Solmujen sisäinen konnektiviteetti ja topologiset roolit toiminnallisissa aivoverkoissa

Many real-life phenomena consist of a number of interacting elements and can thus be modeled as a complex network. The human brain is an example of such a system where the neuronal information processing of the brain is characterized by interaction and information exchange between different brain regions. In this Thesis, we examine functional brain networks estimated from functional magnetic resonance imaging (fMRI) data. When defining network nodes, the small measurement units, voxels, are grouped to larger entities that represent supposedly functionally homogeneous brain regions referred to as Regions of Interest (ROIs). Despite their assumed homogeneity, it has been demonstrated that the voxels within a ROI exhibit spatially and temporally varying correlation structure. This gives rise to a concept referred to as internal connectivity. On the larger scale, the ROIs form a brain network where each ROI has its role in the structure of the network topology, i.e., a topological role. Topological roles have been suggested to be indicative of the node's functional specialization. On the other hand, it has been argued that internal connectivity may relate to the mechanisms the ROI uses to interact with its neighbors in the functional brain network. This Thesis combines these two ideas. To this end, we aim to predict the ROI's topological role from its internal connectivity features. We find that using internal connectivity features as model variables increases the classification accuracy in comparison to a baseline classifier. These results suggest that there is a relationship between internal connectivity and the ROI's topological role. This link provides a basis for faster and more computationally efficient topological role estimation. Further, it helps to better understand the mechanisms brain regions use to interact with each other. Both of these factors importantly increase our knowledge on brain function under different tasks and circumstances.Monet todellisen maailman ilmiöt koostuvat useista vuorovaikutuksessa olevista elementeistä, ja niitä voidaan mallintaa kompleksisina verkostoina. Ihmisaivot ovat esimerkki tällaisesta järjestelmästä, jossa aivojen hermosolutason tiedonkäsittely perustuu aivoalueiden väliseen vuorovaikutukseen ja tiedonvaihtoon. Diplomityössäni tutkin toiminnallisesta magneettikuvausdatasta rakennettuja toiminnallisia aivoverkkoja. Verkon solmuja määritettäessä pienet mittauselementit, vokselit, ryhmitellään isommiksi kokonaisuuksiksi, jotka edustavat toiminnallisesti yhtenäisiksi oletettuja aivoalueita (engl. Region of Interest, ROI). On kuitenkin osoitettu, että oletetusta yhtenäisyydestään huolimatta ROIden sisällä on monimuotoisia sekä paikallisesti että ajallisesti vaihtelevia korrelaatiorakenteita. Tästä syntyy sisäisen konnektiviteetin käsite, joka kuvaa ROI:n sisäistä korrelaatiorakennetta ja sen vaihtelua. Laajemmassa mittakaavassa ROI:t muodostavat aivoverkon, jossa jokaisella ROI:lla on verkon rakenteessa oma roolinsa, n.s. topologinen rooli. Topologisten roolien ajatellaan liittyvän ROI:den toiminnalliseen erikoistumiseen. On myös esitetty, että sisäinen konnektiviteetti liittyy niihin mekanismeihin, joiden avulla ROI vuorovaikuttaa naapureidensa kanssa toiminnallisessa aivoverkossa. Tämä diplomityö yhdistää nämä kaksi ajatusta: ROI:n topologista roolia pyritään ennustamaan sen sisäisen konnektiviteetin tekijöiden avulla. Tulokset osoittavat, että sisäisen konnektiviteetin tekijät parantavat ennustustarkkuutta verrattuna valistuneeseen arvaukseen perustuvaan pohjatasoluokittimeen. Tulokset osoittavat, että ROI:n sisäisen konnetiviteetin ja topologisten roolien välillä on yhteys. Tämä yhteys tarjoaa pohjan topologisten roolien nopeammalle ja laskennallisesti tehokkaammalle määrittämiselle ja lisää ymmärrystä niistä mekanismeista, joita ROI:t käyttävät vuorovaikuttaakseen toistensa kanssa. Nämä tekijät lisäävät tietoa aivojen toiminnasta eri tilanteissa ja tehtävissä

Regions of Interest as nodes of dynamic functional brain networks

The properties of functional brain networks strongly depend on how their nodes are chosen. Commonly, nodes are defined by Regions of Interest (ROIs), predetermined groupings of fMRI measurement voxels. Earlier, we demonstrated that the functional homogeneity of ROIs, captured by their spatial consistency, varies widely across ROIs in commonly used brain atlases. Here, we ask how ROIs behave as nodes of dynamic brain networks. To this end, we use two measures: spatiotemporal consistency measures changes in spatial consistency across time and network turnover quantifies the changes in the local network structure around an ROI. We find that spatial consistency varies non-uniformly in space and time, which is reflected in the variation of spatiotemporal consistency across ROIs. Furthermore, we see time-dependent changes in the network neighborhoods of the ROIs, reflected in high network turnover. Network turnover is nonuniformly distributed across ROIs: ROIs with high spatiotemporal consistency have low network turnover. Finally, we reveal that there is rich voxel-level correlation structure inside ROIs. Because the internal structure and the connectivity of ROIs vary in time, the common approach of using static node definitions may be surprisingly inaccurate. Therefore, network neuroscience would greatly benefit from node definition strategies tailored for dynamical networks

Regions of Interest as nodes of dynamic functional brain networks

doi: 10.1162/netn_a_00047The properties of functional brain networks strongly depend on how their nodes are chosen. Commonly, nodes are defined by Regions of Interest (ROIs), predetermined groupings of fMRI measurement voxels. Earlier, we demonstrated that the functional homogeneity of ROIs, captured by their spatial consistency, varies widely across ROIs in commonly used brain atlases. Here, we ask how ROIs behave as nodes of dynamic brain networks. To this end, we use two measures: spatiotemporal consistency measures changes in spatial consistency across time and network turnover quantifies the changes in the local network structure around an ROI. We find that spatial consistency varies non-uniformly in space and time, which is reflected in the variation of spatiotemporal consistency across ROIs. Furthermore, we see time-dependent changes in the network neighborhoods of the ROIs, reflected in high network turnover. Network turnover is nonuniformly distributed across ROIs: ROIs with high spatiotemporal consistency have low network turnover. Finally, we reveal that there is rich voxel-level correlation structure inside ROIs. Because the internal structure and the connectivity of ROIs vary in time, the common approach of using static node definitions may be surprisingly inaccurate. Therefore, network neuroscience would greatly benefit from node definition strategies tailored for dynamical networks.Peer reviewe

Social perspective-taking shapes brain hemodynamic activity and eye movements during movie viewing

Putting oneself into the shoes of others is an important aspect of social cognition. We measured brain hemodynamic activity and eye-gaze patterns while participants were viewing a shortened version of the movie 'My Sister's Keeper' from two perspectives: that of a potential organ donor, who violates moral norms by refusing to donate her kidney, and that of a potential organ recipient, who suffers in pain. Inter-subject correlation (ISC) of brain activity was significantly higher during the potential organ donor's perspective in dorsolateral and inferior prefrontal, lateral and inferior occipital, and inferior-anterior temporal areas. In the reverse contrast, stronger ISC was observed in superior temporal, posterior frontal and anterior parietal areas. Eye-gaze analysis showed higher proportion of fixations on the potential organ recipient during both perspectives. Taken together, these results suggest that during social perspective-taking different brain areas can be flexibly recruited depending on the natureof the perspective that is taken.Peer reviewe

Differential inter-subject correlation of brain activity when kinship is a variable in moral dilemma

Previous behavioural studies have shown that humans act more altruistically towards kin. Whether and how knowledge of genetic relatedness translates into differential neurocognitive evaluation of observed social interactions has remained an open question. Here, we investigated how the human brain is engaged when viewing a moral dilemma between genetic vs. non-genetic sisters. During functional magnetic resonance imaging, a movie was shown, depicting refusal of organ donation between two sisters, with subjects guided to believe the sisters were related either genetically or by adoption. Although 90% of the subjects self-reported that genetic relationship was not relevant, their brain activity told a different story. Comparing correlations of brain activity across all subject pairs between the two viewing conditions, we found significantly stronger inter-subject correlations in insula, cingulate, medial and lateral prefrontal, superior temporal, and superior parietal cortices, when the subjects believed that the sisters were genetically related. Cognitive functions previously associated with these areas include moral and emotional conflict regulation, decision making, and mentalizing, suggesting more similar engagement of such functions when observing refusal of altruism from a genetic sister. Our results show that mere knowledge of a genetic relationship between interacting persons robustly modulates social cognition of the perceiver.Peer reviewe

A drama movie activates brains of holistic and analytical thinkers differentially

10.1093/scan/nsy099People socialized in different cultures differ in their thinking styles. Eastern-culture people view objects more holistically by taking context into account, whereas Western-culture people view objects more analytically by focusing on them at the expense of context. Here we studied whether participants, who have different thinking styles but live within the same culture, exhibit differential brain activity when viewing a drama movie. A total of 26 Finnish participants, who were divided into holistic and analytical thinkers based on self-report questionnaire scores, watched a shortened drama movie during functional magnetic resonance imaging. We compared intersubject correlation (ISC) of brain hemodynamic activity of holistic vs analytical participants across the movie viewings. Holistic thinkers showed significant ISC in more extensive cortical areas than analytical thinkers, suggesting that they perceived the movie in a more similar fashion. Significantly higher ISC was observed in holistic thinkers in occipital, prefrontal and temporal cortices. In analytical thinkers, significant ISC was observed in right-hemisphere fusiform gyrus, temporoparietal junction and frontal cortex. Since these results were obtained in participants with similar cultural background, they are less prone to confounds by other possible cultural differences. Overall, our results show how brain activity in holistic vs analytical participants differs when viewing the same drama movie.Peer reviewe