Background & Aims

Both sustained pleasure and pain show dynamic changes in the emotional states (e.g., how pleasant vs. unpleasant) for long time scales. Because these brain states slowly emerge and fluctuate over time, it is hard to specify the timings of the emergence and recovery of these state-related brain responses (Robinson et al., 2010). Here, we conducted a functional Magnetic Resonance Imaging (fMRI) experiment in which participants received the oral delivery of chocolate and capsaicin fluids. While experiencing multiple combinations of sustained pleasure and pain conditions, participants continuously rated their subjective pleasantness and unpleasantness. To model these brain responses more effectively, we used spline functions as flexible basis sets in the GLM analysis to identify encoding models of dynamic affective states.

Methods

We collected 14-minute fMRI data from 61 participants (28 females, mean of age = 22.74, SD of age = 2.66). We delivered capsaicin or chocolate fluids to participants’ oral cavity through the MR-compatible fluid delivery system (Octaflow II, ALA Scientific Instruments Inc., Westbury, NY). We delivered capsaicin or sweet chocolate fluids sequentially across the ‘Capsaicin-Capsaicin (CC)’, ‘Capsaicin-Sweet (CS), and ‘Sweet-Sweet (SS)’ conditions, while we delivered capsaicin fluid only once in the ‘Capsaicin-water (C0)’ condition. During scans, we asked participants to continuously rate their subjective pleasantness or unpleasantness, based on the general Labeled Magnitude Scale (Bartoshuk et al., 2004). Using these fMRI data, we conducted the spline-based univariate analysis that flexibly captures the fMRI signals related to sustained pleasant and unpleasant states. We obtained the areas under the curve (AUCs) that indicate the overall activation magnitudes for each brain voxel.

Results

We found different brain regions that showed significant AUCs for initial sustained pain (i.e., the first capsaicin deliveries across CC, C0, and CS conditions) and initial sustained pleasure (i.e., the first chocolate deliveries in SS condition). We found the dorsal posterior insula, cingulate cortex, and somatosensory cortex for the initial capsaicin deliveries, and the amygdala, ventral anterior insula, and medial prefrontal cortex for the initial chocolate deliveries. These results showed additional brain regions compared to the results of the conventional GLM analysis. There were overlapping brain regions that showed significant AUCs for both initial pain and initial pleasure. The insula regions showed positive AUCs for both initial pain and pleasure, largely part of the salience and somatomotor networks. The orbitofrontal cortex and temporal pole showed negative AUCs for initial pain and positive AUCs for initial pleasure, largely part of the limbic and subcortical networks.

Conclusions

This study characterized brain regions based on their response patterns toward sustained pleasure and pain, showing dynamic changes in the brain responses over time. We will further examine these dynamic changes in more detail, such as whether the response patterns differ between experiencing pain after pain and pleasure after pain.

References

Bartoshuk, L. M., Duffy, V. B., Green, B. G., Hoffman, H. J., Ko, C.-W., Lucchina, L. A., Marks, L. E., Snyder, D. J., & Weiffenbach, J. M. (2004). Valid across-group comparisons with labeled scales: the gLMS versus magnitude matching. Physiology & behavior, 82(1), 109-114.
Robinson, L. F., Wager, T. D., & Lindquist, M. A. (2010). Change point estimation in multi-subject fMRI studies. Neuroimage, 49(2), 1581-1592.

Presenting Author

Soo Ahn Lee

Poster Authors

Soo Ahn Lee

OTHR

Sungkyunkwan University

Lead Author

E-mail

Topics

  • Pain Imaging