V4 color area) As of 2015, the neurological correlates of<unk>  had not been established. Dedicated regions of the brain are specialized for given functions. Increased cross-talk between regions specialized for different functions may account for the many types of<unk> . For example, the additive experience of seeing color when looking at graphemes might be due to cross-activation of the grapheme-recognition area and the color area called V4 (see figure). This is supported by the fact that grapheme–color synesthetes can identify the color of a grapheme in their peripheral vision even when they cannot consciously identify the shape of the grapheme. An alternative possibility is disinhibited feedback, or a reduction in the amount of inhibition along normally existing feedback pathways. Normally, excitation and inhibition are balanced. However, if normal feedback were not inhibited as usual, then signals feeding back from late stages of multi-sensory processing might influence earlier stages such that tones could activate vision. Cytowic and Eagleman find support for the disinhibition idea in the so-called acquired forms of<unk>  that occur in non-synesthetes under certain conditions: temporal lobe epilepsy, head trauma, stroke, and brain tumors. They also note that it can likewise occur during stages of meditation, deep concentration, sensory deprivation, or with use of psychedelics such as LSD or mescaline, and even, in some cases, marijuana. However, synesthetes report that common stimulants, like caffeine and cigarettes do not affect the strength of their<unk> , nor does alcohol. A very different theoretical approach to<unk>  is that based on ideasthesia. According to this account,<unk>  is a phenomenon mediated by the extraction of the meaning of the inducing stimulus. Thus,<unk>  may be fundamentally a semantic phenomenon. Therefore, to understand neural mechanisms of<unk>  the mechanisms of semantics and the extraction of meaning need to be understood better. This is a non-trivial issue because it is not only a question of a location in the brain at which meaning is "processed" but pertains also to the question of understanding epitomized in e.g., the Chinese room problem. Thus, the question of the neural basis of<unk>  is deeply entrenched into the general mind–body problem and the problem of the explanatory gap. Genetics Due to the prevalence of<unk>  among the first-degree relatives of people affected, there may be a genetic basis, as indicated by the monozygotic twins studies showing an epigenetic component.<unk>  might also be an oligogenic condition, with locus heterogeneity, multiple forms of inheritance, and continuous variation in gene expression. While the exact genetic loci for this trait haven't been identified, research indicates that the genetic constructs underlying<unk>  are most likely more complex than the simple X-linked mode of inheritance that early researchers believed it to be. Further, it remains uncertain as to whether<unk>  perseveres in the genetic pool because it provides a selective advantage, or because it has become a byproduct of some other useful selected trait. Women have a higher chance of developing<unk> , as demonstrated in the UK where females are 8 times more likely to have it. As technological equipment continues to advance, the search for clearer answers regarding the genetics behind<unk>  will become more promising. Although often termed a "neurological condition,"<unk>  is not listed in either the DSM-IV or the ICD since it usually does not interfere with normal daily functioning. Indeed, most synesthetes report that their experiences are neutral or even pleasant. Like perfect pitch,<unk>  is simply a difference in perceptual experience. Reaction times for answers that are congruent with a synesthete's automatic colors are shorter than those whose answers are incongruent. The simplest approach is test-retest reliability over long periods of time, using stimuli of color names, color chips, or a computer-screen color picker providing 16.7 million choices. Synesthetes consistently score around 90% on the reliability of associations, even with years between tests. In contrast, non-synesthetes score just 30–40%, even with only a few weeks between tests and a warning that they would be retested. Many tests exist for<unk> . Each common type has a specific test. When testing for grapheme–color<unk> , a visual test is given. The person is shown a picture that includes black letters and numbers. A synesthete will associate the letters and numbers with a specific color. An auditory test is another way to test for<unk> . A sound is turned on and one will either identify it with a taste or envision shapes. The audio test correlates with chromesthesia (sounds with colors). Since people question whether or not<unk>  is tied to memory, the "retest" is given. One is given a set of objects and is asked to assign colors, tastes, personalities, or more. After a period of time, the same objects are presented and the person is asked again to do the same task. The synesthete can assign the same characteristics because that person has permanent neural associations in the brain, rather than memories of a certain object. The automaticity of synesthetic experience. A synesthete might perceive the left panel like the panel on the right. Grapheme–color synesthetes, as a group, share significant preferences for the color of each letter (e.g., A tends to be red; O tends to be white or black; S tends to be yellow, etc.) Nonetheless, there is a great variety in types of<unk> , and within each type, individuals report differing triggers for their sensations and differing intensities of experiences. This variety means that defining<unk>  in an individual is difficult, and the majority of synesthetes are completely unaware that their experiences have a name. Neurologist Richard Cytowic identifies the following diagnostic criteria for<unk>  in his first edition book. However, the criteria are different in the second book: #<unk>  is involuntary and automatic. # Synesthetic perceptions are spatially extended, meaning they often have a sense of "location." For example, synesthetes speak of "looking at" or "going to" a particular place to attend to the experience. # Synesthetic percepts are consistent and generic (i.e., simple rather than pictorial). #<unk>  is highly memorable. #<unk>  is laden with affect. Cytowic's early cases mainly included individuals whose<unk>  was frankly projected outside the body (e.g., on a "screen" in front of one's face). Later research showed that such stark externalization occurs in a minority of synesthetes. Refining this concept, Cytowic and Eagleman differentiated between "localizers" and "non-localizers" to distinguish those synesthetes whose perceptions have a definite sense of spatial quality from those whose perceptions do not.
Synesthesia
Dysesthesia
Hypoesthesia
Hyperalgesia
Hypoalgesia
Pallesthesia
Chromesthesia
Neuralgia