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Surface-based searchlight mapping of modality-independent responses to semantic categories using high-resolution fMRI


Hagoort,  Peter
Donders Institute for Brain, Cognition and Behaviour, External Organizations;
Neurobiology of Language Department, MPI for Psycholinguistics, Max Planck Society;

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Simanova, I., Hagoort, P., Oostenveld, R., & van Gerven, M. (2014). Surface-based searchlight mapping of modality-independent responses to semantic categories using high-resolution fMRI. Poster presented at the Sixth Annual Meeting of the Society for the Neurobiology of Language (SNL 2014), Amsterdam, the Netherlands.

Cite as: http://hdl.handle.net/11858/00-001M-0000-002B-9C72-7
Previous studies have shown the possibility to decode the semantic category of an object from the fMRI signal in different modalities of object presentation. Furthermore, by generalizing a classifier across different modalities (for instance, from pictures to written words), cortical structures that process semantic information in an amodal fashion have been identified. In this study we employ high-resolution fMRI in combination with surface-based searchlight mapping to further explore the architecture of modality-independent responses. Stimuli of 2 semantic categories (animals and tools) were presented in 2 modalities: photographs and written words. Stimuli were presented in 40-seconds blocks with 10-seconds intervals. Subjects (N=3) were instructed to judge whether each stimulus within a block was semantically consistent with the others. The experiment also included 8 free recall blocks, in which name of a category appeared on the screen for 2 seconds, followed by 40 seconds of a blank screen. In theses blocks subjects were instructed to covertly recall all entities from the probed category that they had seen during the experiment. Subjects were scanned with 7 Tesla MRIscanner, using 3D EPI sequence with isotropic resolution of 1.5 mm. In each subject, reconstruction of cortical surface was performed. After that, for each vertex on the surface, a set of adjacent voxels in the functional volume was assigned. Subsequently, a linear support vector machine classifier was used to decode object category in each surface-based patch. Generalization analysis across picture and written word presentation was performed, where the classifier was trained on the fMRI data from blocks of written words, and tested on the data from picture blocks, and vice versa. The second analysis was performed on the free recall blocks, where the classifier was trained on merged data from pictures and written words blocks, and tested on the free recall blocks. Further, we explored how the decoding accuracy in the inferior temporal cortex changes with the diameter of the searchlight patch. Since surface-based voxel grouping takes into account the cortical folding and ensures that voxels belonging to different gyri do not fall in the same searchlight group, it allows answering the question, at what spatial scale is the modality-independent information is represented. The cross-modal analysis in all three subjects revealed a cluster of voxels in inferior temporal cortex (lateral fusiform and inferotemporal gyri) and posterior middle temporal gyrus. The topography of significant clusters also suggested involvement of the inferior frontal gyrus, lateral prefrontal cortex, and medial prefrontal cortex. Interestingly, these areas were the most evident in the free recall test, although the searchlight maps of the three subjects showed substantial individual differences in this analysis. Overall, the data yield a similar picture as previous research, highlighting the role of IT/pMTG and prefrontal cortex in the crossmodal semantic representation. We further extended previous research, by showing that the classification accuracy in these areas decreases with the increase of the searchlight patch size. These results indicate that the modality-independent categorical activations in the IT cortex are represented on the spatial scale of millimetres.