10 Imaging Techniques in Synesthesia Research

The fact that the results of imaging techniques seem to be very heterogeneous, constitutes one of the problems of current synesthesia research. On the one hand, this is because the number of test persons is often not representative, and the test designs can be very different. On the other hand, because the intensity of perception varies according to the individual, it is hardly possible to investigate a homogenous group. Despite the large discrepancy among the results, there is evidence that the color region (V4), the posterior interior temporal cortex (PIT), and the transitional region between the parietal and the occipital cortex are fundamentally involved in synesthetic perception (see, e.g., Nunn et al. 2002, Rouw and Scolte 2007, Sperling et al. 2006). There is also evidence for the involvement of the prefrontal cortex (see, e.g., Paulesu et al. 1995, Beeli et al. 2007, Sperling et al. 2006), to which general functions such as working memory, attention, and personality are ascribed. The occipital cortex primarily processes visual information, and the parietal cortex is responsible for, among other things, spatial perception, orientation, as well as somatosensory functions. There are various association fields in the parieto-occipital transitional region which, among other things, carry out the integration of visual information. The PIT cortex is also known as the integrative region of visual information, in particular of color and form. Because up to now mainly grapheme-color synesthetes have been examined using imaging techniques, it cannot be ruled out that other regions of the brain are involved in other forms of synesthesia.