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Changes in cortical short latency somatosensory evoked potentials with stimulus frequency and interfering stimulation
SIO1 P 7.05 SCALP T O P O G R A P H Y AND D I S T R I B U T I O N OF CORTICAL S O M A T O S E N S O R Y EVOKED POTENTIALS (SEPS) TO MEDIAN NERVE STIMULATION.
S. Tsuji and Y. Murai
N I 8 of the scalp recording. These findings suggest that there are at least three distinct potentials between PI5 and N20. The ascending limb of N20 of SEP is not a simple gradient from P15 to N20 but a compound potential that originates in the thalamus.
(Kitakyushu City, Japan) Topographies and distributions of cortical SEPs to median nerve stimulation were studied in 8 normal adults and 3 neurological patients in order to investigate the generator mechanism of the cortical SEPs. Twelve recording electrodes were placed over the scalp. Grid II electrodes were placed on the linked ears, each ear and shoulder. SEPs recorded from the scalp using three different reference electrodes showed similar morphologies. SEPs recorded from C4-A1A2 derivation to left median nerve stimulation were composed of 3 negative (N16, NI8, N30) and 3 positive components (P12, P23, P44) within 50 msec analysis, whereas, those recorded from frontal electrodes had 2 negative (N16, N24) and 3 positive components (P12, P20, P34). The polarities of NI8 and P20, and those of P23 and N24 were completely reverse, and there were no significant differences in their peak latencies. P12 and N16 were recorded all over the scalp. Topographies of N18 and P23 were localized over the posterior quadrant areas contralateral to stimulation, whereas, P20 and N24 were localized over the frontal areas. Topographies of N30 and P44 were localized around C4. These findings suggest that components recorded from posterior quadrant and frontal electrodes could be generated by a dipole at the somatosensory cortex.
P 7.06 O R I G I N OF ASCENDING LIMB OF N20 OF S O M A T O S E N S O R Y EVOKED POTENTIAL IN HUMAN. AN ANALYSIS ON DIRECT RECORDING FROM THE MIDBRAIN.
T. Taira and K. Kitamura
P7.07 EFFECTS ON MEDIAN NERVE SEPS OF CONT I N U O U S LIGHT T O U C H APPLIED TO ADJACENT AND REMOTE AREAS OF THE BODY SURFACE.
R. Kakigi and S.J. Jones (London, UK) SEPs following electrical median nerve stimulation were recorded with and without continuous light touch applied to adjacent or remote areas of the body surface. The nature and distribution of the interference effect was demonstrated by subtracting the 'interference' from the 'control' response, to derive a 'difference' waveform. Tactile stimulation of the ipsilateral thumb resulted in a difference waveform in which there was apparent phase reversal between pre- and post-central regions for 2 complexes, at latencies of 20 and 30 msec. This. it was surmised, might be due to partial 'saturation' of a generator in the posterior bank of the central sulcus (S I, area 3b) which was then unable to respond fully to the median nerve volley. A similar effect was observed in association with tactile stimulation of the little finger, possibly due to a process analogous to that of 'surround inhibition'. Tactile stimulation of more remote regions, principally the face and contralateral hand, resulted in consistent difference waveforms in which the distribution of early components suggested generators in the facial representation in S 1 a n d / o r S lI and the hand representation in S ll, respectively. Later components of all difference waveforms were distributed in accordance with a possible generator in the parietal 'association" cortex.
(Tokyo, Japan) The N18 component of somatosensory evoked potential (SEP) can be recorded as a small shoulder on the ascending limb of N20. This potential is thought to be of thalamic origin because of its wide distribution over the scalp and being recorded even with suprathalamic lesions. However, there is no unanimous conclusion concerning the components of the ascending limb of N20. We recorded SEP from the trajectory of rostral mesencephalic reticulotomy for pain relief. Four positive potentials followed by one negative wave were recorded at the level of the ventral thalamus and the rostral midbrain. The first two positive wa~es corresponded to P13 and P15 of the scalp recording. The latter two positive waves changed their polarity at the suprathalamic level and formed the ascending limb of N20. The last negative wave was largest at the level of nucleus ventrocaudalis externus of the thalamus and corresponded to
P 7.08 CHANGES IN CORTICAL S H O R T LATENCY S O M A T O S E N S O R Y EVOKED P O T E N T I A L S W I T H S T I M U L U S FREQUENCY AND INTERFERING STIMULATION.
R. Powell, S.G. Bayliss and N.L. Robinson (Guildford, UK) The early cortical SEP can be considered as a series of positive/negative/positive wavelets (P15, N20, P22, N23, P25, N38). Since the stimulus undoubtedly recruits sensory fibres of different modalities such as cutaneous, muscle tension and joint position, it is possible that different components of the waveform may reflect different sensory inputs. The ability to separate the various sensory responses may have both scientific and clinical value. In the present study, we have compared the
S102 effect of 4 forms of interfering stimulus with changes in stimulus frequency upon the somatosensory response. Interfering stimuli always cause a reduction in the cortical response; however, different types of stimuli affect different components of the evoked potential. Whereas cutaneous stimulation causes a reduction in the N23-P25 component, stimulation of muscle afferents using a vibration stimulus results in a reduction of the P15-N20-P22 component. Changes in stimulus frequency also result in reductions in the cortical response with the N23-P25 component being the most affected. On the basis of these results, it is suggested that the P15-N20-P22 component may reflect the input from muscle afferents whereas the N23-P25-N30 component may be more related to cutaneous inputs.
P 7.09 CENTRAL AND PERIPHERAL S U B C O M P O N E N T S O F T H E HALF-FIELD (HF) PATTERN REVERSAL VEP: SPATIOTEMPORAL MAPPING, SOURCE DERIVATION AND DIPOLE MODELLING.
thalamotomies, in patients suffering from severe dyskinesias prevalent in the right arm. The operations were performed under local anaesthesia. The patients were awake and were tested by verbal tasks throughout the procedure. The target nucleus in all procedures was represented by the VL. Location of the target was obtained by the Talairach method. A posterior (occipital) approach was employed with a stainless steel electrode, insulated but for the tip, being manually advanced toward the target. PR-VEPs were recorded from this electrode against a non-cephalic reference. The first recording was obtained at the cortical level and the following ones at 5 10 mm advances in depth. ERG was recorded from a corneal lens electrode. A LED checkerboard pattern (black and red) was used as stimulator: spatial frequency 75 rain of arc, temporal frequency 1Hz. The ERG and PR-VEP signals were evaluated by an AMPLAID MK10 Multisensory System. Our recordings confirm that the generator of the P100 wave is entirely located in the calcarine cortex, while an electrical field recordable in the surroundings of the lateral geniculate body precedes the beginning of the occipital response by 10-15 msec.
W.M. Carroll, G.W. Thickbroom and F.L. Mastaglia (P~rth, W. Australia) VEPs to left HF stimulation were recorded from 30 scalp sites in 11 normal subjects. Spatiotemporal maps were drawn from reference-independent waveforms obtained by source derivation and dipoles were modelled for the principal components. The dominant response was a parieto-occipital negative-positive-negative complex ipsilateral (IL) to the HF with peak latencies at 68, 96 and 136 msec and a contralateral (CL) parietal complex of opposite polarities P68, N96, P136). Dipole modelling showed a right postero-medial occipital origin for each of the negative and positive components. With occlusion of the central 50-7.50 , the IL complex was markedly attenuated in the occipital region but relatively preserved in the temporo-parietal region; the CL temporo-parietal complex was unchanged or enhanced. There was a corresponding shift in the axis of the P96 dipole. It is concluded that: (i) all of the major HF VEP components originate in the CL visual cortex; (ii) the 1L N68, P96, N136 components arise mainly from cortex on the occipital pole representing the central 50-70 of the visual field; (iii) the CL P68, N96, P136 components arise from more anterior visual cortex subserving the peripheral field and do not only represent the opposite ends of the centrally-derived N68, P96, N136 dipoles.
P 7.10 INTRACEREBRAL R E C O R D I N G S O F P R - V E P S IN THREE AWAKE PATIENTS.
E. Fava, A. Ducati, E.D.F. Motti and F. Marossero (Milan, Italy) Pattern reversal visual evoked potentials (PR-VEPs) and electroretinogram (ERG) were recorded during three stereotaxic
P 7.11 INTRACRANIAL S T U D I E S ON SURFACE AND DEPTH POTENTIAL GENERATORS IN MAN: 1. AUDITORY BRAIN-STEM POTENTIALS. M. Velasco and F. Velasco (Mexico City, Mexico) Bipolar and referential EEG and multiple unit activity (MUA) responses correlated to the surface auditory brain-stem potentials (components I to VII and MSW) were studied in different subcortical structures of patients with implanted electrodes used as an electrophysiological procedure for surgical treatment. Two types of bipolar responses were recorded within two circumscribed subcortical regions: one biphasic A response, correlated to surface components V and VI-VII, was recorded from the mesencephalic reticular formation (MRF), the medial (MG) and lateral (LG) geniculate thalamic nuclei; with polarity inversion between MRF and MG and concomitant MUA discharges at contralateral MG. Other monophasic B response, correlated to the surface slow negative wave following VII, was recorded from the hippocampal formation (CnA) bilaterally, with neither polarity reversals nor associated MUA discharges. Two referential responses N8 and N15 correlated to bipolar responses A and B were recorded within a wide subcortical area. Referential N8 and NI5 responses attained maximal amplitude and minimal latency at contralateral MG and bilateral CnA, respectively. From here, their amplitude decreased, except those at the ventro-basal thalamus where no amplitude-latency gradients were observed.
S. Tsuji and Y. Murai
N I 8 of the scalp recording. These findings suggest that there are at least three distinct potentials between PI5 and N20. The ascending limb of N20 of SEP is not a simple gradient from P15 to N20 but a compound potential that originates in the thalamus.
(Kitakyushu City, Japan) Topographies and distributions of cortical SEPs to median nerve stimulation were studied in 8 normal adults and 3 neurological patients in order to investigate the generator mechanism of the cortical SEPs. Twelve recording electrodes were placed over the scalp. Grid II electrodes were placed on the linked ears, each ear and shoulder. SEPs recorded from the scalp using three different reference electrodes showed similar morphologies. SEPs recorded from C4-A1A2 derivation to left median nerve stimulation were composed of 3 negative (N16, NI8, N30) and 3 positive components (P12, P23, P44) within 50 msec analysis, whereas, those recorded from frontal electrodes had 2 negative (N16, N24) and 3 positive components (P12, P20, P34). The polarities of NI8 and P20, and those of P23 and N24 were completely reverse, and there were no significant differences in their peak latencies. P12 and N16 were recorded all over the scalp. Topographies of N18 and P23 were localized over the posterior quadrant areas contralateral to stimulation, whereas, P20 and N24 were localized over the frontal areas. Topographies of N30 and P44 were localized around C4. These findings suggest that components recorded from posterior quadrant and frontal electrodes could be generated by a dipole at the somatosensory cortex.
P 7.06 O R I G I N OF ASCENDING LIMB OF N20 OF S O M A T O S E N S O R Y EVOKED POTENTIAL IN HUMAN. AN ANALYSIS ON DIRECT RECORDING FROM THE MIDBRAIN.
T. Taira and K. Kitamura
P7.07 EFFECTS ON MEDIAN NERVE SEPS OF CONT I N U O U S LIGHT T O U C H APPLIED TO ADJACENT AND REMOTE AREAS OF THE BODY SURFACE.
R. Kakigi and S.J. Jones (London, UK) SEPs following electrical median nerve stimulation were recorded with and without continuous light touch applied to adjacent or remote areas of the body surface. The nature and distribution of the interference effect was demonstrated by subtracting the 'interference' from the 'control' response, to derive a 'difference' waveform. Tactile stimulation of the ipsilateral thumb resulted in a difference waveform in which there was apparent phase reversal between pre- and post-central regions for 2 complexes, at latencies of 20 and 30 msec. This. it was surmised, might be due to partial 'saturation' of a generator in the posterior bank of the central sulcus (S I, area 3b) which was then unable to respond fully to the median nerve volley. A similar effect was observed in association with tactile stimulation of the little finger, possibly due to a process analogous to that of 'surround inhibition'. Tactile stimulation of more remote regions, principally the face and contralateral hand, resulted in consistent difference waveforms in which the distribution of early components suggested generators in the facial representation in S 1 a n d / o r S lI and the hand representation in S ll, respectively. Later components of all difference waveforms were distributed in accordance with a possible generator in the parietal 'association" cortex.
(Tokyo, Japan) The N18 component of somatosensory evoked potential (SEP) can be recorded as a small shoulder on the ascending limb of N20. This potential is thought to be of thalamic origin because of its wide distribution over the scalp and being recorded even with suprathalamic lesions. However, there is no unanimous conclusion concerning the components of the ascending limb of N20. We recorded SEP from the trajectory of rostral mesencephalic reticulotomy for pain relief. Four positive potentials followed by one negative wave were recorded at the level of the ventral thalamus and the rostral midbrain. The first two positive wa~es corresponded to P13 and P15 of the scalp recording. The latter two positive waves changed their polarity at the suprathalamic level and formed the ascending limb of N20. The last negative wave was largest at the level of nucleus ventrocaudalis externus of the thalamus and corresponded to
P 7.08 CHANGES IN CORTICAL S H O R T LATENCY S O M A T O S E N S O R Y EVOKED P O T E N T I A L S W I T H S T I M U L U S FREQUENCY AND INTERFERING STIMULATION.
R. Powell, S.G. Bayliss and N.L. Robinson (Guildford, UK) The early cortical SEP can be considered as a series of positive/negative/positive wavelets (P15, N20, P22, N23, P25, N38). Since the stimulus undoubtedly recruits sensory fibres of different modalities such as cutaneous, muscle tension and joint position, it is possible that different components of the waveform may reflect different sensory inputs. The ability to separate the various sensory responses may have both scientific and clinical value. In the present study, we have compared the
S102 effect of 4 forms of interfering stimulus with changes in stimulus frequency upon the somatosensory response. Interfering stimuli always cause a reduction in the cortical response; however, different types of stimuli affect different components of the evoked potential. Whereas cutaneous stimulation causes a reduction in the N23-P25 component, stimulation of muscle afferents using a vibration stimulus results in a reduction of the P15-N20-P22 component. Changes in stimulus frequency also result in reductions in the cortical response with the N23-P25 component being the most affected. On the basis of these results, it is suggested that the P15-N20-P22 component may reflect the input from muscle afferents whereas the N23-P25-N30 component may be more related to cutaneous inputs.
P 7.09 CENTRAL AND PERIPHERAL S U B C O M P O N E N T S O F T H E HALF-FIELD (HF) PATTERN REVERSAL VEP: SPATIOTEMPORAL MAPPING, SOURCE DERIVATION AND DIPOLE MODELLING.
thalamotomies, in patients suffering from severe dyskinesias prevalent in the right arm. The operations were performed under local anaesthesia. The patients were awake and were tested by verbal tasks throughout the procedure. The target nucleus in all procedures was represented by the VL. Location of the target was obtained by the Talairach method. A posterior (occipital) approach was employed with a stainless steel electrode, insulated but for the tip, being manually advanced toward the target. PR-VEPs were recorded from this electrode against a non-cephalic reference. The first recording was obtained at the cortical level and the following ones at 5 10 mm advances in depth. ERG was recorded from a corneal lens electrode. A LED checkerboard pattern (black and red) was used as stimulator: spatial frequency 75 rain of arc, temporal frequency 1Hz. The ERG and PR-VEP signals were evaluated by an AMPLAID MK10 Multisensory System. Our recordings confirm that the generator of the P100 wave is entirely located in the calcarine cortex, while an electrical field recordable in the surroundings of the lateral geniculate body precedes the beginning of the occipital response by 10-15 msec.
W.M. Carroll, G.W. Thickbroom and F.L. Mastaglia (P~rth, W. Australia) VEPs to left HF stimulation were recorded from 30 scalp sites in 11 normal subjects. Spatiotemporal maps were drawn from reference-independent waveforms obtained by source derivation and dipoles were modelled for the principal components. The dominant response was a parieto-occipital negative-positive-negative complex ipsilateral (IL) to the HF with peak latencies at 68, 96 and 136 msec and a contralateral (CL) parietal complex of opposite polarities P68, N96, P136). Dipole modelling showed a right postero-medial occipital origin for each of the negative and positive components. With occlusion of the central 50-7.50 , the IL complex was markedly attenuated in the occipital region but relatively preserved in the temporo-parietal region; the CL temporo-parietal complex was unchanged or enhanced. There was a corresponding shift in the axis of the P96 dipole. It is concluded that: (i) all of the major HF VEP components originate in the CL visual cortex; (ii) the 1L N68, P96, N136 components arise mainly from cortex on the occipital pole representing the central 50-70 of the visual field; (iii) the CL P68, N96, P136 components arise from more anterior visual cortex subserving the peripheral field and do not only represent the opposite ends of the centrally-derived N68, P96, N136 dipoles.
P 7.10 INTRACEREBRAL R E C O R D I N G S O F P R - V E P S IN THREE AWAKE PATIENTS.
E. Fava, A. Ducati, E.D.F. Motti and F. Marossero (Milan, Italy) Pattern reversal visual evoked potentials (PR-VEPs) and electroretinogram (ERG) were recorded during three stereotaxic
P 7.11 INTRACRANIAL S T U D I E S ON SURFACE AND DEPTH POTENTIAL GENERATORS IN MAN: 1. AUDITORY BRAIN-STEM POTENTIALS. M. Velasco and F. Velasco (Mexico City, Mexico) Bipolar and referential EEG and multiple unit activity (MUA) responses correlated to the surface auditory brain-stem potentials (components I to VII and MSW) were studied in different subcortical structures of patients with implanted electrodes used as an electrophysiological procedure for surgical treatment. Two types of bipolar responses were recorded within two circumscribed subcortical regions: one biphasic A response, correlated to surface components V and VI-VII, was recorded from the mesencephalic reticular formation (MRF), the medial (MG) and lateral (LG) geniculate thalamic nuclei; with polarity inversion between MRF and MG and concomitant MUA discharges at contralateral MG. Other monophasic B response, correlated to the surface slow negative wave following VII, was recorded from the hippocampal formation (CnA) bilaterally, with neither polarity reversals nor associated MUA discharges. Two referential responses N8 and N15 correlated to bipolar responses A and B were recorded within a wide subcortical area. Referential N8 and NI5 responses attained maximal amplitude and minimal latency at contralateral MG and bilateral CnA, respectively. From here, their amplitude decreased, except those at the ventro-basal thalamus where no amplitude-latency gradients were observed.