Datensatz

Zusammenfassung

Recent studies have shown that global information about shapes is processed in both early ventral (i.e. V1, V2, Vp, V4) and higher occipitotemporal visual areas (i.e. Lateral Occipital Complex/LOC). However, the temporal and spatial properties of shape processing across visual areas in the human brain are largely unknown. The present thesis addressed this question in a combined fMRI and MEG study, that made use of the complimentary spatial and temporal resolution of the two techniques. An event-related adaptation paradigm was applied, in which lower neural responses are observed for two identical than two different consecutivelypresented stimuli. The stimuli were shapes, that consisted of collinear Gabor elements. To investigate the temporal properties of shape processing, the interstimulus interval between the two consecutively-presented stimuli was manipulated (ISI: 100 vs. 400 msec). The results showed adaptation for both the short and the long ISI in the LOC, but only for the short ISI in early visual areas. Further, the spatial properties (Local vs. Global) were tested by changes in the local orientation of the Gabor elements or different global changes. Strong fMRI adaptation effects to local changes were observed in early visual areas (V1, V2, VP and V4) and to a smaller extent also in LOC. In contrast, fMRI adaptation effects to global changes were found only for the LOC, but not the early visual areas. In summary, the findings indicated, that processing of shape information in early visual areas is transient and restricted to a local neighbourhood around the receptive fields of their cells resulting in an analysis at high spatiotemporal resolution in early visual areas. In contrast, a rather coarse spatiotemporal resolution is implemented in the processing of shape information in higher visual areas resulting in sustained analysis. Further, recurrent processing between early and higher visual areas via feedforward and feedback projections might play a critical role in local-to-global and global-to-local mechanisms in shape analysis. In addition, the experiments confirmed the role of event-related fMRI adaptation paradigms as a sensitive tool to study shape analysis at different spatial and temporal scales across visual areas in the human brain and finally indicated that combined fMRI and MEG studies allow the investigation at high temporal and spatial resolution.