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The longtime explanation of Grapheme-Color Synesthesia has been that it is caused by an increase in the connections of the areas surrounding the visual cortex. To understand the connections refer to the picture of the visual cortex above. The primary visual cortex is the main visual processing area of the brain. It is located at the back of the head and is labeled on the image above as V1. The other areas that play a role in synesthesia are V4 and V8. Areas V4 and V8 are both higher processing areas of the occipital lobe that are focused on color and grapheme processing.
Before continuing it is advisable to know what a neuron is. A neuron is the basic transmission unit in the brain and spinal cord. It uses electrical gradients to send information from different parts of the body to the brain and to transfer information with in the brain. Looking at the picture of a neuron, we see that there are 3 main parts: the dendrites, the axon, and the terminal button. Information from other neurons is sent to this neuron through the dendrites. The information passing chemicals are called neurotransmitters. They bind to the dendrites and then an electrical charge is sent down the axon to the terminal button, where the cell releases its own neurotransmitters to the next cell. This is how information is passed through out the body. An increase in connections means that there are more neuron fibers connecting parts of the brain than usual in synesthetes.
Much of the research available supports the model that an increase in connections between the color processing areas V4 and V8 causes synesthesia. (Hubbard, 2003) An increase in connections means that when compared to an average brain the synesthetic brain has more neurons connecting areas V4, V8, and V1. These areas of the brain communicate normally, but in a synesthete these communications are increased. These increased connections are hypothesized to be caused by a genetic mutation that leads to defective pruning of the connections with areas V4 and V8. (Ramachandran, 2001) This defective pruning leads to there being more neurons and these neurons connect to other areas in the brain that are associated with whatever type of synesthesia you have. In our case the connections are to the right fusiform gyrus, which is used in grapheme recognition, and the left intraparietal sulcus and frontal cortex, which are used in binding and consciousness. When these areas are connected, as in synesthesia, the individual may have different colors bound to different graphemes. (Red-letter, 2007) (Rouw, 2007)
This information comes from a study done by Romke Rouw and Steven Scholte from the University of Amsterdam in the Netherlands. These used diffusion tensor imaging (DTI) to look at brain differences in synesthetes. DTI is a brain imaging technique, using a large expensive machine that allows for non-invasive visualizations of the connections in the brain. This means they were able to view synesthetic brains without having to operate and were able to conduct experiments while observing the patient. This allowed Rouw and Scholte to observe an increase in connections between areas V4 and V8, with the fusiform gyrus, the intraparietal sulcus, and the frontal cortex. As mentioned earlier 90% of synesthetes perceive the color in their “mind’s eye” and the other 10% project the colors. Rouw and Scholte found that the connections were more plentiful as the strength of the synesthetic reaction increased. (i.e. the synesthetes that project had the most connections.) (Red-letter, 2007) (Rouw, 2007)The neural cause of synesthesia is based on an increase in connections between different parts of the brain. In the case of grapheme-color synesthesia area, V4 and V8 are the primary locations of the increase of connections.