Biomedical subjects

Riitta Hari

Publications and source records attributed to Riitta Hari.

At least 19 recordsLinked to original sources

PubMed2003

Enhanced extrastriate activation during observation of distorted finger postures.

Hand and finger postures of other people are important body language cues that strongly contribute to the observer's decision about the person's intentions, thoughts, and attentional state. We compared neuromagnetic cortical activation elicited by color images of natural and distorted finger postures. The distorted postures contained computer-deformed joint angles and thereby easily caught the observer's attention. From about 260 msec onwards, extrastriate occipital areas of both hemispheres were activated more strongly by distorted than natural finger postures. We interpret this result as an early top-down effect of emotional valence on the processing of unusual hand shapes in the extrastriate visual cortex.

Adult↗

PubMed2003

Phase locking between human primary and secondary somatosensory cortices.

Unilateral stimulation of human peripheral nerves activates the primary somatosensory cortex (SI) contralaterally and the secondary somatosensory cortex (SII) bilaterally. We aimed at characterizing phase locking between SI and SII in response to electric stimuli applied once every 3 s to the right median nerve at the wrist; phase locking between brain regions has been proposed to either reflect joined processing or information exchange. Ongoing neuromagnetic activity of healthy volunteers was recorded with 204 planar gradiometers covering the whole scalp. After selecting a sensor maximally sensitive to activity in the left (contralateral) SI, phase locking between this sensor and the other 203 sensors was examined from single trial data. Statistically significant phase locking was found at approximately 20 Hz, 80-90 ms after the stimuli between the left SI and the right SII in 9 of 10 subjects. Sensors with high phase-locking values over the left SI and right SII were separated by sensors with no phase-locked activity over the scalp midline, indicating that the phase locking was not caused by the sensors seeing activity from the same sources. The observed SI-SII phase locking would not be reflected in the evoked responses because a considerable part of it was not time-locked to the stimuli. Thus, our finding reveals a unique interaction in the sensorimotor system.

Adult↗

PubMed2003

Impaired mirror-image imitation in Asperger and high-functioning autistic subjects.

Imitation is crucial for proper development of social and communicative skills. Here, we argue that, based on an error analysis of a behavioral imitation task, adult Asperger and high-functioning autistic subjects suffer from an intriguing deficit of imitation: they lack the natural preference for imitation in a mirror-image fashion. The imitation task consisted of a simple movement sequence of putting a pen with the left or right hand into a green or a blue cup using one of two possible grips. The subjects were asked to imitate the experimenter's hand movements either using the crossed hand (e.g., the subject's right hand corresponding to the experimenter's right hand) for imitation or to imitate as if looking in a mirror (e.g., the subject's left hand corresponding to the experimenter's right hand). When people normally view other persons face-to-face, they prefer to imitate as in a mirror, and observation of mirror-image-like movements speeds up performance in nonimitative tasks. However, our autistic subjects, defective in social cognition, did not profit from mirror-image movements of others. These results provide a new insight into the difficulties that autistic subjects face in viewing and understanding actions of others.

Adult↗

PubMed2003

Diminished auditory mismatch fields in dyslexic adults.

Electroencephalographic studies have demonstrated smaller auditory responses to infrequent deviances of speech and nonspeech sounds in dyslexic than normal-reading subjects. We used a whole-scalp neuromagnetometer to study selectively reactivity of the auditory cortices to sound deviances in 8 dyslexic and 11 normal-reading adults. Within a monotonous sequence of 50-millisecond 1000 Hz binaural tones, tones of 920 and 1080 Hz occurred with 7% probability each. Magnetic mismatch fields, elicited by the stimulus deviances, were diminished in the left hemisphere of the dyslexic subjects. The results indicate deficient change detection in the left auditory cortex of right-handed dyslexic adults.

Adult↗

PubMed2003

Binaural interaction in the human auditory cortex revealed by neuromagnetic frequency tagging: no effect of stimulus intensity.

Frequency tagging of magnetoencephalographic signals was recently introduced as a new tool to study binaural interaction in the human auditory cortex [Fujiki et al., J. Neurosci. 22 (2002) RC205]. As the method has potential value for assessing brain plasticity in patients with unilateral hearing deficits, we studied binaural interaction in 10 healthy adults at different intensity levels. Cortical steady-state fields were measured with a 306-channel whole-scalp neuromagnetometer to amplitude-modulated sounds (carrier frequency 1 kHz), presented monaurally or binaurally at 45, 60 and 75 dB SL. The modulation frequencies were 39.1 Hz for the right ear and 41.1 Hz for the left. During binaural stimulation, the ipsilateral responses were suppressed more than the contralateral ones in both hemispheres, and the hemispheric balance shifted towards the contralateral hemisphere for inputs from both ears. The patterns of binaural interaction were similar at all three stimulus intensities. These data could be useful in examining patients who suffer from auditory disorders as well as in revealing basic mechanism of human auditory processing.

Acoustic Stimulation↗

PubMed2003

Comparison of BOLD fMRI and MEG characteristics to vibrotactile stimulation.

The characteristics of blood oxygenation level-dependent (BOLD) fMRI and magnetoencephalographic (MEG) responses to vibrotactile stimuli in humans were studied and compared. The stimuli, presented with interstimulus intervals (ISIs) ranging from 1 to 5 s, yielded highly reproducible MEG responses, with current dipoles in the primary somatosensory (SI) cortex in all subjects. BOLD fMRI responses to similar stimuli showed substantial intrasubject variation in the activation sites around the SI cortex. BOLD responses were detected in all subjects in the secondary somatosensory (SII) cortices as well, with comparable BOLD response amplitudes to those in the SI cortex. Current dipoles, used to model the MEG signals, were stronger at longer ISIs than shorter ISIs. The BOLD response amplitudes did not show a similar dependence on ISI, but the activated brain area was larger when longer ISIs or longer stimuli were applied. Our results support the view that combined use of brain mapping methods provides complementary information and should be considered in functional brain examinations.

Adult↗

PubMed2003

Magnetoencephalographic analysis of bilaterally synchronous discharges in benign rolandic epilepsy of childhood.

The purpose of this study was to examine the spatial and temporal relationship between bilateral foci of bilaterally synchronous discharges in benign rolandic epilepsy of childhood (BREC) using a whole-scalp neuromagnetometer. We simultaneously recorded interictal magnetoencephalographic (MEG) and electroencephalographic (EEG) signals in six children with BREC. Interictal spikes were classified into three groups: bilaterally synchronous discharges (BSDs), unilateral discharges on right side (UD-R), and unilateral discharges on left side (UD-L). We used equivalent current dipole (ECD) modelling to analyse the cortical sources of interictal spikes. Both BSDs and UDs were found in Patients 1-4, whereas only UDs were identified in Patients 5 and 6. The ECDs of interictal spikes were located in rolandic regions, 10-20mm anterior and lateral to hand somatosensory cortices. Multi-dipole analysis of BSDs showed two ECDs in homotopic motor areas of the hemispheres. During BSDs, the right-sided activation preceded the left-sided activation by 15-21 milliseconds in Patients 1 and 2. In Patients 3 and 4, the activation occurred 17-20 milliseconds earlier in the left than the right hemisphere. Within the same hemisphere, the sources of BSDs and UDs were located in similar areas. In conclusion, our results imply the cortical epileptogenicity in bilateral perirolandic areas in BREC. The sequential activation during BSDs in both hemispheres suggest the existence of synaptic connections, possibly via the corpus callosum, between bilateral irritative foci.

Brain Mapping↗

PubMed2003

Effects of interstimulus interval on cortical responses to painful laser stimulation.

Short laser pulses applied to the skin are used increasingly in both clinical and basic assessment of nociceptive brain mechanisms. The authors aimed to characterize further the cortical responses to noxious laser stimuli and to define the interstimulus interval (ISI) for the optimum signal-to-noise ratio during a fixed measurement time. Three hundred six-channel whole-scalp magnetoencephalographic (MEG) and midline EEG signals were recorded from nine healthy adults during painful thulium laser stimulation. The stimuli were delivered on the dorsum of the left hand at ISIs of 0.5, 1, 2, 4, 8, and 16 seconds. The MEG responses peaked at 160 to 195 msec around the contralateral primary somatosensory (SI) cortex, at 150 to 190 msec in the contralateral secondary somatosensory (SII) cortex, and at 160 to 205 msec in the ipsilateral SII cortex. The simultaneously measured electrical vertex potentials peaked at 190 to 230 msec and 310 to 330 msec (N200-P300). All these responses showed rather similar refractory times: The amplitudes increased strongly from 0.5 to 4-second ISIs and thereafter saturated at ISIs of 8 to 16 seconds. On the basis of the time constants of the recovery cycles, the optimum ISI for obtaining the best signal-to-noise ratio for laser-evoked MEG and EEG responses during a fixed measurement interval is 4 to 5 seconds.

Adult↗

PubMed2002

Viewing lip forms: cortical dynamics.

Viewing other persons' actions automatically activates brain areas belonging to the mirror-neuron system (MNS) assumed to link action execution and observation. We followed, by magnetoencephalographic cortical dynamics, subjects who observed still pictures of lip forms, on-line imitated them, or made similar forms in a self-paced manner. In all conditions and in both hemispheres, cortical activation progressed in 20-70 ms steps from the occipital cortex to the superior temporal region (where the strongest activation took place), the inferior parietal lobule, and the inferior frontal lobe (Broca's area), and finally, 50-140 ms later, to the primary motor cortex. The signals of Broca's area and motor cortex were significantly stronger during imitation than other conditions. These results demonstrate that still pictures, only implying motion, activate the human MNS in a well-defined temporal order.

Adult↗

PubMed2002

Facilitation of the spinal H-reflex by auditory stimulation in dyslexic adults.

Dyslexic subjects show a variety of mild sensory and motor deficits that have been assumed to reflect dysfunction of the large-diameter 'magnocells' in different parts of the brain. Hearing as a warning sense relies on rapidly-conducting fibers, and on the basis of the magnocellular deficit theory, we wondered whether auditory alerting would be weakened in dyslexic adults. We quantified the strength of sound-induced spinal facilitation in seven dyslexic and eight normal-reading adults by measuring the amplitudes of H-reflex, a monosynaptic spinal reflex, after loud binaural sounds. The audiospinal facilitation was of similar strength in dyslexic and control adults, indicating normal auditory alerting via cerebrospinal pathways. The slightly prolonged facilitation in dyslexics agrees with the dyslexics' general sluggishness of sensorimotor processing.

Acoustic Stimulation↗

PubMed2002

Cortico-muscular coupling in a human subject with mirror movements--a magnetoencephalographic study.

We studied cortico-muscular coupling in a 15-year-old male suffering from congenital mirror movements (MMs) of hands. Cortex-muscle coherence was analyzed between magnetoencephalographic signals and the electromyograms (EMGs) recorded from both hands and feet during uni- and bilateral isometric contractions. Regardless of the side of the intended contraction, the motor cortex contralateral to the contraction was coupled to the muscles of both hands at 20-25 Hz. No coupling was found from the other, ipsilateral hemisphere. EMGs of the two hands were coupled during both intended uni- and bilateral contractions, but only during unilateral contractions could the coupling solely be explained by cortical activation. We suggest that our subject's MMs result from activation of an ipsilateral corticospinal projection, with involvement of additional synchronizing mechanisms at the subcortical, brainstem, or spinal level during bilateral contraction.

Adolescent↗

PubMed2002

Auditory cortical responses to speech-like stimuli in dyslexic adults.

Auditory cortical processing of speech-like sounds was studied in 9 dyslexic and 11 normal-reading adults. Noise/square-wave sequences, mimicking transitions from a fricative consonant to a vowel, were presented binaurally once every 1.1 sec and the cortical responses were recorded with a whole-scalp neuromagnetometer. The auditory cortices of both hemispheres were less reactive to acoustical changes in dyslexics than in controls, as was evident from the weaker responses to the noise/square-wave transitions. The results demonstrate that dyslexic adults are deficient in processing acoustic changes presented in rapid succession within tens to hundreds of milliseconds. The observed differences could be related to insufficient triggering of automatic auditory attention, resulting, for instance, from a general deficiency of the magnocellular system.

Acoustic Stimulation↗

PubMed2002

Neuromagnetic responses to frequency-tagged sounds: a new method to follow inputs from each ear to the human auditory cortex during binaural hearing.

Binaural cortical responses are mixtures of inputs from both ears. We introduce here a novel method that allows, for the first time, to selectively follow these inputs in humans up to the cortex during binaural hearing. We recorded neuromagnetic cortical responses to amplitude-modulated continuous tones, with different modulation frequencies at each ear. During binaural hearing, the left- and right-ear inputs competed strongly in both auditory cortices: the right-hemisphere responses were symmetrically suppressed, compared with monaural stimulation, for sounds of both ears, whereas the left-hemisphere responses were suppressed significantly more for ipsilateral than contralateral sounds, thereby intensifying the right-ear dominance of the left auditory cortex. This type of hemisphere- and ear-selective information on cortical binaural interaction could have important applications in human auditory neuroscience.

Acoustic Stimulation↗

PubMed2002

Left-hemisphere-dominant SII activation after bilateral median nerve stimulation.

We used bilateral median nerve stimuli to find out possible hemispheric dominance in the activation of the second somatosensory cortex, SII. Somatosensory evoked fields (SEFs) were recorded from 14 healthy adults (7 right-handed, 7 left-handed) with a 306-channel neuromagnetometer. Electrical stimuli were applied once every 3 s simultaneously either to the left and right median nerves at the wrists or to the palmar skin of both thumbs. Sources of SEFs were modeled with four current dipoles, located in the SI and SII cortices of both hemispheres. The SI activation strengths did not differ between the hemispheres, whereas the SII responses were significantly stronger in the left than in the right hemisphere. In right-handers, the left/right SII ratios were 1.9 and 1.8 for wrist and thumb stimuli, respectively. The corresponding values were 1.5 and 1.7 in left-handers. The results indicate handedness-independent functional specialization of the human SII cortices.

Adult↗

PubMed2002

Visually evoked gamma responses in the human brain are enhanced during voluntary hyperventilation.

Hypocapnia induced by hyperventilation (HV) has powerful effects on neuronal excitability and synaptic transmission. We have studied the effect of hyperventilation on the phase-locked oscillatory components of the evoked responses in the human brain. We recorded visually evoked magnetoencephalographic responses before, during, and after voluntary hyperventilation to pattern-reversal checkerboard stimuli. Gamma-band (30-45 Hz) responses phase-locked to the stimuli were generated in the occipital visual cortex. A wavelet-based time-frequency analysis revealed that the gamma responses increased during HV whereas their frequency did not change significantly. A recent in vitro study in the rat hippocampus demonstrated that the stability of spontaneous gamma activity increases during hypocapnia as a result of enhanced GABAergic transmission. To test if a similar mechanism could account for our findings, we performed simulations on a network of 100 Hodgkin-Huxley neurons connected by inhibitory synapses. We found that enhanced GABA(A) transmission, paired with enhanced excitability, can explain the increase in evoked gamma activity without changing the frequency.

Adult↗

PubMed2002

Modulated activation of the human SI and SII cortices during observation of hand actions.

Neurons in area F5 of the monkey premotor cortex are activated during both execution and observation of hand actions. A similar "mirror-neuron system" seems to exist also in the human brain, including at least the superior temporal sulcus region, Broca's area, and the primary motor cortex. We recorded somatosensory evoked fields in response to median nerve stimulation from nine healthy subjects during (i) rest, (ii) manipulation of a small object, and (iii) observation of the same action to find out to what extent the somatosensory cortices display behavior similar to the human mirror-neuron system. SI signals were enhanced and SII signals suppressed during both manipulation and observation, except when the right manipulating hand was stimulated. Our results suggest that the SI and SII cortices contribute to the human mirror-neuron system, possibly providing information necessary for preserving the sense of self during action observation.

Adult↗

PubMed2002

Mind's ear in a musician: where and when in the brain.

The temporospatial pattern of brain activity during auditory imagery was studied using magnetoencephalography. Trained musicians were presented with visual notes and instructed to imagine the corresponding sounds. Brain activity specific to the auditory imagery task was observed, first as enhanced activity of left and right occipital areas (average onset 120-150 ms after the onset of the visual stimulus) and then spreading to the midline parietal cortex (precuneus) and to such extraoccipital areas that were not activated during the visual control condition (e.g., the left temporal auditory association cortex and the left and right premotor cortices). The latest activations, with average onset latencies of 270-400 ms clearly separate from the earliest ones, occurred in the left sensorimotor cortex and the right inferotemporal visual association cortex. These data imply a complex temporospatial activation sequence of multiple cortical areas when musicians recall firmly established audiovisual associations.

Adult↗

PubMed2002

Comparison of minimum current estimate and dipole modeling in the analysis of simulated activity in the human visual cortices.

Magnetoencephalographic(MEG) data are typically interpreted using source models because of the nonunique inverse problem. Although single current dipoles, adequately representing local active areas, can be identified accurately, multiple and overlapping sources form a challenge for MEG modeling. We tested the performances of multidipole modeling and minimum current estimate (MCE) in the analysis of complicated source configurations. Simulated current sources were placed to physiologically meaningful areas of the human visual cortices. Ten volunteers from the laboratory staff analyzed four different simulations with both dipole modeling and MCE without prior information of the sources. In general, the same sources were found using both modeling methods. The subjects tended to report more false sources with MCE than with dipole model, in part due to their inexperience with the method. Dipole model was more accurate than MCE both in time and space for nonsimultaneous sources but both methods performed similarly when sources overlapped in time. For all source configurations, considerably smaller source amplitudes were reported with MCE than with dipole model.

Computer Simulation↗