Summary.-The reliability of perception of a series of torque pulses and the influ- ... Analysis showed distinctive cumulative frequency curves with the threshold of.
Percephral and Motor Skills, 1991, 72, 1223-1227. O Perceprual and Motor SkiUs 1991
T H E PERCEPTION O F TORQUE PULSES' A. WALMSLEY AND L. R. T. WILLIAMS University of Otago, New Zeaknd Summary.-The reliability of perception of a series of torque pulses and the influence of practice on the perceptual threshold were assessed. Eleven subjects were tested. Analysis showed distinctive cumulative frequency curves with the threshold of perception ranging from 0.016 Nm to 0.049 Nm. The reliability of individual differences over 100 trials was ,998 and, when the number of trials was reduced to 25, reliability was ,992. There was little evidence of a practice effect. I t was clear that individual difference~in torque perception could be accurately measured.
There is considerable evidence emphasizing the importance of force, and its rotational equivalent torque, in the control of human movement. For example, Marsden, Merton, and Morton (1972) made the following points about the control of voluntary movement: "(1) that there is servo action, sensitive, brisk, and so early as clearly to be automatic, in voluntary movements of the thumb, (2) that the gain of the servo loop is proportional to the force exerted, . . ." (p. 143). Ten years later Abend, Bizzi, and Morasso (1982) identified the potential control parameters used to produce hand trajectories in arm movements with two degrees of freedom. Their investigations led them to write: We speculate that for curved paths, the control system generates a series of force vectors, acting at the hand, the orientation of each vector roughly paralleling the intended curved trajectory (p. 345).
Almost another decade has passed, and the dynamics of limb movement is still a major focus for those involved in human movement science, albeit in the nonlinear domain of limit-cycle oscillators rather than the linear domain of the mass-spring models (Kay, Kelso, Saltzman, & Schoner, 1987). In contrast, however, to the wealth of information on force and torque as system variables, there is little known about the abllity to perceive consciously a change of torque. Russell (1981) and Russell and Marteniuk (1974) studied torque perception from an informational analysis standpoint and determined that torque is only of limited use as a hnesthetic feedback parameter. An important contribution was made by Henry (1953), who directly addressed the conscious perception of force changes. H e found that the sensory acuity for force changes, as measured by the Weber ratio, was 'Supported b Grant No. 14-598 from the University of Otago Research Committee, and Grant No. 32-610 l o r n the University Grants Committee. Address correspondence to either author, School of Ph sical Education, Division of Sciences, University of Otago, PO Box 56, Dunedin, New zealandl
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A. WALMSLEY & L. R. T. WILLIAMS
dependent on the rate of change of force, being most acute when the force changed at a rate of 3.6 x lo5 dyne/s (3.6 N/s) or greater. The Weber ratio reported varies from 0.046 to 0.089, with a mean of 0.061. The subjects did not mention any perception of joint motion and reported their perception of changes in the force applied to the hand to be diffuse: Pressure changes, when they were perceived, were most frequently localized as tension changes in the triceps and next most frequently In the anterior deltoid, often but not invariably associated with an awareness of altered pressure agnlnst the distal part of the hand (p. 183).
While Henry quantified the average force-increment necessary for conscious perception in a group of subjects, he did not report any analysis of individual differences. The purposes of the present study were to measure individual torque perception thresholds for a s m d group of subjects using a newly developed torque motor system (Walmsley & Bahr, 19901,~to determine whether differences between individuals are reliably quantifiable, and to assess the effect of repeated exposure on perceptual threshold.
Informed consent was obtained from 11 subjects (I woman and 10 men) who ranged in age from 19 to 47 years. All subjects were right-handed, and all tests were carried out on the index finger of the hand indicated as the preferred one. Each subject was given 100 trials with a rest period of 2 min. between blocks of 20 trids. During testing, the subject was seated with the forearm resting on a foam wedge on a horizontal platform adjusted so that the angle between the upper arm and trunk was approximately 30°. A pistol grip was grasped lightly in the right hand, with the index finger extended forward so that the meta~arpophalan~eal joint was positioned vertically over the axis of the torque motor shaft (see Fig. 1). The middle phalanx of the finger was attached to the torque arm with a velcro strap so that torque pulses would be applied to the finger in the direction of finger flexion. The pulses produced no discernable movement of the finger. Testing was carried out with the subject blindfolded and wearing ear muffs to d d extraneous stimuli and allow the subject to concentrate fully on the perception of the torque pulses. Following a random foreperiod of two, 'The tor ue motor system is designed to allow an experimenter to manipulate the torque output of a smaa DC rinted armature motor (Servalco G9M4) in real time. The controller is based on a single b o a r 1 microcomputer using the National Semiconductor NSC8OO CMOS microprocessor which implements the 2-80 instruction set completely. The NSC800 CPU has expanded interrupt capability compared with the 2-80 which makes it ideal for experimental control ap lications. For these specific torque-perturbation experiments, it has been necessary to design a n j build a special-purpose signal conditioning card to convert a digital command word to an analog control voltage and provide power amplification and voltage to current conversion. Thus, the microcomputer may control the DC motor current and hence its torque out ut, directly. The system is set up to deliver a maximum torque of 93.0 x 10.' Nm in steps o r 7.4 x Nm.
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three, or four seconds, the train of torque pulses was applied to the finger. The subject indicated perception of a pulse by pressing a push button switch held in the left hand, at which point the applied torque was stored as the threshold for that trial. After a rest of five seconds the next trial was started.
PISTOL GRIP
n
FIG. 1. Diagram showing location of finger in relation to the torque motor shaft and the attachment to the torque arm
The experimental design employed a train of evenly spaced torque pulses of uniformly increasing magnitude. I n view of the rate dependence of perception of force changes reported by Henry (1953), it was decided to present the torque stimulus as a series of pulses in which the rate of change of torque was as high as ~ossible.I n this situation, the subject's perception of the perturbation will be most acute. The effective rate of change of torque was determined by the maximum rate of change of current in the motor, found to be greater than lo4 A/s (Walmsley & Bahr, 1990). Consequently, the maximum rate of change of torque was > 950 Nmlsec. The torque pulses were of 200 msec. duration and were separated by intervals of 200 msec. The torque pulses had a minimum magnitude of 7.4 x Nm, which was also the increment between pulses.
RESULTS AND DISCUSSION The frequency of perception at a particular torque level was determined, and these were summed from the lowest to the highest torque step to obtain a cumulative frequency over 100 trials for each subject. The curves of cumulative frequency against torque level (see Fig. 2) show a distinct sigmoid shape which defines the critical range of torque over which perception occurs. The threshold for perception is taken as the level at which a response occurred in 50% or more of the trials and ranged from 16 x lo-' Nm
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&
L. R. T WILLIAMS
(SD = 5.10-' Nm) to 49 x lo-' Nm (SD = 9.10-' Nm). It is interesting to note that the two subjects who showed higher thresholds were 20 years older than the average age of the remaining subjects.
Torque /Nm
FIG.2. Cumulative frequency of perception curves for all subjects ( N = 11)
The split-half method using odd and even scores was applied to estimate the reliability of the measurement. This was done for the full complement of 100 trials and also for blocks of the first 60, 30, and 25 trials. The coefficients, with the Spearman-Brown correction applied, ranged from .992 for the first 25 trials to ,998 for all 100 trials (see Table 1). This indicates that the mechanism of perception is stable and consistent and that the instrumentation system used is especially powerful in quantifying perceptual thresholds and distinguishing individual differences in threshold. TABLE 1 RELIABILITY OF TORQUE PERCEPTION THRESHOLD POOLED OVERALLSUEIJECTS ( N = 11) Number of Trials
Odd Trials Mean (Nm x 10.')
Even Trials Mean
(Nmx 10.')
Corrected Split-half Reliability Coeff~clent
100 GO 30 25
31.05 30.58 30.29 29.76
30.69 30.11 30.06 29.30
,998 ,995 ,996 ,992
The effect of practice was assessed by taking the difference between the block means of the first and last 30 trials for each subject. Group mean thresholds for the first and last blocks were 30 x 10.' Nm (SD = 9 x lo-' Nm)
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and 32 x lo-' Nm (SD= 11.10-' Nm), respectively. These values confirm that there were no order effects caused by the repeated observations. The minimum detectable magnitude of a short duration rectangular torque pulse applied to the index finger of an individual may be measured with a high reliability. The threshold of detection is stable over repeated observations and remains reliable with as few as 25 trials. Although age was not systematically manipulated as a variable, there was an indication that higher thresholds were associated with older subjects. There is still a need to clarify what the subject perceives. While it is likely that the perceptual system integrates information from a number of sources, including the muscle spindle and cutaneous pressure receptors, it is notable that most subjects referred to diffuse muscular rather than cutaneous sensation, which accords with Henry's 1753 report. The present technique shows promise for investigating the processes involved in perception of mechanical stimuli such as torque pulses. For example, in addition to age effects, the temporal structure of torque stimuli could be manipulated to define the role of dynamic parameters such as rotational impulse in the perception of torque. REFERENCES
ABEND,W. K., BUZI, E., & MORASSO, P. (1982) Human a m trajectory formation. Brain, 105, 331-348. (1953) Dynamic kinesthetic perception. Research Quarterly, 24, 176-187. G.~(1987) , Space-time behaviour KAY, B. A,, KELSO, A. S., SALTZMAN,E. L., & S C M O N of single and'bimanud rhythmical movements: data and limit cycle model. Journal o/ Experimental Psychology: Human Perception and Performance, 13, 178-192. MARSDEN, C. D., MERTON,P. A., & MORTON, H. B. (1972) Servo action in human voluntary movement. Nature, 238, 140-143. RUSSELL, D. G. (1981) Channel capacity for kinaesthetic t o q u e information. Perceptual and Motor Skills, 52, 387-390. RUSSELL,D. G., & MARTEN~JK, R. G. (1974) An informational analysis of absolute judgements of torque. Perception and Psychopbysics, 16, 443-448. WALMSLEY, A , , & BAHR,J. L. (1990) A computer controlled torque motor. Measurement Science & Technology, 1, 539-543.
HENRY, F. M.
Accepted May 24, 1991.
