Papers by Antti Korvenoja
The adult mammalian nervous system has the ability to reorganize itself in an activity- dependent... more The adult mammalian nervous system has the ability to reorganize itself in an activity- dependent manner. The aim of this study was to investigate the influence of short-term repetitive electrical stimulation (rES, training session) on somatosensory evoked fields (SEF) as recorded with MEG. 6 healthy subjects were investigated, including 1 control. The SEFs were recorded right before the training, immediately after it, and after 25 min. Left median /3 subjects/ and right median /2 subjects/ nerve (MN) were stimulated. The interstimulus interval was 3 sec, the stimulus intensity was set to 1.3 times motor threshold of APB muscle. The training session consisted of a burst of 5 pulses with a frequency of 200 Hz, repeated every sec. for a duration of 30 min; the intensity of the stimulation was set to 2 times the motor threshold. The following SEF components were analyzed: N20m, P35m and P60m (responses in the primary somatosensory SI- area), as well as responses in the secondary somato...
The use of physiologicalconstraints in the solution of the inverse problem of brain electromagnet... more The use of physiologicalconstraints in the solution of the inverse problem of brain electromagnetic fields has received increasing attention in recent years. A priori information is needed to constrain the solution of the electromagnetic inverse problem; other imaging modalities, such as fMRI, can provide information of this kind. The goal is to combine complementary,information from different imaging modalities and
Lecture Notes in Computer Science, 1999
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Duodecim; lääketieteellinen aikakauskirja, 2000
ABSTRACT Conventional imaging techniques, such as computed tomography (CT) and magnetic resonance... more ABSTRACT Conventional imaging techniques, such as computed tomography (CT) and magnetic resonance imaging (MR), are of immeasurable assistance in the diagnosis and characterization of primary intracranial tumors. Factors which can be accurately deduced via these techniques includes the location, size, mass effect and edema associated with brain tumors; usually a differential diagnosis of tumor type can be generated based on characteristics on CT and MR imaging. However, factors relating to the growth potential of brain tumors cannot be elucidated with standard imaging techniques alone. Functional imaging techniques may provide the information necessary for accurate analysis of brain tumor behavior before and after therapy. There is a well- known association between the regional supply of energy substrates to the brain in accordance to its metabolic needs. This relationship has been exploited in the development of techniques which measure regional brain activity in qualitative and quantitative terms so that we may better understand cerebral function in normal and disease processes. Of particular importance is the need to determine the presence of residual or recurrent tumor after therapy. The present treatment of high-grade astro- cytomas (anaplastic astrocytomas or glioblastoma multiforme) consists of tumor debulking followed by high-energy local radiotherapy. These treatments are designed to deposit very high radiation doses within the tumor mass while minimizing damage to the adjacent neuropil. Nevertheless, radiation-induced reactive change and gliosis (radiation necrosis) may occur in otherwise healthy tissue within weeks to months after radiotherapy. This condition is manifested by expanding masses with peripheral enhancement and central necrosis, surrounded by edema, in the region of the treated tumor which mimics recurrent tumor growth clinically and by standard radiographic techniques. Furthermore, in patients with recurrence, the tumor cell density correlates with aggressiveness of tumor regrowth and therefore patient prog- nosis, but this also cannot be elucidated on standard imaging techniques. The
Journal of neurophysiology, 1999
A sudden change in the direction of motion is a particularly salient and relevant feature of visu... more A sudden change in the direction of motion is a particularly salient and relevant feature of visual information. Extensive research has identified cortical areas responsive to visual motion and characterized their sensitivity to different features of motion, such as directional specificity. However, relatively little is known about responses to sudden changes in direction. Electrophysiological data from animals and functional imaging data from humans suggest a number of brain areas responsive to motion, presumably working as a network. Temporal patterns of activity allow the same network to process information in different ways. The present study in humans sought to determine which motion-sensitive areas are involved in processing changes in the direction of motion and to characterize the temporal patterns of processing within this network of brain regions. To accomplish this, we used both magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI). The fMRI data w...
Somatosensory & Motor Research, 2004
Somatosensory point localization is a clinical test evaluating spatial accuracy of the somatosens... more Somatosensory point localization is a clinical test evaluating spatial accuracy of the somatosensory system. Possible effects of the interstimulus interval (ISI) on point localization threshold have not been previously examined. In the present set of experiments the effect of time delay on somatosensory point localization was studied using ISIs of 1, 3, 5, 7, and 9 s, and applying a newly developed computer-controlled application method of a Semmes-Weinstein monofilament. It was found that the point localization threshold was not significantly affected by the ISI length. However, the response time was shorter and response accuracy better at the shorter (1 and 3 s) than at the longer (5, 7, and 9 s) ISIs, suggesting a change in the mechanism underlying point localization decision criteria in ISIs longer than 3 s.
Radiology, 2006
To prospectively evaluate magnetoencephalography (MEG) and functional magnetic resonance (MR) ima... more To prospectively evaluate magnetoencephalography (MEG) and functional magnetic resonance (MR) imaging, as compared with intraoperative cortical mapping, for identification of the central sulcus. Fifteen patients (six men, nine women; age range, 25-58 years) with a lesion near the primary sensorimotor cortex (13 gliomas, one cavernous hemangioma, and one meningioma) were examined after institutional review board approval and written informed consent from each patient were obtained. At MEG, evoked magnetic fields to median nerve stimulation were recorded; at functional MR imaging, hemodynamic responses to self-paced palmar flexion of the wrist were imaged. General linear model analysis with contextual clustering (P < .01) was used to analyze functional MR imaging data, and dipole modeling was used to analyze MEG data. MEG and functional MR localizations were compared with intraoperative cortical mappings. The distance from the area of functional MR imaging activation to the tumor margin was compared between the patients with discordant and those with concordant intraoperative mapping findings by using unpaired t testing. MEG depicted the central sulcus correctly in all 15 patients, as verified at intraoperative mapping. The functional MR imaging localization results agreed with the intraoperative mappings in 11 patients. In all four patients with a false localization, the primary activation was in the postcentral sulcus region, but it did not differ significantly from the primary activation in the patients with correct localization with respect to proximity to the tumor (P = .38). Furthermore, at functional MR imaging, multiple nonprimary areas were activated, with considerable interindividual variation. Although both MEG and functional MR imaging can provide useful information for neurosurgical planning, in the present study, MEG proved to be superior for locating the central sulcus. Activation of multiple nonprimary cerebral areas may confound the interpretation of functional MR imaging results.
NeuroReport, 1996
The functional significance of the second somatosensory cortex (SII) is poorly understood. Howeve... more 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.
NeuroReport, 2010
This study investigated the characteristics of event-related potentials using somatosensory go/no... more This study investigated the characteristics of event-related potentials using somatosensory go/no-go paradigms. We manipulated the interstimulus interval and analyzed its effect on the peak amplitude and latency of the N140 and P300 components. The amplitude of N140 increased as the interstimulus interval increased, and was significantly larger in no-go than in go trials at the 1-s and 2-s interstimulus intervals, but not the 4-s and 6-s interstimulus intervals. The amplitude of P300 also increased with the interstimulus interval, and was significantly larger in no-go than in go trials at all interstimulus intervals. The reaction time in go trials was longer with increasing interstimulus interval. This study suggests that brain activities associated with go/no-go decisional processes are influenced by the interstimulus interval.
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Papers by Antti Korvenoja