Browsing by Subject "MEG"

Aims of the present study: Attention-deficit/hyperactivity disorder (ADHD), beginning in childhood and often continuing into adulthood, is a neurodevelopmental condition that impairs an individual’s functioning in everyday life. The disorder is characterized by impairments in attention regulation or impulse control, or both. Many of the symptoms are related to disturbances in attention, the ability to select behaviorally relevant stimuli and to filter out irrelevant information among the overwhelming amount of sensory data. Neural mechanisms of attention have been linked to oscillations in electophysiological brain activity at alpha frequencies (8–13 Hz), but information on alpha oscillations in adult ADHD has remained scarce. The aim of the present study was to examine differences in attention and distractibility related alpha oscillations between adult ADHD patients and neurotypical controls. Methods: Participants were instructed to attend moving spherical objects and to report color changes in the objects. Number of attended objects varied from one (in right or left visual hemifield) to two (one in both hemifields). In addition to the attended objects, participants were at times presented with distractors which they were instructed to ignore. Brain activity during task performance was measured with magnetoencephalography (MEG). Results and discussion: Behavioral performance was similar between the groups. However, alpha oscillations related to distractor processing differed in a statistically significant manner between ADHD patients and controls. Main differences were related to inter-hemispheric interactions, suggesting that attentional deficits in ADHD might be related to abnormalities between inter-hemispheric communication.

Epilepsy is one of the most common neurological diseases worldwide. Around one-fifth of people with epilepsy are diagnosed with drug-resistant epilepsy. Some of them benefit from epilepsy surgery. Before surgical treatment, the location of the epileptogenic zone (EZ) must be carefully identified to achieve a good surgical outcome. New methods for locating the EZ are constantly being developed. The aim of this study was to investigate whether long-range temporal correlations (LRTC) can be used for non-invasive localization of the EZ in patients with drug-resistant epilepsy. The study involved 10 patients with epilepsy who had previously undergone epilepsy surgery and an MRI. We used the detrended fluctuation analysis (DFA) method to quantify the LRTCs in four different frequency bands in 200 cortical regions based on the patients' individual source reconstructions. We correlated the DFA values with distances from the resection areas and cortical locations of interictal epileptiform discharges (IEDs). In addition, DFA maps were visually inspected by three clinicians to identify the most likely EZs. The study found that DFA maps were significantly correlated with distance from the location of the resection only in the patients with type II focal cortical dysplasia (FCD II) and were associated with IED sites only in the FCD II patients. In addition, visual analysis of the DFA maps showed that in the FCD patients, there was a high interobserver agreement and accuracy in defining the affected hemisphere and lobe. These findings suggest that LRTCs could provide a non-invasive method to identify EZ and facilitate the preoperative evaluation of epilepsy patients.

Tässä pro gradu -tutkielmassa on tarkasteltu aivosähkö- ja aivomagneettikäyrien amplitudien vaihteluiden vastaavuussuhteita koehenkilön suoriutumiseen audiovisuaalisten ärsykkeiden tarkkaavaisuustehtävissä. Aikaisemmista tutkimuksista tiedetään, että koehenkilön osumatarkkuus ei pysy vakiona koko tehtävän ajan, vaan on monesti jaksottunut valppauden ja herpaantumisen jaksoihin. Lisäksi osumatarkkuus koko kokeen ajalta on alhaisempi kuin lyhyen kalibraatiojakson ajalta mitattuna. Tämän intuitiiviseltä tuntuvan keskittymiskyvyn järkkymisen taustalla on esitetty olevan henkilön introspektiiviset ja mielenvaelteluun liittyvät kognitiiviset toiminnot. Ennen tätä tutkimusta on jäänyt kuitenkin osoittamatta osumatarkkuuden ailahtelun yhteys aivokuoren hermostollisen aktiivisuuden pitkällä ajalla autokorreloiviin muutoksiin lähdemallintamisella. Tämän pro gradun tutkimustulokset osoittavat, että näiden kahden lajin välillä on olemassa merkittävä korrelaatioyhteys. Lisäksi lepovaiheen aivotoiminnasta modaliteettispesifeillä tarkkaavaisuus- ja oletustilan verkoston alueilla voidaan ennustaa psykofyysisen suoriutumisen vaihteluja jatkuvan audiovisuaalisen ärsykekynnyksen tarkkaavaisuustehtävän aikana. Keskittymiskyvyn vaihtelun muutoksia hermostollisella tasolla ja näitä mahdollisesti ilmentäviä käyttäytymisen ailahteluja psykofyysisinä parametreinä, kuten osumatarkkuutena ja reaktionopeutena, voidaan luonnehtia skaalauslakianalyysilla. Ilmiön skaalaton käyttäytyminen heijastelee monimutkaisen järjestelmän taipumusta luoda sisäisiä vastaavuussuhteita eli autokorrelaatioita, jotka heikkenevät hitaammin ja ulottuvat kauemmaksi ajassa ja/tai paikassa kuin mitä alla piilevistä mekanismeista voidaan suoraan ennustaa. On havaittu, että osumatarkkuuden jaksottuminen ja spontaani aivotoiminta noudattavat potenssilain skaalauskäyttäytymistä ajan suhteen. Psykofyysisen ja hermostollisen skaalauslain mukaisen käyttäytymisen kvantifioimiseksi tässä opinnäytetyössä on käytetty vaihtelun ikkunallista autokorrelaatioanalyysiä, DFA:ta. DFA paljastaa ilmiön sisällä olevien peräkkäisten tapahtumien autokorrelaatioiden kestävyyden tarkasteluvälin kasvaessa. Skaalausluvut eli DFA-eksponentit on johdettu tässä kokeessa jatkuvan audiovisuaalisen ärsykekynnyksen tarkkaavaisuustehtävän ja levon aikana rekisteröidyistä aivosähkö- ja aivomagneettikäyräsignaalien verhokäyrästä sekä psykofyysisen osuma/huti -binäärisekvenssistä rakennetusta keinotekoisesta satunnaiskulun kaltaisesta käyrästä. Jatkuvat ärsykekynnystehtävät soveltuvat hyvin tarkkaavaisuuden top-down mekanismien tutkimiseen, koska heikoista, vain juuri ja juuri havaintokyvyn säteellä olevista ärsykkeistä seuraa verraten heikko bottom-up hermostovaste. Näin keskittymiskykyyn vaikuttavat top-down säätelymekanismit kuten motivaatio, päämäärät tai mielenvaeltelu eli spontaanilta vaikuttava aivotoiminta edustuu selkeämmin aivosähkö- ja -magneettikäyrissä. Aivokuoren kokonaisvaltaisen skaalautumisen lisäksi ollaan kiinnostuneita psykofyysisten ja hermostollisten vastaavuussuhteiden jakaumamallista tietyille aivoalueille. Mitattujen hermostollisten signaalien paikantaminen tarkalleen tietyille aivokuoren alueille aiheuttaa käänteisen ongelman, joka on ratkaistu tässä MNE -lähdemallintamisella. Lähdemallintamisen algoritmit tuottavat todennäköisimmän mallin aivokuoren alueista, joiden aktiivisuudella voidaan selittää mitatut MEEG signaalit. Mallintaminen on työn kriittinen vaihe, koska sillä yhdistetään neuroanatominen tieto fysiologisen ja psykofyysisen tiedon kanssa. Yksilötason data on käsitelty lopuksi ryhmätasolla tilastollisin menetelmin korrelaatiotulosten merkittävyyksien arvioimiseksi.

The aim of this study is to examine the learning mechanisms and acquisition of non-native phoneme contrasts in young adults using neurophysiological and behavioral methods. According to the traditional view, acquiring novel phonemes after the sensitive periods in the early childhood is very difficult. However, later findings have shown that foreign phoneme contrasts can be learned at a later age, too. Acquiring new phonemic categories requires neuroplastic changes in the brain. Neurophysiological studies have examined the brain’s ability to differentiate between closely related phonemic categories at the early stage of spoken language processing by measuring, for example, event-related mismatch negativity responses (MMN). MMN, or its magnetic equivalent MMNm, is elicited when the brain registers a difference in a repetitive sensory stimulus. Studies have shown that even a moderate amount of auditory training with closely related foreign phonemes improves the brain’s ability to discriminate between them resulting in enhanced MMN or MMNm responses. In this experiment the neural mechanisms of foreign language phoneme acquisition and the learning-related neuroplastic changes were studied using magnetoencephalography (MEG) and neuromagnetic evoked responses (MMNm in particular). 20 Finnish subjects were measured in the experiment. Their task was to learn to differentiate between acoustically closely related Russian fricatives Ш /ʂ/ and Щ /ɕ(ː)/. The subjects’ differentiation skills were first tested in a behavioral task where Russian pseudoword minimal pairs were presented to them auditorily. The first phoneme in the word pairs was varied and the subjects had to report whether they heard a difference between the words or not. The same stimuli were then presented in a passive MEG task where the brain’s change detection responses were tested in an unattended situation as the subjects were watching a silent film. After the measurement the subjects practiced the phonemes at home for approximately one week by playing a learning game by computer. After training they were measured again. Structural magnetic resonance images of the subjects’ brain were also measured for MEG source localization purposes. Behavioral discrimination ability of the experimental phonemes was considerably worse than with familiar control phonemes. The discrimination skills seemed to improve by training, but the difference was not statistically significant. Contrary to the hypotheses, statistically significant MMNm responses were not found before or after training. No significant differences were found in other auditory MEG responses or their neural source current distributions between the measurements either. However, individual differences in learning were sizeable. For the subjects who improved their performance in the behavioral task a modest training-related boost in the auditory responses supporting the hypotheses could be observed. Although very small and statistically insignificant, the effect was opposite for control stimuli and did not exist in the non-learner group suggesting some sort of change in neural processing in the learner group. This study was not able to replicate the findings from various previous studies on phoneme acquisition in adulthood. Although it is likely that certain methodological limitations (e.g. small number of stimulus repetitions, challenging stimuli) affected the significance of the results, based on this study the generalizability of some of the previous findings can be called into question.