Does Location of Tonic Pain Differentially Impact Motor Learning and Sensorimotor Integration?

Abstract

Recent work found that experimental pain appeared to negate alterations in cortical somatosensory evoked potentials (SEPs) that occurred in response to motor learning acquisition of a novel tracing task. The goal of this experiment was to further investigate the interactive effects of pain stimulus location on motor learning acquisition, retention, and sensorimotor processing. Three groups of twelve participants (n = 36) were randomly assigned to either a local capsaicin group, remote capsaicin group or contralateral capsaicin group. SEPs were collected at baseline, post-application of capsaicin cream, and following a motor learning task. Participants performed a motor tracing acquisition task followed by a pain-free retention task 24⁻48 h later while accuracy data was recorded. The P25 (p < 0.001) SEP peak significantly decreased following capsaicin application for all groups. Following motor learning acquisition, the N18 SEP peak decreased for the remote capsaicin group (p = 0.02) while the N30 (p = 0.002) SEP peaks increased significantly following motor learning acquisition for all groups. The local, remote and contralateral capsaicin groups improved in accuracy following motor learning (p < 0.001) with no significant differences between the groups. Early SEP alterations are markers of the neuroplasticity that accompanies acute pain and motor learning acquisition. Improved motor learning while in acute pain may be due to an increase in arousal, as opposed to increased attention to the limb performing the task.

Keywords: local pain; motor learning; remote pain; somatosensory evoked potentials (SEPs).

Figure 1

Schematic of the protocol. Somatosensory evoked potentials (SEPs), Numeric pain rating scale (NPRS).

Figure 2

Photographs of the computer monitor showing one version of the motor tracing task performed by each participant task and a participant’s hand on the wireless touchpad that was used to perform the tracing task. The traces consist of a series of dots which the participant tracked when the trace passed a horizontal line. The orange dot shows the location of the participant’s cursor along the horizontal line. Motor performance error was calculated as the average distance that the cursor was from the “perfect” trace.

Figure 3

Bar-graph of averaged normalized (to pre-motor learning values) SEP ratios showing capsaicin versus remote versus contralateral groups post-application (A), and post-motor learning acquisition (B). A: The P25 (p < 0.001) SEP peaks significantly decreased following the application of capsaicin cream for all groups as indicated by asterisks. B: Following motor learning acquisition, significantly different changes from baseline are indicated by asterisks for the remote group N18 SEP peak (p = 0.02) and for all three groups the N30 (p = 0.002) SEP peaks were significantly increased. Error bars represent the standard deviation.

Figure 4

Data from a representational remote participant (average of 1000 traces) indicating SEP peaks including A (2.47 Hz) and B (4.98 Hz) sessions. Note the significant differences for the P25 SEP peak following capsaicin application and for the N18 and N30 SEP peaks following motor learning acquisition as indicated by asterisks.

Figure 5

Bar graph depicting the percent error by group. The remote, local, and contralateral and capsaicin groups improved in accuracy following motor learning acquisition (p < 0.001) and at retention (p < 0.001) as indicated by asterisks. Error bars represent the standard deviation.

Figure 6

Bar-graph depicting averaged NPRS ratings of participants by group. Significant differences in subjective pain levels relative to baseline were observed for all three groups 5 min post-application (p < 0.001), 20 min post-application (p < 0.001), post-motor learning acquisition (35 min mark) (p < 0.001) and post-motor learning acquisition (45 min mark) (p < 0.001) as indicated by an asterisk. At 20 min post application there was a significant difference between the local and contralateral groups (p = 0.02) as indicated by a double asterisk. Error bars represent the standard deviation.