The Disinhibited Feedback Theory of Synesthesia

The Model It Follows

The brain works by having neural signals propagate down to certain brain areas through multiple pathways. These signals are always sent back through feedback connections. But sometimes, these top-down signals are inhibited in order to prevent synesthetic experiences from occurring. Information propagates down the inducer pathway and hits the convergence area. In synesthetes, due to the disinhibition of the signal, information continues to propagate down through the concurrent pathway.

So, what is suggested is that feedforward signaling in the inducer pathway activates neurons in the brain area where the inducer and concurrent pathways converge. Then feedback signaling from this area propagates down and activates a concurrent representation. In other words, feedforward origination of the inducer results in the feedback activation of the concurrent. This feedback representation of the concurrent is the synesthetic result that the individual experiences.

The Theory

The disinhibited feedback theory states that no abnormal neural connections exist in the human brain, and proposes that synesthesia is caused internally by neural connections that exist entirely in the normal human brain. Other theories like cross activation, assume that infants are born with unique connections between cortical sensory systems, but later encounter a failure in the neural pruning process of the brain which normally eliminates these connections. Since these connections are never eliminated, there remains these horizontal shortcut-like connections that allow for cross activation to occur. Disinhibited feedback denies the existence of these unique horizontal connections from birth, and demonstrates a way synesthesia can be achieved through the model.

Support for the Theory

The ability for drugs and medications to induce synesthetic experiences in non-synesthetic people is evidence for this theory. It's possible that certain drugs manipulate these connections to allow for the feedforwarding of the inducer to cause the feedback activation of the concurrent. Often, congenital (people born with synesthesia) synesthetic experiences are usually usually very generic, of just color and/or movement. But when psychedelics (drug induced synesthetes) have an experience, it is usually complex. Psychedelics often see animals or complex scenes.

In many studies involving color -> grapheme synesthesia, in addition to the V4 area being activated, the anterior lingual gyrus and the intraparietal sulcus regions were activated as well. Since the intraparietal sulcus is responsible for multisensory processing in the brain, this adds support for this theory and re-entrant processing as well.