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Biomedical subjects

M Kajola

Publications and source records attributed to M Kajola.

At least 19 recordsLinked to original sources
PubMed1997

Cortical control of human motoneuron firing during isometric contraction.

We recorded whole scalp magnetoencephalographic (MEG) signals simultaneously with the surface electromyogram from upper and lower limb muscles of six healthy right-handed adults during voluntary isometric contraction. The 15- to 33-Hz MEG signals, originating from the anterior bank of the central sulcus, i.e., the primary motor cortex, were coherent with motor unit firing in all subjects and for all muscles. The coherent cortical rhythms originated in the hand motor area for upper limb muscles (1st dorsal interosseus, extensor indicis proprius, and biceps brachii) and close to the foot area for lower limb muscles (flexor hallucis brevis). The sites of origin corresponding to different upper limb muscles did not differ significantly. The cortical signals preceded motor unit firing by 12-53 ms. The lags were shortest for the biceps brachii and increased systematically with increasing corticomuscular distance. We suggest that the motor cortex drives the spinal motoneuronal pool during sustained contractions, with the observed cortical rhythmic activity influencing the timing of efferent commands. The cortical rhythms could be related to motor binding, but the rhythmic output may also serve to optimize motor cortex output during isometric contractions.

Adult↗
PubMed1996

Neural net identification of thumb movement using spectral characteristics of magnetic cortical rhythms.

Neural nets have shown great promise as tools for reducing and examining multi-dimensional data. When carefully tuned with selected data sets of individual subjects neural nets have indisputable potential in identifying distinct stages of voluntary finger movements. However, robust, automatized data description methods would be needed to eventually extend the use of neural networks into visualization of brain activity during more complex, multimodal tasks where the cortical processes are not equally well understood. We explored the suitability of a self-organizing map (SOM) in the widely studied case of voluntary finger movements (left and right thumb), using as input such spectral characteristics that showed systematic task-dependent changes when averaged over repeated movements. SOMs constructed without individual fine-tuning and with generally chosen training parameters from these spectral features identified correctly 85% of the ongoing movements but, somewhat surprisingly, not the side of thumb movement. Even for this inclusive choice of input, the neural nets were sensitive to transient signals, but focused fine tuning, based on a priori known subgroups in the data, is clearly required for more detailed classification. Thus, a neural net visualization is likely not the most attractive first approach for characterization of cortical processing during complex multimodal tasks.

Brain Mapping↗
PubMed1996

Movement-related slow cortical magnetic fields and changes of spontaneous MEG- and EEG-brain rhythms.

Cortical activity was recorded from 5 healthy adults with a 122-channel whole-head magnetometer while the subjects performed during unilateral finger movements at self-paced intervals exceeding 6 s. The readiness field (RF) started over the contralateral somatomotor area 0.3-1 s prior to the movement onset in subjects (Ss) 1, 2, and 4, and culminated in the motor field (MF) 30 ms after it (Ss 1-4). These signals were followed by movement evoked fields MEFI (Ss 1-5) and MEFII (Ss 1-4) at 100-150 ms and 200-250 ms after the movement onset, respectively. One subject showed clear RF over the ipsilateral hemisphere as well. The contralateral dominance of the RF contrasted the more symmetric distribution of the simultaneously recorded electric Bereitschaftspotential (BP). The RF onset never preceded the BP onset. We suggest that BP receives contribution from the early bilateral activation of the crown of the precentral gyrus, whereas RF reflects later activity of the fissural motor cortex. Spontaneous oscillations in the background activity (spontaneous activity) of approximately 10 Hz started to dampen 2-3 s prior to the movement onset in the somatomotor areas of both hemispheres with contralateral predominance (S1 and S3), and returned to a steady level 0.8-2 s after the movement onset in all subjects. Higher frequency bands in the same area displayed a prominent rebound about 1 s after the movement onset in 4 subjects. Execution of self-paced movements is evidently expressed differently in the slow movement-related fields and in the cortical spontaneous activity.

Adult↗
PubMed1995

Visual cortex activation in blind humans during sound discrimination.

We used a whole-scalp magnetometer with 122 planar gradiometers to study the activity of the visual cortex of five blind humans deprived of visual input since early infancy. Magnetic responses were recorded to pitch changes in a sound sequence when the subjects were either counting these changes or ignoring the stimuli. In two of the blind subjects, magnetic resonance images were also obtained, showing normal visual cortex macroanatomy. In these subjects, the magnetic responses to counted pitch changes were located at visual and temporal cortices whereas ignored pitch changes activated the temporal cortices almost exclusively. Also in two of the other three blind, the visual-cortex activation was detectable in the auditory counting task. Our results suggest that the visual cortex of blind humans can participate in auditory discrimination.

Acoustic Stimulation↗
PubMed1995

Functional segregation of movement-related rhythmic activity in the human brain.

Multiple synaptic interconnections in the human brain support concerted rhythmic activity of a large number of cortical neurons, typically close to 10 and 20 Hz. Our present neuromagnetic data provide evidence for distinct functional roles of these spectral components in the somatomotor cortex. The sites of suppression during movement and the subsequent rebound of the 20-Hz rhythm followed, along the motor cortex, the representation of fingers, toes, and mouth, as opposed to the stable origin of the 10-Hz rhythms close to the hand somatosensory cortex. The 20-Hz activity appears to be a signature of active immobilization following movement, whereas the reactive 10-Hz signals likely reflect lack of relevant sensory input from the important upper limbs.

Adult↗
PubMed1995

Signal-space projections of MEG data characterize both distributed and well-localized neuronal sources.

We describe the use of signal-space projection (SSP) for the detection and characterization of simultaneous and/or sequential activation of neuronal source distributions. In this analysis, a common signal space is used to represent both the signals measured by an array of detectors and the underlying brain sources. This presents distinct advantages for the analysis of EEG and MEG data. Both highly localized and distributed sources are characterized by the components of the field patterns which are measured by the detectors. As a result, a unified description of arbitrary source configurations is obtained which permits the consistent implementation of a variety of analysis techniques. The method is illustrated by the application of SSP to auditory, visual and somatosensory evoked-response MEG data. Single-trace evoked responses obtained by SSP of spontaneous activity demonstrate that a considerable discrimination against both system noise and uncorrelated brain activity may be achieved. Application of signal-space projections determined in the frequency domain to spontaneous activity illustrates the possibility of including temporal relationships into the analysis. Finally, we demonstrate that SSP is particularly useful for the description of multiple sources of distributed activity and for the comparison of the strengths of specific neuronal sources under a variety of different paradigms or subject conditions.

Brain↗
PubMed1995

Reactivity of magnetic parieto-occipital alpha rhythm during visual imagery.

Spontaneous MEG signals were recorded during visual imagery from 13 healthy adults with a whole-scalp neuromagnetometer. The parieto-occipital 7-14 Hz alpha activity was suppressed strongly while subjects visualized and evaluated letters. The act of forming a visual image caused a smaller suppression than did inspection of the imaged pattern for a named property. The maximum suppression depended on the baseline alpha level and, for the majority of the subjects, occurred close to the area with the strongest alpha, showing no systematic hemispheric asymmetry. Sources for the alpha activity, modeled with equivalent current dipoles, clustered in the parietal and occipital lobes. The strongest suppression of the activity occurred near the parieto-occipital sulcus.

Adult↗
PubMed1994

Activation of the human posterior parietal cortex by median nerve stimulation.

We recorded somatosensory evoked magnetic fields from ten healthy, right-handed subjects with a 122-channel whole-scalp SQUID magnetometer. The stimuli, exceeding the motor threshold, were delivered alternately to the left and right median nerves at the wrists, with interstimulus intervals of 1, 3, and 5 s. The first responses, peaking around 20 and 35 ms, were explained by activation of the contralateral primary somatosensory cortex (SI) hand area. All subjects showed additional deflections which peaked after 85 ms; the source locations agreed with the sites of the secondary somatosensory cortices (SII) in both hemispheres. The SII responses were typically stronger in the left than the right hemisphere. All subjects had an additional source, not previously reported in human evoked response data, in the contralateral parietal cortex. This source was posterior and medial to the SI hand area, and evidently in the wall of the postcentral sulcus. It was most active at 70-110 ms.

Adult↗
PubMed1994

Cortical reactivity in progressive myoclonus epilepsy.

We studied 4 patients with progressive myoclonus epilepsy (Unverricht-Lundborg disease; ULD). Somatosensory evoked fields (SEFs), auditory evoked fields (AEFs), and spontaneous activity over the somatomotor cortex were recorded with a 24-channel SQUID gradiometer. All patients had "giant" 20-45 msec median nerve SEFs at the first somatomotor cortex, with 2-6 times larger amplitudes than the healthy control subjects. Later deflections were not similarly enhanced. The dependence of SEF amplitudes on interstimulus interval (0.2-4 sec) and on successive ulnar-median nerve stimulation (stimulus interval 40 msec) was comparable to that in controls. Cortical AEFs were attenuated and delayed. In 3 patients, the spontaneous activity consisted of 6-8 Hz mu rhythm, which originated within 2 cm from the sources of SEFs and was abolished by clenching of the contralateral fist. Control subjects had major spectral peaks around 10 and 20 Hz. The SEF amplitudes and the strength of the 6-10 Hz mu correlated strongly, suggesting that some components of evoked and spontaneous activity obtain contributions from overlapping neuronal populations. The results imply that ULD is associated with thalamo-cortical hyperreactivity in the sensorimotor but not in the auditory system.

Acoustic Stimulation↗
PubMed1993

Parietal epileptic mirror focus detected with a whole-head neuromagnetometer.

Whole-head magnetoencephalographic recordings revealed two parietal epileptic foci in homotopic areas of the hemispheres. The discharges occurred 17-20 ms later on the left than on the right hemisphere, implying the existence of a left-sided mirror focus. The foci were about 1 cm posterior to the hand primary somatosensory area, identified by evoked response measurements, and thus suggested epileptic activity at the parietal association cortex, in agreement with the observed callosal conduction time.

Adolescent↗
PubMed1993

A comparison of the localization of spontaneous neuromagnetic activity in the frequency and time domains.

We investigated the localization of current sources for spontaneous magnetoencephalographic data in the frequency and time domains. The two analysis techniques yielded complementary information about the underlying neuronal generators. Phase-coherent sources for occipital 10 Hz alpha band activity were identified in the frequency domain from the dependence of the equivalent current dipole strengths on the phase of the Fourier transform. Source localization in both the frequency and time domains was used to analyze data containing interictal spike and slow-wave activity. Predominantly 2-6 Hz spectral components localized in the frequency domain were found within an 8 cm3 volume centered at the time-domain source location of the spike. In general, the characteristics of the noise sources in magnetoencephalographic systems favor the use of frequency-domain analysis for rhythmic spontaneous activity.

Brain↗
PubMed1992

Magnetoencephalographic localization of epileptic cortex--impact on surgical treatment.

A 24-channel, planar, superconducting quantum interference device gradiometer, sampling a fourth of the head surface over brain tissue, was used to determine the site of an epileptic focus in a 36-year-old woman with intractable complex partial epilepsy. The other presurgical findings appeared divergent: a large arachnoid cyst over the right parietal convexity, dissimilar interictal electroencephalographic patterns, and several neuropsychological dysfunctions. The equivalent current sources of magnetoencephalographic spikes were in the right posterior temporal region of the cortex, 4 cm apart from the cyst. Surgical exploration of the area pinpointed by magnetoencephalography revealed a pachygyric patch of cortex displaying focal discharges on the electrocorticogram. After resection, a dramatic reduction of seizures occurred. The good agreement between electrocorticography and magnetoencephalography warrants future investigation of multichannel magnetoencephalography as a potential alternative to invasive presurgical recordings.

Adult↗
PubMed1992

Generator sites of spontaneous MEG activity during sleep.

We have recorded spontaneous magnetoencephalographic (MEG) activity during overnight natural sleep in 4 healthy adults with a 24-channel SQUID gradiometer, mainly over the sides of the head. All sleep stages were obtained. The MEG wave forms resembled the EEG phenomena recorded simultaneously from the scalp midline, but the electric and magnetic signals did not always coincide. The source locations of different signals were studied by using a current dipole model. The equivalent sources of magnetic transients, resembling and often coinciding with the electric vertex waves and K-complexes, as well as the transients during REM sleep, were concentrated within a volume of 4 x 4 x 3 cm3 in the inferior parietal lobe. For spindles and slow waves, no such focal generators were found.

Adult↗
PubMed1992

Modified activity of the human auditory cortex during auditory hallucinations.

Previous reports have shown abnormalities in brain metabolism and evoked responses of schizophrenic patients with hallucinations. The authors recorded electric and magnetic auditory responses during transitory auditory hallucinations in two patients. Small but replicable response delays occurred during hallucinations. The results suggest that the effect of hallucinations on auditory cortex activity is similar to the effect of real sounds.

Acoustic Stimulation↗
PubMed1991

Magnetoencephalographic 10-Hz rhythm from the human auditory cortex.

Spontaneous magnetoencephalographic activity was recorded with a 24-SQUID gradiometer over the lateral aspects of the head in 3 healthy adults. All subjects displayed 8-10 Hz rhythmic activity which was not affected by opening of the eyes but was occasionally dampened by auditory stimuli. The equivalent sources of the rhythm were in the supratemporal auditory cortex, and the activity may therefore represent 'idling' of the auditory cortex. Obviously each sensory projection cortex has its own local spontaneous rhythm.

Acoustic Stimulation↗
PubMed1991

Landau-Kleffner syndrome: epileptic activity in the auditory cortex.

The Landau-Kleffner syndrome (LKS) is characterized by electroencephalographic spike discharges and verbal auditory agnosia in previously healthy children. We recorded magnetoencephalographic (MEG) spikes in a patient with LKS, and compared their sources with anatomical information from magnetic resonance imaging. All spikes originated close to the left auditory cortex. The evoked responses were contaminated by spikes in the left auditory area and suppressed in the right--the latter responses recovered when the spikes disappeared. We suggest that unilateral discharges at or near the auditory cortex disrupt auditory discrimination in the affected hemisphere, and lead to suppression of auditory information from the opposite hemisphere, thereby accounting for the two main criteria of LKS.

Aphasia↗