Jc. Makous et al., EFFECTS OF A DORSAL COLUMN LESION ON TEMPORAL PROCESSING WITHIN THE SOMATOSENSORY SYSTEM OF PRIMATES, Experimental Brain Research, 112(2), 1996, pp. 253-267
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.