Analysis of physiological responses during the expectations period

As for the case of the stimulus events we analyzed the SCR and average HR associated with the expectancy period between the presentation of the cue and the stimulus delivery. In particular, we measured the artifact-free amplitude of SCRs evoked by cues, by considering a reliable SCR amplitude the one exceeding a threshold of 0.02 μS that started between 1 and 4 s after the cue presentation, and peaked in the period between this onset time and the delivery of the stimulation. The resulting amplitude value was log-transformed. As for the HR, differentially from the analysis of stimulus induced physiological changes in which the respiration cycle was constrained, we calculated average HR from the cue presentation to the delivery of the stimulus. For each of these measures, we run a repeated measures ANCOVAs with Cue Unpleasantness (High, Low) and Cue Modality (Pain, Disgust) as fixed factors and individual reliance of the cue as covariate. These analyses led to no significant effects (Fs(1,16) ≤1.51, [n.s.]).

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2 Chapter 2

Neural substrates for modality-specific expectancy effects for Pain and Disgust

Sharvit, G.

Vuilleumier, P.

Corradi-Dell’Acqua, C.

(Under review).

2.1 Abstract

Expectations modulate the subjective experience of pain by increasing sensitivity to nociceptive inputs. Several brain structures are involved in expectancy modulation of pain, including the anterior insula (AI) and middle cingulate cortex (MCC)). However, it is still unknown whether the neural representations triggered by pain expectancy hold sensory-specific information or, alternatively, code for modality-independent features (e.g., unpleasantness). To address this question, we used Functional Magnetic Resonance Imaging (fMRI) to investigate neural activity underlying the expectation of different, but comparably unpleasant, events: thermal pain and olfactory disgust. We presented participants with predictive cues informing them about the modality (thermal or olfactory) and unpleasantness (high or low) of an upcoming stimulus, and investigated how this affected the subjective experience and brain response elicited by either thermal painful or olfactory disgusting stimuli of matched unpleasantness. We found that stimulus-induced activity in the right dorsal AI and MCC was positively modulated by the expected unpleasantness of an event of the same modality.

Further, this activity was negatively modulated by the expected unpleasantness of an event of different modality. Our data suggest that these areas are sensitive to both sensory modalities, but keep track of the modality of unpleasant events and of whether they match prior expectations.

2.2 Introduction

Prior predictions can alter the subjective experience of perceptual events, as observed for several sensory modalities including vision^512,312, audition⁵¹³, touch³⁰², and olfaction^300,301. In vision, expectancy improves perceptual sensitivity by selectively enhancing neurons that process the expected stimulus and its features such as color, orientation⁴³², or even semantic category⁵¹⁴.

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Furthermore, expectations can increase the activity of sensory cortices to the relevant stimulus modality, while attenuating activity in the irrelevant modalities⁴³⁴.

Expectancy effects are also observed in the domain of pain, with stronger subjective experiences reported when individuals predict painful events^87,91,515. Similarly to other sensory modalities, pain expectancy have been found to modulate activity of brain regions (e.g. insula, cingulate cortex) held to underlie the primary experience of pain (for an overview see⁸⁸). However, unlike other sensory modalities, the exact role of these different neural structures in pain expectancy is still unresolved.

Indeed, pain is a multi-component experience with sensory-specific features, but also affective features, possibly related to amodal properties shared with other aversive experiences (e.g.

unpleasantness, arousal). Although the insular and cingulate cortex receive sensory-specific information about pain from thalamic nuclei118,194,188, their sensitivity to nociceptive signals is not exclusive, as they process a wide range of other sensory and affective events^96,488,494, including disgust498,246,264,273. Thus, recent studies have proposed that pain-evoked activity in insular and cingulate cortex may reflect a combination of neural processes similarly engaged by non-nociceptive events, arguing against the presence of any pain-specific signal in these regions122,516–518.

Here, we adopted an experimental paradigm developed in Chapter 1 to investigate whether the neural representations triggered by pain expectancy hold sensory-specific information or, alternatively, code for modality-independent features (e.g., unpleasantness). We used functional Magnetic Resonance Imaging (fMRI) to identify neural structures underlying the expectation of two different, but comparably unpleasant, events: thermal pain and olfactory disgust. Disgust represents the ideal control condition, as it shares many important features with pain: both are unpleasant, arousing, relevant for one’s survival, intimately linked to interoceptive processing²⁸², and eliciting similar facial expressions⁴⁸⁷. Furthermore, both activate the insula and cingulate cortex^127,488.

We presented our participants with cues predicting either painful or disgusting events (factor Cue Modality) of either high or low unpleasantness (factor Cue Unpleasantness), and measured how these cues affected the subjective experience and brain activity in response to moderately painful or disgusting stimuli of matched unpleasantness (factor Stimulus Modality). Based on earlier studies, we expected that insula and cingulate cortex should activate following both painful and disgusting stimuli^127,488. We also expected that pain cues of high (relative to low) unpleasantness should further modulate insular and/or cingulate activity^87,89,490. Our key question, however, was whether a similar expectancy effect could also be observed in cross-modal conditions, that is, when cues predicting disgust precede painful stimuli, or when cues predicting pain precede disgusting stimuli. If cues elicit a sensory-specific representation of the upcoming event, then a stronger expectancy effect should be

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observed when the cue and stimulus are of the same (compared to different) modality. Conversely, if they elicit amodal aversive signals in these regions, similar expectancy effects should be observed when predicted and experienced events are from different modalities.

2.3 Materials & Methods

2.3.1 Participants

We recruited 20 participants (10 females; aged 19–36 mean 26.7 SD 4.53 years). None had any history of neurological/psychiatric illness, reported any olfactory deficit. Written informed consent was obtained from all participants, who were naïve to the purpose of the experiment. The study was approved by the local ethics committee and conducted according to the declaration Helsinki.

2.3.2 Olfactory stimulation

Similar to Experiment 1, odorants were delivered to the participants’ nostrils by means of rubber cannulas connected to a computer-controlled, multi-channel, custom-built olfactometer. The olfactometer is able to reliably release several kinds of compounds over multiple trials, without contamination from one trial to the other, at known times, and without additional noise or tactile stimulation in the nose (see Ischer et al., 2014 for technical details of this apparatus). The odorous substances were diluted at variable concentrations in odorless dipropylene glycol. Odors were embedded in 1 l/min constant and filtered airstream. Odorants were provided by Firmenich, SA (Geneva) and selected on the basis of previous evaluations500,501,519. Isovaleric acid (reminiscent of dirty socks), ghee (rotten food), and Sclarymol (sweat) were chosen to elicit disgust in the participants.

Each of these substances was diluted in different concentrations (0.1%, 0.5%, 1%, 5% and 10% for Isovaleric acid, ghee and Sclarymol). In addition, lilac (10%) and soap (10%) odors were also used to elicit positively-valenced sensations in participants. These positive odors were used in order to give to participants an olfactory relief from the disgusting odors and to reduce any putative habituation or sensitization effects.

The experimental set-up comprised 17 olfactory stimuli (Isovaleric acid, gee and Sclarymol, each at 5 different concentrations, plus lilac and soap), and an 18^th odorless solution of dipropylene glycol that served as a control. In the main experiment, each participant was presented with 4 out of the 17 possible olfactory stimulations. These comprised one odorant expected to elicit disgust, at three different concentrations: low (unpleasantness ratings ~ − 5), medium (~ − 20), and high (~ − 40).

Additionally, a fourth odorant was expected to elicit a pleasant experience (> 0). Those four odors were selected on an individual basis according to a pleasantness-rating task carried out at the beginning of the experimental session.

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In this preselection task, all 17 odors (plus the odorless control) were delivered as follows: each trial began with a 1 s fixation cross shown in the center of the computer screen; then the instruction

“Breathe-out” was presented together with a numerical 3 s countdowns. During the countdown, participants were instructed to expire and empty their lungs. When the countdown reached 0, participants had to breath in evenly while the text string “Breathe-in” instruction was presented and the odorant delivered (2 s). This trial structure allowed to minimize the intra- and inter participant breathing pattern variability (see also, Experiment 1, and Delplanque et al., 2009) and to synchronize the respiration cycle with the odorant delivery regardless of its nature. After each stimulus, a visual analogic scale (VAS) was presented. Participants were asked to rate the degree of subjective unpleasantness/pleasantness evoked by the odorant by marking the corresponding position on the scale with a mouse device held in their right hand. The 18 stimuli (17 odors plus the odorless control) were presented twice in an equally distributed and pseudorandomized order. The selection session lasted approximately 15 minutes.

2.3.3 Thermal stimulation

Thermal stimuli were delivered through a computer-controlled thermal stimulator with an MRI-compatible 25 x 50 mm fluid-cooled Peltier probe (MSA, Thermotest), attached to participants’ left leg. For each participant, we selected three temperatures, each evoking different degrees of unpleasantness (low, medium, and high), comparable to the three odors selected for the same participant. These three temperatures were individually calibrated for each participant based on a brief thermal-stimuli selection session.

In line with Experiment 1, in this selection phase, unpleasant temperature levels were determined through a modified double random staircase (DRS) algorithm aimed at identifying stimuli with unpleasant ratings similar to the highly unpleasant odor (measured with the same VAS as used for the odor selection session). Our DRS procedure selected a given temperature on each successive trial according to the previous response of the participant. Trials rated as more unpleasant than the given cut-off (subject-specific, based on ratings for the highly unpleasant odor) led to a subsequent lowered temperature in the next trial; whereas trials rated as less unpleasant than the given cut-off led to a subsequent higher temperature. This resulted in a sequence of temperatures that rapidly ascended towards, and subsequently converged around, a subjective unpleasantness threshold, which was in turn calculated as the average value of the first 4 temperatures leading to a direction change in the sequence. In order to prevent participants to anticipate a systematic relationship between their rating and the subsequent temperature, two independent staircases were presented randomly. Initial thermal stimulations for the two staircases were 41°C and 43 °C. Within each staircase, stimulus

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temperatures increased or decreased with steps of 3°C, while smaller changes (1°C) occurred following direction flips in the sequence. None of our subjects was stimulated at temperature larger than 52°C.

The thermal stimuli were delivered in the following way: participants first saw a 1 s long fixation-cross, followed by the text string “Temperature is changing” and concomitant delivery of the heat stimulation. Each thermal event was composed of 3 s of rise time, 2 s of plateau at the target-temperature, and 3 s of return to baseline (37°C). The speed of the temperature rise and the temperature return was automatically adjusted according to the plateau in order to maintain both a rise time and a return time of approximately 3 s each. The unpleasantness scale was presented just after the 2 s of plateau stimulation, when the temperature started to return to baseline, and lasted until participant provided a response.

The DRS approach allowed us to determine temperatures eliciting three distinct levels of unpleasantness (corresponding to different levels of pain): low, moderate, and high. This led to a highly unpleasant temperature, which varied on a participant-by-participant basis, but with an average value of 48.47°C (SD 2.46). On the basis of this painful temperature, we selected two additional temperatures associated with medium (1/1.5°C less than the highly unpleasant temperature, corresponding to an average of 47.38°C, SD 2.59) and low unpleasantness (2/3°C less than the highly unpleasant temperature, corresponding to an average 45.93°C, SD 2.54). This session lasted approximately 10 minutes.