A custom designed system to measure corticospinal tract jitter

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Electroencephalography and clinical Neurophysiology 109 (1998) 194–197

Technical note

A custom designed system to measure corticospinal tract jitter Marjan Mihelin a ,*, Rajka M. Liscic b a

Institute of Clinical Neurophysiology, University Medical Centre Ljubljana, SI-1525 Ljubljana, Slovenia b Institute for Medical Research and Occupational Health, Zagreb, Croatia Accepted for publication: 14 November 1997

Abstract Typical latency of an individual limb muscle response to magnetic or electric stimulation of the human cortex is in the range of 10–50 ms. For the latency variability, i.e., jitter studies, a resolution of at least 20 ms is needed. Commercially available EMG equipment needs custom-designed upgrading to allow for such studies. Two solutions were designed: (i) a hardware unit allowing an adjustable delay of data acquisition after the delivered stimuli; and (ii) diverting of the amplified biological signal and the EMG equipment trigger to an external computer equipped with an analogue-to-digital conversion (ADC) module. Custom-designed software made fast ADC possible during the whole period of data acquisition. Both concepts were applied to a Vickers Medical Mystro electromyograph, and have been successfully used in the Ljubljana (Slovenia) Institute of Clinical Neurophysiology for the last 2 years.  1998 Elsevier Science Ireland Ltd. Keywords: Cortical stimulation; Data acquisition; Jitter; Long latency responses; Measurement system

1. Introduction The basis of any electrophysiologic measurement is amplification of a rather weak biological signal. The rest of the equipment has become more and more digitised and is nowadays actually an application-oriented computer. Such equipment allows convenient routine work, but demonstrates rather poor flexibility when used for research purposes. New demands when studying long latency responses, as in the case of various cortical stimulation measurements, could only be met by an upgrade of the existing device. Since jitter measurements became a routine neurophysiological procedure (Sta˚lberg and Trontelj, 1979), commercially available EMG equipment regularly offers high resolution jitter studies, but mostly of short latency responses (up to 10 ms; Trontelj et al., 1988). Newer techniques, i.e., stimulation of the human brain by electric or magnetic stimulation, evoke responses of much longer latencies, typically between 10 and 50 ms, where acquisi-

* Corresponding author. Tel.: +386 61316152; fax: +386 61302771; e-mail: [email protected]

0924-980X/98/$19.00  1998 Elsevier Science Ireland Ltd. All rights reserved PII S0924-980X (97)0007 9-9

tion time up to 100 ms is needed (Caramia et al., 1991; Thompson et al., 1991; Prior et al., 1993). We present two modifications of commercial EMG equipment for research on the corticospinal jitter of longer latency responses from two limb muscles. Based on previous experience (Zidar et al., 1987, 1989; Zarola et al., 1989; Liscic et al., 1994), electric and magnetic transcranial motor cortex stimulation was used and responses detected by a single fibre EMG technique (Sta˚lberg and Trontelj, 1994).

2. Methods and materials Magnetic transcranial stimulation was performed with a MAGSTIM 200 stimulator with maximal magnetic flow density of 1.5 T (Liscic and Mihelin, 1996). First dorsal interosseus (FDI) hand muscle motor units were activated with the coil centred over the vertex. Orbicularis oris (OR) muscle activation was achieved with the coil centred 4 cm laterally from the vertex on a line joining the vertex and the external auditory meatus, and tilted towards the stimulated hemisphere.

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Electrical stimuli were applied through the Ag–AgCl electrodes fixed to the scalp with collodium. The cathode was placed on the vertex and the anode 9–10 cm laterally on a line joining the vertex and the external auditory meatus, contralaterally to the muscle under study (Rothwell et al., 1987). Digitimer D 180 stimulator with a maximum pulse amplitude of 1500 V and with a fixed stimulus width of 50 ms was used. Stimulus intensity was expressed on a linear scale as a percentage of the maximum output of the device. Single motor unit potentials were detected by single fibre needle electrodes Medelec SF-25. The responses were displayed on electromyograph Vickers Medical-Medelec Mystro. Stimuli were delivered randomly with a mean frequency of 0.2 Hz. Identification of the single motor unit action potential as one and the same on consecutive stimulation was based on constancy of its shape, amplitude, latency and its threshold for activation. Series of 100 responses were consistently achieved with magnetic stimulation, while with electric stimuli, a few series were smaller because subjects found the stimulation too unpleasant and the experiment was terminated prematurely. Absolute latencies of the evoked single unit responses and their variability on consecutive stimuli were then measured in the series. Typical latency of an individual response in our study (for OR and FDI muscle) was in the range between 10 and 30 ms, suggesting an analysis time of 50 ms; a latency resolution of at least 20 ms was needed, but not offered from Mystro or other available commercial systems. In the case of detection in the lower limbs muscles, latencies of around 50 ms would be expected, suggesting 100 ms acquisition time. To overcome this limitation, we have designed, developed and tested two solutions. 2.1. Measurement system with a hardware delay unit Being identical to acquisition time, sweep time can be set freely in the Medelec Mystro electromyograph, but the rate of sampling is fixed at 1000 points per sweep. Sampling time = Sweep time=number of digitised data Sampling time is 1/1000 of the chosen sweep time (i.e., if a biologic response has a latency close to 50 ms, it can be quantified with an accuracy of 50 ms at the most). This drawback could be overcome by a hardware delay unit (Tektronix oscilloscope). Taking into account the distance from the stimulating to the detecting point, the time for which an external unit could safely delay acquisition of data can be predicted. A major benefit could thus be achieved: recording only in the epoch of the expected event with a much better resolution per sweep, since the time of interest is quite short; a sweep time of 10 or 20 ms, which is quite sufficient for the response duration, means a sampling time of 10 or 20 ms (for Medelec Mystro). Of course, the fixed delay of the delay unit should be added to the readings from the screen when measuring latencies of

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the response components (Fig. 1). The drawbacks of this system are the inability to store the acquired sweeps for offline and further re-analysis, and the impossibility of evaluating the trace in the epoch preceding the response, which may be of interest in some physiological studies. 2.2. Measurement system with an external personal computer For studying biologic responses consisting of two or more separated components, the measurement system with a hardware delay unit proved inadequate, as each component should be studied separately in its own time window. This drawback was overcome by introducing into the system an external personal computer (PC) incorporating a proficient analogue-to-digital converter (Meilhaus PC-30D ADC) and 32 MB RAM, of which nearly all was used as RAM disk. Thus, very fast storing of individual responses digitised at 50 kHz, 100 kHz or at the highest available sampling rate of 200 kHz, and at 12-bit amplitude resolution was achieved. In addition to magnetic/electric stimulus, a simultaneous signal was generated by either stimulator to trigger the beam of the electromyograph and the AD conversion. A sweep time of 100 ms, quite sufficient for monitoring the whole response on the screen, was set on the electromyograph. Meanwhile, AD conversion was performed throughout the sweep time at a rate of 50, 100 or 200 kHz (20, 10 or 5 ms resolution), more than sufficient for the study of the 100 ms response component variability. To synchronise both equipments regarding their sweep time, an IEEE 488 parallel interface was used and ADC analysis time adapted automatically to the manually adjusted sweep time of the electromyograph (Fig. 2). After determining the electromyograph settings, PC software for sampling was used, so that the number of digitised data at any sweep time was equivalent to the product of the sweep time and sampling frequency. Number of digitised data = Sweep time × sampling frequency At a sampling rate of 200 kHz and with a sweep time of 100 ms, 20 000 points were acquired with every stimulus. Digitised data were stored on the RAM disk during the recording and thereafter transferred to the regular hard disk. To ensure fastest possible repetition rate of the stimulus, fast transfer of data from RAM to the hard disk was needed and achieved by a Tekram disk cache controller with 4 MB of memory. In the system with a hardware delay unit, latencies and variability of the responses were studied by means of the Mystro ‘Specialist Application Module’ (SAM). After each series of 100 stimuli, both the time window limits and the amplitude analysis level were manually adjusted so that the analysis level was higher than any artefact and lower than the lowest amplitude response. Results of the analysis were

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M. Mihelin, R.M. Liscic / Electroencephalography and clinical Neurophysiology 109 (1998) 194–197

Fig. 1. Measurement system with a hardware delay unit. The trace on the screen of the electromyograph starts after a delay defined by the hardware delay unit. The acquisition time of the electromyograph is defined according to the range of expected latencies of the recorded signals. Signals are presented in the chart.

printed out and later retyped into a data base for further analysis. Every component of an eventual multi-component response had to be studied separately as they could not all be registered in one sweep. Per unit, 1 h was needed for the recording procedure and 0.5 h for the analysis. The system with an external computer proved far more flexible. The programme for the display and analysis of responses was sophisticated enough to enable studying of

individual as well as of superimposed response components. The analysis level and time window of the whole multicomponent response can be set manually; all components of a series of responses can thus be optimally included into analysis (Fig. 3). In every response, intervals between the time of stimulus and intersection between the analysis level and any individual component were measured and stored in a special file. Mean latency and the variability of every

Fig. 2. Measurement system with an external PC computer. The electromyograph and ADC in the PC start their sweeps simultaneously. Mystro is used for safe and appropriate amplification of signals. The acquisition time of the ADC is automatically adapted to the manually adjusted Mystro sweep time by means of a 488 parallel interface (bold connection line).

M. Mihelin, R.M. Liscic / Electroencephalography and clinical Neurophysiology 109 (1998) 194–197

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Fig. 3. Simultaneous analysis of all components of a multi-component response. Analysis level and time windows are manually adjusted from the picture of all superimposed responses.

component could hence be easily calculated according to the standard ‘jitter’ algorithms (Mihelin et al., 1975; Sta˚lberg and Trontelj, 1979).

3. Discussion The described measurement systems have been used for 2 years in the Ljubljana Institute of Clinical Neurophysiology. Both concepts have proved capable of overcoming the limitations set by conventional systems. The main advantage of the system with the hardware delay unit is simple enhancement of the resolution of a commercially available electromyograph, of which all modules operate and are made use of as before the adaptation. However, for practical research work, the system with the external computer proved much more flexible owing to the elaborated custom-designed software. As all but the amplifying and monitoring function of a conventional electromyograph are taken over by a specially equipped PC, the system is not limited only to use in connection with the original electromyograph; it can be applied with any such equipment, providing its amplifier outputs are accessible.

References Caramia, M.D., Cicinelli, P., Paradiso, C., Mariorenzi, R., Zarola, F., Bernardi, G. and Rossini, P.M. Excitability changes of muscular responses to magnetic brain stimulation in patients with central motor disorders. Electroenceph. clin. Neurophysiol., 1991, 81: 243–250. Liscic, R.M., Zidar, J. and Prevec, T.S. Activation of the corticobulbar tract to the lower facial muscles by magnetic stimulation: a study of motor unit responses. Neurol. Croat., 1994, 43: 65–74.

Liscic, R.M. and Mihelin, M. Electric and magnetic stimulation of the human motor cortex: an EMG study of single motor unit responses. In: 11th Int. Symp. on Biomedical Engineering, Zagreb (Croatia), 7–9 November 1996, pp. 85–88. Mihelin, M., Trontelj, J.V. and Trontelj, J.K. Automatic measurement of random interpotential intervals in single fibre electromyography. Int. J. Biomed. Comput., 1975, 6: 181–191. Prior, A., Bertolasi, L., Dressler, D., Rothwell, J.C., Day, B.L., Thompson, P.D. and Marsden, C.D. Transcranial electric and magnetic stimulation of the leg area of the human motor cortex: single motor unit and surface EMG responses in the tibialis anterior muscle. Electroenceph. clin. Neurophysiol., 1993, 89: 131–137. Rothwell, J.C., Thompson, P.D., Day, B.L., Dick, J.P.R., Kachi, T., Cowan, J.M.A. and Marsden, C.D. Motor cortex stimulation in intact man. 1. General characteristics of EMG responses in different muscles. Brain, 1987, 110: 1173–1190. Sta˚lberg, E. and Trontelj, J.V. Single Fibre Electromyography. The Mirvalle Press, Old Woking, Surrey, UK, 1979. Sta˚lberg, E. and Trontelj, J.V. Single Fiber Electromyography. Studies in Healthy and Diseased Muscle. Raven Press, New York, 1994. Thompson, P.D., Day, B.L., Rothwell, J.C., Dressler, D., Maertens de Noordhout, A. and Marsden, C.D. Further observations on the facilitation of muscle responses to cortical stimulation by voluntary contraction. Electroenceph. clin Neurophysiol., 1991, 81: 397–402. Trontelj, J.V., Khuraibet, A. and Mihelin, M. The jitter in stimulated orbicularis oculi muscle: technique and normal values. J. Neurol. Neurosurg. Psychiatr., 1988, 51: 814–819. Zarola, F., Caramia, M.D., Paradiso, C., Mariorenzi, R., Martino, G., Traversa, R. and Rossini, P.M. Single fibre motor evoked potentials to brain, spinal roots and nerve stimulation. Comparison of the ‘central’ and ‘peripheral’ response jitter to magnetic and electric stimuli. Brain Res., 1989, 495: 217–224. Zidar, J., Trontelj, J.V. and Mihelin, M. Percutaneous stimulation of human corticospinal tract: a single fibre EMG study of individual motor unit responses. Brain Res., 1987, 422: 196–199. Zidar, J., Zgur, T. and Kiprovski, K. Transcranial electrical and magnetic motor cortex stimulation: studies in intact man. Neurologia, 1989, 38: 271–283.

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