Background & Aims
The modulation of ongoing neural oscillations could be related to pain perception, as these oscillations have been shown to be modulated by painful stimuli in different frequency bands [1]. This study aims to explore their functional significance by modulating pain perception using expectation – a factor known to influence the perception of painful stimuli, while recording brain activity using scalp electroencephalography (EEG). Cue-based modulation of pain expectation can change participants’ perception of thermonociceptive stimuli, allowing us to compare the responses to stimuli with the same intensity but that are perceived differently [2]. We expect that a cue indicating a high intensity stimulus will increase perceived stimulus intensity compared to a cue that indicates a low intensity stimulus. If ongoing oscillations and pain perception are functionally related, we expect a larger modulation of these oscillations following a high intensity cue compared to a low intensity cue.
Methods
40 healthy subjects were recruited (23.6 ± 3.5 years old, 17 males). Thermonociceptive stimuli were delivered in a slow sustained periodic manner at 0.2 Hz over a duration of 50s per trial. Temperatures varied from baseline (35°C) to one of 3 target temperatures. Participants’ stimulus expectations were modulated using 2 different visual cues, informing them that either a low or a high intensity stimulation would follow. First, the appropriate cue was shown with either the low temperature (LL, 44°C) or the high temperature (HH, 50°C). Then, unknown by the participants, the same cues would be presented with the medium stimulation temperature (LM and HM, 47°C). Ratings of expected and perceived stimulus intensity were collected on a Visual Analog Scale (VAS) before and after each trial. 64-channel EEG measured the neural response related to the 50s of stimulation. Linear mixed models were used to analyze the ratings and amplitudes in the alpha, beta and theta frequency bands.
Results
Participants correctly identified the visual cues and adapted their stimulus expectations accordingly (F(1,1239)= 6108, p<0.001). These expectations significantly modulated the perception of the thermonociceptive stimuli that were delivered at the same stimulation intensity (post-hoc pairwise comparison of the VAS ratings following condition HM vs LM: t(1237)=25.950, p<0.001). While a significant periodic modulation was found at the frequency of interest for condition HH, no differences were found in the modulation of ongoing oscillations between condition HM and LM. Moreover, the only difference that sustained the post-hoc pairwise comparison was a larger modulation in condition HH compared to condition HM in all frequency bands (t(117)=3.6, p<0.001).
Conclusions
The mismatch between the collected VAS ratings and the amplitudes at the frequency of interest suggests a dissociation between the perception of a nociceptive stimulus and the modulation of ongoing oscillations (measured using scalp EEG). Our results suggests that the intensity at which a stimulus is delivered is the main contributing factor to the amplitude measured at the frequency of interest.
References
1.Liberati, G., et al., Tonic thermonociceptive stimulation selectively modulates ongoing neural oscillations in the human posterior insula: Evidence from intracerebral EEG. NeuroImage, 2019. 188: p. 70-83.
2.Atlas, L.Y., et al., Brain Mediators of Predictive Cue Effects on Perceived Pain. The Journal of Neuroscience, 2010. 30(39): p. 12964-12977.