Biomedical subjects

I M Tarkka

Publications and source records attributed to I M Tarkka.

At least 19 recordsLinked to original sources

PubMed1999

Sources of movement-related cortical potentials derived from foot, finger, and mouth movements.

Movement-related cortical potentials (MRCPs) register brain electrical activity before and during movement execution. In an attempt to delineate the components of MRCPs that reflect common sources to various movements and that are movement-specific, simple self-paced voluntary foot, finger, and mouth movements were studied. MRCPs were recorded in eight healthy volunteers with 30 electrodes placed on the scalp. Data were analyzed using Brain Electric Source Analysis software, and multiple equivalent dipole models were developed to separate spatial and temporal aspects of brain activity related to the execution of voluntary movements. Independent models were separately developed for the grand average data and for the individual subjects' data for each movement type. MRCPs derived from foot movements were accounted for using a 5-dipole model, finger movements using an 8-dipole model, and mouth movements with a 7-dipole model, yielding the grand average residual variances of 3%, 2%, and 6%, respectively. Based on individual models, intersubject variability of dipole locations was less than 10 mm (+/- SD). Overlaying the mean dipole coordinates onto the stereotaxic atlas provided proof that the sensorimotor cortical areas, supplementary motor area, and also cerebellum and thalamus were active in all three movements. Locations of the dipoles in the contralateral sensorimotor area clearly implied well-known medial to lateral somatotopic organization of foot, finger, and mouth movements. Temporal separation of the activity spread over different brain areas was demonstrated by evolution in the moments of dipole source potentials. The authors' models support the view of simultaneous activation of the primary motor cortex and supplementary motor area at the time of movement execution. Multiple equivalent dipole models developed in this study implied the activity originating in corresponding brain areas as previously detected by positron emission tomography or functional magnetic resonance imaging. However, MRCPs provided additional information regarding the temporal evolution of the brain activity related to the execution of voluntary movements. Thus, the concurrent use of MRCPs and other imaging techniques may provide complementary information not easily obtained by the other imaging techniques themselves.

Adult↗

PubMed1998

Source localization of P300 from oddball, single stimulus, and omitted-stimulus paradigms.

Three different auditory stimulus paradigms were used to elicit P300 potentials. Normal subjects were tested on the classical rare target stimulus, single-stimulus and omitted-stimulus conditions. Noninvasive identification of the cerebral sources of the event-related potentials (ERPs) was performed using spatio-temporal multiple dipole modeling (BESA software) with individually sized spherical head models. The grand average data of each condition was first independently modeled and these models were used as starting values for modeling each individual subject's data. Models for the rare-stimulus condition and single-stimulus condition both consisted of 6 dipoles. Models for the omitted-stimulus condition consisted of 2 dipoles. The dipole locations of the final individual 6-dipole models for the rare and single-stimulus conditions did not differ significantly from each other or from one previous result obtained from a another group of subjects (Tarkka et al. 1995). Super-imposition of the dipole coordinates on the sterotaxic brain atlas suggests that bilateral deep medial temporal lobe structures are the major contributors to rare and single-stimulus P300s. Because both the wave form morphology and the source model of the P300 elicited by single stimulus were close to those of the rare-stimulus P300 it may be that the underlying neural mechanisms eliciting these P300 potentials are essentially the same.

Acoustic Stimulation↗

PubMed1997

Magnetic fields from human prefrontal cortex differ during two recognition tasks.

The present study represents our second successful use of magnetoencephalography to identify different sources of human prefrontal activity corresponding to subjects' engagement in different tasks. We used two visual recognition tasks: a familiar person recognition and an abstract pattern recognition task in the context of a design suitable for eliciting Contingent Negative Variations (CNVs) and their concurrent slow magnetic fields in this preliminary study of 5 subjects. Each trial of either task was started by one of two specific warning symbols (S1), indicating whether a person's picture or an abstract pattern should be attended during the presentation of a second stimulus (S2), and compared to the corresponding person's picture or pattern contained in the third stimulus, (S3) that followed. The S2 and S3 stimuli were common to both tasks, and were composed of patterns made with four line traces superimposed on photographs of persons familiar to each subject. Subjects responded with a right hand button press, following S3, indicating their judgments regarding the identity of the patterns or persons' pictures contained in the S2 and the S3 stimuli, for the two tasks, respectively. Results showed that the sources of the CNV equivalent magnetic fields were localized in different cortical regions depending on the task and that this difference was consistent across all subjects. The sources were localized in the right hemisphere, in medial areas of the prefrontal cortex for the person recognition task and in the dorsolateral prefrontal cortex for the pattern recognition task. The same degree of consistency was not found for the left hemisphere sources. Moreover, as in our previous study, we found no difference between the sources active during the first and the second CNV periods (occurring during the S1-S2 and the S2-S3 intervals, respectively), within each task condition.

Adult↗

PubMed1996

Dissociation of cortical areas responsible for evoking excitatory and inhibitory responses in the small hand muscles.

Noninvasive transcranial magnetic stimulation (TMS) of the brain using a focal eight-shaped coil with 100% stimulation output was performed in eleven healthy subjects to find out if excitatory and inhibitory responses in the small hand muscles could be dissociated. Motor evoked potentials (MEP) as well as silent periods (SP) were recorded from the right abductor pollicis brevis (APB), and first dorsal interosseus (FDI) muscles at rest and during weak voluntary contraction. Mapping of the cortical representation area was performed over different scalp locations on the left hemisphere. The cortical representation maps for ABP and FDI recorded during contraction covered much larger area and were more elongated in the anterior-posterior than in the medial-lateral direction compared to maps obtained during relaxation. The distribution maps for SPs covered larger scalp areas compared to the maps of MEPs obtained during voluntary contraction. Also during voluntary contraction the locations for evoking the longest SPs were not identical to locations for evoking the peak MEP amplitudes; the longest SPs were observed during stimulation of more medial and frontal locations compared to peak MEPs. Interestingly, stimulation of some locations resulted in the appearance of an isolated MEP without the following SP and in other locations an isolated SP was recorded. The areas for evoking isolated MEPs were in the center, whereas the areas for isolated SPs were located in the periphery of the map. Features such as exclusive locations for MEPs and SPs, and different locations for peak MEP amplitudes and longest SPs, suggest dissociation of the excitatory and inhibitory cortical processes evoked by transcranial magnetic stimulation during voluntary contraction.

Adult↗

PubMed1996

Generators for human P300 elicited by somatosensory stimuli using multiple dipole source analysis.

Cognitive event-related potentials, such as P300, are sensitive to manipulations of psychological variables and may provide evidence to support theories of brain mechanisms involved in cognition. However, the relationship between event-related potentials and the active neural structures is not yet understood. Electrical stimulation of the index and little fingers of the left hand in the context of a somatosensory target discrimination task, performed by healthy human subjects, elicited the middle-latency component of somatosensory evoked potentials, N60, the long-latency component, N140, and the P300 component. Identification of the generators for both the earlier components and P300, using equivalent electrical dipole modeling, was performed. Individual spatiotemporal seven-dipole models were developed in order to suggest locations of the sources generating each subject's scalp-recorded wave forms. Three dipoles with fairly weak moments, located in the primary and secondary sensory areas, explained the middle- and long-latency somatosensory evoked potential components, and the remaining four dipoles (4-7), with stronger dipole moments, were active during P300. There was a clear temporal separation of dipole activity between the somatosensory evoked potential components and the P300 component. Dipoles 4 and 5 were found quite symmetrically in the parahippocampal areas of the two hemispheres, while dipoles 6 and 7 were slightly asymmetrical. Dipole 7 was found in the left hippocampal area. Dipole 6 appeared in the right insular cortex. The locations of the four dipoles implicated in the generation of the somatosensory P300 were compared with the locations of four dipoles accounting for the auditory evoked P300 described in our previous paper [Tarkka et al. (1995) Electroenceph. clin. Neurophysiol. 96, 538-545]. No substantial difference in source locations of the P300 was found between auditory and somatosensory modality other than an asymmetrical activity in the somatosensory modality contralateral to the stimulated hand.

Adult↗

PubMed1996

Middle latency somatosensory evoked potentials: noninvasive source analysis.

The sources of the components of short latency somatosensory evoked potential in humans have been identified using electrical source localization, magnetoencephalography, and electrocorticography, but the sources of the middle latency components are less understood. The purpose of this study was to identify the source of the middle latency component N60 of the electrically elicited somatosensory evoked potential (SEP). Noninvasive equivalent electrical multiple dipole source localization technique (BESA) was used. SEPs were recorded from nine healthy subjects with 30 electrodes placed on the scalp. Median nerve at wrist (bipolar electrode), index, and little fingers (ring electrodes) were successively stimulated with 2.1 Hz frequency. A window of 100 ms was analyzed. Spatiotemporal equivalent electrical dipole models, each consisting of four dipoles, explained the three different SEPs, from median, index, and little fingers, with residual variances below 9%. The sources generating N20, P30, and N60 all appeared to locate in the posterior bank of the central sulcus contralateral to the stimulated hand. The location of the dipole number 2, most active around N20 component, differed between the index and little finger models, suggesting that somatotopic organization of the primary somatosensory area of the digits was demonstrated by these models. A radial source (dipole 4) was most active around 60 ms in all three models, thus explaining the N60 component. The location of this dipole did not suggest somatotopic organization specific to the stimulated finger for the 60 ms component.

Adult↗

PubMed1995

Early and late motor evoked potentials reflect preset agonist-antagonist organization in lower limb muscles.

A single transcranial magnetic stimulus can evoke two involuntary muscle responses in lower limb muscles of healthy humans. The purpose of the present study was to find out if these responses, when evoked during the processing period of a simple or choice reaction time task, such as ankle dorsiflexion, have specific characteristics related to the task. During the auditory reaction time, a transcranial magnetic stimulus was delivered to observe changes in the excitability of the central nervous system. A dual-cone coil was used, which effectively stimulated the fairly deep-lying lower limb motor cortex. Stimuli were delivered in a random order with 20-300-ms delays from the auditory go-signal. Motor evoked potentials (MEP) in right and left anterior tibial and soleus muscles were analyzed while early MEPs were observed invariably in both muscles; late MEPs occurred consistently only in soleus muscles. Both early and late MEP amplitudes were larger in simple reaction time trials than in choice reaction time trials. The late MEP appeared earlier in the simple reaction time task than in the choice reaction time task, reflecting faster central processing of simple reaction time tasks. The amplitude of the soleus late MEP in the simple reaction time task followed closely the amplitude of anterior tibial early MEP, suggesting a preset agonist-antagonist organization. This relationship was not present in the choice reaction time task.

Adult↗

PubMed1995

Electric source localization of the auditory P300 agrees with magnetic source localization.

The event-related cortical potential elicited in the context of auditory target detection tasks includes the N1, P2 and P3 components. The aim of the present study was to identify the sources of these scalp-recorded components using an electrical multiple dipole model. Nine healthy adults volunteered for the study. An auditory oddball paradigm was used. Stimuli (18% target and 82% non-target tones) were delivered through ear-phones and subjects were required to silently count the targets. Event-related potentials (ERPs) to these stimuli were recorded by 30 electrodes placed on the scalp. The identification of the sources of the ERP was attempted using the brain electric source analysis (BESA) program. The instantaneous source locations of N1, P2 and P3 reported in magnetoencephalographic (MEG) literature were used as initial starting locations for the spatio-temporal multiple dipole modeling of the EEG data. First the auditory long latency responses were modeled separately. Bilateral superior temporal plane sources with almost vertical orientations explained the first 250 msec window of the non-target tone recording including N1/P2 complex. This agrees with MEG source localization of N1m/P2m. Two slightly deeper dipoles in superior temporal gyri and bilateral dipoles in hippocampi or parahippocampal areas explained P3 (analysis window 250-600 msec). The final model explained the complete epoch of 600 msec with 6 dipoles and the residual variances of individual models ranged from 3.83% to 7.77%. The concordance between MEG and BESA source localization results supports the notion of generators in temporal lobes for the N1/P2 complex and generators in temporal and hippocampal areas for the P3 component.

Adult↗

PubMed1995

Asymmetry in walking performance and postural sway in patients with chronic unilateral cerebral infarction.

The asymmetrical nature of hemiparetic gait is well known; however, the role of walking asymmetry for speed performance is unclear. The purpose of the present study was to determine whether the range of walking speeds in chronic hemiparetic patients is associated with their gait asymmetry and postural sway. Twenty ambulatory patients with chronic unilateral supratentorial infarction were studied. Foot-ground contact patterns during swing and stance phases at various self-selected walking speeds were analyzed. The magnitude and direction of asymmetry in durations of stride phases were evaluated and compared with healthy subjects. Posturographic studies were performed to estimate the postural sway during quiet standing. Hemiparetic patients walked slower, more asymmetrically, and swayed more laterally favoring their nonaffected leg than did healthy persons. Although there was variability in durations of stride phases when comparing the two sides, a prolonged swing on the affected side and a prolonged stance on the nonaffected side were observed in all patients. The magnitude of asymmetry in stride phases varied among the patients; however, it was significantly higher than in controls (p < 0.03). Increased mean lateral sway during quiet standing was indicative of restricted velocity performance during walking. Patients with higher swing asymmetry achieved their maximum speed performance at lower velocity levels. However, the ability of patients to ambulate with a number of self-selected speeds was not associated with the magnitude of their overall gait asymmetry. Patients with right hemisphere lesions appeared to have less ambulatory ability than patients with left hemisphere lesions.

Adult↗

PubMed1994

Electrical source localization of human movement-related cortical potentials.

Movement-related cortical potentials (MRCP) of healthy humans were analyzed with a brain electrical source analysis program for modelling equivalent electrical dipoles in order to suggest the generators of the MRCP. Both right and left voluntary self-paced index finger movements were performed by 14 subjects and MRCP were recorded on the scalp with 29 electrodes. A spatiotemporal three-dipole model was developed and it explained 94.1% of the right-hand data and 96.4% of the left-hand data. The models for right- and left-hand data were almost similar except for the different hemispheres. A midline dipole was most active during the preparatory period before the onset of voluntary movement and two contralaterally locating dipoles were most active during and immediately after the motor act. The present model did not support recently suggested bilateral activation of primary motor cortices preceding the voluntary movement. The locations, orientations and the time course of activation of the dipoles suggest the involvement of the supplementary motor area in preparation of the movement and the contralateral primary motor cortex during the execution of the movement and the contralateral primary sensory cortex following the movement.

Adult↗

PubMed1994

Characteristics of the silent period after transcranial magnetic stimulation.

Transcranial magnetic stimulation (TMS) of human cortex during voluntary muscle contraction produces a transient period of inhibition (i.e., silent period, SP) in the electromyographic (EMG) activity. The duration of the SP in relation to the level of muscle force (10%, 50% and 100% of maximum voluntary contraction) as well as possible cumulative effects of sequential TMS on the SP were studied. Methodologic problems were encountered in defining the SP and thus the duration of both an absolute (complete EMG silence) and relative (return of uninterrupted EMG activity) SP was measured. In all subjects, shortening of the SP duration occurred in relation to an increase in force when the criterion for absolute SP was used. Conversely, the relative SP duration suggested a trend toward prolongation with increasing force of contraction. No cumulative effects of TMS were observed on the absolute SP duration, whereas two subjects showed a cumulative effect of TMS on the relative SP. We conclude that the effect of muscle force and sequential TMS on the SP duration is dependent on the methods used to measure the SP. It is therefore essential to agree on methodology before SP measurements are clinically useful.

Adult↗

PubMed1993

Equivalent electrical source analysis of pain-related somatosensory evoked potentials elicited by a CO2 laser.

The purpose of this study was to localize possible neural sources of pain-related cortical evoked potentials. A brain electrical source analysis was performed on late somatosensory evoked potential data (500-ms window was analyzed) elicited by short heat pulses produced by a CO2 laser. These stimuli activate pain and temperature pathways. The first, fairly small, negative response can be recorded on the scalp about 160 ms from the stimulus (N1 component). The major negativity on the scalp has its peak about 240 ms from the stimulus and is followed by a positivity (N2 and P2 components). A four-dipole model was developed using data following left-hand stimulation. Spatiotemporal source analysis suggested that the N1 component could be generated by contralateral activity in both primary and secondary somatosensory cortices and that N2 could be generated by bilateral activity mainly in secondary somatosensory cortices. A deep dipole in the midline was active during the time range of the negative potentials and seemed to be mostly responsible for the P2 component. This dipole was located too frontally to be thalamic, but it corresponded well to the location of the anterior cingulate gyrus. The model also yielded good fits for right-hand and left-foot stimulation data and, in addition, another set of left-hand data obtained with different electrode spacing in a different group of subjects (residual variances from 2.8% to 3.3%). The model explaining data sets from different body part stimulations varied very little, except with respect to the location of the dipole representing the activity of the primary somatosensory area.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1993

Movement-related cortical potentials in patients with cerebellar degeneration.

We studied the topographic distribution of scalp-recorded, averaged movement-related cortical potentials occurring immediately before and after the onset of voluntary movements in six patients with cerebellar degenerative disease. We placed 26 electrodes on the scalp overlying the sensorimotor area and recorded cortical potentials related to abduction of the index finger. The amplitudes and latencies of the potentials were normal in all patients except two, in whom the negative slope (NS') was absent. All patients had an abnormal topographic pattern of potentials compared with normal subjects. The initial slope of motor potential (isMP), which was focal and contralateral in the normal subjects, was diffuse and bilateral in the patients. The topography of the frontal peak of motor potential (fpMP) was more posterior in the patients than in normal subjects. The patterns found in this preliminary study indicate a derangement of sensorimotor cortex activity in voluntary movement as a consequence of cerebellar dysfunction.

Adult↗

PubMed1992

Sensory and movement-related cortical potentials in nociceptive and auditory reaction time tasks.

The processing of a sensory stimulus leading to a simple motor command was studied with scalp-recorded long latency cortical potentials in humans. Two sensory modalities were tested in their ability to activate descending motor pathways: auditory stimuli and painful cutaneous stimuli produced by a CO2 laser. Subjects were asked to react to stimuli with voluntary index finger movements. The stimulus-related and movement-related cortical potentials were recorded simultaneously with five midline electrodes on the scalp. The auditory reaction time, measured from the stimulus to the onset of electromyogram (EMG), was faster (150 ms) than the laser reaction time (350 ms). The onset of EMG of finger movements occurred only after the first negative components following auditory or laser stimuli but before the positive components. The latency from the auditory negativity to the onset of EMG was about 50 ms and the latency from the laser negativity to the onset of EMG was about 110 ms. This finding indicates that not only the peripheral afferent conduction but also central processing takes longer in a pain-related somatosensory task than in an auditory task. The frontal peak of Motor Potential (fpMP), a cortical potential related to the sensory feedback from movement, occurred with a constant latency after the onset of EMG (100 ms) and was unaffected by the task.

Adult↗

PubMed1991

The cortical potential related to sensory feedback from voluntary movements shows somatotopic organization of the supplementary motor area.

In topographic EEG mapping, the peak negativity of movement-related cortical potentials (MRCPs) occurs after the onset of movement and appears anterior to motor cortex, over the region of the supplementary motor area (SMA). This peak, referred to as the frontal peak of the motor potential (fpMP), may well be related to sensory feedback from the movement. The somatotopic organization of the SMA is such that the upper extremity is anterior to the lower extremity. We mapped the MRCPs close to the onset of EMG activity relating to finger and toe movements. The fpMP of finger movements mapped more anteriorly than that of toe movements. These maps offer additional evidence that fpMP originates in the SMA.

Adult↗

PubMed1991

Topography of scalp-recorded motor potentials in human finger movements.

Four distinct negative events were identified in the averaged, scalp-recorded EEGs of normal subjects before and after the onset of self-paced, voluntary finger movements; reaction-time movements and passive movements were also studied. These events are the peak of the negative slope (NS'), the initial slope of motor potential (isMP), the parietal peak of motor potential (ppMP), and the frontal peak of motor potential (fpMP). For self-paced movements, NS' and isMP occurred before the onset of electromyographic (EMG) activity, and ppMP and fpMP occurred after the onset of EMG activity. NS' had a wide distribution, covering the parietal region with slight contralateral predominance. The isMP mapped focally over the contralateral hand motor area on the scalp. The location of ppMP was similar to that of isMP. The fpMP was localized anterior and medial to motor cortex with a contralateral preponderance and possible location over the supplementary motor area. The isMP and fpMP also were identified in the recordings of reaction-time movements, but only the fpMP persisted in the recordings of passive movements. The isMP appears to reflect activation of the cortical cells in the hand area of motor cortex for the execution of voluntary movement, and the fpMP appears to reflect proprioceptive feedback from the periphery.

Adult↗

PubMed1990

Topography of movement-related cortical potentials is abnormal in Parkinson's disease.

We studied the scalp-recorded, movement-related cortical potentials occurring immediately before and after the onset of movement in 5 patients with asymmetric Parkinson's disease. The topographic distribution of the initial slope of motor potential (isMP) was diffuse for voluntary finger movements of the more affected hand but normal for movements of the less affected hand. The frontal peak of motor potential (fpMP) was located more posterior in patients than in normal subjects. The peak amplitudes of the potentials were normal in all patients. The topographic abnormalities might reflect inadequate excitatory activity from the basal ganglia to the primary motor cortex and the supplementary motor area.

Adult↗

PubMed1990

Cortical topography of premotor and motor potentials preceding self-paced, voluntary movement of dominant and non-dominant hands.

The cortical potentials preceding movement, negative slope (NS'), premovement positivity (PMP), and the initial slope of motor potential (MP), were studied in detail with a 29-channel averaged EEG mapping technique in normal subjects. Self-paced, voluntary movements of the right and left index fingers were performed up to 150 times, and topographic color maps were created from the averaged wave forms. The maps revealed NS' of the dominant hand on the vertex and NS' of the non-dominant hand on the contralateral centroparietal area. PMP appeared on the ipsilateral precentral area, and the initial slope of MP appeared on a small, distinct contralateral precentral area, presumably the hand motor area. The amplitudes of the potentials did not show significant differences between dominant and non-dominant hands. PMP and the initial slope of MP appeared significantly earlier preceding non-dominant hand movement as compared with dominant movement. The findings indicate some difference in cortical activity relating to dominant and non-dominant hand movement.

Adult↗