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 discriminations between spatial attributes of tactile stimuli. In contrast, performance on certain tests involving stimulus sequences is substantially impaired, such as tactile direction sensitivity and frequency discrimination. The present study extends these findings to show that a repetitive cutaneous stimulus is undetectable following complete interruption of the ipsilateral dorsal column. Macaca arctoides monkeys were trained to discriminate between different durations of 10 Hz indentation of the glabrous skin of one foot. Preoperatively, these animals could discriminate reliably between three pulses (the standard stimulus duration of 200 ms) and comparison trains of six or more pulses (500 ms or more). Following incomplete interruption of the ipsilateral dorsal column of one monkey, discrimination of the duration of stimulation was unimpaired. However, complete lesions of the ipsilateral dorsal column eliminated performance above the criterion of 75% correct responses for approximately 1 year of postoperative testing of three monkeys. Comparison stimuli of as many as 38 pulses (3.7 s) were utilized during postoperative testing. The inability to detect repetitive stimulation is hypothesized to be related to abnormal intracortical inhibition that has been demonstrated to occur within the primary somatosensory cortex (SI) of monkeys after interruption of the contralateral dorsal column.