Responses to tonic heat pain in the ongoing EEG under conditions of controlled attention

Abstract

To confirm the existence of an ongoing electroencephalogram (EEG) pattern that is truly suggestive of pain, tonic heat pain was induced by small heat pulses at 1 °C above the pain threshold and compared to slightly less intense tonic non-painful heat pulses at 1 °C below the pain threshold. Twenty healthy subjects rated the sensation intensity during thermal stimulation. Possible confounding effects of attention were thoroughly controlled for by testing in four conditions: (1) focus of attention directed ipsilateral or (2) contralateral to the side of the stimulation, (3) control without a side preference, and (4) no control of attention at all. EEG was recorded via eight leads according to the 10/20 convention. Absolute power was computed for the frequency bands delta (0.5–4 Hz), theta (4–8 Hz), alpha1 (8–11 Hz), alpha2 (11–14 Hz), beta1 (14–25 Hz), and beta2 (25–35 Hz). Ratings were clearly distinct between the heat and pain conditions and suggestive for heat and pain sensations. Manipulation of attention proved to be successful by producing effects on the ratings and on the EEG activity (with lower ratings and lower EEG activity (theta, beta1, 2) over central areas for side-focused attention). During pain stimulation, lower central alpha1 and alpha2 activity and higher right-parietal and right-occipital delta power were observed compared to heat stimulation. This EEG pattern was not influenced by the manipulation of attention. Since the two types of stimuli (pain, heat) were subjectively felt differently although stimulation intensities were nearby, we conclude that this EEG pattern is clearly suggestive of pain.

• EEG
• experimental pain
• ongoing EEG
• tonic pain
• visuospatial attention

Acknowledgments

This study was supported by a FNK grant of the University of Bamberg.

Annotations

1. It has to be acknowledged that even when selecting temperatures that are very close in intensity but also differ in their perceptual quality (pain vs. no-pain), one cannot exclude that possible changes are mainly due to intensity differences and do not reflect exclusively the change in quality from “no-pain” to “pain”. In order to prove a truly pain-specific EEG pattern, identical forms of physical stimulation leading to different perceptual qualities are required, which might be produced by devices like the “thermal grill”.

2. The blindfolded condition “no control of attention” was not included in the EEG analysis given that the lack of visual input makes it difficult to compare it with the other conditions. Thus, leaving the visuospatial attentional control once completely inactivated—as intended—was bought by a general lack of activation of the visual system, which was helpful to assess the efficacy of the visuospatial attentional control on pain but hampered the study of EEG changes.