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Biomedical subjects

V Jousmäki

Publications and source records attributed to V Jousmäki.

At least 19 recordsLinked to original sources
PubMed2003

Dorsal penile nerve stimulation elicits left-hemisphere dominant activation in the second somatosensory cortex.

Activation of peripheral mixed and cutaneous nerves activates a distributed cortical network including the second somatosensory cortex (SII) in the parietal operculum. SII activation has not been previously reported in the stimulation of the dorsal penile nerve (DPN). We recorded somatosensory evoked fields (SEFs) to DPN stimulation from 7 healthy adults with a 122-channel whole-scalp neuromagnetometer. Electrical pulses were applied once every 0.5 or 1.5 sec to the left and right DPN. For comparison, left and right median and tibial nerves were stimulated alternatingly at 1.5-sec intervals. DPN stimuli elicited weak, early responses in the vicinity of responses to tibial nerve stimulation in the primary somatosensory cortex. Strong later responses, peaking at 107-126 msec were evoked in the SII cortices of both hemispheres, with left-hemisphere dominance. In addition to tactile processing, SII could also contribute to mediating emotional effects of DPN stimuli.

Adult↗
PubMed2002

Cortical activation associated with passive movements of the human index finger: an MEG study.

We recorded somatosensory evoked fields to passive extensions of the left and right index fingers in eight healthy adults. A new nonmagnetic device was designed to produce calibrated extensions of 19 degrees, with a mean angular velocity of 630 degrees/s. The responses, recorded with a 306-channel neuromagnetometer, were modeled with current dipoles. The earliest activation was in the primary somatosensory cortex, with peaks at 36-58 and 30-82 ms for left and right index finger extensions, respectively. Later signals were observed in the left second somatosensory (SII) cortex in six of eight subjects at 75-175 and 75-155 ms for left- and right-sided extensions, respectively; three subjects showed bilateral SII activation in at least one condition. Our results suggest a predominant role for the human left SII cortex in proprioceptive processing.

Adult↗
PubMed2001

Evidence for a 7- to 9-Hz "sigma" rhythm in the human SII cortex.

Electrical activity of the human brain features several rhythmical components which can be readily studied with whole-scalp neuromagnetometers. We describe a new 7- to 9-Hz "sigma" rhythm in the human second somatosensory cortex, distinct from both the mu rhythm of the primary sensorimotor cortex and the tau rhythm of the supratemporal auditory cortex. Sigma shows rate-selective responsiveness to rhythmical median nerve stimulation and is enhanced by stimulation at the rhythm's dominant frequency. Single stimuli may trigger several periods of the rhythm. The functional significance of the sigma rhythm remains to be investigated.

Adult↗
PubMed2001

Functional overlap of finger representations in human SI and SII cortices.

We aimed to find out to what extent functional representations of different fingers of the two hands overlap at the human primary and secondary somatosensory cortices SI and SII. Somatosensory evoked fields (SEFs) were recorded with a 306-channel neuromagnetometer from 8 subjects. Tactile stimuli, produced by diaphragms driven by compressed air, were delivered to the fingertips in three different conditions. First, the right index finger was stimulated once every 2 s. Then two other stimuli were interspersed, in different sessions, to right- or left-hand fingers (thumb, middle finger, or ring finger) between the successive right index finger stimuli. Strengths of the responses to right index finger stimuli were evaluated in each condition. Responses to right index finger stimuli were modeled by three current dipoles, located at the contralateral SI and the SII cortices of both hemispheres. The earliest SI responses, peaking around 65 ms, were suppressed by 18% (P < 0.05) when the intervening stimuli were presented to the same hand; intervening stimuli to the other hand had no effect. The SII responses were bilaterally suppressed by intervening stimuli presented to either hand: in the left SII, the suppression was 39 and 42% (P < 0.01) and in the right SII 67 and 72% (P < 0.001) during left- and right-sided intervening stimuli, respectively. Left- and right-sided intervening stimuli affected similarly the SII responses and had no effect on the response latencies. The results indicate a strong and symmetric overlap of finger representations for both hands in the human SII cortices, and a weaker functional overlap for fingers of the same hand in the SI cortex.

Adult↗
PubMed2000

Speaking modifies voice-evoked activity in the human auditory cortex.

The voice we most often hear is our own, and proper interaction between speaking and hearing is essential for both acquisition and performance of spoken language. Disturbed audiovocal interactions have been implicated in aphasia, stuttering, and schizophrenic voice hallucinations, but paradigms for a noninvasive assessment of auditory self-monitoring of speaking and its possible dysfunctions are rare. Using magnetoencephalograpy we show here that self-uttered syllables transiently activate the speaker's auditory cortex around 100 ms after voice onset. These phasic responses were delayed by 11 ms in the speech-dominant left hemisphere relative to the right, whereas during listening to a replay of the same utterances the response latencies were symmetric. Moreover, the auditory cortices did not react to rare vowel changes interspersed randomly within a series of repetitively spoken vowels, in contrast to regular change-related responses evoked 100-200 ms after replayed rare vowels. Thus, speaking primes the human auditory cortex at a millisecond time scale, dampening and delaying reactions to self-produced "expected" sounds, more prominently in the speech-dominant hemisphere. Such motor-to-sensory priming of early auditory cortex responses during voicing constitutes one element of speech self-monitoring that could be compromised in central speech disorders.

Adult↗
PubMed2000

Cognitive response profile of the human fusiform face area as determined by MEG.

Activation in or near the fusiform gyrus was estimated to faces and control stimuli. Activation peaked at 165 ms and was strongest to digitized photographs of human faces, regardless of whether they were presented in color or grayscale, suggesting that face- and color-specific areas are functionally separate. Schematic sketche evoked approximately 30% less activation than did face photographs. Scrambling the locations of facial features reduced the response by approximately 25% in either hemisphere, suggesting that configurational versus analytic processing is not lateralized at this latency. Animal faces evoked approximately 50% less activity, and common objects, animal bodies or sensory controls evoked approximately 80% less activity than human faces. The (small) responses evoked by meaningless control images were stronger when they included surfaces and shading, suggesting that the fusiform gyrus may use these features in constructing its face-specific response. Putative fusiform activation was not significantly related to stimulus repetition, gender or emotional expression. A midline occipital source significantly distinguished between faces and control images as early as 110 ms, but was more sensitive to sensory qualities. This source significantly distinguished happy and sad faces from those with neutral expressions. We conclude that the fusiform gyrus may selectively encode faces at 165 ms, transforming sensory input for further processing.

Adult↗
PubMed2000

Extraction of event-related signals from multichannel bioelectrical measurements.

Independent component analysis (ICA) is a powerful tool for separating signals from their mixtures. In this field, many algorithms were proposed, but they poorly use a priori information in order to find the desired signal. Here, we propose a fixed point algorithm which uses a priori information to find the signal of interest out of a number of sensors. We particularly applied the algorithm to cancel cardiac artifacts from a magnetoencephalogram.

Algorithms↗
PubMed2000

Independent component approach to the analysis of EEG and MEG recordings.

Multichannel recordings of the electromagnetic fields emerging from neural currents in the brain generate large amounts of data. Suitable feature extraction methods are, therefore, useful to facilitate the representation and interpretation of the data. Recently developed independent component analysis (ICA) has been shown to be an efficient tool for artifact identification and extraction from electroencephalographic (EEG) and magnetoencephalographic (MEG) recordings. In addition, ICA has been applied to the analysis of brain signals evoked by sensory stimuli. This paper reviews our recent results in this field.

Algorithms↗
PubMed1999

Task-dependent modulation of 15-30 Hz coherence between rectified EMGs from human hand and forearm muscles.

1. Recent reports have shown task-related changes in oscillatory activity in the 15-30 Hz range in the sensorimotor cortex of human subjects and monkeys during skilled hand movements. In the monkey these oscillations have been shown to be coherent with oscillatory activity in the electromyographic activity of hand and forearm muscles. 2. In this study we investigated the modulation of oscillations in the electromyogram (EMG) of human volunteers during tasks requiring precision grip of two spring-loaded levers. 3. Two tasks were investigated: in the 'hold' task, subjects were required to maintain a steady grip force (ca 2.1 N or 2.6 N) for 8 s. In the 'ramp' task, there was an initial hold period for 3 s (force ca 2.1 N) followed by a linear increase in grip force over a 2 s period. The task ended with a further steady hold for 3 s at the higher force level (ca 2.6 N). 4. Surface EMGs were recorded from five hand and forearm muscles in 12 subjects. The coherence of oscillatory activity was calculated between each muscle pair. Frequencies between 1 and 100 Hz were analysed. 5. Each subject showed a peak in the coherence spectra in the 15-30 Hz bandwidth during the hold task. This coherence was absent during the initial movement of the levers. During the ramp task the coherence in the 15-30 Hz range was also significantly reduced during the movement phase, and significantly increased during the second hold period, relative to the initial hold. 6. There was coherence between the simultaneously recorded magnetoencephalogram (MEG) and EMG during steady grip in the hold task; this coherence disappeared during the initial lever movement. Using a single equivalent current dipole source model, the coherent cortical activity was localized to the hand region of the contralateral motor cortex. This suggests that the EMG-EMG coherence was, therefore, at least in part, of cortical origin. 7. The results are discussed in terms of a possible role for synchrony in the efficient recruitment of motor units during maintained grip.

Adult↗
PubMed1999

Somatosensory evoked fields to large-area vibrotactile stimuli.

We describe a method to apply large-area vibrotactile stimuli, based on a vibrating balloon, on the palms of both hands during evoked response studies. Magnetoencephalographic (MEG) signals were recorded with a whole-scalp neuromagnetometer from six healthy subjects while they held their hands on a balloon which was made to vibrate by delivering tones to it through a loudspeaker and a tube. The 200 Hz stimuli, presented once every 1 or 2 s in separate sessions, elicited prominent and replicable somatosensory evoked fields (SEFs) and also auditory evoked fields (AEFs) due to the concomitant sound. Source modelling allowed reliable differentiation between bilateral activation of the primary somatosensory (SI) cortices (peaks at 46-61 ms after the stimulus onset) and of the supratemporal auditory cortices (peaks at 104-126 ms). These simple vibrotactile stimuli could be useful for rapid and reliable identification of the somatosensory and auditory cortices, for example in presurgical evaluation of children.

Acoustic Stimulation↗
PubMed1999

Magnetoencephalography in presurgical evaluation of children with the Landau-Kleffner syndrome.

PURPOSE: Our aim was (a) to localize the primary epileptogenic cortex for possible multiple subpial transsection in four children with the Landau-Kleffner syndrome (LKS), and (b) to evaluate the impact of magnetoencephalography (MEG) in the localizing process. METHODS: We used EEG to detect the overall epileptiform activity and MEG for selective recording of fissural spikes. The cortical generators of MEG spikes were modeled with dipoles, and their activation order was determined. The voltage distribution, consistent with the earliest MEG sources, was then identified during the course of the patient's EEG spikes to determine the relative timing between stereotypic EEG and MEG spikes and to distinguish the earliest (primary) source area among the secondary ones. RESULTS: In all patients, the earliest spike activity originated in the intrasylvian cortex, spreading in one subject to the contralateral sylvian cortex within 20 ms. Secondary spikes occurred within 10-60 ms in ipsilateral perisylvian, temporooccipital, and parietooccipital areas. A single intrasylvian pacemaker initiated all epileptic activity in two patients, whereas the other two had independent left- and right-hemisphere circuits or focal spikes. MEG source dynamics predicted the results of the methohexital suppression test in two patients and was confirmed by surgery outcome in one patient, in whom all epileptic activity ceased after a small transsection of the sylvian pacemaker. CONCLUSIONS: (a) The intrasylvian cortex is a likely pacemaker of epileptic discharges in LKS, and (b) MEG provides useful presurgical information of the cortical spike dynamics in LKS patients.

Auditory Cortex↗
PubMed1998

Effects of stimulus intensity on signals from human somatosensory cortices.

We recorded somatosensory evoked magnetic fields (SEFs) to left median nerve electric stimulation from seven healthy subjects. The stimulus intensity was varied in three sessions: sensory stimuli evoked a clear tactile sensation without any movement, weak motor stimuli exceeded the motor threshold, and strong motor stimuli caused a vigorous movement. Responses were modelled with sources in the contralateral primary somatosensory cortex (SI), the contralateral and ipsilateral secondary somatosensory cortices (SIIs) and the contralateral posterior parietal cortex (PPC). The amplitude of the 20 ms response from the SI cortex and the subjective magnitude estimations followed the stimulus intensity whereas signals from the three other areas saturated already at the level of the motor threshold. The results implicate differential roles for various somatosensory cortices in intensity coding.

Adult↗
PubMed1998

Vibration-induced auditory-cortex activation in a congenitally deaf adult.

Considerable changes take place in the number of cerebral neurons, synapses and axons during development, mainly as a result of competition between different neural activities [1-4]. Studies using animals suggest that when input from one sensory modality is deprived early in development, the affected neural structures have the potential to mediate functions for the remaining modalities [5-8]. We now show that similar potential exists in the human auditory system: vibrotactile stimuli, applied on the palm and fingers of a congenitally deaf adult, activated his auditory cortices. The recorded magnetoencephalographic (MEG) signals also indicated that the auditory cortices were able to discriminate between the applied 180 Hz and 250 Hz vibration frequencies. Our findings suggest that human cortical areas, normally subserving hearing, may process vibrotactile information in the congenitally deaf.

Adult↗
PubMed1998

Sensorimotor integration in human primary and secondary somatosensory cortices.

We measured somatosensory evoked fields (SEFs) to electric median nerve stimuli from eight healthy subjects with a whole-scalp 122-channel neuromagnetometer in two different conditions: (i) 'rest', with stimuli producing clear tactile sensation without any motor movement, and (ii) 'contraction' with exactly the same stimuli as in 'rest', but with the subjects maintaining sub-maximal isometric contraction in thenar muscles of the stimulated hand. The aim was to study the role of the primary (SI) and secondary somatosensory (SII) cortices in sensorimotor integration. The amplitude of the SI response N20m did not change with coincident isometric contraction, whereas P35m was significantly reduced. On the contrary, activation of contra- and ipsilateral SII cortices was significantly enhanced during the contraction. We suggest that isometric contraction facilitates activation of SII cortices to tactile stimuli, possibly by decreasing inhibition from the SI cortex. The enhanced SII activation may be related to tuning of SII neurons towards relevant tactile input arising from the region of the body where the muscle activation occurs.

Adult↗
PubMed1998

Three hands: fragmentation of human bodily awareness.

We describe patient E.P. who occasionally perceives a 'ghost' hand which copies the previous positions of the left hand with a 0.5-1 min time lag, but follows the movement patterns of the right hand. The symptoms started after an operation of a ruptured aneurysm, followed by an infarction of the right frontal lobe; E.P. also has a previously lesioned corpus callosum. Neuromagnetic recordings revealed that activity of the left secondary somatosensory cortex was strongly suppressed during the ghost arm percept, thereby providing an objective correlate for E.P.'s sensations. We conclude that simultaneous mental contents about body scheme may be based on neural information extracted at considerably different times, resulting in fragmentation of bodily awareness.

Adult↗
PubMed1998

Neural processing of human faces: a magnetoencephalographic study.

This is a whole head magnetoencephalographic (MEG) study of the neural processing of briefly presented images of human faces in 14 normal subjects. The experiments involved three tasks of increasing complexity, involving image categorisation, image comparison and the identification of emotion. The analyses were based on average responses to repeated stimuli in the different image categories. These averages were processed to give numerical measures of the power within defined regions and latency spans. The only statistically significant difference in these data between the response to faces and other images is in the right occipito-temporal channels at a latency of 140 ms. The face-specific response is largely independent of the task. Source modelling suggests an extended source in the ventral occipito-temporal region. The analysis supports the notions of both face-specificity and right hemisphere dominance for all image types at early latencies.

Adult↗
PubMed1997

Activation trace lifetime of human cortical responses evoked by apparent visual motion.

Visually evoked magnetoencephalographic responses were recorded from 11 healthy humans to 1.1 x 1.1 degrees oblique gratings moving quickly 0.2 degree rightwards and back once every 0.2-6.4 s. The aim was to study the duration of sensory memory in the motion-specific visual cortex called V5. Responses from the V5 region peaked at 140-180 ms after stimulus onset. Signal-to-noise ratio allowed source identification in eight subjects: bilaterally in four and unilaterally in four. The response strength as a function of interstimulus interval determined an activation trace lifetime, reflecting how long the preceding stimuli affect the response to the following stimulus, i.e. how long the V5 cortex "remembers' each stimulus. The lifetimes varied interindividually from 0.4 to 1.4 s, but were within 0.1 s in the hemispheres of the four subjects with bilaterally identified sources.

Adolescent↗