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Effects of practice on some aspects of human tracking performance under two sensory feedback conditions
Journal of the neurological Sciences
515
Elsevier Publishing Company, Amsterdam-Printed in The Netherlands
Effects of Practice on some Aspects of Human Tracking Performance under Two Sensory Feedback Conditions J. R. G. CARRIE Institute of Psychiatry, The Maudsley Hospital, Denmark Hill, London SE5 (Great Britain)
(Received 10 January, 1967)
INTRODUCTION SUTTON (1957) studied human operator performance during a simple manual task in which each subject was required to maintain a fixed force with the aid of a visual display of the accuracy of his performance. He reported the results of a study in which 3 subjects carried out this tracking task during a 60-see period, and the total (i.e. unrestricted bandwidth) error "power" recorded in each of the six successive 10-see intervals within this period was measured. No "practice" or "fatigue" effects were detected. In the study described in this paper, an experimental arrangement similar to that of SUTTON was employed. It differed from that of SUTTON in two main respects: (1) a slightly different experimental procedure was used; (2) the measurements have been restricted to error activity occurring at above 4 c/s. Changes in the characteristics of this type of activity were probably obscured by SUTTON'S"total error" measurements, since components at above 4 c/s probably contribute only about 30 ~o of the total tracking error. Though these high-frequency, low-amplitude error components have not been investigated in detail in most experiments on tracking behaviour, other studies have shown that activity in this frequency range markedly changes its characteristics in response to certain procedures, even though the mean effort exerted against a load is kept constant (SUTTON AND SYKES 1956; CARRIE 1966). Initial investigations indicated that substantial changes in error amplitude in the 4-12 c/s range could be detected during six successive 10-sec periods of measurement. The aim of the present experiment was, therefore, to define the changes in error amplitude which occurred in this frequency range with repeated performance. Measurement was carried out under two test conditions: (i) when the subjects performed the motor task with the aid of a visual display of error; and (ii) when they carried out the task with their eyes closed. Present address of the author: Rochester State Hospital and Mayo Graduate School of Medicine, Rochester, Minn. (U.S.A.). J. neurol. Sci. (1967) 5:515-522
516
J.R.G. CARRIE METHODS
Each subject was required to perform a compensatory tracking task in which he had to keep his index finger in a fixed reference position as accurately as possible. This therefore constituted a tracking task with zero (i.e. totally predictable) input. Details of the methods used for the detection of limb movement, the visual presentation of error information, and the frequency analysis of error have been given elsewhere (CARRIE 1964, 1965, 1966).
Subjects Thirty young adult subjects were studied, 19 males and 11 females. None had had previous practice in a tracking task of the type used in the test situation. Procedure Each subject was allowed 60 sec continuous tracking, using the visual error display, prior to the start of the main experiment, in order to allow familiarisation with the test situation. There was then a rest interval of 3 min. The procedure subsequently followed has been described in detail elsewhere (CARRIE 1965, 1966). Twelve 10-sec "trials" were carried out, separated by 3-min rest intervals. In each "trial" the finger was initially positioned in the "zero-error" reference alignment with the aid of the visual display of error, but on each alternate "trial" the subject was required to close his eyes in response to a verbal command 2-3 sec before the start of the period of measurement, and to maintain the finger in the zero-error position without the aid of the visual display while the frequency analysis was carried out. The frequency analysis gave a measure, in arbitrary units, of the mean amplitude of movement of the finger per unit bandwidth at each frequency between 4 c/s and 12 c/s inclusive under two conditions: (i) during each of six 10-sec "trials" when the subjects were monitoring the visual display of the alignment of the finger on the oscilloscope; and (ii) during a similar number of "trials" when the eyes were closed. RESULTS
Inspection of the measurements obtained as described above showed that the mean error amplitude displayed in the first three 10-sec "trials" was greater than that occurring during the second three "trials" in the results from 26 subjects when the visual display was used, and 22 subjects when the eyes were closed. Therefore, in making a quantitative assessment of practice effects, the error observed during the first three "trials" has been compared with that occurring in the second three.
Frequency analysis In displaying the results, the mean error amplitude per unit bandwidth has been plotted as a function of frequency using a log-log scale. The design of the equipment used was such that squaring of the error was not necessary, as errors in either direction away from the reference alignment were scored as being of the same sign. Fig. 1 shows the pooled results from the whole group of 30 subjects when they were provided with the visual display of the position of the index finger in relation to the J. neurol. Sci. (1967) 5:515-522
ASPECTS
OF
HUMAN
TRACKING
PERFORMANCE
UNDER
TWO
FEEDBACK
CONDITIONS
517
zero-error alignment. The error amplitude diminishes with increasing frequency, except between 7 and 10 c/s. In this region there is a large peak in the spectrum, which shows a maximum at 8 c/s. Comparison of the averaged measurements from the first three "trials" with those from the second three in the series of six shows that the error amplitude is reduced in the latter three "trials" throughout the 4-12 c/s band. There is an approximately equal proportional difference in amplitude at each frequency (equal ratios are represented by equal lengths on a logarithmic scale). Though the 8 c/s and 9 c/s components are relatively slightly more attenuated than the others, the peak in this range of activity is still easily visible in the second half of the series of six test measurements.
2oot
200
.y ~2
~IOC b
L~IOC
9 E W
lal
i L I i .I 4 6 8 Frequency (c/s)
i
i ~1/2 10
I
4
t
I
~
I
6 8 F r e q u e n c y (c/s)
I
I
10
i I
12
Fig. 1. Mean error amplitude in arbitrary units plotted as a function of frequency on a log-log scale. Pooled results obtained from 30 subjects who were provided with a visual display of the position of the index finger in relation to the zero-error alignment, o - - o : Averaged results from trials 1-3 inclusive; ~ O: averaged results from trials 4-6 inclusive. Fig. 2. Results obtained from 30 subjects during tracking with the eyes closed. Symbols as in Fig. 1. Fig. 2 shows the averaged results from the whole group when the subjects were required to maintain the finger in the zero error alignment during the period of measurement with the eyes closed. It is immediately seen that the spectra in Fig. 2 differ strikingly from those in Fig. 1 in that the peak at 7-10 c/s is of much smaller amplitude. An upward deflection away from the downward trend at lower frequencies with a small peak at 8 c/s can be seen in both curves in Fig. 2, but no clearly-defined " h u m p " can be seen. In addition, the amplitude of each point in Fig. 2 outside the 7-10 c/s range is slightly less than that of the corresponding point in Fig. 1. Eye closure therefore sharply modified the characteristics of the error in the frequency range studied. There is an approximately equal difference in the relative amplitudes of error at each frequency in the first as compared with the second half of the series of six measurements. The proportional changes in error amplitude in the 4-12 c/s range associated with practice illustrated in Fig. 2 are similar in amplitude to those in Fig. 1. Statistical analysis
A statistical analysis of the results has been carried out in order to provide quantitaJ. neurol. Sci. (1967) 5:515-522
518
J. R. G . C A R R I E
tive clarification of two questions. (i) Are the practice effects observed separately under the two conditions, visual and non-visual, both statistically significant? (ii) Do the magnitudes of the practice effects observed separately under the two conditions differ significantly from one another? (i) Significance o f practice effect under each o f the two test conditions. The observed average error amplitudes at the frequencies studied (i.e. the ordinates of the amplitudefrequency plots) were separately summed to give an "error score" for each subject for the first three and the second three trials in the series. This procedure was carried out separately for the two test conditions. The arbitrary conversion of output measurements from high " Q " filters to " b r o a d - b a n d " scores as described above is unlikely to have resulted in the loss of important information in view of the similarity in the general shape of the amplitude-frequency curves obtained from different individuals. Thus 30 paired scores for the "trials" carried out when the subjects were supplied with visual information regarding the accuracy of performance, and a similar number of paired scores for observations when the eyes were closed were obtained. No further transformation was necessary prior to statistical analysis; t-tests for correlated means were carried out to determine whether the means of the error scores for the first half differed from those of the second half of the series of tests under each of the two conditions. TABLE
1
ERROR AMPLITUDE SCORES
Test conditions With visuallypresented error display With eyes closed
Mean error amplitude (arbitrary units) trials trials 1-3 4-6
t
Significance
1306
1010
4.022
< 0.001
1052
887
2.920
< 0.01
The results of this analysis are shown in Table 1. It is concluded that the mean error amplitude was significantly less in the second three as compared with the first three of the series of six 10-sec "trials", both when the subjects used an amplified visual display of error to aid the control of the limb, and when they carried out the task with the eyes closed. (ii) Comparison o f the practice effects observed under the two test conditions. In order to obtain a quantitative measure of the practice effects observed in each subject the ratio of the mean amplitude score (obtained by summation of the amplitude-frequency plots) for the first three "trials" to the score for the second three in the series of six was calculated. This calculation was carried out for each of the 30 subjects for the two test conditions to give two series of "practice ratio" scores reflecting changes in performance under each of the two test conditions. The significance of the difference J. neurol. Sci. (1967) 5:515-522
ASPECTS OF H U M A N T R A C K I N G P E R F O R M A N C E U N D E R T W O FEEDBACK C O N D I T I O N S
519
between the means of the two series of practice ratios was then determined by means of a t-test for correlated means. The results are summarised in Table 2. It is concluded that there was no significant difference between the practice effects observed when the subjects were aided by a visual display of the accuracy of their performance and those which occurred when the subjects carried out the task with their eyes closed. TABLE 2 RATIOS OF ERROR AMPLITUDE SCORES (TRIALS 1-3: TRIALS WITH AND WITHOUT VISUAL CUES
With visuallypresented error display
1.39
With eyes closed
1.28
4-6)
t
Significance
1.10
>0.05
DISCUSSION
These results differ from those of SUTTON(1957), who detected no significant practice effects during six consecutive 10-sec periods of measurement in a simple motor task of the type studied in this experiment. It is possible that this discrepancy is due to the fact that the conditions of SUTTON'S experiment differed in certain respects from those employed in the present study. For example, the magnitude of the muscle effort exerted by the subjects was much greater in SUTTON'Sexperiment than in the present investigation and in his study fatigue effects may have cancelled changes occurring in association with practice. However, a more likely explanation of the discrepancy between the results of the two investigations is that the subjects studied by SUTTON were more practised in the test situation than those who participated in the present investigation. SUTTON'Sgroup therefore showed less modification in performance with repeated testing than the subjects studied in the experiment described in this paper. SUTTONdoes not say exactly how experienced his subjects were in the task, but he does state that they had had "considerable initial practice". The present investigation therefore shows that marked practice effects can be observed in the high-frequency components of tracking error when subjects carry out a simple motor task in which they are fairly inexperienced. Further experiments will be required to determine how rapidly these effects diminish to an insignificant level. There is a striking difference in the prominence of the peak in the spectrum at 7-10 c/s when the subjects were required to perform with and without visual information about the accuracy of performance of the task. When the controlled muscle effort was carried out while the subjects were provided with an amplified visual display of the position of the index finger in relation to the "zero-error" alignment there was a prominent peak at 7-10 c/s, but after they had closed their eyes this feature was much less obvious. The detailed characteristics of this effect, and its possible physiological basis, have been discussed elsewhere (CARRIE1966). This phenomenon constitutes a rather special case of the general situation discussed by HORN (1965), in which a ~neurol. Sci. (1967) 5:515-522
520
J.R.G.
CARRIE
stimulus (in the present instance a visual stimulus) sent in through one sensory channel may influence the performance of a task, when task information (in the present instance kinaesthetic information) is already arriving over another channel. Evidence has been presented in other studies (CARRIE 1966) which suggests that the relative size of the peak in the frequency spectrum at 7-10 c/s and the amplitude of the " r a n d o m " tremor component which is reflected in the height of the "base-line" on which this peak is superimposed are determined by mechanisms which are to some extent functionally independent. The results of the present experiment tend to confirm this view. Practice in a task involving a controlled muscle effort produced a fairly uniform attenuation of error amplitude under both test conditions over the whole 4-12 c/s frequency range, and therefore presumably affected the mechanism determining the amplitude of the "random" component. Eye closure on the other hand, had a relatively specific effect on the mechanism determining movement amplitude at 7-10 c/s, producing marked attenuation of tremor occurring in this frequency range. In general, the rate of improvement in performance of a task is roughly proportional, other things being equal, to the amount of information that is made available about how well it is being carried out. For example, LASZLO (1966) had described the considerable impairment of performance in a simple motor task associated with kinaesthetic sense loss. MACPHERSON et al. (1949) found that when subjects were provided with "visual knowledge" of the accuracy with which they were performing a simple motor task, their performance improved more rapidly than when this information was not available. Furthermore, when the task was performed under kinaesthetic control only, the error was increased, mainly due to the subjects overshooting the target. It was therefore expected that the error would be larger with a kinaesthetic input only, and that the proportional rate of reduction of error amplitude would be greater when the subjects received both visual and kinaesthetic information regarding performance than when kinaesthetic signals alone were available. However, the error amplitude has been found to be smaller throughout the 4-12 c/s range when kinaesthetic information only was available, particularly at 7-10 c/s (though there was clearly a relatively large, slow, DC drift when the eyes were closed). Furthermore the rates of reduction in error amplitude were similar whether "visual knowledge" of the results of performance was available or not. It is possible that the test procedure used in this experiment resulted in some interaction between performance under the two conditions, and that if the tests with visual monitoring of performance and with eyes closed had been carried out separately, the magnitude of the practice effects would have differed in the two series. This possibility has not been systematically investigated at the present time, but preliminary results on a few subjects suggest that no such interaction occurred. On the other hand, the similarity of the practice effects observed under the two test conditions may be due to the fact that the measurements made in this experiment did not include frequencies below 4 c/s. Most of the tracking error (about 70 ~o) is concentrated below 4 c/s. The usual visual reaction time is 250 msec or more, and so it would be expected that most of the tremor activity at above about 4 c/s would be involuntary in nature and hence not modifiable in response to visual cues. Thus it may be that it is only in the very low frequency range below 4 c/s that corrective reactions specifically related to a visual display of error are detectable. However, the significance of reaction J. neurol. Sci.
(1967) 5:515-522
ASPECTS OF HUMAN TRACKINGPERFORMANCEUNDER TWO FEEDBACK CONDITIONS 521 time duration in determining the characteristics of tracking behaviour is uncertain (ADAMS 1961; SMITH 1962). The error amplitude in the 4-12 c/s range may be determined mainly by the "tonic" activity of mechanisms located in supraspinal centres. On the other hand, it may reflect the m o m e n t to moment modification of neuromuscular activity in response to incoming sensory information. In the present experiment, removal of visual cues did not significantly alter the rate of change in error amplitude occurring with repetition of the test task. Thus whatever may be the nature of the change in the activity of the neural structures determining tremor amplitude in the 4-12 c/s range that occurs with practice, it does not appear to be associated with increasingly effective processing of visual information about the accuracy of performance of a task.
ACKNOWLEDGEMENTS The author wishes to thank Mr. G. H. Cox for help with the electronics. This work was carried out with the aid of grants from the Ford Foundation and Bethlem Royal and Maudsley Hospitals Research Fund.
SUMMARY The accuracy with which 30 subjects performed a compensatory tracking task which had a zero input was studied under two conditions: (i) while the limb was related under visual control to the "zero-error" alignment; and (ii) when they had their eyes closed. Frequency analysis was carried out so as to give a measurement of the mean amplitude of the error per unit bandwidth between 4 c/s and 12 c/s inclusive. The subjects were required to perform the task during a series of six 10-sec "trials". The error was of relatively lower amplitude when the eyes were closed, especially at 7-10 c/s, where a distinct peak occurred in the spectrum when visual knowledge of the results of performance was available. Under each test condition the mean amplitude of the error at 4-12 c/s was significantly less during the second three than the first three of the series of six "trials" under both test conditions, as measured by a quantitative index. The proportional reductions in the error associated with practice that occurred under the two conditions did not differ significantly in terms of this index. The significance of these findings, and possible reasons for discrepancies between the results of this study and those of previous investigations have been discussed.
REFERENCES
ADAMS,J. A. (1961) Human tracking behavior, Psychol. Bull., 58: 55-79. CARRIE,J. R. G. (1964)Physiological Tremor in Normal Subjects and Psychiatric Patients, M.D. Thesis, Edinburgh University. CARRm, J. R. G. (1965) Finger tremor in alcoholic patients, J. Neurol. Neurosurg. Psychiat., 28: 529-532. CARRXE,J. R. G. (1966) Finger tremor with an amplified visual display of position and following eye closure, J. neurol. Sci., 3: 329-339. HORN, G. (1965) In: D. S. LEHRMAN,R. A. HINDEAND E. SHAW(Eds.), Advances in the Study of Behavior, Vol. 1, Academic Press, New York and London. J. neurol. Sci. 0967) 5:515-522
522
J . R . G . CARRIE
LASZLO, J. I. (1966) The performance of a simple motor task with kinaesthetic sense loss, Quart. J. exp. Psychol., 18 : 1-8. MACPHERSON, S. J., V. DEES AND G. C. GRINDLEY (1949) The effect of knowledge of results on learning and performance, Part 2 (Some characteristics of very simple skills), Quart. J. exp. Psychol., 1 : 68-78. SMITH, K. U. (1962) Delayed Sensory Feedback and Behavior, Saunders, Philadelphia and London. SUTTON, G. G. (1957) The error power spectrum as a technique for assessing the performance of the human operator in a simple task, Quart. J. exp. Psychol., 9: 42-51. SLrlTON,G. G. AND K. SYKES (1956) The reduction of hand tremor by removal of visual stimulus, Statistical Advisory Unit Report, Ministry of Aviation Library, London, 4/58: 28-36.
J. neurol. Sci. (1967) 5:515-522
515
Elsevier Publishing Company, Amsterdam-Printed in The Netherlands
Effects of Practice on some Aspects of Human Tracking Performance under Two Sensory Feedback Conditions J. R. G. CARRIE Institute of Psychiatry, The Maudsley Hospital, Denmark Hill, London SE5 (Great Britain)
(Received 10 January, 1967)
INTRODUCTION SUTTON (1957) studied human operator performance during a simple manual task in which each subject was required to maintain a fixed force with the aid of a visual display of the accuracy of his performance. He reported the results of a study in which 3 subjects carried out this tracking task during a 60-see period, and the total (i.e. unrestricted bandwidth) error "power" recorded in each of the six successive 10-see intervals within this period was measured. No "practice" or "fatigue" effects were detected. In the study described in this paper, an experimental arrangement similar to that of SUTTON was employed. It differed from that of SUTTON in two main respects: (1) a slightly different experimental procedure was used; (2) the measurements have been restricted to error activity occurring at above 4 c/s. Changes in the characteristics of this type of activity were probably obscured by SUTTON'S"total error" measurements, since components at above 4 c/s probably contribute only about 30 ~o of the total tracking error. Though these high-frequency, low-amplitude error components have not been investigated in detail in most experiments on tracking behaviour, other studies have shown that activity in this frequency range markedly changes its characteristics in response to certain procedures, even though the mean effort exerted against a load is kept constant (SUTTON AND SYKES 1956; CARRIE 1966). Initial investigations indicated that substantial changes in error amplitude in the 4-12 c/s range could be detected during six successive 10-sec periods of measurement. The aim of the present experiment was, therefore, to define the changes in error amplitude which occurred in this frequency range with repeated performance. Measurement was carried out under two test conditions: (i) when the subjects performed the motor task with the aid of a visual display of error; and (ii) when they carried out the task with their eyes closed. Present address of the author: Rochester State Hospital and Mayo Graduate School of Medicine, Rochester, Minn. (U.S.A.). J. neurol. Sci. (1967) 5:515-522
516
J.R.G. CARRIE METHODS
Each subject was required to perform a compensatory tracking task in which he had to keep his index finger in a fixed reference position as accurately as possible. This therefore constituted a tracking task with zero (i.e. totally predictable) input. Details of the methods used for the detection of limb movement, the visual presentation of error information, and the frequency analysis of error have been given elsewhere (CARRIE 1964, 1965, 1966).
Subjects Thirty young adult subjects were studied, 19 males and 11 females. None had had previous practice in a tracking task of the type used in the test situation. Procedure Each subject was allowed 60 sec continuous tracking, using the visual error display, prior to the start of the main experiment, in order to allow familiarisation with the test situation. There was then a rest interval of 3 min. The procedure subsequently followed has been described in detail elsewhere (CARRIE 1965, 1966). Twelve 10-sec "trials" were carried out, separated by 3-min rest intervals. In each "trial" the finger was initially positioned in the "zero-error" reference alignment with the aid of the visual display of error, but on each alternate "trial" the subject was required to close his eyes in response to a verbal command 2-3 sec before the start of the period of measurement, and to maintain the finger in the zero-error position without the aid of the visual display while the frequency analysis was carried out. The frequency analysis gave a measure, in arbitrary units, of the mean amplitude of movement of the finger per unit bandwidth at each frequency between 4 c/s and 12 c/s inclusive under two conditions: (i) during each of six 10-sec "trials" when the subjects were monitoring the visual display of the alignment of the finger on the oscilloscope; and (ii) during a similar number of "trials" when the eyes were closed. RESULTS
Inspection of the measurements obtained as described above showed that the mean error amplitude displayed in the first three 10-sec "trials" was greater than that occurring during the second three "trials" in the results from 26 subjects when the visual display was used, and 22 subjects when the eyes were closed. Therefore, in making a quantitative assessment of practice effects, the error observed during the first three "trials" has been compared with that occurring in the second three.
Frequency analysis In displaying the results, the mean error amplitude per unit bandwidth has been plotted as a function of frequency using a log-log scale. The design of the equipment used was such that squaring of the error was not necessary, as errors in either direction away from the reference alignment were scored as being of the same sign. Fig. 1 shows the pooled results from the whole group of 30 subjects when they were provided with the visual display of the position of the index finger in relation to the J. neurol. Sci. (1967) 5:515-522
ASPECTS
OF
HUMAN
TRACKING
PERFORMANCE
UNDER
TWO
FEEDBACK
CONDITIONS
517
zero-error alignment. The error amplitude diminishes with increasing frequency, except between 7 and 10 c/s. In this region there is a large peak in the spectrum, which shows a maximum at 8 c/s. Comparison of the averaged measurements from the first three "trials" with those from the second three in the series of six shows that the error amplitude is reduced in the latter three "trials" throughout the 4-12 c/s band. There is an approximately equal proportional difference in amplitude at each frequency (equal ratios are represented by equal lengths on a logarithmic scale). Though the 8 c/s and 9 c/s components are relatively slightly more attenuated than the others, the peak in this range of activity is still easily visible in the second half of the series of six test measurements.
2oot
200
.y ~2
~IOC b
L~IOC
9 E W
lal
i L I i .I 4 6 8 Frequency (c/s)
i
i ~1/2 10
I
4
t
I
~
I
6 8 F r e q u e n c y (c/s)
I
I
10
i I
12
Fig. 1. Mean error amplitude in arbitrary units plotted as a function of frequency on a log-log scale. Pooled results obtained from 30 subjects who were provided with a visual display of the position of the index finger in relation to the zero-error alignment, o - - o : Averaged results from trials 1-3 inclusive; ~ O: averaged results from trials 4-6 inclusive. Fig. 2. Results obtained from 30 subjects during tracking with the eyes closed. Symbols as in Fig. 1. Fig. 2 shows the averaged results from the whole group when the subjects were required to maintain the finger in the zero error alignment during the period of measurement with the eyes closed. It is immediately seen that the spectra in Fig. 2 differ strikingly from those in Fig. 1 in that the peak at 7-10 c/s is of much smaller amplitude. An upward deflection away from the downward trend at lower frequencies with a small peak at 8 c/s can be seen in both curves in Fig. 2, but no clearly-defined " h u m p " can be seen. In addition, the amplitude of each point in Fig. 2 outside the 7-10 c/s range is slightly less than that of the corresponding point in Fig. 1. Eye closure therefore sharply modified the characteristics of the error in the frequency range studied. There is an approximately equal difference in the relative amplitudes of error at each frequency in the first as compared with the second half of the series of six measurements. The proportional changes in error amplitude in the 4-12 c/s range associated with practice illustrated in Fig. 2 are similar in amplitude to those in Fig. 1. Statistical analysis
A statistical analysis of the results has been carried out in order to provide quantitaJ. neurol. Sci. (1967) 5:515-522
518
J. R. G . C A R R I E
tive clarification of two questions. (i) Are the practice effects observed separately under the two conditions, visual and non-visual, both statistically significant? (ii) Do the magnitudes of the practice effects observed separately under the two conditions differ significantly from one another? (i) Significance o f practice effect under each o f the two test conditions. The observed average error amplitudes at the frequencies studied (i.e. the ordinates of the amplitudefrequency plots) were separately summed to give an "error score" for each subject for the first three and the second three trials in the series. This procedure was carried out separately for the two test conditions. The arbitrary conversion of output measurements from high " Q " filters to " b r o a d - b a n d " scores as described above is unlikely to have resulted in the loss of important information in view of the similarity in the general shape of the amplitude-frequency curves obtained from different individuals. Thus 30 paired scores for the "trials" carried out when the subjects were supplied with visual information regarding the accuracy of performance, and a similar number of paired scores for observations when the eyes were closed were obtained. No further transformation was necessary prior to statistical analysis; t-tests for correlated means were carried out to determine whether the means of the error scores for the first half differed from those of the second half of the series of tests under each of the two conditions. TABLE
1
ERROR AMPLITUDE SCORES
Test conditions With visuallypresented error display With eyes closed
Mean error amplitude (arbitrary units) trials trials 1-3 4-6
t
Significance
1306
1010
4.022
< 0.001
1052
887
2.920
< 0.01
The results of this analysis are shown in Table 1. It is concluded that the mean error amplitude was significantly less in the second three as compared with the first three of the series of six 10-sec "trials", both when the subjects used an amplified visual display of error to aid the control of the limb, and when they carried out the task with the eyes closed. (ii) Comparison o f the practice effects observed under the two test conditions. In order to obtain a quantitative measure of the practice effects observed in each subject the ratio of the mean amplitude score (obtained by summation of the amplitude-frequency plots) for the first three "trials" to the score for the second three in the series of six was calculated. This calculation was carried out for each of the 30 subjects for the two test conditions to give two series of "practice ratio" scores reflecting changes in performance under each of the two test conditions. The significance of the difference J. neurol. Sci. (1967) 5:515-522
ASPECTS OF H U M A N T R A C K I N G P E R F O R M A N C E U N D E R T W O FEEDBACK C O N D I T I O N S
519
between the means of the two series of practice ratios was then determined by means of a t-test for correlated means. The results are summarised in Table 2. It is concluded that there was no significant difference between the practice effects observed when the subjects were aided by a visual display of the accuracy of their performance and those which occurred when the subjects carried out the task with their eyes closed. TABLE 2 RATIOS OF ERROR AMPLITUDE SCORES (TRIALS 1-3: TRIALS WITH AND WITHOUT VISUAL CUES
With visuallypresented error display
1.39
With eyes closed
1.28
4-6)
t
Significance
1.10
>0.05
DISCUSSION
These results differ from those of SUTTON(1957), who detected no significant practice effects during six consecutive 10-sec periods of measurement in a simple motor task of the type studied in this experiment. It is possible that this discrepancy is due to the fact that the conditions of SUTTON'S experiment differed in certain respects from those employed in the present study. For example, the magnitude of the muscle effort exerted by the subjects was much greater in SUTTON'Sexperiment than in the present investigation and in his study fatigue effects may have cancelled changes occurring in association with practice. However, a more likely explanation of the discrepancy between the results of the two investigations is that the subjects studied by SUTTON were more practised in the test situation than those who participated in the present investigation. SUTTON'Sgroup therefore showed less modification in performance with repeated testing than the subjects studied in the experiment described in this paper. SUTTONdoes not say exactly how experienced his subjects were in the task, but he does state that they had had "considerable initial practice". The present investigation therefore shows that marked practice effects can be observed in the high-frequency components of tracking error when subjects carry out a simple motor task in which they are fairly inexperienced. Further experiments will be required to determine how rapidly these effects diminish to an insignificant level. There is a striking difference in the prominence of the peak in the spectrum at 7-10 c/s when the subjects were required to perform with and without visual information about the accuracy of performance of the task. When the controlled muscle effort was carried out while the subjects were provided with an amplified visual display of the position of the index finger in relation to the "zero-error" alignment there was a prominent peak at 7-10 c/s, but after they had closed their eyes this feature was much less obvious. The detailed characteristics of this effect, and its possible physiological basis, have been discussed elsewhere (CARRIE1966). This phenomenon constitutes a rather special case of the general situation discussed by HORN (1965), in which a ~neurol. Sci. (1967) 5:515-522
520
J.R.G.
CARRIE
stimulus (in the present instance a visual stimulus) sent in through one sensory channel may influence the performance of a task, when task information (in the present instance kinaesthetic information) is already arriving over another channel. Evidence has been presented in other studies (CARRIE 1966) which suggests that the relative size of the peak in the frequency spectrum at 7-10 c/s and the amplitude of the " r a n d o m " tremor component which is reflected in the height of the "base-line" on which this peak is superimposed are determined by mechanisms which are to some extent functionally independent. The results of the present experiment tend to confirm this view. Practice in a task involving a controlled muscle effort produced a fairly uniform attenuation of error amplitude under both test conditions over the whole 4-12 c/s frequency range, and therefore presumably affected the mechanism determining the amplitude of the "random" component. Eye closure on the other hand, had a relatively specific effect on the mechanism determining movement amplitude at 7-10 c/s, producing marked attenuation of tremor occurring in this frequency range. In general, the rate of improvement in performance of a task is roughly proportional, other things being equal, to the amount of information that is made available about how well it is being carried out. For example, LASZLO (1966) had described the considerable impairment of performance in a simple motor task associated with kinaesthetic sense loss. MACPHERSON et al. (1949) found that when subjects were provided with "visual knowledge" of the accuracy with which they were performing a simple motor task, their performance improved more rapidly than when this information was not available. Furthermore, when the task was performed under kinaesthetic control only, the error was increased, mainly due to the subjects overshooting the target. It was therefore expected that the error would be larger with a kinaesthetic input only, and that the proportional rate of reduction of error amplitude would be greater when the subjects received both visual and kinaesthetic information regarding performance than when kinaesthetic signals alone were available. However, the error amplitude has been found to be smaller throughout the 4-12 c/s range when kinaesthetic information only was available, particularly at 7-10 c/s (though there was clearly a relatively large, slow, DC drift when the eyes were closed). Furthermore the rates of reduction in error amplitude were similar whether "visual knowledge" of the results of performance was available or not. It is possible that the test procedure used in this experiment resulted in some interaction between performance under the two conditions, and that if the tests with visual monitoring of performance and with eyes closed had been carried out separately, the magnitude of the practice effects would have differed in the two series. This possibility has not been systematically investigated at the present time, but preliminary results on a few subjects suggest that no such interaction occurred. On the other hand, the similarity of the practice effects observed under the two test conditions may be due to the fact that the measurements made in this experiment did not include frequencies below 4 c/s. Most of the tracking error (about 70 ~o) is concentrated below 4 c/s. The usual visual reaction time is 250 msec or more, and so it would be expected that most of the tremor activity at above about 4 c/s would be involuntary in nature and hence not modifiable in response to visual cues. Thus it may be that it is only in the very low frequency range below 4 c/s that corrective reactions specifically related to a visual display of error are detectable. However, the significance of reaction J. neurol. Sci.
(1967) 5:515-522
ASPECTS OF HUMAN TRACKINGPERFORMANCEUNDER TWO FEEDBACK CONDITIONS 521 time duration in determining the characteristics of tracking behaviour is uncertain (ADAMS 1961; SMITH 1962). The error amplitude in the 4-12 c/s range may be determined mainly by the "tonic" activity of mechanisms located in supraspinal centres. On the other hand, it may reflect the m o m e n t to moment modification of neuromuscular activity in response to incoming sensory information. In the present experiment, removal of visual cues did not significantly alter the rate of change in error amplitude occurring with repetition of the test task. Thus whatever may be the nature of the change in the activity of the neural structures determining tremor amplitude in the 4-12 c/s range that occurs with practice, it does not appear to be associated with increasingly effective processing of visual information about the accuracy of performance of a task.
ACKNOWLEDGEMENTS The author wishes to thank Mr. G. H. Cox for help with the electronics. This work was carried out with the aid of grants from the Ford Foundation and Bethlem Royal and Maudsley Hospitals Research Fund.
SUMMARY The accuracy with which 30 subjects performed a compensatory tracking task which had a zero input was studied under two conditions: (i) while the limb was related under visual control to the "zero-error" alignment; and (ii) when they had their eyes closed. Frequency analysis was carried out so as to give a measurement of the mean amplitude of the error per unit bandwidth between 4 c/s and 12 c/s inclusive. The subjects were required to perform the task during a series of six 10-sec "trials". The error was of relatively lower amplitude when the eyes were closed, especially at 7-10 c/s, where a distinct peak occurred in the spectrum when visual knowledge of the results of performance was available. Under each test condition the mean amplitude of the error at 4-12 c/s was significantly less during the second three than the first three of the series of six "trials" under both test conditions, as measured by a quantitative index. The proportional reductions in the error associated with practice that occurred under the two conditions did not differ significantly in terms of this index. The significance of these findings, and possible reasons for discrepancies between the results of this study and those of previous investigations have been discussed.
REFERENCES
ADAMS,J. A. (1961) Human tracking behavior, Psychol. Bull., 58: 55-79. CARRIE,J. R. G. (1964)Physiological Tremor in Normal Subjects and Psychiatric Patients, M.D. Thesis, Edinburgh University. CARRm, J. R. G. (1965) Finger tremor in alcoholic patients, J. Neurol. Neurosurg. Psychiat., 28: 529-532. CARRXE,J. R. G. (1966) Finger tremor with an amplified visual display of position and following eye closure, J. neurol. Sci., 3: 329-339. HORN, G. (1965) In: D. S. LEHRMAN,R. A. HINDEAND E. SHAW(Eds.), Advances in the Study of Behavior, Vol. 1, Academic Press, New York and London. J. neurol. Sci. 0967) 5:515-522
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J. neurol. Sci. (1967) 5:515-522