Gustatory sensation in response to tactile stimulation has been reported to occur in only 0.6% of synesthetes and has rarely been formally described.

Case Study: A 29-year-old female with schizoaffective disorder – depressed subtype, two months prior to presentation, began noticing that when she touched objects, she would experience an abnormal taste in her mouth. When she would touch specific items with her fingertips, for instance wood, bedsheets, tables, cups, plates, silverware, metal, trash cans, and toothpaste would induce a bitter taste. Touching wood caused the most intense bitter taste. Touching glass did not precipitate a taste.

Mental Status Examination: Disheveled and guarded with minimal eye contact. Anxious and agitated. She believes people are stalking her and that cell phones, radios, and computers emit radiation which makes her head burn as if it is on fire. She perceived that inanimate objects, such as furniture, were staring at her. Other: CT scan of brain with and without contrast: normal.

This could be due to neural overactivation between brain regions due to a neighborhood effect, with cross-firing of connecting fibers. The cortical taste area of the brain lies within the Brodmann area 43 of the parietal lobe which includes the pre- as well as post-central gyri extending into the parietal operculum. Brodmann areas 3, 1, and 2 in the postcentral gyrus is where the primary sensation for touch is mediated. The stimulation in adjacent and overlapping anatomical structures may be explained by ephaptic transmission with firing of one causing discharge of the second, yclept the adjacency principle.This co-localization hypothesis is further supported by the occurrence of pathological state of seizures which manifest both tactile and gustatory sensation. Such a hypothesis would be supported by Anderson’s 1886 observation discussing a peculiar sensation in the right arm and hand elicited concurrently with a bitter sensation in the mouth. Through an overflow phenomenon, touch may induce a diffuse serotonergic discharge to more remote regions of the brain that may mediate their functions through serotonin, such as taste. Possibly, other pathology may be invoked to explain this synesthesia. A deprived sensory system may be hyper-receptive to stray or overflow of sensations which are then interpreted within the sensorily deprived sensory system, as in Charles Bonnet syndrome. In the chemosensory sphere, sensory deprivation is associated with chemosensory hallucinations. If the patient manifests gustatory deficit, it may have predisposed her to sensitivity to such stray or overflowing sensory stimulus from touch manifesting as gustation. In those who present with synesthesia, investigation of pathology in the sensory system of the receiving concurrent sensation is warranted.

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