Interstimulus interval effects on habituation of flexor withdrawal activity mediated by the functionally transected human spinal cord.

The habituation of flexor withdrawal activity mediated by the functionally transected human spinal cord was investigated as a function of a wide range of different temporal intervals among the repetitive cutaneous stimuli. Nine patients with a chronic, transverse lesion of the cervical spinal cord were submitted to four daily habituation training series involving interstimulus intervals of either 1, 5, 25, or 125 seconds. The stimulus consisted of a 40-msec pulse train applied through surface electrodes to a midplantar site, and the response was expressed in terms of the goniometrically recorded dorsiflexion of the ankle. A reliable degree of habituation was found for the series involving the 1- and 5-second intervals. However, holding constant the number of stimuli involved in the comparisons, no habituation was found for the series involving intervals of 25 and 125 seconds.