Acquired synesthesia in retinitis pigmentosa

Patient PH developed retinitis pigmentosa in childhood and progressively lost his vision until he became completely blind at 40 years old. At age 42, he started experiencing vivid 'synesthesia'; tactile stimuli on the hand evoked a vivid visual sensation of 'movement: 'expansion' or 'jumping'. Intriguingly, the synesthesia was much more vivid when the hand was in front of the face rather than behind. The effect is unlikely to be confabulatory since touch thresholds (Semmes Monofilaments) were normal and identical for hand in front versus hand behind the head, while 'thresholds' for evoked visual sensations were significantly higher for the 'behind' condition. Also, the critical fusion frequency for the tactile sensation was much higher than the visually evoked ones. We propose three explanations. (i) 'Remapping' or 'cross-talk'. As a result of de-afferentation, sensory input to the somatosensory pathways (e,g. insular cortex) also innervates extrastriate visual areas. (ii) When a person is touched, there may be spontaneously evoked tactile associative 'memories' that would not normally evoke actual visual qualia because of competing 'spontaneous activity' from the visual pathways. However, upon de-afferentation, the associations may be experienced as synesthesia. (iii) After de-afferentation, the 'back projections' to somatosensory areas from visual areas may be strengthened.