Effects of spinal lesions on temporal resolution of cutaneous sensations

In order to test the possibility that the dorsal columns (DCs) contribute to temporal resolution of tactile stimuli, Macaca speciosa monkeys were trained to discriminate different frequencies of stimulation delivered to the glabrous surface of the left foot. Brief (11-msec) pulses of 550-micron indentation from the skin surface were presented for 1 sec at a standard rate of 10 pulses per second (pps), and subsequent trains of the same duration either were replications of the standard or consisted of a higher (comparison) frequency of up to 35 pps. Correct performance consisted of a lever press in response to the comparison stimulus when it occurred as the second or third train in a trial. Signal detection analysis of response tendencies in the second stimulus interval revealed difference threshold values of 2.8 pps, on the average, for 75% correct responding. Transections of the ipsilateral dorsolateral column or the contralateral anterolateral column or both did not significantly affect discrimination of the frequency of cutaneous stimulation. However, following interruption of the ipsilateral DC, two monkeys could not discriminate 10 from 35 pps throughout testing for more than 1 year. These results contrast with a preservation of spatiotactile resolution that has been demonstrated repeatedly for animals following lesions of the DCs or the lateral columns, indicating that a unique function of the DC pathway relates to temporal coding.