IMPAIRED DETECTION OF REPETITIVE STIMULATION FOLLOWING INTERRUPTION OF THE DORSAL SPINAL COLUMN IN PRIMATES

Transection of the dorsal spinal column in monkeys has been previously shown to spare detection, localization and a variety of discriminatio ns between spatial attributes of tactile stimuli. In contrast, perform ance on certain tests involving stimulus sequences is substantially im paired, such as tactile direction sensitivity and frequency discrimina tion. The present study extends these findings to show that a repetiti ve cutaneous stimulus is undetectable following complete interruption of the ipsilateral dorsal column. Macaca arctoides monkeys were traine d to discriminate between different durations of 10 Hz indentation of the glabrous skin of one foot. Preoperatively, these animals could dis criminate reliably between three pulses (the standard stimulus duratio n of 200 ms) and comparison trains of six or more pulses (500 ms or mo re). Following incomplete interruption of the ipsilateral dorsal colum n of one monkey, discrimination of the duration of stimulation was uni mpaired. However, complete lesions of the ipsilateral dorsal column el iminated performance above the criterion of 75% correct responses for approximately 1 year of postoperative testing of three monkeys. Compar ison stimuli of as many as 38 pulses (3.7 s) were utilized during post operative testing. The inability to detect repetitive stimulation is h ypothesized to be related to abnormal intracortical inhibition that ha s been demonstrated to occur within the primary somatosensory cortex ( SI) of monkeys after interruption of the contralateral dorsal column.