display screen located in front of a seated subject. The target dot jumped horizontally to a different screen location once every 5 s. 3. During a 120-s visual tracking run, both the target dot and the subject's joystick response dot were visible and the electrocutaneous stimulation circuits were turned off. The visual tracking task required the subject to keep his/her response dot atop the target dot as continuously, quickly, and accurately as possible. 4. To generate the tactile stimulus, the computer encoded the random target signal into a stimulus control signal that the external ESIU hardware then converted into the stimulus waveform delivered at the silver concentric electrode. The stimulus waveform's frequency and pulse width characteristics depended on the frequency range, transfer function shape, and pulse width compensation characteristics of the code being tested. After hydrating the stimulation site, an
654
A.Y.J. Szeto and G.R. Farrenkopf Target Signal
D/A X - Channel Y - Channel
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Tactile Display F I G U R E 4. S y s t e m block diagram of the electrocutaneous stimulation interface u n i t (ESIU). For electrocutaneous tracking of two different codes, both X and Y channels would be used, For t h i s study, the tracking of just one code was i m p l e m e n t e d u s i n g t h e X - c h a n n e l .
5.
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elastic cloth bandage held the electrode against the ventral surface of the subject's nondominant forearm. The subject's other hand manipulated the joystick controller. During training, both the target dot and the subject's response dot were displayed so that a subject could learn to associate the tactile sensations with the target dot's different screen positions. The stimulus frequency of the ventral electrode decreased when the target dot moved to the right and increased when it moved to the left. During a 90-s electrocutaneous tracking run, only the subject's response dot was visible on the display screen. Thus the subject had to track the target dot based solely on the perceived frequency of the tactile sensations at the electrode. The subject estimated location of the unseen target dot (i.e., informational signal) by moving a joystick to place his/her response dot at that location. The subject's tracking errors were found by subtracting the target dot's voltage from the subject's response dot's (i.e., joystick's) voltage. The error voltage then passed through a fullwave rectifier and voltage to frequency converter before being summed by a counter to yield the integrated absolute mean error (AME) score for that tracking trial. To permit off-line detailed analysis of the tracking performance, the target signal and subject's response signal were also digitized at 20 Hz and stored on a floppy data disk.
Single Electrode Codes
655 Experimental Design
The study used a repeated measures, balanced incomplete block design involving 30 subjects, each of whom was randomly assigned to test a pair of codes. Each of the six electrocutaneous codes was tested by 10 different subjects, five times during the first test session and five times during the second test session. To mitigate against possible learning effects, each pair of codes tested by one subject was also tested by a second subject in the reverse order. Eighteen males and 12 females, ranging in age from 18 to 33 years, were recruited from students at San Diego State University. Each subject completed two test sessions of 1.5 to 2 hours each and was paid $20 for his/her participation (see Table 2).
Experimental Protocol One electrocutaneous code was used per test session. Before a code was tested, the subject needed to learn the characteristic set of tactile sensations associated with the various horizontal screen positions of the target dot (informational signal). The train-
TABLE 2. Experimental design. Subject
Sex
Age
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
M F F M M F M M M M F M F M M F M M M M M F F F F M F M M F
33 23 18 24 22 24 25 24 26 22 24 26 24 20 20 22 24 22 23 21 20 22 19 19 18 20 22 19 20 20
Sequence of Codes Tested Hyb-lOOC Exp-50 Hyb-50 Exp-50 Hyb-lOOC Hyb-lO0 Hyb-lOOC Hyb-50 Hyb-lO0 Hyb-lOOC Exp-lO0 Exp-50C Exp-100 Hyb-50 Exp- 1O0 Exp-50 Hyb-50 Hyb-100 Exp-50C Exp-lO0 Exp-50 Hyb-lO0 Exp-50C Exp-50 Exp-lO0 Exp-50C Hyb-50 Hyb-lO0 Exp-50C Hyb-lOOC
Hyb-lO0 Exp-lO0 Exp-lO0 Hyb-lO0 Hyb-50 Hyb-50 Exp-lO0 Exp-50 Exp-50C Exp-50C Exp-50C Exp-50 Exp-50 Hyb-100C Hyb- 1O0 Exp-50C Exp-50C Exp-lO0 Hyb-lOOC Hyb-50 Hyb-50 Exp-50 Hyb-lO0 Hyb-lOOC Hyb-lOOC Hyb-50 Hyb-lO0 Hyb-lOOC Exp-lOO Exp-50
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ing period began with explanations of the tracking task and followed by systematic presentations o f the target dot at various basic screen positions and the corresponding tactile sensations. Initially the subject felt the tactile sensations while seeing the target dot's position on the display screen. Later, the target dot was removed from the display screen, and the subject had to position the response dot where he/she believed the unseen target dot to be. During training, the experimenter provided verbal and visual feedback as to the correctness of the subject's estimates. Training was concluded and formal electrocutaneous tracking began when the subject's estimates of the target dot's locations reached the criterion level of task mastery, about 10070 error or less. This training procedure allowed the subjects to reach a similar level of task proficiency at their own rates o f learning. To ensure consistency from the first test session to the second, the same stimulation site was used in both test sessions. A cross modality check o f the stimulation intensity was made by asking all subjects to mark the perceived level of stimulation intensity on a 10 cm long visual analog scale (VAS). The VAS spanned the intensity range from barely perceptible to barely tolerable. The experimenter instructed the subject to set the stimulus current so that the resulting tactile sensations were clear and comfortable. In summary, the following data were collected during every test session: 9 9 9 9 9 9 9 9 9 9 9 9
test session's date and time; subject's name, age, and gender; electrocutaneous code being tested; integrated AME scores for three visual tracking trials of 120 s each; digitized tracking error data for the visual tracking trials; integrated AME error scores for 7-9 electrocutaneous tracking trials o f 90 s each; digitized tracking error data for each of the electrocutaneous tracking trials; training time needed to achieve the criterion mastery o f the electrocutaneous code under test; stimulus currents needed to produce clear and comfortable tactile sensations and the location of the intensity marks on the VAS; experimenter's observations; subject's verbal and written comments; and questionnaire responses. ANALYSIS OF DATA
The data collected during the 60 test sessions were first consolidated into a form suitable for analysis using S P S S / P C + , a statistical software package designed for a personal computer. For each visual and electrocutaneous tracking trial performed during a test session, four tracking performance indices were calculated: zero time shift percent absolute mean error (070AME); zero time shift percent root mean squared (070RMS) error; response latency; and time shifted minimum 070RMS. The calculated indices for individual tracking trials were then averaged together to form the composite tracking performance indices for that particular test session. The percentage of fullscale AME (070AME) for a given 90-s or 120-s trial reflected the absolute difference in the horizontal distance between the target and response dots on the screen during a tracking run. The 070AME score was obtained by normalizing
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657
the error score displayed on the on-line tracking error monitor (Fig. 4) at the end o f a tracking trial. The percentage of full scale RMS tracking error (for a given time shift) for a tracking trial was computed by the Apple computer using the following equation: 070RMS(t) - 100~176x ~ [target(i) - response(i + t)] 2 31 i=0 n
(2)
where n is the number of data samples, t is the time shift between the target and response data, i denotes the particular time sample, and 100~ is a conversion factor. Thus, if a subject's responses were maximally wrong for the entire tracking run (i.e., the average difference between the target and response equaled _+31), then his/her RMS error for that run would be 100070. The third index o f tracking performance was the average response latency for each tracking trial. Because human reaction time depends on the complexity and clarity of the stimulus signal (13), changes in the tactile sensation generated by the target signal should be more slowly detected for electrocutaneous codes that were less clear. Hence response latency should reflect the clarity o f an electrocutaneous code and thus could be useful in discriminating among the various codes. The response latency for a given tracking trial was estimated by shifting the response data (in increments of 0.05 s or one data sample) relative to the target signal. The time shift (or delay) that yielded the lowest 07oRMS was considered to be an unbiased estimate of the average response latency for that tracking trial. The corresponding time shifted minimum 070RMS was also used in the statistical analysis of the data. Table 3 lists the mean and standard deviation for the nonshifted 070AME, the non-
TABLE 3. Summary of tracking data (mean _+ S.D.|.
%AME
%RMS
Min %RMS
Response Latency (s)
Exp-50
15.20 _+1.58
22.54 +2.09
15.56 + 1.57
1.36 +0.29
17.96 _+8.00
Hyb-50
15.65 _+2.49
23.96 +2.93
16.66 _+2.60
1.37 _+0.54
15.40 _+5.38
Exp-1 O0
14.27 _+1.29
21.95 -+ 1.39
14.72 _+1.92
1.30 _+0.46
15.07 _+4,91
Hyb-1 O0
14.63 -+3.70
23.81 _+4.71
17.24 _+4.95
1.34 _+0.68
14.98 _+8.47
Exp-50C
15.52 +_2.23
22.73 _+3.00
15.66 _+2.88
1.44 _+0.40
19.51 +4.00
Hyb- 100C
14.34 _+2.30
21.54 _+2.95
14.92 _+2.55
1.13 _+0.26
16.66 _+4.10
Electrocutaneous Code
Training Time (min)
%AME = percentage of fullscale absolute mean electrocutaneous tracking error. %RMS = percentage of fullscale root mean square electrocutaneous tracking error. Min %RMS = minimum %RMS electrocutaneous tracking error with time shifting between target and response. Response latency is the time shift which resulted in minimum electrocutaneous RMS error.
658
A. ZJ. Szeto and G.R. Farrenkopf
shifted ~ error, the time shifted minimum %RMS, and the response latency (or time shift) for each of the codes tested. The %RMS errors are graphed in Fig. 5 by code and gender of the test subjects. Figure 6 displays the visual and electrocutaneous tracking latencies as a function of gender and test session. STATISTICAL ANALYSES
In analyzing the collected data, the factors or independent variables under consideration were the test session, gender, and electrocutaneous code. The dependent variables were %AME, %RMS, time shifted minimum %RMS, electrocutaneous tracking response latency, and training time. In addition to the dependent variables described above, two other dependent variables were calculated. Each subject's tracking errors (%AME and %RMS) were adjusted to compensate for their individual differences in manual tracking skills. Since visual tracking scores measured each subject's manual tracking skills, the electrocutaneous %AME and %RMS tracking performance scores were adjusted by subtracting out the corresponding %AME and %RMS visual tracking scores. Hence, the resulting adjusted %AME and %RMS error scores were expected to be less depen-
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ELECTROCUTANEOUS CODE FIGURE 5. Average root mean square tracking error graphed as a function of the tested code and test subjects" gender.
Single Electrode Codes
659
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Tracking response latency for visual and electrocutaneous tracking in test sessions l and2.
dent on the reaction times of the subjects and more dependent on the interpretability of the stimulus codes. A number of two-tailed and one-tailed t-test comparisons were made on the experimental data based on common characteristics (such as range of stimulus frequencies, transfer function shape, and the use of pulse width compensation). Tables 4 and 5 list the comparisons made, the data used in the analysis, the degrees of freedom, the tvalue, and the significance level (p-value). In some of the t-tests, the assumption of that the two comparison groups had equal variances could not be made. Hence a tvalue based on a separate variance estimate was used together with the corresponding fractional degrees of freedom. DISCUSSION OF RESULTS The first group of t-tests compared tracking performances between sessions 1 and 2. Despite the relative simplicity of the visual tracking task, the subjects tracked the target dot significantly better (i.e., achieved a smaller tracking error) in the second session. The training time data for the two sessions showed that the subjects also required significantly less time in the second session to reach the criterion level of tac-
A.Y.J. Szeto and G.R. Farrenkopf
660
TABLE 4. Summary of two-tail t-test statistical results. Comparison
Data Used
df
t
p$
1. Tracking performances in sessions 1 vs. 2
VAME VRMS %AME %RMS Adj %AME Adj %RMS Min %RMS Resp lat Training time
58 58 58 58 58 58 53. 12 58 58
2.53 1.45 1.08 0.64 0.15 0.04 0.74 1.01 1.86
** NS NS NS NS NS NS NS *
2. Tracking performances of males vs. females
VAME VRMS %AME %RMS Adj %AME Adj %RMS Min %RMS Resp lat Training time
58 58 38.69 37.79 38.31 58 58 29.23 58
-3.13 -2.46 -2.79 -1.97 -1.96 -1.20 -1.33 -3.55 O. 15
*** ** *** * * NS NS *** NS
3. E x p - l O 0 vs. Hyb-50
VAME VRMS %AME %RMS Adj %AME Adj %RMS Min %RMS Resp lat Training time
18 18 13.51 12.87 18 18 18 18 18
0.56 -0.27 -1.56 -1.97 -2.24 - 1.99 -1.90 -0.33 -0.14
NS NS NS * ** * * NS NS
~t = p-value for a two-tail t-test statistic. * =p
