Electroencephalography and Clinical Neurophysiology/Evoked Potentials Section
Median and tibial nerve somatosensory evoked potentials: middle-latency components from the vicinity of the secondary somatosensory cortex in humans
Introduction
Studies of the scalp topography of the somatosensory evoked potentials (SEPs) following median nerve and finger stimulation have yielded a relatively clear concept about the cortical areas that generate the early cortical components. The parietal N20 and P30 are thought to be generated by a tangential dipole in area 3b of the primary somatosensory cortex (SI); other components in this latency range may be due to radial dipoles in area 1 and 2 or in pre-rolandic cortex (Deiber et al., 1986; Desmedt et al., 1987; Allison et al., 1989a). These conclusions have been supported by dipole source modelling, magnetencephalography (MEG) and subdural recordings (Allison et al., 1991; Baumgartner et al., 1991; Buchner et al., 1995). In contrast to the large number of studies about median nerve SEPs there are only a few reports about SEPs following stimulation of lower limb nerves. The initial cortical component of the tibial nerve SEP (P40) is also thought to be generated by a dipole in area 3b (Kakigi et al., 1995). Due to dipole orientation perpendicular to the cortical surface within the interhemispheric fissure the P40 shows a paradoxical lateralization; that is, its amplitude is consistently higher over ipsilateral than contralateral central electrode positions (Cruse et al., 1982; Lesser et al., 1987).
The generator sources for middle-latency SEPs are understood less well. Over the past decade evidence has accumulated that median nerve and finger SEPs in the latency range around 100 ms are generated near the secondary somatosensory cortex (SII). The first description of SEP components from human SII was derived from MEG recordings of dipolar field patterns near the Sylvian fissure that were observed at 95–125 ms following stimulation of median and peroneal nerves (Hari et al., 1983).
In previous studies, we have reported a low temporal negativity at 110 ms peak latency (N110) and a simultaneous midline positivity (Kunde and Treede, 1993). This N110 was recorded with stimulation of both mixed and cutaneous nerves in the upper limb (Treede and Kunde, 1995). Since the scalp maxima of SEP components do not necessarily overly the active areas and the hand area of SI is relatively far lateral, it is possible that the different topographies of N20 and N110 reflect different orientations rather than locations of the underlying sources. For a better differentiation between the components of the primary and secondary somatosensory cortex we now stimulated the lower limb for which the representation in SI and SII exhibits maximal separation. The aim of the present study was to confirm generation of N110 in the vicinity of the lateral sulcus by identifying a homologous component in the tibial nerve SEP.
Section snippets
Subjects
This study was performed on 12 healthy volunteers who gave written informed consent and were paid for participation (age 21–29 years, mean 24 years, 6 females and 6 males). All subjects were acquainted with the experimental surrounding in earlier sessions. They were comfortably seated in a noise- and light-reduced room, which had a temperature of 24°C and was electrically shielded.
Stimuli
Left and right median and tibial nerves were transcutaneously stimulated with electrical constant current square
Results
Fig. 1 shows original recordings from one subject following stimulation of the left median and left tibial nerve. Fig. 1A revealed the typical features of the primary cortical components (N20 and P40). The N20 exhibited a polarity reversal across the central sulcus. Its maximum negativity was close to the Pz and P4 electrodes while the maximum positivity was located between Fz and Cz. Corresponding to the sensory homunculus the polarity reversal of the P40 occurred across the interhemispheric
Discussion
Electrical stimulation applied to left and right tibial nerves elicited a negative potential with a peak latency of 131 ms and a maximal amplitude in contralateral temporal electrode positions. The ipsilateral N130 appeared somewhat later and with significantly smaller amplitude than the contralateral one. This N130 showed a stable topography for at least 20 ms. The best derivation to record the N130 was from T3/T4 versus Fz or Pz. After stimulation of the left and right median nerve a similar
Acknowledgements
The authors would like to thank Dr. W. Magerl for help with the statistics, J. Knappen for installing the mapping software and G. Günther for assistance with the figures. This publication contains essential parts of the dissertation of C. Kany, which will be submitted to the Faculty of Medicine of the Johannes-Gutenberg University, Mainz. The study was supported by Deutsche Forschungsgemeinschaft (Tr 236/6-1).
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2022, NeuroImageCitation Excerpt :Specifically, the N20 component of the SEP, a negative deflection after 20 ms at centro-parietal electrode sites in response to median nerve (i.e., “hand”) stimulation (Allison et al., 1991), is thought to be exclusively generated by excitatory post-synaptic potentials (EPSPs) of the first thalamo-cortical volley (Bruyns-Haylett et al., 2017; Nicholson Peterson et al., 1995; Wikström et al., 1996), and therefore represents a direct measure of instantaneous cortical excitability. Assuming a homologous neural circuitry for the somatosensory foot region, we related this measure to the first cortical component of the SEP in response to tibial nerve (i.e., “foot”) stimulation, the P40 component (Allison et al., 1996; Kany and Treede, 1997). This way, we sought to dissociate local from domain-wide fluctuations of neural excitability across spatially distinct sites within the somatosensory cortex.
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2016, Clinical NeurophysiologyCitation Excerpt :It is not far to speculate that neuronal facilitation as demonstrated in the present study in the context of train stimuli may contribute to such findings. The feedback loops involved in the generation of the late SSEP responses may cover different cortical and subcortical centres (Kany and Treede, 1997; Mima et al., 1997; Brázdil et al., 2003; Kublik, 2004; Nieuwenhuis et al., 2005). The decay slope of the N1 decline with extended ISI values (Fig. 3) resembles that of an EPSP curve.
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2013, Neuroscience LettersCitation Excerpt :The decrease in amplitude with the faster SRR was especially strong for N1 and N2. These late SEP components are known to be controlled by feedback loops [5,11,12,14,16]. Therefore, slow SRR are superior to fast SRR when short-term synaptic plasticity be studied.
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2012, NeuroscienceCitation Excerpt :There have been fewer studies of SEPs to lower limb (tibial nerve) stimulation than to upper limb (median nerve) stimulation but it is known that the latency and topography of tibial nerve SEP components differs from median nerve SEPs, because of longer signal conduction times (given the greater ankle to brain than wrist to brain distance) and because of the different locations of leg and arm representation in primary somatosensory cortex (Jones and Small, 1978; Kany and Treede, 1997). Thus, for tibial stimulation the P40 component has generally been considered to be the first cortical potential (short-latency component) and is generally recorded 20–30 ms later than the first cortical potential – the N20 – to median nerve stimulation in the same participant (Kany and Treede, 1997). Like the N20, the P40 is thought to be generated in area 3b of SI (Kakigi et al., 1995), but is characterised by ‘paradoxical lateralization’, i.e. tibial nerve SEP amplitude is greater in the ipsilateral rather than contralateral hemisphere (Cruse et al., 1982).
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2012, NeuroImageCitation Excerpt :Second, the four fitted dipoles explained the scalp distribution of SEPs with RV of 6.8% and 11.4 ± 3.4% in the group level and single-subject level respectively. Third, the obtained source results, i.e., that the neural sources of N60, N120, and P240 in SEPs were mainly located at the contralateral SI, bilateral SII, and CC respectively, were consistent with a large number of previous studies (Blatow et al., 2007; Kany and Treede, 1997; Mouraux and Iannetti, 2009; Shimojo et al., 2000). As the neural activity of any source would be expected to be widely spread at the scalp electrodes because of volume conduction in EEG (Nunez and Srinivasan, 2006), it would be difficult to accurately estimate effective connectivity between neural sources using pairs of electrodes.
Temporal window of integration in the somatosensory modality: An MEG study
2011, Clinical Neurophysiology