Heidi Wikström

Effects of interstimulus interval on somatosensory evoked magnetic fields (SEFs): a hypothesis concerning SEF generation at the primary sensorimotor cortex

Cerebral responses evoked by peripheral stimuli are known to depend critically on the interstimulus interval (ISI). Here we report on the effects of ISI on somatosensory evoked magnetic fields (SEFs) to right median nerve stimulation, obtained in 9 healthy adults with ISIs of 0.15 0.3, 1,3 and 5 s. At the contralateral (left) primary sensorimotor cortex (SMI), the first cortical response, N20m, was stable between the ISIs 0.3 and 5 s, but slightly attenuated at the shortest ISI of 0.15 s. In contrast, the P35m and P60m deflections were very sensitive to changes of the ISI, declining steadily with shortening of the ISI throughout the entire range. These deflections were frequently undetectable at the shortest ISI of 0.15 s. Concomitant with the reductions of P35m and P60m, an N45m deflection was enhanced toward the short ISIs. Responses from second somatosensory cortex (SII) and posterior parietal cortex (PPC) were seen only with ISIs of 1 s or greater, being strongest at the 5 s ISI. Based on known effects of the ISI on intracellular evoked potentials, we present the following tentative model for the generation mechanism of the SMI response: N20m represents early excitatory postsynaptic potentials (EPSPs), P35m early inhibitory postsynaptic potentials (IPSPs), N45m secondary EPSPs and P60m late IPSPs in pyramidal neurones of area 3b. For practical purposes, SEFs from SMI can be obtained with short ISIs, while responses from SII and PPC require an ISI of at least 1 s.

Significance of the second somatosensory cortex in sensorimotor integration : enhancement of sensory responses during finger movements

THE functional significance of the second somatosensory cortex (SII) is poorly understood. However, lesion and cortical stimulation studies indicate that SII may be involved in sensory aspects of tactile learning and in movement control. In the present study, we explored a possible role of SII in sensorimotor integration in humans using a multichannel magnetometer. Somatosensory evoked fields (SEFs) from SII to electrical stimulation of left and right median nerves were recorded in six healthy volunteers during rest and in different test conditions. Continuous cutaneous stimulation of the right hand or face reduced the SEFs to both left and right median nerve stimulation. Right-sided finger movements increased the SEFs to right, but not left, median nerve stimulation. The responses were equally enhanced by simple finger flexion movement and by a complex finger sequence. The suppression of SEFs by competing cutaneous inputs from different areas of the body indicates that the neurones underlying the responses receive inputs from large, bilateral receptive fields. The enhancement of sensory reactions to signals from the actively moving limb but not to those from the opposite limb indicates a spatial tuning of the SII neurones to behaviourally relevant input channels, also suggesting that SII is important for the integration of sensory information to motor programmes.

Activation of ipsilateral primary sensorimotor cortex by median nerve stimulation

WE report evidence for activation of ipsilateral primary sensorimotor cortex (SMI) after median nerve stimulation recorded with magnetoencephalography (MEG). We measured somatosensory evoked magnetic fields (SEFs) to median nerve stimulation with a 122-channel helmet-shaped magnetometer in 10 healthy subjects. In five, the magnetic field patterns suggested long-latency activation of the ipsilateral SMI. Source locations found by current dipole fitting corresponded to the SMI hand area, as determined by contralateral stimulation. Further evidence for the origin of the ipsilateral responses in SMI was provided by the suppression of these responses during movement of the contralateral fingers. Sensory input to ipsilateral SMI could play a role in sensorimotor integration of bilateral movements.

Dynamics of mu-rhythm suppression caused by median nerve stimulation: a magnetoencephalographic study in human subjects

We studied event-related desynchronization (ERD) of the 8-13 Hz rhythm (mu rhythm) of the primary somatosensory cortex (SI) caused by contra- and ipsilateral median-nerve stimulation. We used whole-head magnetoencephalography (MEG) and wavelet analysis together with our newly developed color-coded single-trial ERD display. The somatosensory stimuli suppressed mu rhythm at both contra- and ipsilateral SI, but the attenuation was clearly lateralized, being at least 20% stronger contra- than ipsilaterally. Moreover, repeated stimulation significantly reduced mu-rhythm ERD in the ipsilateral but not in the contralateral hemisphere in the course of the experiment. The observed lateralization is in agreement with the classical concept of a dominant role of the contralateral hemisphere in the processing of somatosensory information. The strong ipsilateral ERD in the beginning of the experiment may reflect the presence of non-specific arousal-like activation, which attenuates toward the end of the experiment.

Somatosensory evoked potentials and magnetic fields elicited by tactile stimulation of the hand during active and quiet sleep in newborns.

OBJECTIVE Our objective was to characterize the effects of sleep stages on tactile somatosensory evoked responses in full-term newborns. METHODS Somatosensory evoked potentials (SEPs) and magnetic fields (SEFs) to tactile stimulation of the tip of the index finger and/or thenar eminence were measured from 14 healthy newborns. The stimulus was a gentle tap produced by a moving membrane driven by an air-pressure pulse. RESULTS SEPs and SEFs to tactile stimulation of the skin were similar in waveform and latency to SEPs known to be produced by electrical stimulation of the fingertip of neonates. The two most distinguishable positive deflections of SEPs, P1 and P2, within 300 ms of the stimulation, and their magnetic counterparts were clearly smaller in active compared to quiet sleep. CONCLUSIONS Our study demonstrates for the first time that it is possible to record SEFs in neonates, and that clear late cortical somatosensory responses are produced by tactile stimulation. In addition, the effect of sleep stage on these responses indicates differences in the processing of the incoming information, at least in the somatosensory modality, in active and quiet sleep. SIGNIFICANCE Tactile stimulation may be useful as a completely non-invasive technique for studying the physiology of the somatosensory system in neonates. Methodologically, since the effect of sleep stage is profound, one must carefully monitor the sleep stages in studies of event-related responses in newborns, or else this effect may confound the phenomena being studied.

Specific changes in somatosensory evoked magnetic fields during recovery from sensorimotor stroke.

We studied recovery‐induced changes in the responsiveness of the primary somatosensory cortex in stroke patients with sensory and/or motor symptoms. Somatosensory evoked magnetic fields, in response to median nerve stimulation, were recorded in 14 patients with their first symptomatic unilateral stroke 1 to 15 days from the first symptoms and again 2 to 3 months later. Neuronal activity at the contralateral primary somatosensory cortex was modeled with equivalent current dipoles at the peak latencies of the first two cortical deflections at about 20 msec (N1m) and at 28 to 91 msec (P1m). Twenty‐three age‐matched healthy volunteers, 9 of whom were tested also in serial recordings, served as control subjects. At follow‐up, 6 patients showed a significant increase of P1m amplitude, whereas N1m increased only in 1. Clinical improvement of two‐point discrimination ability, but not of other basic somatosensory skills, was significantly correlated with the increase of P1m. We conclude that the recovery of discriminative touch after stroke is paralleled by the growth of the P1m somatosensory evoked magnetic field deflection, and we propose that this may reflect re‐establishment of lateral inhibitory functions at the primary somatosensory cortex. Ann Neurol 2000;47:353–360

Smoking Increases the Incidence of Complicated Diverticular Disease of the Sigmoid Colon

Backround and Aims: The aim of this study was to establish whether smoking is associated with complicated diverticular disease and adverse outcomes of operative treatment of diverticular disease. Smoking has been associated with increased rate of perforations in acute appendicitis as well as failure of colonic anastomosis in patients resected for colonic tumours. It has also been suggested that smoking is a risk factor for complicated diverticular disease of the colon. Material and Methods: Retrospective investigation of records of 261 patients electively operated for diverticular disease in Helsinki university Central Hospital during a period of five years. Results: The smokers underwent sigmoidectomy at a younger age than the non-smokers (p = 0.001) and they had an increased rate of perforations (p = 0.040) and postoperative recurrent diverticulitis episodes (p = 0.019). Conclusions: We conclude that smoking increases the likelihood of complications in diverticulosis coli. The development of complicated disease also seems to proceed more rapidly in smokers.

Somatosensory evoked magnetic fields to median nerve stimulation: interhemispheric differences in a normal population.

The objective of the present study was to evaluate the normal interhemispheric variability of the locations and activation strengths of the somatosensory cortices. Somatosensory evoked magnetic fields (SEFs) were recorded with a 122-channel magnetometer in 23 healthy subjects (mean age 57 years) to stimulation of left and right median nerves. Equivalent current dipole (ECD) strengths and locations were determined for the main SEF deflections at the contralateral primary sensorimotor (SMI) and secondary somatosensory (SIIc) cortices. In a Cartesian co-ordinate system, defined by the preauricular points and the nasion, the SMI sources were slightly but significantly more laterally and anteriorly located in the right than in the left hemisphere. No systematic co-ordinate asymmetries were found for the SIIc sources. In individual subjects, the interhemispheric differences in the ECD co-ordinates averaged less than 6 mm at both SMI and SIIc. The group means of the source strengths did not differ between the hemispheres, but individual differences were on average 20% for the SMI and 65% for the SIIc sources. We conclude that at the individual level, the median nerve SEFs from SMI can be used to detect abnormally large interhemispheric asymmetries of source locations in the centimetre scale.

Effects of voluntary hyperventilation on cortical sensory responses. Electroencephalographic and magnetoencephalographic studies.

Abstract It is well established that voluntary hyperventilation (HV) slows down electroencephalographic (EEG) rhythms. Little information is available, however, on the effects of HV on cortical responses elicited by sensory stimulation. In the present study, we recorded auditory evoked potentials (AEPs) and magnetic fields (AEFs), and somatosensory evoked magnetic fields (SEFs) from healthy subjects before, during, and after a 3- to 5-min period of voluntary HV. The effectiveness of HV was verified by measuring the end-tidal CO2 levels. Long-latency (100–200 ms) AEPs and long-latency AEFs originating at the supratemporal auditory cortex, as well as long-latency SEFs from the primary somatosensory cortex (SI) and from the opercular somatosensory cortex (OC), were all reduced during HV. The short-latency SEFs from SI were clearly less modified, there being, however, a slight reduction of the earliest cortical excitatory response, the N20m deflection. A middle-latency SEF deflection from SI at about 60 ms (P60 m) was slightly increased. For AEFs and SEFs, the center-of-gravity locations of the activated neuronal populations were not changed during HV. All amplitude changes returned to baseline levels within 10 min after the end of HV. The AEPs were not altered when the subjects breathed 5% CO2 in air in a hyperventilation-like manner, which prevented the development of hypocapnia. We conclude that moderate HV suppresses long-latency evoked responses from the primary projection cortices, while the early responses are less reduced. The reduction of long-latency responses is probably mediated by hypocapnia rather than by other nonspecific effects of HV. It is suggested that increased neuronal excitability caused by HV-induced hypocapnia leads to spontaneous and/or asynchronous firing of cortical neurones, which in turn reduces stimulus-locked synaptic events.

Immaturity of somatosensory cortical processing in human newborns.

The development of the early component of somatosensory evoked potentials (SEPs) from the neonatal N1 to adult N20 response has previously been described. The main emphasis has been on the change in the response latency during maturation. We used magnetoencephalography (MEG) to characterize the cortical generators of the N1 and the subsequent response in healthy human newborns. Furthermore, we studied the maturation of tactile processing according to responses evoked by tactile stimulation of the index finger in newborns, 6-month-old babies and adults. This study provides evidence of specific differences in the somatosensory processing in neonates compared to that in adults. Although the initial cortical response to electrical median nerve stimulation in the newborns was similar in field distribution to the corresponding N20m in adults, the subsequent major deflection in the response waveform had the opposite polarity. Similar immaturity in cortical processing was seen in the tactile evoked fields in both the newborns and the 6-month-old infants compared with the adults. Our results indicate that although the somatosensory pathway in full-term newborns is sufficiently developed to supply the brain with tactile information, the cortical neuronal networks for processing the input may not function in the same way as in adults.