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Abstract:
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]).
