Datensatz

Zusammenfassung

In Transcranial Magnetic Stimulation (TMS), strong magnetic pulses delivered by a coil placed over the subject’s head are used to induce neural activity in a focal area of the brain. TMS can be used to demonstrate a causal relationship between behavior and the neural processing in a brain structure of interest by showing that a subject’s task performance is diminished during TMS stimulation of that structure (i.e., the “virtual lesion” approach [1]). We addressed two questions in the current study: 1) How well does the position of the maximal TMS effect coincide with the brain activation pattern observed during the task using other neuroimaging techniques such as fMRI or PET? 2)Which visual area is most critical for conscious perception of a visual stimulus, i.e. which visual area has to be disturbed after stimulus presentation to diminish the recognition performance significantly? (“visual suppression” effect [2,3]). In all subjects, the spatial pattern of the TMS effect was smooth and the coil positions at which the maximal suppression occurred were located next to each other. This indicates that the TMS target was a single continuous brain structure and not, e.g. two or more separate sub-areas. The Center of Gravity (CoG) of the TMS map was consistently positioned over the inferior part of the superior occipital gyrus. As expected, the fMRI activation pattern was rather extended and covered several visual areas. The TMS CoG was consistently located over the medial-inferior part of the fMRI activation. Visual mapping [4] delineated the TMS CoG being significantly closer to the CoG of V2 than to any other visual area. Mean deviation of TMS CoG from V2 CoG obtained with fMRI was 5.1mm (SE 0.6mm, n=7), showing a good spatial congruence between these two neuroimaging techniques. Several control studies were performed to test for possible involvement of other visual areas. The findings suggest that V2 and not primary visual cortex V1 is the brain area primarily targeted in visual suppression. In consequence, our data does not support the special role of V1 in conscious visual perception as previously suggested by several authors (for review see [5]).