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

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.reviewe