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The Re-entrant Processing Theory

The Theory

The Re-entrant processing theory is a hybrid theory that includes some of the same points made by the disinhibited feedback theory and also does not allow for unique horizontal connections. This theory states that in addition to the forward activation of the V1 through V4 regions to the posterior (PIT) and anterior (AIT) inferior temporal regions, abnormal neural activity from the AIT gets sent back as feedback to representations in the PIT and V4 regions. This feedback from AIT to the PIT and V4 is what results in the synesthetic experiences of the individual.

Support for the Theory

Evidence used to argue for this theory over the cross-activation theory is that within this model, visual context and the meaning of their perceptions are what influences their synesthetic experiences. The cross-activation theory is based only on the perception process, the physical and symbolic levels, but does not include a semantic level influence like re-entrant processing does. As you can see, with the existence of this theory, it is hard to distinguish between the 3 theories due to neuroimaging being not defined, detailed, and precise enough. Studies that apply EEG (electroencephalograms) and MEGs (magnetoencephalograms) to time course activation within different synesthesias might allow us to untangle the theories. Since the distance between the brain regions focused on in grapheme -> color synesthesia is relatively small, MEGs might be the ideal study needed to disentangle the theories.

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