Cortical dynamics of visual feature and object-level processing in the human occipitotemporal cortex : MEG source analysis and evaluation of conductivity models

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dc.contributor Aalto-yliopisto fi
dc.contributor Aalto University en Tarkiainen, Antti 2012-01-24T14:39:53Z 2012-01-24T14:39:53Z 2003-03-28
dc.identifier.isbn 951-22-6377-7
dc.description.abstract The computer simulations carried out in this thesis demonstrated that a spherically symmetric conductor model is in most cases an adequate model for the conductivity geometry of the human head and the use of more sophisticated head models does not contribute considerably to the source estimation accuracy of magnetoencephalography. The main reason for this is the noise that is typically present in the measured signals and masks effectively the differences between different head conductor models. The brain activation studies carried out in this thesis concentrated on the early cortical processing of two special cases of behaviourally highly relevant and frequently encountered visual stimuli: letter-strings and faces. The results show that the early visual processing of both image types consists of at least two separable processes taking place in the occipital and occipitotemporal cortices within 200 ms after the stimulus presentation. The first process, associated with low-level visual feature analysis, occurred in the midline occipital cortex at about 100 ms after image onset. This processing stage was common to the analysis of both letter-strings and faces. From the occipital cortex the activity advanced to the inferior occipitotemporal areas bilaterally, reaching the maximum at about 150 ms after stimulus onset. Although both letter-strings and faces activated largely overlapping areas in the inferior occipitotemporal cortex, the hemispheric distribution of these areas was different. Letter-string processing concentrated to the left hemisphere, whereas face processing occurred more bilaterally, apparently with slight right-hemisphere dominance. This processing stage is likely to represent a more general object-level analysis phase that takes place after the common low-level analysis and acts as a gateway to higher processing areas. When these analysis stages were evaluated in dyslexic subjects a significant underactivation in the occipitotemporal cortex was found to letter-strings but not to faces. This result suggests that the dysfunction of occipitotemporal cortex previously reported in dyslexia is likely to be limited to the processing of letters or at least letter-like objects. en
dc.format.extent 51, [101]
dc.format.mimetype application/pdf
dc.language.iso en en
dc.publisher Helsinki University of Technology en
dc.publisher Teknillinen korkeakoulu fi
dc.relation.haspart Tarkiainen, A., Liljeström, M., Seppä, M. and Salmelin, R., 2003. The 3D topography of MEG source localization accuracy: Effects of conductor model and noise. TKK report TKK-KYL-008.
dc.relation.haspart Tarkiainen, A., Helenius, P., Hansen, P. C., Cornelissen, P. L. and Salmelin, R., 1999. Dynamics of letter string perception in the human occipitotemporal cortex. Brain 122, pages 2119-2131.
dc.relation.haspart Helenius, P., Tarkiainen, A., Cornelissen, P., Hansen, P. C. and Salmelin, R., 1999. Dissociation of normal feature analysis and deficient processing of letter-strings in dyslexic adults. Cerebral Cortex 9, pages 476-483.
dc.relation.haspart Tarkiainen, A., Cornelissen, P. L. and Salmelin, R., 2002. Dynamics of visual feature analysis and object-level processing in face versus letter-string perception. Brain 125, pages 1125-1136.
dc.relation.haspart Tarkiainen, A., Helenius, P. and Salmelin, R., Category-specific occipitotemporal activation during face perception in dyslexic individuals: An MEG study. NeuroImage. Accepted for publication.
dc.relation.haspart Cornelissen, P., Tarkiainen, A., Helenius, P. and Salmelin, R., Cortical effects of shifting letter-position in letter-strings of varying length. Journal of Cognitive Neuroscience. Accepted for publication.
dc.subject.other Medical sciences en
dc.subject.other Physics en
dc.title Cortical dynamics of visual feature and object-level processing in the human occipitotemporal cortex : MEG source analysis and evaluation of conductivity models en
dc.type G5 Artikkeliväitöskirja fi
dc.description.version reviewed en
dc.contributor.department Department of Engineering Physics and Mathematics en
dc.contributor.department Teknillisen fysiikan ja matematiikan osasto fi
dc.subject.keyword magnetoencephalography en
dc.subject.keyword MEG en
dc.subject.keyword conductor model en
dc.subject.keyword spherically symmetric model en
dc.subject.keyword realistically shaped model en
dc.subject.keyword current dipole en
dc.subject.keyword letter-string en
dc.subject.keyword word en
dc.subject.keyword face en
dc.subject.keyword object en
dc.subject.keyword noise masking en
dc.subject.keyword occipitotemporal en
dc.subject.keyword extrastriate en
dc.subject.keyword visual processing en
dc.subject.keyword dyslexia en
dc.identifier.urn urn:nbn:fi:tkk-000349
dc.type.dcmitype text en
dc.type.ontasot Väitöskirja (artikkeli) fi
dc.type.ontasot Doctoral dissertation (article-based) en
dc.contributor.lab Low Temperature Laboratory, Brain Research Unit en
dc.contributor.lab Kylmälaboratorio, Aivotutkimusyksikkö fi

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