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Neural basis of synesthesia : ウィキペディア英語版 | Neural basis of synesthesia Synesthesia is a neurological condition in which two or more bodily senses are coupled. For example, in a form of synesthesia known as Grapheme → color synesthesia, letters or numbers may be perceived as inherently colored. In another, called number → form synesthesia, numbers are automatically and consistently associated with locations in space. In yet another form of synesthesia, called ordinal linguistic personification, either numbers, days of the week, or months of the year evoke personalities. In other forms of synesthesia, music and other sounds may be perceived as colored or having particular shapes. Recent research has begun to explore the neural basis of these experiences, drawing both on neuroscientific principles and on functional neuroimaging data. Based on these data, two major theories have been proposed concerning the neural basis of synesthesia. Both theories start from the observation that there are dedicated regions of the brain that are specialized for certain functions. For example, the part of the human brain involved in processing visual input, called the visual cortex can be further subdivided into regions that are preferentially involved in color-processing (the fourth visual area, V4) or with motion processing, called V5 or MT. Based on this notion of specialized regions, some researchers have suggested that increased cross-talk between different regions specialized for different functions may account for different types of synesthesia. ==Cross-activation==
Since regions involved in the identification of letters and numbers lie adjacent to a region involved in color-processing (V4), the additional experience of seeing colors when looking at graphemes might be due to "cross-activation" of V4 . This cross-activation may arise due to a failure of the normal developmental process of pruning, which is one of the key mechanisms of synaptic plasticity, in which connections between brain regions are partially eliminated with development. Similarly, lexical → gustatory synesthesia may be due to increased connectivity between adject regions of the insula in the depths of the lateral sulcus involved in taste processing that lie adjacent to temporal lobe regions involved in auditory processing. Similarly, taste → touch synesthesia may arise from connections between gustatory regions and regions of the somatosensory system involved in processing touch. However, not all forms of synesthesia are easily explained by adjacency.
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