Extinction and synesthesia in patients with spinal cord injuries

Abstract

Extinction and synesthesia were studied in 50 patients with spinal cord injury with various levels and extents. Extinction was found in 20 (40 per cent) and synesthesia in 6 (12 per cent) of the 40 males and 10 females. No correlations were found between either of the two phenomena and parameters of patient's age, lesion's age, lesion's level and extent, or accompanying head injury. The latter occurred in 22/50 patients (44 per cent), diagnosed by a history of amnesia. Extinction was tested by synchronous double stimulation unilaterally and bilaterally, symmetrically and asymmetrically. It was more frequent unilaterally than bilaterally. The modalities of superficial sensibility were prone to be extinct but those of deep sensibility (pallesthesia from bone conduction and kinesthesia) were not. Extinction of tactile responses did not imply that all other skin modalities (pain, temperature, pressure, skin pallesthesia) will also be extinct in an all or nothing fashion. Modalities could become extinct either singly or in combination. Extinction in spinal man was presumed to be caused by a reduction of perception in hypesthetic areas and was of diagnostic value insofar as its segmental distribution suggested the longitudinal extent of a cord lesion. This concept is in accord with the results of monkey experiments by Eidelberg and Schwartz (1971). Synesthia in spinal man is not identical with synchiria of the monkey. It is not in a reciprocal relationship with extinction and both phonemena co-existed in four of the six patients. Synesthesia is elicited by a stimulus delivered to the normesthetic skin above the level of the lesion with two responses: one well localised at the site of stimulation and the other unilaterally or bilaterally in anaesthetic parts of the body. Five of the six patients reported volitional phantom movements of somatic (toes) and/or visceral (micturition-defaecation) structures. Such sensations are elicited by remembering the engrams of pre-traumatic experiences in areas of the body image connected with motor function. Synesthesia could be understood as a sensory counterpart in areas of the body image when ascending impules of actual perception are mixed with impulses modulated at the artifical synapse of the rostral cord stump creating a phantom sensation.