- Email: [email protected]
Conditioned H-reflexes prior to movement
564
Brahl Research, 192 (1980) 564 5 6 9 ~) E l s e v i e r / N o r t h - H o l l a n d Biomedical Press
Conditioned H-reflexes prior to movement
S. J O H N S U L L I V A N
Department of K#zesiology, University of Waterloo, Waterloo, Ont. N2L 3G I (Canada) (Accepted F e b r u a r y 21st, 1980)
Key words: H-reflex - - c o n d i t i o n i n g - - late facilitation - - t u n i n g - - v o l u n t a r y response
The pattern of excitability of spinal reflexes prior to a voluntary response provides information on the organizational processes involved in the initiation of movementl,3,14,xT, 23,25. Although some disagreement exists regarding the pattern of excitation during the classic reaction time (RT) foreperiod 3,15,23,25, the finding of an increase in excitability beginning some 40-60 msec prior to the onset of voluntary movement has been well established1,11,15, 22. Studies dealing with this phenomenon have generally employed a constant foreperiod in order to increase the temporal certainty of the response signal and the demonstrated changes in spinal organization have been interpreted as evidence of preparatory 'set'. The establishment of such a preparatory 'set' is thought to involve both the spinal and supraspinal components of the stretch reflex6,1°,lL In addition, the preparatory 'set' has been shown to bring about an alteration in the firing pattern of pyramidal tract neurons6, 31. Hayes and Clarke 1° recently reported an increased occurrence and a decreased latency of the electrically elicited 'late reflex' recorded from the soleus muscle during a RT foreperiod prior to a visual stimulus. This finding was interpreted as being suggestive of an increase in the excitability of the supraspinal reflex pathway, and possibly related to the previously reported changes in cortical cell activity. In the present study an alternative method of investigating these preparatory changes in neural organization was employed. The technique of conditioning an Hreflex with a preceding subthreshold stimulus was used to determine whether or not there are any changes in the 'late facilitation' phase of the H-reflex recovery curve between interstimulus intervals of 75-250 msec. Such facilitation has previously been attributed, albeit tentatively, to long-loop reflex influences16,29,34-36. The experimental manipulations involved 11 healthy adults of both sexes who were tested while lying in a semi-reclined manner on a padded testing table with their backs supported. The testing posture, with the knee flexed, the right foot supported by a rigidly fixed foot plate, and the head stabilized and looking straight forward, ensured an optimum reflex responsO 2,~8. About 75 cm in front of the subject, at eye level, was a small light-emitting diode (3.5 mcd luminous intensity) which served as the reaction
565 signal (RS). The response to this brief (100 msec) signal was a rapid isometric plantar flexion of the right foot. A short (100 msec) auditory tone (WS) informed the subject of the forthcoming RS. This WS-RS interval was held constant at 1000 msec. The eliciting stimulus (ES) was delivered to the posterior tibial nerve in the region of the popliteal fossa of the subject's right leg through a 5 mm diameter surface electrode. The electrode was positioned in such a manner as to obtain an H-reflex without a concomitant direct muscle response. The anode consisted of a moist pad firmly secured over the patella. The H-reflex was elicited by a square wave pulse of 1 msec duration generated by a two-channel nerve stimulator (Grass, model $88). Each channel was connected in series to a stimulus isolation unit (Grass, model SIU5), and a single constant current device (Grass, model CCU1). Two bipolar (silver/silverchloride) electrodes placed 3 cm apart over the midline of the right soleus muscle recorded the evoked H-reflexes. The recorded muscle action potentials were amplified (Bicom differential amplifier, model 2122) and filtered (3 dB down at 10 and 1 k Hz) prior to being displayed on a Hewlett Packard storage oscilloscope (model 1201A). The peak-to-peak amplitude of the control H-reflexes and the conditioned test Hreflexes was recorded directly from the storage oscilloscope, as was the latency of the voluntary response. The unconditioned H-reflexes were elicited at 6 delay intervals during the foreperiod and prior to the voluntary response. These delays, expressed relative to WS, were at 400, 800, 950, 1000, 1100 and 1150 msec. The conditioned test H-reflexes were elicited at the following intervals subsequent to WS; 500, 900, 1050, 1100 msec. Each of these test H-reflexes was preceded 100 msec earlier by a subthreshold conditioning H-reflex stimulus 30. These 10 delay intervals, together with two probe H-reflex (an Hreflex elicited without an accompanying WS or RS) trials composed one cycle of trials. The administration of all trials was randomized within each cycle, with at least 15 sec separating adjacent trials. Immediately prior to each of the 4 experimental cycles, the intensity of the unconditioned H-reflex stimulus was adjusted to produce a response of 50 ~o of the maximum H-reflex amplitude. Another H-reflex control condition, where no response was required to RS and yet an H-reflex was elicited 800 msec following WS, was evaluated just prior to the first cycle of trials. The value of the conditioned H-reflex obtained prior to the first cycle of trials served as the control amplitude for the conditioned H-reflexes. In addition, volitional RT responses were performed in the WS-RS situation when the subject knew that no reflex would be elicited. The experimental session lasted 1.5 h. All subjects had previously undergone a short familiarization session. Shown in Fig. 1 are the peak-to-peak amplitudes for both the unconditioned Hreflexes and the conditioned H-reflexes obtained during the foreperiod and prior to the onset of the voluntary response. Also shown is the value of the control H-reflex (5.02 mV) obtained under the condition where no voluntary response was required to RS. The control value (2.87 mV) of the conditioned H-reflex is also shown. The latency to the onset of muscle activity corresponding to the voluntary response was 190.1 msec. The pattern of the unconditioned H-reflexes recorded between 400 and 1150 msec following WS was similar to that obtained by previous researchersg,11,14,17, 22.
566 8
--
A
> E v
6 ta.I
a
F-"-i rt
4
-
2
--
t
X b.l
._I b_ ,
-r
1
0
,
200
I 400
i
I 600
r
J
I
800
~
I
I000
1200
WS
RS WS -
ES
INTERVAL
(rnsec)
Fig. 1. Conditioned ( x ) and unconditioned (O) H-reflex amplitude during a visual RT foreperiod and prior to the initiation of the voluntary response. The vertical bars correspond to the S.E. of the mean of the reflex amplitude. The upper (-- -) line represents the value of the control H-reflex obtained in the condition where the subject was not required to respond to RS. The lower (--) line is the amplitude of the unconditioned H-reflex recorded prior to the start of the experiment. WS, auditory warning signal; RS, visual reaction signal, ES, eliciting (conditioned or unconditioned H-reflex) stimulus. Analysis of variance verified that the amplitude of the unconditioned H-reflex obtained at 1150 msec was reliably different (P < 0.001) from the other 5 intervals. This elevated value of the unconditioned H-reflex which occurred some 40 msec prior to the onset of the volitional response is in keeping with previously reported values (40-90 msec) of an increase in spinal excitability prior to response onsetl,ll,14,2L This sudden increase in the level of the unconditioned H-reflex represents an increase in the excitability of the motoneuronal pool in preparation for the forthcoming response. There are at least two known mechanisms which could be responsible for this modulation of the spinal centers. Kots 15 suggested that the spinal preparatory changes could be due to a facilitation of the agonist motoneuronal pool as a result of descending pyramidal tract input. A second mechanism whereby the Ia afferent terminals are released from a state of presynaptic inhibition could also lead to an increase in the amplitude of the unconditioned H-reflex 8,2~,24. The present data (400-1100 msec) provide some evidence of the occurrence of a state of inhibition (conceivably presynaptic) both during the foreperiod and the early stages of the RS-voluntary response interval. With the exception of the 1150 msec interval, the H-reflexes recorded at the remaining intervals, together with those of the probe H-reflex (4.78 mV), are reduced relative to the control condition (5.06 mV). Although these comparisons were not significant, they are consistent with the findings of Requin 23 and Requin et al. ~'5, and Gerilowsky and Tsekov 7, who found a strong depression of the test H-reflex recorded during the foreperiod. The importance of such a state of inhibition may be to protect the alpha motoneuron from Ia input due to possible postural changes ~2,25. However, the present data do not reflect the systematic depression which was found to occur as the W S - R S interval increasedT, 23,25. The mean amplitude of the subthreshold conditioned H-reflexes was reduced re-
567 lative to the independently elicited control (unconditioned) H-reflex. The extent of this depression is known to be a function of the interstimulus interval 26,3°. The interval used here (100 msec) corresponds to the period of 'late' or 'phase IV' facilitation which is usually seen in the H-reflex recovery curve superimposed upon the background of depressed test H-reflexeszl,zg,3°. This period of facilitation was originally thought to be mediated through a long-loop pathway involving the cerebellum 5,8°, but more recent evidence 16 implicates a pathway that also involves the motor cortex z7,35. The increase in amplitude of the conditioned H-reflex of 20.9 % (0.6 mV) between WS-ES intervals of 500 and 1100 msec was analyzed by an analysis of variance procedure. Although the difference between means was substantial, the resulting test statistic was not significant (P > 0.05). Thus little support could be found for the increased gain in supraspinal pathways during the RT foreperiod postulated by Hayes and Clarke 10 on the basis of observations of 'late reflex' activity. The 'late reflex' examined in that instance was considered to be the composite effect of long-loop facilitation coupled with the Ia afferent discharge that occurs during the relaxation phase of the H-reflex twitch contraction 4,2s. By employing a subthreshold conditioning stimulus, the present study effectively eliminated the possible contribution of the twitch-evoked sensory inputs. It would appear, therefore, that the facilitation of the 'late reflex' noted by Hayes and Clarke 10 was probably the result of an increased Ia afferent discharge; with only a small possibility of a weak preparatory increase in the supraspinal component. Previous attempts to identify alterations in the excitability of spinal and supraspinal reflex responses prior to movement have had similarly conflicting results. On the one hand, evidence obtained from studies involving torque motor perturbations has shown an initial facilitation followed by a powerful inhibition for the M1 (spinal) and a systematic facilitation of the M2 (spinal) reflex during the RT foreperiod~. Conversely, Mortimer et al. TM reported no change in the M1 and M2 amplitudes until 10 msec prior to the voluntary RT response. While the present data do not resolve this issue, they do indicate that there is no obvious preparatory modulation of the excitability of the pathways mediating the phase IV facilitation 21 during the RT foreperiod. This work was funded through Grant A0063 from the National Sciences and Engineering Research Council of Canada. The author wishes to express his thanks to Dr. Keith C. Hayes for his valuable advice and criticism throughout all phases of this project.
1 Blair-Thomas, C. A. and Luschei, E., Increases in reflex excitability of monkey masseter motoneurons before a jaw-bite reaction time response, J. Neurophysiol., 38 (1975) 981-989. 2 Bonnet, M. et Requin, J., Evolution des r6sponses musculaires/~ la surcharge pendant Ia p6riode pr6paratoire ~ un mouvement orient& InProc. intern. Congr., phys. Educ., Quebec, 1980, in press. 3 Brunia, C. H. M., Motor preparation recorded on the spinal level. In J. Requin (Ed.), Anticipation et Comportement, Editions du C.N.R.S., Paris, 1980, in press. 4 Burg, D., Szumski, A. J., Struppler, A. and Velho, F., Afferent and efferent activation of human muscle receptors involved in reflex and voluntary contraction, Exp. NeuroL, 41 (1973) 754-768.
568 5 Eccles, J. C., Long-loop reflexes from the spinal cord to the brain stem and cerebellum, Atti. Accad. med. lombarda, 21 (1966) 1-19. 6 Evarts, E. V. and Tanji, J., Reflex and intended responses in motor cortex pyramidal tract neurons of monkeys, J. Neurophysiol., 39 (1976) 1069-1080. 7 Gerilowsky, L. and Tsekov, T. S., Changes in the excitability of the segmental apparatus for reciprocal inhibition of agonists during a fixed waiting period, Agressologie, 16 (1975) 233-236. 8 Gottlieb, G. L., Agarwal, G. C. and Stark, L., [nteractions between voluntary and postural mechanisms of the human motor system, J. Neurophysiol., 33 (1970) 365 -381. 9 Gurfinkel, V. S. and Pal'tsev, Ye. I., Effect of the state of segmental apparatus of the spinal cord on the execution of a simple motor task, Biophysics, 10 (1965) 855-860. l0 Hayes, K. C. and Clarke, A. M., Facilitation of late reflexes during the preparatory period of voluntary movement, Brain Research, 153 (1978) 176-182. 11 Hammond, P. H., The influence of prior instruction to the subject on an apparently involuntary neuromuscular response, J. PhysioL (Lond.), 132 (1956) 17P 18P. 12 Hayes, K. C. and Sullivan, J., Tonic neck reflex influence on tendon and Hoffman reflexes in man, Electromyograph. clin. Neurophysiol., 16 (1976) 251--261. 13 Hugon, M., Methodology of the Hoffmann reflex in man. In J. E. Desmedt (Ed.), New Developments in Electromyography and Clinical Neurophysiology, Vol. 3, Karger, Basel, 1973, pp. 328-335. 14 Kots, Ya. M., Supraspinal control of the segmental centers of muscle antagonists in man - - 1. Reflex excitability of the motor neurons of muscle antagonists in the period of organization of voluntary movement, Biophysics, 14 (1969) 176-183. 15 Kots, Ya. M., The Organization of Voluntary Movement, Plenum Press, New York, 1977. 16 Lachat, J. M., R~egg, D. G. and Wiesendanger, M., Transcortical facilitation of the H-(monosynaptic)-reflex in monkeys, Experientia (Basel), 33 (1977) 781. 17 Michie, P. T., Clarke, A. M., Sinden, J. D. and Glue, L. C. T., Reaction time and spinal excitability in a simple reaction time task, Physiol. Behav., 16 (1976) 311-315. 18 Mortimer, J. A., Webster, D. A. and Dukich, T. A., Changes in short and long-loop reflexes before voluntary movements in man, Neurosci. Abstr., (1979). 19 Newsom Davis, J. and Sears, T. A., The proprioceptive reflex control of the intercostal muscles during their voluntary activation, J. Physiol. (Lond.), 209 (1975) 711-738. 20 Newton, R. A. and Price, D. C., Modulation of cortical and pyramidal tract induced motor responses by electrical stimulation of the basal ganglia, Brain Research, 85 (1975) 403-422. 21 Paillard, J., Functional organization of afferent innervation of muscle studied in man by monosynaptic testing, Amer. J. phys. Med., 38 (1959) 239-247. 22 Pierrot-Deseilligny, E. and Lacert, P., Amplitude and variability of monosynaptic reflexes prior to various voluntary movements in normal and spastic man. In J. E. Desmedt (Ed.), New Developments in Electromyography and Clinical Neurophysiology, Fol. 3, Karger, Basel, 1973, pp. 328-335. 23 Requin, J. Some data on neurophysiological processes involved in the preparatory motor activity to reaction time performance. In Acta Psychologica, Vol. 30, Attention and Performance, H, NorthHolland, Amsterdam, 1969, pp. 358-367. 24 Requin, J. and Paillard, J., Depression of spinal monosynaptic reflexes as a specific aspect of preparatory motor set in visual reaction time. In Visual Information Processing and Control of Motor Activity, Bulg. Acad. Sci., Sofia, 1970. 25 Requin, J., Bonnet, M. and Semjen, A., Is there specificity in the supraspinal control of motor structures during preparation? In S. Dornic (Ed.), Attention and Performance FL, Lawrence Erlbaum, Hillsdale, 1977. 26 Robinson, K. L., Mcllwain, J. S. and Hayes, K. C., Effects of H-reflex conditioning upon the contralateral alpha motoneuron pool, Electroenceph. clin. Neurophysiol., 37 (1978) 589-597. 27 Stein, R. B. and Bawa, P., Reflex responses of human soleus muscle to small perturbations, J. Neurophysiol., 39 (5) (1976) 1105-1116. 28 Szumski, A. J., Burg, D., Struppler, A. and Velho, F., Activity of muscle spindles during twitch and clonus in normal and spastic human subjects. Electroenceph. clin. NeurophysioL, 37 (1974) 589-597. 29 Tfiboftkovfi, H., Supraspinal influences on H-reflexes. In J. E. Desmedt (Ed.), New Developments in Electromyography and Clinical Neurophysiology, FoL 3, Karger, Basel, 1973, pp. 328-335. 30 Tfibo~ikov~., H. and Sax, D., Conditioning of H-reflex by a preceding subthreshold H-reflex stimulus, Brain, 92 (1969) 203-212. 31 Tanji, J. and Evarts, E. V., Anticipatory activity of motor cortex neurons in relation to direction of an intended movement, J. Neurophysiol., 39 (1976) 1062-1068.
569 32 Tanji, J. and Taniguchi, K., Does the supplementary motor area play a part in modifying motor cortex reflexes? J. Physiol. (Paris), 74 (1978) 317-318. 33 Tatton, W. G. and Lee, R. G., Evidence for abnormal long-loop reflexes in rigid Parkinsonian patients, Brain Research, 100 (1975) 671-676. 34 Wiesendanger, M., Why transcortical reflexes?, Canad. J. Neurol. Sci., 2 (1975) 295-301. 35 Wiesendanger, M., Comments on the problem of transcortical reflexes, J. PhysioL (Paris), 74 (1978) 325-330. 36 Yap, C. B., Spinal segmental and long-loop reflexes on spinal motoneurone excitability in spasticity and rigidity, Brain, 90 (1969) 887-896.
Brahl Research, 192 (1980) 564 5 6 9 ~) E l s e v i e r / N o r t h - H o l l a n d Biomedical Press
Conditioned H-reflexes prior to movement
S. J O H N S U L L I V A N
Department of K#zesiology, University of Waterloo, Waterloo, Ont. N2L 3G I (Canada) (Accepted F e b r u a r y 21st, 1980)
Key words: H-reflex - - c o n d i t i o n i n g - - late facilitation - - t u n i n g - - v o l u n t a r y response
The pattern of excitability of spinal reflexes prior to a voluntary response provides information on the organizational processes involved in the initiation of movementl,3,14,xT, 23,25. Although some disagreement exists regarding the pattern of excitation during the classic reaction time (RT) foreperiod 3,15,23,25, the finding of an increase in excitability beginning some 40-60 msec prior to the onset of voluntary movement has been well established1,11,15, 22. Studies dealing with this phenomenon have generally employed a constant foreperiod in order to increase the temporal certainty of the response signal and the demonstrated changes in spinal organization have been interpreted as evidence of preparatory 'set'. The establishment of such a preparatory 'set' is thought to involve both the spinal and supraspinal components of the stretch reflex6,1°,lL In addition, the preparatory 'set' has been shown to bring about an alteration in the firing pattern of pyramidal tract neurons6, 31. Hayes and Clarke 1° recently reported an increased occurrence and a decreased latency of the electrically elicited 'late reflex' recorded from the soleus muscle during a RT foreperiod prior to a visual stimulus. This finding was interpreted as being suggestive of an increase in the excitability of the supraspinal reflex pathway, and possibly related to the previously reported changes in cortical cell activity. In the present study an alternative method of investigating these preparatory changes in neural organization was employed. The technique of conditioning an Hreflex with a preceding subthreshold stimulus was used to determine whether or not there are any changes in the 'late facilitation' phase of the H-reflex recovery curve between interstimulus intervals of 75-250 msec. Such facilitation has previously been attributed, albeit tentatively, to long-loop reflex influences16,29,34-36. The experimental manipulations involved 11 healthy adults of both sexes who were tested while lying in a semi-reclined manner on a padded testing table with their backs supported. The testing posture, with the knee flexed, the right foot supported by a rigidly fixed foot plate, and the head stabilized and looking straight forward, ensured an optimum reflex responsO 2,~8. About 75 cm in front of the subject, at eye level, was a small light-emitting diode (3.5 mcd luminous intensity) which served as the reaction
565 signal (RS). The response to this brief (100 msec) signal was a rapid isometric plantar flexion of the right foot. A short (100 msec) auditory tone (WS) informed the subject of the forthcoming RS. This WS-RS interval was held constant at 1000 msec. The eliciting stimulus (ES) was delivered to the posterior tibial nerve in the region of the popliteal fossa of the subject's right leg through a 5 mm diameter surface electrode. The electrode was positioned in such a manner as to obtain an H-reflex without a concomitant direct muscle response. The anode consisted of a moist pad firmly secured over the patella. The H-reflex was elicited by a square wave pulse of 1 msec duration generated by a two-channel nerve stimulator (Grass, model $88). Each channel was connected in series to a stimulus isolation unit (Grass, model SIU5), and a single constant current device (Grass, model CCU1). Two bipolar (silver/silverchloride) electrodes placed 3 cm apart over the midline of the right soleus muscle recorded the evoked H-reflexes. The recorded muscle action potentials were amplified (Bicom differential amplifier, model 2122) and filtered (3 dB down at 10 and 1 k Hz) prior to being displayed on a Hewlett Packard storage oscilloscope (model 1201A). The peak-to-peak amplitude of the control H-reflexes and the conditioned test Hreflexes was recorded directly from the storage oscilloscope, as was the latency of the voluntary response. The unconditioned H-reflexes were elicited at 6 delay intervals during the foreperiod and prior to the voluntary response. These delays, expressed relative to WS, were at 400, 800, 950, 1000, 1100 and 1150 msec. The conditioned test H-reflexes were elicited at the following intervals subsequent to WS; 500, 900, 1050, 1100 msec. Each of these test H-reflexes was preceded 100 msec earlier by a subthreshold conditioning H-reflex stimulus 30. These 10 delay intervals, together with two probe H-reflex (an Hreflex elicited without an accompanying WS or RS) trials composed one cycle of trials. The administration of all trials was randomized within each cycle, with at least 15 sec separating adjacent trials. Immediately prior to each of the 4 experimental cycles, the intensity of the unconditioned H-reflex stimulus was adjusted to produce a response of 50 ~o of the maximum H-reflex amplitude. Another H-reflex control condition, where no response was required to RS and yet an H-reflex was elicited 800 msec following WS, was evaluated just prior to the first cycle of trials. The value of the conditioned H-reflex obtained prior to the first cycle of trials served as the control amplitude for the conditioned H-reflexes. In addition, volitional RT responses were performed in the WS-RS situation when the subject knew that no reflex would be elicited. The experimental session lasted 1.5 h. All subjects had previously undergone a short familiarization session. Shown in Fig. 1 are the peak-to-peak amplitudes for both the unconditioned Hreflexes and the conditioned H-reflexes obtained during the foreperiod and prior to the onset of the voluntary response. Also shown is the value of the control H-reflex (5.02 mV) obtained under the condition where no voluntary response was required to RS. The control value (2.87 mV) of the conditioned H-reflex is also shown. The latency to the onset of muscle activity corresponding to the voluntary response was 190.1 msec. The pattern of the unconditioned H-reflexes recorded between 400 and 1150 msec following WS was similar to that obtained by previous researchersg,11,14,17, 22.
566 8
--
A
> E v
6 ta.I
a
F-"-i rt
4
-
2
--
t
X b.l
._I b_ ,
-r
1
0
,
200
I 400
i
I 600
r
J
I
800
~
I
I000
1200
WS
RS WS -
ES
INTERVAL
(rnsec)
Fig. 1. Conditioned ( x ) and unconditioned (O) H-reflex amplitude during a visual RT foreperiod and prior to the initiation of the voluntary response. The vertical bars correspond to the S.E. of the mean of the reflex amplitude. The upper (-- -) line represents the value of the control H-reflex obtained in the condition where the subject was not required to respond to RS. The lower (--) line is the amplitude of the unconditioned H-reflex recorded prior to the start of the experiment. WS, auditory warning signal; RS, visual reaction signal, ES, eliciting (conditioned or unconditioned H-reflex) stimulus. Analysis of variance verified that the amplitude of the unconditioned H-reflex obtained at 1150 msec was reliably different (P < 0.001) from the other 5 intervals. This elevated value of the unconditioned H-reflex which occurred some 40 msec prior to the onset of the volitional response is in keeping with previously reported values (40-90 msec) of an increase in spinal excitability prior to response onsetl,ll,14,2L This sudden increase in the level of the unconditioned H-reflex represents an increase in the excitability of the motoneuronal pool in preparation for the forthcoming response. There are at least two known mechanisms which could be responsible for this modulation of the spinal centers. Kots 15 suggested that the spinal preparatory changes could be due to a facilitation of the agonist motoneuronal pool as a result of descending pyramidal tract input. A second mechanism whereby the Ia afferent terminals are released from a state of presynaptic inhibition could also lead to an increase in the amplitude of the unconditioned H-reflex 8,2~,24. The present data (400-1100 msec) provide some evidence of the occurrence of a state of inhibition (conceivably presynaptic) both during the foreperiod and the early stages of the RS-voluntary response interval. With the exception of the 1150 msec interval, the H-reflexes recorded at the remaining intervals, together with those of the probe H-reflex (4.78 mV), are reduced relative to the control condition (5.06 mV). Although these comparisons were not significant, they are consistent with the findings of Requin 23 and Requin et al. ~'5, and Gerilowsky and Tsekov 7, who found a strong depression of the test H-reflex recorded during the foreperiod. The importance of such a state of inhibition may be to protect the alpha motoneuron from Ia input due to possible postural changes ~2,25. However, the present data do not reflect the systematic depression which was found to occur as the W S - R S interval increasedT, 23,25. The mean amplitude of the subthreshold conditioned H-reflexes was reduced re-
567 lative to the independently elicited control (unconditioned) H-reflex. The extent of this depression is known to be a function of the interstimulus interval 26,3°. The interval used here (100 msec) corresponds to the period of 'late' or 'phase IV' facilitation which is usually seen in the H-reflex recovery curve superimposed upon the background of depressed test H-reflexeszl,zg,3°. This period of facilitation was originally thought to be mediated through a long-loop pathway involving the cerebellum 5,8°, but more recent evidence 16 implicates a pathway that also involves the motor cortex z7,35. The increase in amplitude of the conditioned H-reflex of 20.9 % (0.6 mV) between WS-ES intervals of 500 and 1100 msec was analyzed by an analysis of variance procedure. Although the difference between means was substantial, the resulting test statistic was not significant (P > 0.05). Thus little support could be found for the increased gain in supraspinal pathways during the RT foreperiod postulated by Hayes and Clarke 10 on the basis of observations of 'late reflex' activity. The 'late reflex' examined in that instance was considered to be the composite effect of long-loop facilitation coupled with the Ia afferent discharge that occurs during the relaxation phase of the H-reflex twitch contraction 4,2s. By employing a subthreshold conditioning stimulus, the present study effectively eliminated the possible contribution of the twitch-evoked sensory inputs. It would appear, therefore, that the facilitation of the 'late reflex' noted by Hayes and Clarke 10 was probably the result of an increased Ia afferent discharge; with only a small possibility of a weak preparatory increase in the supraspinal component. Previous attempts to identify alterations in the excitability of spinal and supraspinal reflex responses prior to movement have had similarly conflicting results. On the one hand, evidence obtained from studies involving torque motor perturbations has shown an initial facilitation followed by a powerful inhibition for the M1 (spinal) and a systematic facilitation of the M2 (spinal) reflex during the RT foreperiod~. Conversely, Mortimer et al. TM reported no change in the M1 and M2 amplitudes until 10 msec prior to the voluntary RT response. While the present data do not resolve this issue, they do indicate that there is no obvious preparatory modulation of the excitability of the pathways mediating the phase IV facilitation 21 during the RT foreperiod. This work was funded through Grant A0063 from the National Sciences and Engineering Research Council of Canada. The author wishes to express his thanks to Dr. Keith C. Hayes for his valuable advice and criticism throughout all phases of this project.
1 Blair-Thomas, C. A. and Luschei, E., Increases in reflex excitability of monkey masseter motoneurons before a jaw-bite reaction time response, J. Neurophysiol., 38 (1975) 981-989. 2 Bonnet, M. et Requin, J., Evolution des r6sponses musculaires/~ la surcharge pendant Ia p6riode pr6paratoire ~ un mouvement orient& InProc. intern. Congr., phys. Educ., Quebec, 1980, in press. 3 Brunia, C. H. M., Motor preparation recorded on the spinal level. In J. Requin (Ed.), Anticipation et Comportement, Editions du C.N.R.S., Paris, 1980, in press. 4 Burg, D., Szumski, A. J., Struppler, A. and Velho, F., Afferent and efferent activation of human muscle receptors involved in reflex and voluntary contraction, Exp. NeuroL, 41 (1973) 754-768.
568 5 Eccles, J. C., Long-loop reflexes from the spinal cord to the brain stem and cerebellum, Atti. Accad. med. lombarda, 21 (1966) 1-19. 6 Evarts, E. V. and Tanji, J., Reflex and intended responses in motor cortex pyramidal tract neurons of monkeys, J. Neurophysiol., 39 (1976) 1069-1080. 7 Gerilowsky, L. and Tsekov, T. S., Changes in the excitability of the segmental apparatus for reciprocal inhibition of agonists during a fixed waiting period, Agressologie, 16 (1975) 233-236. 8 Gottlieb, G. L., Agarwal, G. C. and Stark, L., [nteractions between voluntary and postural mechanisms of the human motor system, J. Neurophysiol., 33 (1970) 365 -381. 9 Gurfinkel, V. S. and Pal'tsev, Ye. I., Effect of the state of segmental apparatus of the spinal cord on the execution of a simple motor task, Biophysics, 10 (1965) 855-860. l0 Hayes, K. C. and Clarke, A. M., Facilitation of late reflexes during the preparatory period of voluntary movement, Brain Research, 153 (1978) 176-182. 11 Hammond, P. H., The influence of prior instruction to the subject on an apparently involuntary neuromuscular response, J. PhysioL (Lond.), 132 (1956) 17P 18P. 12 Hayes, K. C. and Sullivan, J., Tonic neck reflex influence on tendon and Hoffman reflexes in man, Electromyograph. clin. Neurophysiol., 16 (1976) 251--261. 13 Hugon, M., Methodology of the Hoffmann reflex in man. In J. E. Desmedt (Ed.), New Developments in Electromyography and Clinical Neurophysiology, Vol. 3, Karger, Basel, 1973, pp. 328-335. 14 Kots, Ya. M., Supraspinal control of the segmental centers of muscle antagonists in man - - 1. Reflex excitability of the motor neurons of muscle antagonists in the period of organization of voluntary movement, Biophysics, 14 (1969) 176-183. 15 Kots, Ya. M., The Organization of Voluntary Movement, Plenum Press, New York, 1977. 16 Lachat, J. M., R~egg, D. G. and Wiesendanger, M., Transcortical facilitation of the H-(monosynaptic)-reflex in monkeys, Experientia (Basel), 33 (1977) 781. 17 Michie, P. T., Clarke, A. M., Sinden, J. D. and Glue, L. C. T., Reaction time and spinal excitability in a simple reaction time task, Physiol. Behav., 16 (1976) 311-315. 18 Mortimer, J. A., Webster, D. A. and Dukich, T. A., Changes in short and long-loop reflexes before voluntary movements in man, Neurosci. Abstr., (1979). 19 Newsom Davis, J. and Sears, T. A., The proprioceptive reflex control of the intercostal muscles during their voluntary activation, J. Physiol. (Lond.), 209 (1975) 711-738. 20 Newton, R. A. and Price, D. C., Modulation of cortical and pyramidal tract induced motor responses by electrical stimulation of the basal ganglia, Brain Research, 85 (1975) 403-422. 21 Paillard, J., Functional organization of afferent innervation of muscle studied in man by monosynaptic testing, Amer. J. phys. Med., 38 (1959) 239-247. 22 Pierrot-Deseilligny, E. and Lacert, P., Amplitude and variability of monosynaptic reflexes prior to various voluntary movements in normal and spastic man. In J. E. Desmedt (Ed.), New Developments in Electromyography and Clinical Neurophysiology, Fol. 3, Karger, Basel, 1973, pp. 328-335. 23 Requin, J. Some data on neurophysiological processes involved in the preparatory motor activity to reaction time performance. In Acta Psychologica, Vol. 30, Attention and Performance, H, NorthHolland, Amsterdam, 1969, pp. 358-367. 24 Requin, J. and Paillard, J., Depression of spinal monosynaptic reflexes as a specific aspect of preparatory motor set in visual reaction time. In Visual Information Processing and Control of Motor Activity, Bulg. Acad. Sci., Sofia, 1970. 25 Requin, J., Bonnet, M. and Semjen, A., Is there specificity in the supraspinal control of motor structures during preparation? In S. Dornic (Ed.), Attention and Performance FL, Lawrence Erlbaum, Hillsdale, 1977. 26 Robinson, K. L., Mcllwain, J. S. and Hayes, K. C., Effects of H-reflex conditioning upon the contralateral alpha motoneuron pool, Electroenceph. clin. Neurophysiol., 37 (1978) 589-597. 27 Stein, R. B. and Bawa, P., Reflex responses of human soleus muscle to small perturbations, J. Neurophysiol., 39 (5) (1976) 1105-1116. 28 Szumski, A. J., Burg, D., Struppler, A. and Velho, F., Activity of muscle spindles during twitch and clonus in normal and spastic human subjects. Electroenceph. clin. NeurophysioL, 37 (1974) 589-597. 29 Tfiboftkovfi, H., Supraspinal influences on H-reflexes. In J. E. Desmedt (Ed.), New Developments in Electromyography and Clinical Neurophysiology, FoL 3, Karger, Basel, 1973, pp. 328-335. 30 Tfibo~ikov~., H. and Sax, D., Conditioning of H-reflex by a preceding subthreshold H-reflex stimulus, Brain, 92 (1969) 203-212. 31 Tanji, J. and Evarts, E. V., Anticipatory activity of motor cortex neurons in relation to direction of an intended movement, J. Neurophysiol., 39 (1976) 1062-1068.
569 32 Tanji, J. and Taniguchi, K., Does the supplementary motor area play a part in modifying motor cortex reflexes? J. Physiol. (Paris), 74 (1978) 317-318. 33 Tatton, W. G. and Lee, R. G., Evidence for abnormal long-loop reflexes in rigid Parkinsonian patients, Brain Research, 100 (1975) 671-676. 34 Wiesendanger, M., Why transcortical reflexes?, Canad. J. Neurol. Sci., 2 (1975) 295-301. 35 Wiesendanger, M., Comments on the problem of transcortical reflexes, J. PhysioL (Paris), 74 (1978) 325-330. 36 Yap, C. B., Spinal segmental and long-loop reflexes on spinal motoneurone excitability in spasticity and rigidity, Brain, 90 (1969) 887-896.