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Four Macaca nemestrina monkeys were trained to pull a manipulandum to escape electrocutaneous stimulation (ES) applied to either leg. The intensities of stimulation which the monkeys chose to escape were those that humans identify as painful. The duration of escape trials was inversely related to stimulus intensity, and the force of escape responses was directly related to ES intensity. Reflexive responses were elicited by stimulus intensities below and above the escape threshold, and the force of the flexion reflexes was a negatively accelerating function of stimulus magnitude. The monkeys were also trained to respond, for water reinforcement, to cued, low-intensity ES. The stimulus intensities detected for water reinforcement were 50-100 X less than the escape thresholds. Following unilateral, anterolateral chordotomy, all monkeys demonstrated a large reduction in percentage of escape responding to stimulation of the contralateral leg. When the animals did escape contralateral stimulation, the latencies to respond were longer than preoperatively. The percentage and latency of escape responses to stimulation of the ipsilateral leg were not changed following chordotomy. Despite the depression of contralateral pain reactivity, the animals continued to respond to low levels of stimulation on the detection task, demonstrating that anterolateral chordotomy reduced the painfulness of strong stimulation without eliminating sensibility for low levels of stimulation. However, postoperative detection thresholds were consistently higher contralaterally than ipsilaterally. This effect resulted from slight contralateral decreases and significant ipsilateral increases in sensitivity to low levels of ES (relative to preoperative values). Thus, contralateral axons in the anterolateral column contribute to detection of light cutaneous stimulation, and chordotomy appears to disinhibit inputs from large myelinated afferent fibers to ipsilateral neurons in the spinal gray matter caudal to the lesion. The strictly contralateral slowing and reduction in percentage of pain reactivity by chordotomy correlated with reports from human patients. However, reflexive measures did not suffice as indicants of pain sensitivity. The chordotomies produced bilateral attenuations of reflexive amplitudes. Also, the force of operant escape responses was reduced with stimulation of either side. Thus, the disfacilitation of motoric reactions extended both rostral and caudal to the spinal lesions that interrupted propriospinal axons in the vicinity of the ventral horns.

Five Macaca speciosa monkeys were trained to discriminate between the order of two tactile stimulus presentations (proximal-distal vs. distal-proximal) to the lateral calf. Psychophysical thresholds for tactile sequence recognition were obtained by varying the distance between the spots stimulated. Stabilized preoperative thresholds for stimulation on the lateral calf averaged 10.4 mm, which is comparable to point localization thresholds for sequential contact of the proximal limbs of humans. Complete interruption of the ipsilateral fasciculus gracilis produced elevations of thresholds within the first 3 postoperative weeks to an average of 3.6 times the control values. With training over 6 postoperative months, normal discrimination recovered. In contrast to the effects of dorsal column (DC) lesions, interruption of the lateral and ventral columns ipsilaterally or contralaterally produced no deficit or a mild and transient deficit. Monkeys with isolation of either the ipsilateral dorsolateral column (ILC) or the contralateral ventral quadrant (CVQ) exhibited substantial and enduring deficits. Thresholds were elevated an average of 5 times the control values, and recovery was not observed or was incomplete after months of training. In contrast, isolation of the ipsilateral dorsal column produced little or no threshold elevation. The results of isolating lesions complement the series of primary lesions to each cord sector, showing that spatiotactile sequence recognition for the hindlimb is served more critically by the dorsal columns than by the lateral columns.

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