Ultra-high field imaging reveals increased whole brain connectivity underpins cognitive strategies that attenuate pain

The perception of pain activates a number of brain regions and processes that are involved in its sensory, emotional, cognitive, and affective aspects; all of which require a flexible functional connectivity between local and distant brain regions. Here, we investigate how the attenuation of pain with cognitive interventions affects the strength of these connections by pursuing a whole brain approach in order to assess every cortical connection that contributes to successful pain relief. While receiving 40s trials of tonic cold pain, 22 healthy participants were asked to utilise three different pain attenuation strategies: (a) non-imaginal distraction by counting backwards in steps of seven, (b) imaginal distraction by imagining a safe place, and (c) cognitive reappraisal. During a 7T fMRI recording, participants were asked to rate their pain after each single trial. We related the trial-by-trial variability of the attenuation performance to the trial-by-trial functional connectivity of the cortical data. Across all three conditions, we found that a higher performance of pain attenuation was predominantly associated with higher functional connectivity between all regions. Of note, we observed an association between low pain and high connectivity for regions that belong to the core areas of pain processing, i.e. the insular and cingulate cortices. For one of the cognitive strategies (safe place), the performance success of pain attenuation was explained by diffusion tensor imaging metrics of increased white matter integrity. Therefore, successful cognitive interventions to ameliorate pain and improve clinical outcomes would require the strengthening of cortical connections.

pursuing a whole brain approach in order to assess every cortical connection that 23 contributes to successful pain relief. 24 While receiving 40s trials of tonic cold pain, 22 healthy participants were asked to utilise 25 three different pain attenuation strategies: (a) non-imaginal distraction by counting 26 backwards in steps of seven, (b) imaginal distraction by imagining a safe place, and (c) 27 cognitive reappraisal. During a 7T fMRI recording, participants were asked to rate their pain 28 after each single trial. We related the trial-by-trial variability of the attenuation performance to 29 the trial-by-trial functional connectivity of the cortical data. Across all three conditions, we 30 found that a higher performance of pain attenuation was predominantly associated with 31 higher functional connectivity between all regions.

32
Of note, we observed an association between low pain and high connectivity for regions that 33 belong to the core areas of pain processing, i.e. the insular and cingulate cortices. For one of 34 the cognitive strategies (safe place), the performance success of pain attenuation was 35 explained by diffusion tensor imaging metrics of increased white matter integrity. 36 Therefore, successful cognitive interventions to ameliorate pain and improve clinical 37 outcomes would require the strengthening of cortical connections. An increased perception of pain is generally associated with increased cortical activity; this 42 has been demonstrated for a number of brain regions and processes involved in sensory, 43 emotional, cognitive, and affective aspects of pain (1,2). Given the threatening nature of pain, 44 the information processed from these different aspects have to be integrated and assessed 45 to compute an appropriate decision and subsequent action (3). To do so, pain-processing 46 brain regions are required to exchange information, which entails increased functional 47 connectivity between relevant cortical and subcortical regions (4,5). Conversely, less is 48 known about connectivity changes during decreased pain, although many studies highlight 49 decreased neuronal activity with some studies highlighting selective changes in coupling 50 between brain regions (6). was associated with more negative emotions. 59 A further study found that a change in pre-stimulus cortical connectivity patterns from the 60 anterior insula to the periaqueductal grey (PAG), which is part of the descending pain 61 modulatory system (7), determined whether a subsequent nociceptive stimulus was 62 perceived as painful or not (8). Supporting that observation, other investigations have 63 similarly reported increased functional connectivity between the PAG and the perigenual 64 anterior cingulate cortex (pACC) for conditions associated with decreased pain intensity 65 perception (placebo, shift of attention) (9-11). A recent study even showed that the structural 66 integrity, as measured using diffusion tensor imaging (DTI) of white matter tracts between 67 brain regions coupled with this descending pain modulatory system, was significantly 68 correlated to the effectiveness of transcranial direct current stimulation brain stimulation in 69 alleviating pain (12).

70
Therefore, all studies to date point to the relevance of connectivity patterns in pain 71 modulation; yet, excluding an increased connectivity to the descending pain modulatory 72 system's PAG, the precise nature of cortical connectivity during decreased pain is unclear 73 and limited. Using ultra-high field functional magnetic resonance imaging (fMRI) to provide 74 enhanced signal-to-noise ratio (SNR) to facilitate single-trial analysis, we explored the 75 functional connections that contribute to the attenuation of pain by means of three different 76 cognitive interventions: (a) a non-imaginal distraction by counting backwards in steps of 77 seven; (b) an imaginal distraction by imagining a safe place; and (c) reinterpretation of the 78 pain valence (cognitive reappraisal). These cognitive strategies are hypothesised to be 79 represented in the brain by a complex cerebral network that connects a number of brain 80 regions, where:

81
(1) The effective use of a cognitive strategy that is successful for pain attenuation results in 82 an increase of functional connectivity between task-related brain regions.

83
(2) Decreased connectivity is expected between cortical areas that are involved in the 84 processing and encoding of pain intensity, e.g. sub-regions of the insular cortex, the 85 cingulate cortex, somatosensory cortices, and PAG.

86
(3) Increased connectivity is hypothesised for the descending pain control system, 87 particularly for the connection between the pACC and the PAG.

88
(4) Divisions of the insular cortex and their connections to frontal and somatosensory regions 89 play a key role through their high relevance in integrating sensory information.

90
Unlike previous research paradigms, the present experimental procedure aims to 91 approximate clinical treatment procedures by using a novel pain stimulation approach that 92 produces longer lasting pain experiences. Healthy participants were asked to utilise 93 cognitive strategies in order to attenuate the experience of pain during 40s of cold 94 stimulation. We pursued a whole-brain parcellation approach (13) in order to assess every 95 cortical connection that contributes to successful pain relief.

97
Overall, we found an increase of connectivity during pain attenuation: trials rated as low pain 98 as a consequence of utilising a cognitive strategy had stronger connectivity compared with 99 trials of the unmodulated pain condition that were rated as high pain. Therefore trials with 100 high pain are coupled with low connectivity, and trials with low pain are coupled with high 101 connectivity. 102 We pursued a whole-brain approach by subdividing the cortex into 180 regions per 103 hemisphere plus 21 subcortical regions (13) and related cortical connectivity to pain ratings 104 at single trial level. This approach was facilitated by an increased SNR as a result of ultra-105 high field recording, as well as by a more reliable assessment of single trial data from longer 106 lasting painful stimulation and an extended task application. For each of the three conditions, 107 we merged the 11 trials of the cognitive interventions with the 11 unmodulated pain trials, 108 which has two major advantages:

109
(i) First, it takes the within-subjects variable performance of the pain attenuation attempts 110 into account; e.g. a more successful attempt to attenuate pain is considered to cause a 111 different cortical connectivity than a less successful attempt.

112
(ii) Second, we also take into account the more natural fluctuation of the unmodulated pain 113 trials.

114
The findings are represented in confusion matrices, depicting the pain intensity-related 115 connectivity between all brain regions. Positive relationships (red) show connectivities that 116 were increased in particularly effective trials (performance encoding). For all tasks, we 117 confirmed our first hypothesis by showing that an increased connectivity of task-processing 118 brain regions is related to particularly successful attempts to attenuate pain. However, 119 contrary to our second hypothesis, we found increased brain activity for successful single 120 trials also in pain-processing regions. The increased connectivities are therefore suggested 121 to initiate mechanisms of cortical activity suppression, such as shown in our previous 122 publication (14). Negative relationships (blue) represent cortical connections that are 123 disrupted in successful trials to attenuate pain. Disruptions were hypothesised to occur for 124 the core regions of pain processing, such as for the various subregions of the insular, 125 cingulate, and somatosensory cortices. However, we found that these regions predominantly 126 showed increased connectivity during successful trials of pain attenuation (see above).

127
(1) Counting. We aimed to detect patterns of connectivity changes that are related to 128 successful trials during counting. The attenuation of pain during counting is predominantly 129 related to an increase of cortical connectivity in several brain regions, with the exception of 130 decreased connections involving the right temporo-parieto-occipital junction ( Figure 1A). The 131 detailed matrix of statistical results can be found in the supplementary material 132 (Supplementary Spreadsheet 1). 133 We found that some regions show a particularly strong connectivity: the right insula, the left 134 and right temporal cortices, the left parietal cortex, as well as higher order visual regions in 135 occipito-temporal areas. The best connected area is the right middle insula ( Figure 1C

158
(2) Safe place. During the imagining condition, we found an increase of connectivity across

206
(4) Conjunction analysis. We did not find any pain-related connectivity changes present in all 207 three conditions.

209
Here, we aimed to explore how functional and structural connections in the brain contribute 210 to executing cognitive tasks that attenuate pain (14,15) by utilising a single-trial analysis 211 approach afforded by ultra-high field imaging. Across three experimental conditions, 20 212 healthy participants were asked to (a) count backwards, (b) imagine a safe and happy place, 213 and (c) apply a cognitive reappraisal strategy. All strategies resulted in significant pain relief 214 when compared to the unmodulated pain condition. We applied a whole-brain approach on 215 the basis of brain parcellation definitions (13) and explored connectivity patterns during  Counting. For the cognitively-demanding counting task, we found a number of well-235 connected regions that contribute directly or indirectly to the reduction of pain intensity.  (14)).

245
Disrupted connectivities during the counting task were observed for the right temporo- occipito-temporal cortex connect to and suppress right parietal opercular areas. We also 252 found visual support located in the right occipito-temporal cortex that is functionally 253 connected to parietal areas, which in turn suppress insular activity.

254
Safe place. Similar to the counting condition, we found regions in the left and right parieto-255 occipital cortex to be highly connected to other brain regions. Notably, the parietal cortex is 256 functionally connected without a rise of regional BOLD activity (see (14)). This effect shows 257 that brain regions can play an important role in pain processing via an exchange of reflected by increased functional connectivity between these regions). 262 We found well-connected regions in the precentral gyrus: area 55b has been shown to be 263 active during listening to stories in the language task of the Human Connectome Project 264 dataset (13). Therefore, the increased connectivity in area 55b may be related to the 265 narrative aspects of the imaginary task in which the participants may recall being actively 266 involved in an event of pleasure and happiness. The premotor and motor areas in the 267 precentral gyrus in particular may reflect the motor aspect of the imagination task (23,24).

268
They are connected to orbitofrontal areas which are thought to initiate top-down pain 269 suppression of ascending pathways (9,25).

270
For the safe place condition only, we found that the ability to functionally utilise certain  Reappraisal. The best connected region during cognitive reappraisal is located in the higher 279 order visual cortex, area V6A, which is mainly interconnected with insular and frontal 280 premotor areas. Area V6A is known to contribute to spatial object localisation; a study on 281 monkeys shows that V6A cells are active when executing reaching movements independent 282 of visual or oculomotor processing (31). These cells have also been found to encode body-283 centred spatial localisation (32). The use of V6A and its connection to other brain areas 284 could help the participants -as required by the task -to focus on the stimulated body site.

295
In neuroimaging, functional connectivity is considered a joint phase-locked oscillation of 296 spatially distant cortical regions. Task-based connectivity analyses predominantly utilise a 297 seed-based approach to determine the functional connectivity between a predefined seed 298 region and one or more distant brain regions; such analyses can only take into account the 299 short period during which a task is being executed. However, exact connectivity measures 300 between brain regions would require a sufficient number of samples to quantify the joint in-301 phase increases and decreases of the BOLD response. In order to estimate a reliable 302 measure of connectivity, we applied a relatively long time window (~30s, 15 data points) for 303 inflicting pain, for executing the cognitive task, and for reliably determining the connectivity of

321
Other studies investigated the connectivity in the descending pain control system and 322 observed an increase of connectivity between the perigenual ACC and the PAG during a 323 pain-relieving placebo intervention (9). Given the lower signal-to-noise ratio in mid-brain 324 areas, this finding could not be replicated in any of the present conditions with the current 325 whole-brain approach and a strict correction for multiple comparisons (38). By lowering the 326 statistical threshold, we found a modulation of pain intensity-dependent functional 327 connectivity from the PAG to regions that contribute to pain processing, such as the anterior 328 ventral insula (t>2), the midcingulate cortex (t>2.5), and the nucleus accumbens (t>3),  determined as no pain (0) and the maximum pain the subjects were willing to tolerate (100).

366
Single trial ratings were recorded after each trial.    429 We further analysed whether individual differences in functional connectivity could be 430 explained by individual structural characteristics of the brain. In other words, we analysed 431 whether the functional connectivity that leads to a single subject's successful pain 432 attenuation is facilitated by that subject's high number of fibre tracts. In a similar vein, a poor 433 functional connectivity that is not able to contribute to pain attenuation might be caused by a 434 low number of fibre tracts.