PubMed Health⌕ Search

Biomedical subjects

R Salmelin

Publications and source records attributed to R Salmelin.

At least 19 recordsLinked to original sources
PubMed2000

Single word reading in developmental stutterers and fluent speakers.

Ten fluent speakers and nine developmental stutterers read isolated nouns aloud in a delayed reading paradigm. Cortical activation sequences were mapped with a whole-head magnetoencephalography system. The stutterers were mostly fluent in this task. Although the overt performance was essentially identical in the two groups, the cortical activation patterns showed clear differences, both in the evoked responses, time-locked to word presentation and mouth movement onset, and in task-related suppression of 20-Hz oscillations. Within the first 400 ms after seeing the word, processing in fluent speakers advanced from the left inferior frontal cortex (articulatory programming) to the left lateral central sulcus and dorsal premotor cortex (motor preparation). This sequence was reversed in the stutterers, who showed an early left motor cortex activation followed by a delayed left inferior frontal signal. Stutterers thus appeared to initiate motor programmes before preparation of the articulatory code. During speech production, the right motor/premotor cortex generated consistent evoked activation in fluent speakers but was silent in stutterers. On the other hand, suppression of motor cortical 20-Hz rhythm, reflecting task-related neuronal processing, occurred bilaterally in both groups. Moreover, the suppression was right-hemisphere dominant in stutterers, as opposed to left-hemisphere dominant in fluent speakers. Accordingly, the right frontal cortex of stutterers was highly active during speech production but did not generate synchronous time-locked responses. The speech-related 20-Hz suppression concentrated in the mouth area in fluent speakers, but was evident in both the hand and mouth areas in stutterers. These findings may reflect imprecise functional connectivity within the right frontal cortex and incomplete segregation between the adjacent hand and mouth motor representations in stutterers during speech production. A network including the left inferior frontal cortex and the right motor/premotor cortex, likely to be relevant in merging linguistic and affective prosody with articulation during fluent speech, thus appears to be partly dysfunctional in developmental stutterers.

Adult↗
PubMed2000

Neurophysiology of fluent and impaired reading: a magnetoencephalographic approach.

This article reviews a series of magnetoencephalographic (MEG) experiments aimed at identifying cortical areas and time windows relevant or even critical for fluent reading. The approach was to compare single-word processing in fluent and dyslexic readers. The activations which differed between the two groups were then studied in more detail to determine their functional roles. In fluent reading, overall visual feature processing occurs about 100 milliseconds (ms) after seeing a word, in the posteromedial extrastriate cortex bilaterally. This activation does not differentiate between letters and symbols. The first reading-specific signal is detected about 150 ms after word onset, when the left inferior occipitotemporal cortex responds preferentially to letter strings. After 200 ms, the left superior temporal cortex, in particular, is engaged in semantic processing of single words and their integration with connected text. While visual feature processing seems to be within normal limits in dyslexic subjects, reading is disrupted during the first 200 ms after seeing a word, at the letter-string specific stage. The subsequent activations are weak and delayed as compared with those in fluent readers. Also presented is a case of deep dyslexia, where the same tools were used to demonstrate that reading comprehension was still subserved by the left hemisphere despite severe damage.

Cerebral Cortex↗
PubMed1999

Left-hemisphere dominance for processing of vowels: a whole-scalp neuromagnetic study.

Brain activation of 11 healthy right-handed subjects was studied with magnetoencephalography to estimate individual hemispheric dominance for speech sounds. The auditory stimuli comprised binaurally presented Finnish vowels, tones, and piano notes in groups of two or four stimuli. The subjects were required to detect whether the first and the last item in a group were the same. In the left hemisphere, vowels evoked significantly stronger (37-79%) responses than notes and tones, whereas in the right hemisphere the responses to different stimuli did not differ significantly. Specifically, in the two-stimulus task, all 11 subjects showed left-hemisphere dominance in the vowel vs tone comparison. This simple paradigm may be helpful in non-invasive evaluation of language lateralization.

Acoustic Stimulation↗
PubMed1999

Native language, gender, and functional organization of the auditory cortex.

Whole-head magnetoencephalography was employed in 40 normal subjects to investigate whether the basic functional organization of the auditory cortex varies with linguistic environment. Robust activations of the bilateral supratemporal auditory cortices to 1-kHz pure tones, maximum at about 100 ms after stimulus onset, were studied in Finnish and German female and male subject groups with monolingual background. Activations elicited by the tones were mutually indistinguishable in German and Finnish women. In contrast, German men showed significantly stronger auditory responses to pure tones in the left, language-dominant hemisphere than Finnish men. We discuss the possibility that the prominent left-hemisphere activation in German males reflects higher frequency resolution required for distinguishing between German than Finnish vowels and that the clear effect of native language in male but not in female auditory cortex derives from more pronounced functional lateralization in men. The present data suggest that the influence of native language can extend to auditory cortical processing of pure-tone stimuli with no linguistic content and that this effect is conspicuous in the male brain.

Acoustic Stimulation↗
PubMed1999

Subject's own speech reduces reactivity of the human auditory cortex.

Previous studies on monkeys have shown that uttering-related cortical areas exert an inhibitory effect on the auditory cortex, and cerebral blood-flow analyses on humans have revealed modulation of the activity of the auditory cortex during own speech. To study this modulation on a millisecond time scale, we recorded neuromagnetic evoked responses to short 1-kHz tones while the subjects were reading silently and aloud. The 100-ms response (M100) of the auditory cortex was delayed by 10-21 ms and its amplitude was dampened by 44-71% during reading aloud compared with reading silently. This effect was more prominent in responses to ipsilateral than contralateral tones, possibly due to a sum effect of diminished ipsilateral input to the cortex and decreased transcallosal excitation.

Acoustic Stimulation↗
PubMed1999

Dynamics of letter string perception in the human occipitotemporal cortex.

The inferior occipitotemporal brain areas, especially in the left hemisphere, have been shown to be involved in the processing of written words and letter strings. This processing probably occurs within 200 ms after presentation of the letter string. It has also been suggested that this activation may differ between fluent and dyslexic readers. Using whole-head magnetoencephalography, we studied the spatiotemporal dynamics of brain processes evoked by visually presented letter strings in 12 healthy adult subjects. Our achromatic stimuli consisted of rectangular patches in which single letters, two-letter syllables, four-letter words, or symbol strings of equal length were embedded and to which variable noise was added. This manipulation dissociated three different response patterns. The first of these patterns took place approximately 100 ms after stimulus onset, originated in areas surrounding the V1 cortex and was distributed along the ventral visual stream, extending laterally as far as V4v. This response was systematically modulated by noise but was insensitive to the stimulus content, suggesting involvement in early visual analysis. The second pattern took place approximately 150 ms after stimulus onset and was concentrated in the inferior occipitotemporal region with left-hemisphere dominance. This activation showed a preference for letter strings, and its strength and timing correlated with the speed at which the subjects were able to read words aloud. The third pattern also occurred in the time window approximately 150 ms after stimulus onset, but originated mainly in the right occipital area. Like the second pattern, it was modulated by string length, but showed no preference for letters compared with symbols. The present data strongly support the special role of the left inferior occipitotemporal cortex in visual word processing within 200 ms after stimulus onset.

Adult↗
PubMed1999

Semantic cortical activation in dyslexic readers.

The combined temporal and spatial resolution of MEG (magnetoencephalography) was used to study whether the same brain areas are similarly engaged in reading comprehension in normal and developmentally dyslexic adults. To extract a semantically sensitive stage of brain activation we manipulated the appropriateness of sentence-ending words to the preceding sentence context. Sentences, presented visually one word at a time, either ended with a word that was (1) expected, (2) semantically appropriate but unexpected, (3) semantically anomalous but sharing the initial letters with the expected word, or (4) both semantically and orthographically inappropriate to the sentence context. In both subject groups all but the highly expected sentence endings evoked strong cortical responses, localized most consistently in the left superior temporal cortex, although additional sources were occasionally found in more posterior parietal and temporal areas and in the right hemisphere. Thus, no significant differences were found in the spatial distribution of brain areas involved in semantic processing between fluent and dyslexic readers. However, both timing and strength of activation clearly differed between the two groups. First, activation sensitivity to word meaning within a sentence context began about 100 msec later in dyslexic than in control subjects. This is likely to result from affected presemantic processing stages in dyslexic readers. Second, the neural responses were significantly weaker in dyslexic than in control subjects, indicating involvement of a smaller or less-synchronous neural population in reading comprehension. Third, in contrast to control subjects, the dyslexic readers showed significantly weaker activation to semantically inappropriate words that began with the same letters as the most expected word than to both orthographically and semantically inappropriate sentence-ending words. Thus, word recognition by the dyslexic group seemed to be qualitatively different: Whereas control subjects perceived words as wholes, dyslexic subjects may have relied on sublexical word recognition and occasionally mistook a correctly beginning word for the one they had expected.

Adult↗
PubMed1998

Functional organization of the auditory cortex is different in stutterers and fluent speakers.

Impaired auditory feedback has been suggested to cause stuttering, and subtle irregularities of audition have been reported in behavioural studies. To characterize processing at the auditory cortical level, we recorded neuromagnetic responses to monaural tones in nine stutterers and 10 fluent speakers while the subjects were reading silently, with mouth movements only, aloud, and in chorus with another person. The basic functional organization of the auditory cortices was found to be different in stutterers and controls. The altered interhemispheric balance in stutterers was affected by speech production, due to changes in the left auditory cortical representation, and more severely by self-paced than accompanied speech. This may lead to transient non-optimal interpretation of the auditory input and affect speech fluency.

Adult↗
PubMed1998

Responsiveness of human cortical activity to rhythmical stimulation: a three-modality, whole-cortex neuromagnetic investigation.

We developed numerical indicators to quantify stimulus-related changes in cortical magnetic signals recorded from nine healthy subjects while they received 1- to 2.5-s trains of 15 stimuli (somatosensory, visual, or auditory in separate runs) at rates from 6 to 14 Hz, intermingled with 1.6-s pauses. A locking index (L) was introduced to quantify how well the responses are time locked to the stimuli and a global change factor (GC) to indicate changes in the whole-cortex oscillatory activity in the 5- to 25-Hz frequency range. The responses were visualized with color-coded images illustrating cortical reactivity for all stimulus rates simultaneously. These color maps clearly showed that the modality-specific cortical signals were enhanced at frequencies corresponding to the stimulus rate during the trains. For somatosensory stimulation the activity in the vicinity of the somatosensory hand area was enhanced at most stimulus rates, suggesting mainly superposition of evoked responses. In individuals with strong posterior resting rhythm, visual stimuli typically entrained activity in the parietooccipital sulcus at stimulus rates close to the main frequency of the spontaneous activity, probably reflecting driving of the intrinsic cortical rhythm, whereas in subjects with little spontaneous parietooccipital rhythm the cortical signal appeared to be composed mainly of visual evoked responses. No modality-specific enhancement was observed during auditory stimulation. During the pauses between the trains, the cortical signals were significantly suppressed compared with the resting condition: The peak activity (7-13 Hz) was modulated within, but also outside, the modality-specific areas, and the signals outside the frequency peaks of maximum power were consistently and reproducibly suppressed over the whole cortex by all stimuli.

Adult↗
PubMed1998

Neuromagnetic sequelae of herpes simplex encephalitis.

Spontaneous cortical activity and auditory evoked responses were recorded with a whole-scalp 122-channel neuromagnetometer from 4 patients after left-hemisphere dominant herpes simplex encephalitis and associated memory disorders. Spontaneous activity of one patient contained periodic sharp waves over the left hemisphere; the background activity was attenuated. The sources of periodic sharp waves clustered close to the sources of auditory evoked fields in the temporal lobe. In controls, dominant rhythmic activity over the parieto-occipital region had spectral maximum at 10.6 +/- 0.6 Hz; in patients the dominant rhythmic activity peaked at 8.6 +/- 1.8 Hz. The suppression of the parieto-occipital activity in eyes-open versus eyes-closed condition was smaller in patients than in controls. The patients' peak spectral frequency was correlated with neuropsychological tests reflecting deficient attentional capacity. The observed changes probably reflect decreased subcortical control of the cortical electric activity.

Adult↗
PubMed1998

Modification of neuromagnetic cortical signals by thalamic infarctions.

Auditory evoked responses and spontaneous cortical activity were recorded with a whole-scalp 122-channel neuromagnetometer from 7 patients, who had small thalamic infarctions in the region of the left anterior tuberothalamic artery and associated memory defects. In contrast to healthy control subjects, with dominant rhythmic activity at 10.6 +/- 0.6 Hz in the parieto-occipital region, the spectral maximum in the patients was at 8.9 +/- 0.4 Hz. Abnormal acceleration of rhythmic activity was also observed bilaterally in rolandic areas. Our findings imply that lesions of non-specific thalamic nuclei may disturb human brain rhythms in widespread cortical areas. 'Mismatch responses' to deviant tones (1.1 kHz) among standards (1.0 kHz), suggested to reflect sensory auditory memory in healthy subjects, were absent in 2 patients, markedly decreased in 3, and normal in 2, implying that pathways passing through the anteromedial thalamus contribute to modulation of these responses. We conclude that local unilateral lesions in the anteromedial thalamus may cause extensive, bilateral alterations in the brain's electric activity.

Adult↗
PubMed1998

Distinct time courses of word and context comprehension in the left temporal cortex.

The time course and cortical basis of reading comprehension were studied using magnetoencephalography. The cortical structures implicated most consistently with comprehension were located in the immediate vicinity of the left auditory cortex, where final words totally inappropriate to the overall sentence context evoked enduring activation starting approximately 250 ms and lasting up to 600 ms after word onset. Contextually appropriate but unexpected words produced weaker activation which terminated earlier. Highly anticipated words totally failed to activate this area, suggesting that the conceptual network became involved only if unexpected information was detected during the primary word identification process. We propose that the point in time (350 ms after word onset) where the response to appropriate but unexpected endings started to diverge from those to contextually inappropriate endings reflects the boundary between understanding a single word and the meaning of a whole sentence.

Adult↗
PubMed1998

Global optimization in the localization of neuromagnetic sources.

The locations of active brain areas can be estimated from the magnetic field produced by the neural current sources. In many cases, the actual current distribution can be modeled with a set of stationary current dipoles with time-varying amplitudes. This work studies global optimization methods that find the minimum of the least-squares error function of the current dipole estimation problem. Three different global optimization methods were investigated: clustering method, simulated annealing, and genetic algorithms. In simulation studies, the genetic algorithm was the most effective method. The methods were also applied to analysis of actual measurement data.

Algorithms↗
PubMed1998

An MEG study of picture naming.

The purpose of this study was to relate a psycholinguistic processing model of picture naming to the dynamics of cortical activation during picture naming. The activation was recorded from eight Dutch subjects with a whole-head neuromagnetometer. The processing model, based on extensive naming latency studies, is a stage model. In preparing a picture"s name, the speaker performs a chain of specific operations. They are, in this order, computing the visual percept, activating an appropriate lexical concept, selecting the target word from the mental lexicon, phonological encoding, phonetic encoding, and initiation of articulation. The time windows for each of these operations are reasonably well known and could be related to the peak activity of dipole sources in the individual magnetic response patterns. The analyses showed a clear progression over these time windows from early occipital activation, via parietal and temporal to frontal activation. The major specific findings were that (1) a region in the left posterior temporal lobe, agreeing with the location of Wernicke"s area, showed prominent activation starting about 200 msec after picture onset and peaking at about 350 msec (i.e., within the stage of phonological encoding), and (2) a consistent activation was found in the right parietal cortex, peaking at about 230 msec after picture onset, thus preceding and partly overlapping with the left temporal response. An interpretation in terms of the management of visual attention is proposed.

Adult↗
PubMed1997

Evidence for reactive magnetic 10-Hz rhythm in the human auditory cortex.

We tested the hypothesis that neurons in the human auditory cortex show spontaneous oscillations around 10 Hz, and that this activity ('tau' rhythm) is affected by auditory input. Cortical activity was recorded with a 122-channel whole-scalp neuromagnetometer from healthy adults while they were presented with monaural 500-ms bursts of white noise. The reactivity of spontaneous oscillations was studied over the whole cortex using the Temporal Spectral Evolution method. Oscillatory 6.5-9.5 Hz activity, with sources in the superior temporal lobes, was transiently suppressed by the sounds in eight out of nine subjects. Our results support the existence of a distinct, reactive auditory rhythm in the human temporal cortex.

Adult↗
PubMed1997

Modulation of human cortical rolandic rhythms during natural sensorimotor tasks.

We studied modulation of cortical neuromagnetic rhythms in association with left and right median nerve stimulation, during rest, finger movements, and passive tactile hand stimulation, in seven healthy, right-handed adults. In the rest condition, the amplitude of the rhythmic sensorimotor activity decreased immediately after the median nerve stimuli and increased above the prestimulus level within 0.4 s afterward, especially in the 7- to 25-Hz band. The rebound occurred 100-300 ms earlier for 20 (7-15)-than for 10 (15-25)-Hz activity. Suppressions and rebounds were strongest in the contralateral sensorimotor hand area for the 20-Hz, but not for the 10-Hz, activity. The maximum rebound was on average 22-34% stronger in the left than in the right hemisphere. Active exploration of objects abolished rebounds of both 10- and 20-Hz signals in the contralateral hemisphere and markedly diminished them ipsilaterally. Finger movements without touching an object and passive tactile stimulation produced a weaker effect. The sensorimotor rhythms thus show a characteristic suppression and subsequent rebound after electrical median nerve stimulation. The rebound is left-hemisphere dominant in right-handed subjects and its suppression reveals bilateral cortical activation during both motor tasks and passive tactile stimulation, especially for explorative finger movements.

Adult↗
PubMed1997

Involvement of primary motor cortex in motor imagery: a neuromagnetic study.

Functional brain imaging studies have indicated that several cortical and subcortical areas active during actual motor performance are also active during imagination or mental rehearsal of movements. Recent evidence shows that the primary motor cortex may also be involved in motor imagery. Using whole-scalp magnetoencephalography, we monitored spontaneous and evoked activity of the somatomotor cortex after right median nerve stimuli in seven healthy right-handed subjects while they kinesthetically imagined or actually executed continuous finger movements. Manipulatory finger movements abolished the poststimulus 20-Hz activity of the motor cortex and markedly affected the somatosensory evoked response. Imagination of manipulatory finger movements attenuated the 20-Hz activity by 27% with respect to the rest level but had no effect on the somatosensory response. Slight constant stretching of the fingers suppressed the 20-Hz activity less than motor imagery. The smallest possible, kinesthetically just perceivable finger movements resulted in slightly stronger attenuation of 20-Hz activity than motor imagery did. The effects were observed in both hemispheres but predominantly contralateral to the performing hand. The attempt to execute manipulatory finger movements under experimentally induced ischemia causing paralysis of the hand also strongly suppressed 20-Hz activity but did not affect the somatosensory evoked response. The results indicate that the primary motor cortex is involved in motor imagery. Both imaginative and executive motor tasks appear to utilize the cortical circuitry generating the somatomotor 20-Hz signal.

Adult↗