EFFECTS OF A DORSAL COLUMN LESION ON TEMPORAL PROCESSING WITHIN THE SOMATOSENSORY SYSTEM OF PRIMATES

A dorsal column (DC) lesion has lasting effects on behavioral tasks th at require temporal processing of tactile information (e.g., frequency and duration discrimination). The present experiments describe physio logical correlates of these deficits in temporal discrimination. Compo und action potentials evoked by electrocutaneous stimulation were reco rded from the major white matter subdivisions of the spinal cord in an esthetized monkeys, and relationships between stimulation frequency an d evoked potential (EP) amplitude were determined for the ascending pa thways. At 10 pulses per second (Hz) EPs recorded in the lateral spina l columns were attenuated slightly (by 15% or less, relative to 1.5 Hz ), whereas potentials recorded from the DCs were not attenuated. The a ttenuation increased with stimulation frequencies up to 50 Hz, reachin g 80% for the anterolateral column and 38% for the dorsolateral column , but only 15% for the DC. Epidural EPs were recorded, before and afte r interruption of the contralateral DC, from awake animals with electr odes chronically implanted over primary somatosensory cortex (SI). Fol lowing the lesion, EP responses to 1.5-Hz stimulation were 46% of preo perative responses to the same stimulus. At 10 Hz, EP amplitudes were attenuated even more, to 27% of the preoperative amplitude at 1.5 Hz. Principal components analysis was employed to quantify alterations in EP conformation and stimulus frequency was varied from 1.5 to 10 Hz, b efore and after a DC lesion. Interruption of the DC resulted in a sign ificant decrease in the information provided by the EP about changes i n stimulus frequency. EPs were also recorded from different locations along the anterior-posterior dimension of the hindlimb region of SI in lightly anesthetized animals. Principal components analysis revealed that there was less information present in the EP about changes in sti mulus frequency (1.5-10 Hz) at all recording locations in animals with a DC lesion, compared with the cortex of normal animals. The DC lesio n significantly decreased the amplitude of cortical EPs evoked by repe titive stimulation. At 10 Hz the EP was nearly buried in noise, consis tent with behavioral deficits in discrimination of the duration of 10 Hz stimulation following interruption of the DC. Also, significantly l ess information was present in the cortical EPs about changes in stimu lus frequency in the absence of intact DCs, which is consistent with d eficits in frequency discrimination. This reduction could be explained in part by a lesser capacity of spinal pathways in the lateral column to follow repetitive stimulation above 10 Hz. However, more rostral m anifestations of a DC lesion, at either the thalamus or the cortex, ar e likely to contribute to the reduced capacity of animals with DC lesi ons to make temporal discriminations.