Mental imagery-induced attention modulates pain perception and cortical excitability

Abstract

Background: Mental imagery is a powerful method of altering brain activity and behavioral outcomes, such as performance of cognition and motor skills. Further, attention and distraction can modulate pain-related neuronal networks and the perception of pain. This exploratory study examined the effects of mental imagery-induced attention on pressure pain threshold and cortical plasticity using transcranial magnetic stimulation (TMS). This blinded, randomized, and parallel-design trial comprised 30 healthy right-handed male subjects. Exploratory statistical analyses were performed using ANOVA and t-tests for pain and TMS assessments. Pearson's correlation was used to analyze the association between changes in pain threshold and cortical excitability.

Results: In the analysis of pain outcomes, there was no significant interaction effect on pain between group versus time. In an exploratory analysis, we only observed a significant effect of group for the targeted left hand (ANOVA with pain threshold as the dependent variable and time and group as independent variables). Although there was only a within-group effect of mental imagery on pain, further analyses showed a significant positive correlation of changes in pain threshold and cortical excitability (motor-evoked potentials via TMS).

Conclusions: Mental imagery has a minor effect on pain modulation in healthy subjects. Its effects appear to differ compared with chronic pain, leading to a small decrease in pain threshold. Assessments of cortical excitability confirmed that these effects are related to the modulation of pain-related cortical circuits. These exploratory findings suggest that neuronal plasticity is influenced by pain and that the mental imagery effects on pain depend on the state of central sensitization.

Figure 1

Relationship between pain perception and motor cortex excitability. a: Relationship and pathway of pain perception and motor cortex excitability. b: Transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) can increase motor cortical excitability and pain threshold (= > decrease in pain perception). c: Motor tasks and motor observation increase pain threshold and motor excitability. d: Mental imagery decreases pain threshold and motor cortical excitability. Pictures modified from Flor [8]; Fregni et al. [9]; and Volz et al. [10].

Figure 2

Pain threshold. Pain threshold levels for mental imagery and control groups. Ordinate: Changes in pressure pain threshold level as percentage with respect to baseline value (expressed as: [(t2-t1)/t1] ×100). *p < 0.05 as tested with ANOVA (F(1,27) = 7.40, p = 0.0079) with pressure pain threshold as the dependent variable and group (mental imagery vs control) and time (pre- vs post-intervention) as independent variables. Note that the interaction analyses did not reveal significant results.

Figure 3

Motor-evoked potentials. MEP amplitudes in mV of both groups pre- and postintervention.

Figure 4

Correlation of changes in pain thresholds and MEP amplitude. Correlation between pain threshold of the left hand and MEP amplitudes. Ordinate: Change in pressure pain threshold [in %]. Abscissa: Change of MEP amplitude [in %]; (r = 0.46; p = 0.015). Changes in pressure pain threshold were calculated as follows: t₂ (postintervention value) – t₁ (preintervention value). Changes in MEP amplitude were calculated as follows: t₂ (postintervention amplitude) – t₁ (preintervention amplitude).

Figure 5

Study design. Study design and duration of experiment.