Cross-Activation Theory

This theory hypothesizes that synaesthesia is caused by the hyperconnectivity between the color and number areas of the brain at different stages in processing–which is a result of a genetic mutation that causes defective pruning.

Color & Grapheme Areas of the Brain

“color area”- V4 & V8 (red) areas of the fusiform gyrus “number/grapheme area”-also in fusiform gyrus, adjacent to to V4 (green)

This finding (that the area that processes color and the area that processes numbers/graphemes are adjacent to each other) is what inspired the cross-activation theory.

Evidence for Cross Activation in Synaesthesia

Every time there is an activation of the neurons in the number area, there is also an activation of neurons in the color area. Researchers have observed the connections between these two areas in prenatal brains and in developed brains. The findings show that there are substantially more connections in the prenatal brain than the developed brain. This indicates that these connections usually get pruned out in normal development.

Evidence from the Phantom Limb Evidence from the phantom limb is proof that brain maps are highly malleable (not fixed at birth). The somatosensory area of the cortex that receives sensory information from the hand is adjacent to the area that receives it from the face. When an arm is amputated, signals from the face activate the original hand area of cortex and higher brain centers interpret this activation as arising from the phantom hand.

Evidence from Acquired Cases Evidence from patients with Retinitis Pigmentosa provide further proof that brain maps are highly malleable. Some people with this disorder experience tactile sensations as visual phosphenes. Tactile input takes over for the absence of actual visual input and activates visual areas.

Extent of the Gene Mutation’s Expression

It is proposed to be a single gene mutation that inhibits the pruning process. However, it may be expressed to different extents and in different anatomical locations due to various Transcription Factors.

Higher & Lower Synaesthetes The gene may be expressed selectively in the fusiform gyrus or angular gyrus, where the gene is expressed defines the two major categories of synaesthetes, higher & lower. In lower synaesthetes only actual number grapheme will evoke Synaesthesia (V4 region crossed with the color area). In higher synaesthetes concepts of numbers (roman numerals, clusters of dots representing a value) will evoke a synaesthetic experience. In the higher synaesthetes the area of brain that encodes abstract numerical sequences [angular gyrus] is crossed with a “higher” color processing area [superior temporal gyrus, adjacent to angular gyrus].

Synaesthesia & Creativity

What makes someone creative? Among many other things, creative people have a “remarkable facility to link two seemingly unrelated realms in order to highlight a hidden deep similarity” –Ex. Shakespere ‘It is the East and Juliet is the sun’. Synaesthesia is significantly more common in artists, poets and novelists. Most of this data has been found through surveys that indicate synaesthetes are more commonly fine arts students. Some of this data has also been found through “experimental measures of creativity”.

Concepts are represented in brain maps in the same way that percepts are (as seen earlier, concepts of numbers are mapped in specific locations in the brain). Perhaps many other concepts are also represented in maps in the brain. If this is true, then it follows that we can think of metaphors as involving cross-activation of conceptual maps in a manner analogous to cross-activation of perceptual maps in synaesthesia. If mutation-induced cross-wiring selectively affects the fusiform or angular gyrus someone may experience synaesthesia. However, if this mutation is more diffusely expressed it may produce a more generally cross-wired brain creating a greater propensity and opportunity for creatively mapping from one concept to another. This hypothesis explains why those with more cross-wired connections in their brain are able to generate metaphors, but does NOT address the question of why almost anyone can understand a metaphor…Why is such excess cross-wiring needed only for producing metaphors but not for recognizing them??

Heightened Aversions in Synaesthetic Experiences

Some synaesthetes claim that seeing numbers printed in the “wrong” color is like hearing nails being scratched on a chalkboard. There is experimental validation of this phenomenon, by measuring the SCR in synaesthetes vs. nonsynaesthetes, it was proven that they did have higher heightened aversion during these experiences. This heightened aversion may be due to hyperconnectivity between the sensory cortex and the Limbic System (especially the amygdala and nucleus accumbens).

Objections

Objection from Top-Down Influences Crowding Effect – the number is processed in the fusiform gyrus and evokes the appropriated color before the number is consciously processed. The fact that top down influences are present in experimentation doesn’t disprove the hypothesis that Synaesthesia occurs at a very early level of sensory processing, it just shows that cognitive influences (like attention) can also influence early sensory processing.

Objection of Unidirectional Activation How can you explain that Synaesthesia is unidirectional? Numbers are seen colored but presenting colors doesn’t evoke letters or numbers. Metaphors follow this same pattern of only following one direction (from auditory to visual modalities). People say “Loud Shirt” but never “Red Sound”. This results from strong anatomical [evolutionary?] constraints that permit certain types of cross-activation, but not others. A speculative response from Ramachandran on this matter: there is “something to do with the manner in which certain sensory dimensions like color are represented in brain maps, as opposed to the way in which numbers are mapped”. When a number evokes a color, there is something in the visual image (the number) that a color can be ascribed to. But what if a color evoked a number? Where would the number be, how big would it be?

Objection from The Disinhibited Feedback Theory – In this theory, information is processed up through several levels of the sensory hierarchy to some multi-modal sensory nexus before being fed back to lower areas (V4). Whereas the cross-activation theory suggests connections are more local- they don’t need to go through all the processing before going to the V4 area again.

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