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The capacity to generate and analyze mental visual images is essential for many cognitive abilities. We combined triple-pulse transcranial magnetic stimulation (tpTMS) and repetitive TMS (rTMS) to determine which distinct aspect of mental imagery is carried out by the left and right parietal lobe and to reveal interhemispheric compensatory interactions. The left parietal lobe was predominant in generating mental images, whereas the right parietal lobe was specialized in the spatial comparison of the imagined content. Furthermore, in case of an rTMS-induced left parietal lesion, the right parietal cortex could immediately compensate such a left parietal disruption by taking over the specific function of the left hemisphere.
Mental imagery refers to the experience of a perception in the absence of a corresponding physical stimulus. In our everyday life, mental imagery represents a crucial element of numerous cognitive abilities, such as object recognition, reasoning, language comprehension, and memory. Because of its importance, the exact processes associated with imagery have long occupied cognitive psychologists and been a matter of debate and controversy (1).
Mental imagery is accompanied by the activation of frontoparietal networks (2-5), but the exact brain areas engaged in imagery depend on the specific features of the imagery task (6). When spatial comparisons between imagined objects are required, most functional imaging studies show bilateral parietal activation in homologous intraparietal sulcus areas of the left and right hemispheres (3). However, neuropsychological studies on patients with focal brain lesions generally support a dominant role of the left hemisphere in imagery [(7), but see (8)].
Time-resolved functional magnetic resonance imaging (fMRI) has been used to address this apparent contradiction between functional imaging studies and findings in focal brain injury patients (4). An earlier cluster of activation in both parietal cortices (with left predominance) can be separated from a late cluster confined to the right parietal cortex (Fig. 1). These results support the involvement of both parietal lobes in mental imagery but suggest that each parietal lobe has a distinct functional role at different moments in time. The sequential parietal activation might represent a transition from an earlier more distributed processing stage of image generation to a later right-hemispheric lateralized stage of spatial analysis of the images (4). In a combined fMRI and rTMS study, only rTMS to the...