How someone with<unk>  might perceive (not "see") certain letters and numbers<unk>  or<unk> ed<unk><unk>  is a form of<unk>  in which an individual's perception of numerals and letters is associated with the experience of<unk> s. Like all forms of<unk> ,<unk>  is involuntary, consistent and memorable.<unk>  is one of the most common forms of<unk>  and, because of the extensive knowledge of the visual system, one of the most studied. While it is extremely unlikely that any two synesthetes will report the same<unk> s for all letters and numbers, studies of large numbers of synesthetes find that there are some commonalities across letters (e.g., "A" is likely to be red). Early studies argued that<unk>  was not due to associative learning, such as from playing with<unk> ed refrigerator magnets. However, one recent study has documented a case of<unk>  in which synesthetic associations could be traced back to<unk> ed refrigerator magnets. Despite the existence of this individual case, the majority of synesthetic associations do not seem to be driven by learning of this sort. Rather, it seems that more frequent letters are paired with more frequent<unk> s, and some meaning-based rules, such as ‘b’ being blue, drive most synesthetic associations. There has been a lot more research as to why and how<unk>  occurs with more recent technology and as<unk>  has become more well known. It has been found that<unk><unk>  synesthetes have more grey matter in their brain. There is evidence of an increased grey matter volume in the left caudal intraparietal sulcus (IPS). There was also found to be an increased grey matter volume in the right fusiform gyrus. These results are consistent with another study on the brain functioning of<unk><unk>  synesthetes.<unk><unk>  synesthetes tend to have an increased thickness, volume and surface area of the fusiform gyrus. Furthermore, the area of the brain where word, letter and<unk>  processing are located, V4a, is where the most significant difference in make-up was found. Though not certain, these differences are thought to be part of the reasoning for the presence of<unk> .
Grapheme–color synesthesia
Lexical–gustatory synesthesia
Cutaneous rabbit illusion
Dissociated sensory loss
Meralgia paraesthetica
Baricity
Inverted binocular phenomenon
Chromesthesia