Neuroscience Letters, 116 (1990) 118---122 Elsevier Scientific Publishers Ireland Ltd.
118
NSL 07053
Interlimb coordination of stance in children: divergent modulation of spinal reflex responses and cerebral evoked potentials in terms of age W. Berger, G.A. Horstmann and V. Dietz Department of Clinical Neurology and Neurophysiology, University of Freiburg, Freiburg ( F.R.G.) (Received 19 January 1990; Revised version received 17 April 1990; Accepted 20 April 1990) Key words'. Stance perturbation; Leg muscle EMG response; Cerebral potential; Development; Children EMG responses in the gastrocnemius (GM) and tibialis anterior muscles (TA) of both legs together with cerebral evoked potentials (CP), were recorded following perturbations of stance on a treadmill with split belts, in two age groups of children. Unilateral displacements were followed by ipsilateral short latency and bilateral long latency EMG responses. The CP was similar in both tasks. When displacements were simultaneously induced in opposite directions, a significant reduction in the long latency components of EMG responses occurred, while the amplitude of the CP was maximal in this condition. In the older children the CP and long latency EMG responses were larger and the short latency reflex potentials smaller in all conditions compared to the younger children. It is concluded that (1) CP and EMG responses reflect a divergent modulation of a given somatosensory input; (2) developmental changes are reflected in alterations in the amplitude of CP and EMG responses; (3) there is no evidence of transcortically mediated muscle responses.
M e c h a n i c a l l y e v o k e d cerebral p o t e n t i a l s (CP) have p r o v e d useful in the study o f afferent p a t h w a y s to s u p r a s p i n a l centres a n d their r e l a t i o n s h i p to the c o r r e s p o n d i n g c o m p e n s a t o r y reflex E M G responses. D u r i n g gait, evidence has been p u t f o r w a r d for an inhibition o f g r o u p I afferents on b o t h the spinal a n d s u p r a s p i n a l level [9, 10]. This inhibition is not, at first, a p p a r e n t in small children aged I - 2 years, who u n d e r g o a m a t u r a t i o n a l process involving progressive suppression o f g r o u p I afferents a n d a facilitation o f g r o u p II m e d i a t e d reflexes, with parallel changes in the C P [3]. The CP, t o g e t h e r with corrective E M G reactions to p e r t u r b a t i o n , differed, from those following passive m o v e m e n t s m o r e usually investigated [6, 13, 14] in t h a t the a m p l i t u d e o f the first negative wave was larger. In a recent study on m o n k e y s it was s h o w n that changes in the a m p l i t u d e o f an a p p l i e d stretch h a d an unequal effect on the amplitudes o f the spinal stretch reflex responses a n d cortical e v o k e d p o t e n t i a l s [17]. This suggests a differential m o d u l a t i o n o f afferent i n p u t in reflex responses and CP.
Correspondence: W. Berger, Neurologische Klinik, Hansastr. 9, D-7800 Freibrug, F.R.G. 0304-3940/90/$ 03.50 (t' 1990 Elsevier Scientific Publishers Ireland Ltd.
119
The aim of this study was to elucidate similarities and differences between the CP and E M G response to the same afferent volley induced by perturbation of stance on a treadmill with split belts and to provide information about the gating of a given sensory input in spinal and supraspinal motor centres. Two groups of healthy children, one aged between 3.5 and 5.5 years (n = 19; mean 4.7 years; mean height 111.4 cm) and the other between 5.5 and 11 years (n= 12; mean 8.1 years; mean height 124.5 cm), were examined on a stationary treadmill with split belts (Woodway). Informed consent was obtained from the children and/or their parents. The displacements were applied randomly as follows: (1) unilateral displacement forwards; (2) unilateral displacement backwards; (3) bilateral opposed displacement. The impulse produced a ramp change in velocity from 0 to 8.3 km/h within 25 ms. The amount of horizontal displacement was 5 cm. The technique for recording and analysing the responses of leg muscles and cerebral potentials have been fully described in earlier papers [3, 7, 10]. Briefly, the E M G of the medial head of gastrocnemius (GM) and of the tibialis anterior (TA) muscle was recorded using surface electrodes (sampling rate 500 Hz). CP were recorded in a bipolar montage over the precentral region (left: CI-C3 and right: C2-C4). The electrooculogram (EOG) was recorded from an electrode at the lower canthus of the right eye, the reference electrode being placed above the nasion, in order to exclude artifacts from eye movements. Ankle joint movements were indicated by a goniometer fixed at the lateral aspect of the foot and the lower leg. The onset of the tachometer impulse was used as the trigger for data sampling (n -- 20 for each parameter). Statistical analysis of the data was performed using the SPSS-PC package. Differ-
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Fig. 1. Mean values and S.D. of the averaged (n =20) CP and E M G responses following a unilateral backward perturbation: right leg (A), a unilateral forward perturbation: left leg (B), and an opposing bilateral displacement simultaneously applied: right leg backwards, left leg forwards (C) in the same groups of children; a: 12 children, 5.5-11 years of age, 115-138 cm of height; b: 19 children, 3.5-5.5 years of age, 103-116 cm of height. The arrow indicates the onset of the treadmill impulse.
120
ences between the type of impulses and between the age groups were assessed, respectively, by paired and grouped Student's t-test. In the older children unilateral backward displacement (Fig. 1Aa) caused a bilateral GM response on both sides, preceded on the displaced side by an early reflex response the latency of which is compatible with a monosynaptic linkage. The latency of the later bilateral GM responses was about 60 ms. Unilateral forward displacement (Fig. I Ba) elicited a strong bilateral TA response on both sides. Bilateral opposed displacement, simultaneously applied (Fig. 1Ca) elicited reflex potentials with short latencies on both sides, larger ones in the backwards perturbed right leg than in the forwards perturbed left leg in the GM, and again smaller ones in the TA (right leg). Later responses were drastically reduced (rear leg) or barely detectable (front leg) relative to the unilateral displacement conditions. Onset latency of the CP was constant (36 + 2.7 ms) for all three modes of perturbation. The amplitude of the large, negative and subsequent positive peaks, however, differed depending on the mode of perturbation: the largest negative peaks were elicited following opposed perturbations (mean 51 + 2.0 ~V, Fig. 1Ca). Unilateral backward perturbations elicited a CP with an amplitude of 43_+1.3/IV contralaterally, and 35_+1.4/IV ipsilaterally (Fig. I Aa). This difference was significant (P= 0.008). Unilateral forward perturbation elicited identical bilateral potentials (36.5_+ 1.4/~V, Fig. I Ba). The difference between the latter CP amplitudes and those elicited by simultaneously opposed perturbation was significant (P=0.032). Eye movements (EOG, not displayed here) were clearly discrete from the CP, as has previously been demonstrated [8, 9]. The younger children showed both similarities with and differences to the older age group: the short latency stretch reflexes were larger in the backwards displaced leg (Fig. lAb, Cb) and the later responses were smaller in both the displaced and the non-displaced leg (Fig'. lAb, Bb) during the unilateral backward displacements the gastrocnemius response was negligible in the non-displaced side (Fig. lAb)). Opposed displacements caused a reduction in the later part of the EMG response, as in the older children, while the short latency stretch reflexes were exaggerated (Fig. 1Cb). In this younger age group the CP were generally of smaller amplitude. For simultaneously delivered perturbations, this difference was significant (P = 0.043). The differences between the CP in the 3 conditions were similar to those of the older children: opposing simultaneous displacement produced the largest amplitude (40+2.1 /~V, Fig. ICb). Unilateral backwards directed displacement induced CP with smaller amplitude on both sides (31 _+1.4 and 30_+ 1.0/tV, Fig. 1Ab). Unilateral forward perturbation induced CP of 36-1.8/~V on the contralateral side and 28 + 1.2/tV on the ipsilateral side (Fig. 1Bb), the difference being significant (P= 0.038). The onset latencies were the same in all three conditions (33 _+4.0 ms) after perturbation. The main findings in this new approach with split belts were the bilateral long latency EMG responses following unilateral displacement and the divergent behavior of EMG reflexes and CP following simultaneous displacement in opposite direction. Unilateral displacements were followed by long latency bilateral EMG responses,
121
with short latency reflexes being present only on the displaced side. Bilateral opposed displacements caused a significant reduction in the late EMG responses, whereas reflexes with short latency were preserved. Because of the short latencies involved, the underlying coordination is likely to occur on a spinal level. During unilateral displacement the agonistic muscle of the contralateral side was activated; consequently, reciprocal inhibition occurs between the antagonistic leg muscles during opposing perturbations of both legs [8]. This leads to a reduction in the amplitudes of the long latency response, which would seem to be functionally meaningful, as the body's centre of gravity in this condition falls between the two legs [8]. There is, at present, some controversy about the sources and cortical representation of evoked potentials from the leg [6] and differences have been demonstrated between children and adults [7]. Accordingly, latencies, shapes and amplitudes of the CP investigated in this study were compared with each other only between the different modes of perturbation. They most probably reflect the incoming signals from cutaneous, muscle and joint receptors activated during perturbation in free standing. Nevertheless, the components of the CP show great similarity to those reported for passive dorsiflexion by Starr et al. [14], who suggested, that, in this case, mechanoreceptors were the most probable source for the CP. The main finding concerning CP modulation was that for both age groups the CP amplitudes were largest for bilateral opposed perturbations. This differential behavior of CP with respect to EMG responses indicates a separate processing and gating of a given afferent input to spinal interneuronal centres for the generation of the appropriate compensatory response and the representation of this input to supraspinal centres. Such a gating of afferent information is discussed in animal studies at several sites, including the cuneate nucleus [10, 14], the thalamus [15] and the somatosensory cortex [3]. The different and partly independent processing of afferent and efferent signals is also expressed by the fact that the early large peaks in the cerebral responses were not accompanied by corresponding later EMG responses, which would be expected in the case of a transcortical loop. The main developmentalchanges observed in the EMG responses - reduced short latency responses of presumably monosynaptic origin and increased polysynaptic responses were only partially reflected in the CP, which were generally larger in the older group. Larger CP amplitudes were induced in the older children by backward (stretch of GM), in the younger children by forward (stretch of TA) perturbation on the contralateral side. This finding may reflect the changing significance of flexor and extensor activation during the development: it might represent a differential sensitivity of spinal and supraspinal afferent pathways at different ages. Alternatively, it could reflect the differences seen in the leg muscle activation, with a predominance of TA activity in early childhood [2]. These changes were not very impressive, most probably because of the fact that only children from an age of 3.5 years on were taken for this study, whereas the most dramatic developmental changes in motor control take place earlier [1, 2, 4].
122 W e t h a n k M r s . U . M611inger a n d M r s . U . R 6 m m e l t
for technical assistance and
D r . S. F e l l o w s f o r s c r u t i n i z i n g t h e E n g l i s h text. This work was supported by the Deutsche Forschungsgemeinschaft. I Berger, W., Altenmiiller, E. and Dietz, V., Normal and impaired development of children's gait, Human Neurobiol., 3 (1984) 153-170. 2 Berger, W., Quintern, J. and Dietz, V., Stance and gait perturbations in children: Developmental aspects of compensatory mechanisms, Electroenceph. clin. Neurophysiol., 61 (1985) 385 395. 3 Berger, W., Quintern, J. and Dietz, V., Afferent and efferent control of stance and gait: Developmental changes in children. Electroenceph. Clin, Neurophysiol., 66 (1987) 244252. 4 Brandt, Th., Wenzel, D. and Dichgans, J., Die Entwicklung der visuellen Stabilisation des aufrechten Standes beim Kind: ein Reifezeichen in der Kinderneurologie, Arch. Psychat. Nervenkr., 223 (1976) I 13. 5 Chapin, J.K. and Woodward, D.J., Modulation of sensory responsiveness of single somatosensory cortical cells during movement and arousal behaviours, Exp. Neurol., 72 (1981) 164 178. 6 Cohen, L.G., Starr, A. and Pratt, H., Cerebral somatosensory potentials evoked by muscle stretch, cutaneous taps and electrical stimulation of peripheral nerves in the lower limbs in man, Brain, 108 (1985) 103- I21. 7 Desmedt, J.E. and Bourguet, M., Color imaging of parietal and frontal somatosensory potential fields evoked by stimulation of median or posterior tibial nerve in man, Electroencephagr. clin. Nenrophysiol., 62 (1985) 1 17. 8 Dietz, V., Horstmann, G.A. and Berger, W., Perturbations of human posture; influence of impulse modality on EMG responses and cerebral evoked potentials, J. Mot. Behav., 21 (1989) 357-372. 9 Dietz, V., Quintern, J., Berger, W. and Schenck, E., Cerebral potentials and leg muscle e.m.g, responses associated with stance pertubation. Exp. Brain Res., 57 (1985) 348-354. 10 Dietz, V., Quintern, J. and Berger, W., Afferent control of human stance and gait: Evidence for blocking of group I afferents during gait, Exp. Brain Res., 61 (1985) 153 163. l 1 Dietz, V., Quintern, J. and Sillem, M., Stumbling reactions in man: Significance of proprioceptive and pre-programmed mechanisms, J. Physiol., 386 (1987) 149--163. 12 Ghez, C. and Pisa, M., Inhibition of afferent transmission in cuneate nucleus during movement in the cat, Brain Res., 40 (1972) 145-151. 13 Rushton, D.N., Rothwell, J.C. and Craggs, M.D., Gating of somatosensory evoked potentials during different kinds of movement in man, Brain, 104 (1981) 465-491. 14 Start, A., McKeon, B., Skuse, N. and Burke, D., Cerebral potentials evoked by muscle stretch in man, Brain, 104 (1981) 149-166. 15 Towe, A. and Jabbur, S.J., Cortical inhibition of neurons in dorsal column nuclei of cat, J. Neurophysiol., 24 (1961) 488-498. 16 Ysumoto, T., Nakamura, S. and Iwama, K., Pyramidal tract control over cutaneous and kinesthetic sensory transmission in the cat thalamus, Exp. Brain Res., 22 (1975) 291 294. 17 Wolpaw, J.R. and Dowman, R., Operant conditioning of primate spinal reflexes: effect on cortical SEPs, Electroencephalogr. clin. Neurophysiol., 69 (1988) 398-401.
118
NSL 07053
Interlimb coordination of stance in children: divergent modulation of spinal reflex responses and cerebral evoked potentials in terms of age W. Berger, G.A. Horstmann and V. Dietz Department of Clinical Neurology and Neurophysiology, University of Freiburg, Freiburg ( F.R.G.) (Received 19 January 1990; Revised version received 17 April 1990; Accepted 20 April 1990) Key words'. Stance perturbation; Leg muscle EMG response; Cerebral potential; Development; Children EMG responses in the gastrocnemius (GM) and tibialis anterior muscles (TA) of both legs together with cerebral evoked potentials (CP), were recorded following perturbations of stance on a treadmill with split belts, in two age groups of children. Unilateral displacements were followed by ipsilateral short latency and bilateral long latency EMG responses. The CP was similar in both tasks. When displacements were simultaneously induced in opposite directions, a significant reduction in the long latency components of EMG responses occurred, while the amplitude of the CP was maximal in this condition. In the older children the CP and long latency EMG responses were larger and the short latency reflex potentials smaller in all conditions compared to the younger children. It is concluded that (1) CP and EMG responses reflect a divergent modulation of a given somatosensory input; (2) developmental changes are reflected in alterations in the amplitude of CP and EMG responses; (3) there is no evidence of transcortically mediated muscle responses.
M e c h a n i c a l l y e v o k e d cerebral p o t e n t i a l s (CP) have p r o v e d useful in the study o f afferent p a t h w a y s to s u p r a s p i n a l centres a n d their r e l a t i o n s h i p to the c o r r e s p o n d i n g c o m p e n s a t o r y reflex E M G responses. D u r i n g gait, evidence has been p u t f o r w a r d for an inhibition o f g r o u p I afferents on b o t h the spinal a n d s u p r a s p i n a l level [9, 10]. This inhibition is not, at first, a p p a r e n t in small children aged I - 2 years, who u n d e r g o a m a t u r a t i o n a l process involving progressive suppression o f g r o u p I afferents a n d a facilitation o f g r o u p II m e d i a t e d reflexes, with parallel changes in the C P [3]. The CP, t o g e t h e r with corrective E M G reactions to p e r t u r b a t i o n , differed, from those following passive m o v e m e n t s m o r e usually investigated [6, 13, 14] in t h a t the a m p l i t u d e o f the first negative wave was larger. In a recent study on m o n k e y s it was s h o w n that changes in the a m p l i t u d e o f an a p p l i e d stretch h a d an unequal effect on the amplitudes o f the spinal stretch reflex responses a n d cortical e v o k e d p o t e n t i a l s [17]. This suggests a differential m o d u l a t i o n o f afferent i n p u t in reflex responses and CP.
Correspondence: W. Berger, Neurologische Klinik, Hansastr. 9, D-7800 Freibrug, F.R.G. 0304-3940/90/$ 03.50 (t' 1990 Elsevier Scientific Publishers Ireland Ltd.
119
The aim of this study was to elucidate similarities and differences between the CP and E M G response to the same afferent volley induced by perturbation of stance on a treadmill with split belts and to provide information about the gating of a given sensory input in spinal and supraspinal motor centres. Two groups of healthy children, one aged between 3.5 and 5.5 years (n = 19; mean 4.7 years; mean height 111.4 cm) and the other between 5.5 and 11 years (n= 12; mean 8.1 years; mean height 124.5 cm), were examined on a stationary treadmill with split belts (Woodway). Informed consent was obtained from the children and/or their parents. The displacements were applied randomly as follows: (1) unilateral displacement forwards; (2) unilateral displacement backwards; (3) bilateral opposed displacement. The impulse produced a ramp change in velocity from 0 to 8.3 km/h within 25 ms. The amount of horizontal displacement was 5 cm. The technique for recording and analysing the responses of leg muscles and cerebral potentials have been fully described in earlier papers [3, 7, 10]. Briefly, the E M G of the medial head of gastrocnemius (GM) and of the tibialis anterior (TA) muscle was recorded using surface electrodes (sampling rate 500 Hz). CP were recorded in a bipolar montage over the precentral region (left: CI-C3 and right: C2-C4). The electrooculogram (EOG) was recorded from an electrode at the lower canthus of the right eye, the reference electrode being placed above the nasion, in order to exclude artifacts from eye movements. Ankle joint movements were indicated by a goniometer fixed at the lateral aspect of the foot and the lower leg. The onset of the tachometer impulse was used as the trigger for data sampling (n -- 20 for each parameter). Statistical analysis of the data was performed using the SPSS-PC package. Differ-
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Fig. 1. Mean values and S.D. of the averaged (n =20) CP and E M G responses following a unilateral backward perturbation: right leg (A), a unilateral forward perturbation: left leg (B), and an opposing bilateral displacement simultaneously applied: right leg backwards, left leg forwards (C) in the same groups of children; a: 12 children, 5.5-11 years of age, 115-138 cm of height; b: 19 children, 3.5-5.5 years of age, 103-116 cm of height. The arrow indicates the onset of the treadmill impulse.
120
ences between the type of impulses and between the age groups were assessed, respectively, by paired and grouped Student's t-test. In the older children unilateral backward displacement (Fig. 1Aa) caused a bilateral GM response on both sides, preceded on the displaced side by an early reflex response the latency of which is compatible with a monosynaptic linkage. The latency of the later bilateral GM responses was about 60 ms. Unilateral forward displacement (Fig. I Ba) elicited a strong bilateral TA response on both sides. Bilateral opposed displacement, simultaneously applied (Fig. 1Ca) elicited reflex potentials with short latencies on both sides, larger ones in the backwards perturbed right leg than in the forwards perturbed left leg in the GM, and again smaller ones in the TA (right leg). Later responses were drastically reduced (rear leg) or barely detectable (front leg) relative to the unilateral displacement conditions. Onset latency of the CP was constant (36 + 2.7 ms) for all three modes of perturbation. The amplitude of the large, negative and subsequent positive peaks, however, differed depending on the mode of perturbation: the largest negative peaks were elicited following opposed perturbations (mean 51 + 2.0 ~V, Fig. 1Ca). Unilateral backward perturbations elicited a CP with an amplitude of 43_+1.3/IV contralaterally, and 35_+1.4/IV ipsilaterally (Fig. I Aa). This difference was significant (P= 0.008). Unilateral forward perturbation elicited identical bilateral potentials (36.5_+ 1.4/~V, Fig. I Ba). The difference between the latter CP amplitudes and those elicited by simultaneously opposed perturbation was significant (P=0.032). Eye movements (EOG, not displayed here) were clearly discrete from the CP, as has previously been demonstrated [8, 9]. The younger children showed both similarities with and differences to the older age group: the short latency stretch reflexes were larger in the backwards displaced leg (Fig. lAb, Cb) and the later responses were smaller in both the displaced and the non-displaced leg (Fig'. lAb, Bb) during the unilateral backward displacements the gastrocnemius response was negligible in the non-displaced side (Fig. lAb)). Opposed displacements caused a reduction in the later part of the EMG response, as in the older children, while the short latency stretch reflexes were exaggerated (Fig. 1Cb). In this younger age group the CP were generally of smaller amplitude. For simultaneously delivered perturbations, this difference was significant (P = 0.043). The differences between the CP in the 3 conditions were similar to those of the older children: opposing simultaneous displacement produced the largest amplitude (40+2.1 /~V, Fig. ICb). Unilateral backwards directed displacement induced CP with smaller amplitude on both sides (31 _+1.4 and 30_+ 1.0/tV, Fig. 1Ab). Unilateral forward perturbation induced CP of 36-1.8/~V on the contralateral side and 28 + 1.2/tV on the ipsilateral side (Fig. 1Bb), the difference being significant (P= 0.038). The onset latencies were the same in all three conditions (33 _+4.0 ms) after perturbation. The main findings in this new approach with split belts were the bilateral long latency EMG responses following unilateral displacement and the divergent behavior of EMG reflexes and CP following simultaneous displacement in opposite direction. Unilateral displacements were followed by long latency bilateral EMG responses,
121
with short latency reflexes being present only on the displaced side. Bilateral opposed displacements caused a significant reduction in the late EMG responses, whereas reflexes with short latency were preserved. Because of the short latencies involved, the underlying coordination is likely to occur on a spinal level. During unilateral displacement the agonistic muscle of the contralateral side was activated; consequently, reciprocal inhibition occurs between the antagonistic leg muscles during opposing perturbations of both legs [8]. This leads to a reduction in the amplitudes of the long latency response, which would seem to be functionally meaningful, as the body's centre of gravity in this condition falls between the two legs [8]. There is, at present, some controversy about the sources and cortical representation of evoked potentials from the leg [6] and differences have been demonstrated between children and adults [7]. Accordingly, latencies, shapes and amplitudes of the CP investigated in this study were compared with each other only between the different modes of perturbation. They most probably reflect the incoming signals from cutaneous, muscle and joint receptors activated during perturbation in free standing. Nevertheless, the components of the CP show great similarity to those reported for passive dorsiflexion by Starr et al. [14], who suggested, that, in this case, mechanoreceptors were the most probable source for the CP. The main finding concerning CP modulation was that for both age groups the CP amplitudes were largest for bilateral opposed perturbations. This differential behavior of CP with respect to EMG responses indicates a separate processing and gating of a given afferent input to spinal interneuronal centres for the generation of the appropriate compensatory response and the representation of this input to supraspinal centres. Such a gating of afferent information is discussed in animal studies at several sites, including the cuneate nucleus [10, 14], the thalamus [15] and the somatosensory cortex [3]. The different and partly independent processing of afferent and efferent signals is also expressed by the fact that the early large peaks in the cerebral responses were not accompanied by corresponding later EMG responses, which would be expected in the case of a transcortical loop. The main developmentalchanges observed in the EMG responses - reduced short latency responses of presumably monosynaptic origin and increased polysynaptic responses were only partially reflected in the CP, which were generally larger in the older group. Larger CP amplitudes were induced in the older children by backward (stretch of GM), in the younger children by forward (stretch of TA) perturbation on the contralateral side. This finding may reflect the changing significance of flexor and extensor activation during the development: it might represent a differential sensitivity of spinal and supraspinal afferent pathways at different ages. Alternatively, it could reflect the differences seen in the leg muscle activation, with a predominance of TA activity in early childhood [2]. These changes were not very impressive, most probably because of the fact that only children from an age of 3.5 years on were taken for this study, whereas the most dramatic developmental changes in motor control take place earlier [1, 2, 4].
122 W e t h a n k M r s . U . M611inger a n d M r s . U . R 6 m m e l t
for technical assistance and
D r . S. F e l l o w s f o r s c r u t i n i z i n g t h e E n g l i s h text. This work was supported by the Deutsche Forschungsgemeinschaft. I Berger, W., Altenmiiller, E. and Dietz, V., Normal and impaired development of children's gait, Human Neurobiol., 3 (1984) 153-170. 2 Berger, W., Quintern, J. and Dietz, V., Stance and gait perturbations in children: Developmental aspects of compensatory mechanisms, Electroenceph. clin. Neurophysiol., 61 (1985) 385 395. 3 Berger, W., Quintern, J. and Dietz, V., Afferent and efferent control of stance and gait: Developmental changes in children. Electroenceph. Clin, Neurophysiol., 66 (1987) 244252. 4 Brandt, Th., Wenzel, D. and Dichgans, J., Die Entwicklung der visuellen Stabilisation des aufrechten Standes beim Kind: ein Reifezeichen in der Kinderneurologie, Arch. Psychat. Nervenkr., 223 (1976) I 13. 5 Chapin, J.K. and Woodward, D.J., Modulation of sensory responsiveness of single somatosensory cortical cells during movement and arousal behaviours, Exp. Neurol., 72 (1981) 164 178. 6 Cohen, L.G., Starr, A. and Pratt, H., Cerebral somatosensory potentials evoked by muscle stretch, cutaneous taps and electrical stimulation of peripheral nerves in the lower limbs in man, Brain, 108 (1985) 103- I21. 7 Desmedt, J.E. and Bourguet, M., Color imaging of parietal and frontal somatosensory potential fields evoked by stimulation of median or posterior tibial nerve in man, Electroencephagr. clin. Nenrophysiol., 62 (1985) 1 17. 8 Dietz, V., Horstmann, G.A. and Berger, W., Perturbations of human posture; influence of impulse modality on EMG responses and cerebral evoked potentials, J. Mot. Behav., 21 (1989) 357-372. 9 Dietz, V., Quintern, J., Berger, W. and Schenck, E., Cerebral potentials and leg muscle e.m.g, responses associated with stance pertubation. Exp. Brain Res., 57 (1985) 348-354. 10 Dietz, V., Quintern, J. and Berger, W., Afferent control of human stance and gait: Evidence for blocking of group I afferents during gait, Exp. Brain Res., 61 (1985) 153 163. l 1 Dietz, V., Quintern, J. and Sillem, M., Stumbling reactions in man: Significance of proprioceptive and pre-programmed mechanisms, J. Physiol., 386 (1987) 149--163. 12 Ghez, C. and Pisa, M., Inhibition of afferent transmission in cuneate nucleus during movement in the cat, Brain Res., 40 (1972) 145-151. 13 Rushton, D.N., Rothwell, J.C. and Craggs, M.D., Gating of somatosensory evoked potentials during different kinds of movement in man, Brain, 104 (1981) 465-491. 14 Start, A., McKeon, B., Skuse, N. and Burke, D., Cerebral potentials evoked by muscle stretch in man, Brain, 104 (1981) 149-166. 15 Towe, A. and Jabbur, S.J., Cortical inhibition of neurons in dorsal column nuclei of cat, J. Neurophysiol., 24 (1961) 488-498. 16 Ysumoto, T., Nakamura, S. and Iwama, K., Pyramidal tract control over cutaneous and kinesthetic sensory transmission in the cat thalamus, Exp. Brain Res., 22 (1975) 291 294. 17 Wolpaw, J.R. and Dowman, R., Operant conditioning of primate spinal reflexes: effect on cortical SEPs, Electroencephalogr. clin. Neurophysiol., 69 (1988) 398-401.
