Participants

Forty undergraduates (28 females) with normal or corrected-to-normal vision volunteered to take part in the experiment (M age = 25.04, SD = 3.62). Exclusion criteria were the existence of a chronic or pre-existing pain condition and taking painkillers. The study protocol was approved by the local ethics Committee (Lyon Sud Est 3, 2015-010B, EUDRACT 2014-A01280-47) and carried out in accordance with the relevant guidelines and regulations. Written informed assent and consent were obtained from all the participants. A power analysis based on a pilot study confirmed that, given a desired power of 0.80 and an alpha level of 0.05, a minimum sample size of 33 participants was sufficient to detect a difference.

Material and stimulation procedure

Presentation, timing and control of visual targets were performed on a 21.5-inch Apple IMac using OpenSesame 2.9.715. Each participant was tested individually during a session that lasted approximately 50 minutes. The stimulus was a grey square (RGB code: #646464) viewed under an angle of 9.1° and displayed in the center of the computer screen with variable durations. For the cold pressor test, two containers were used (50 × 39.5 × 32.9 cm), both containing water, one at 12 °C (53,6 °F–pain condition) and the other at room temperature of about 25 °C (77 °F–control condition). The water was continuously in movement to avoid warming around the immersed hand, and its temperature was controlled throughout the test.

Temporal bisection task

During a training phase, participants were introduced to the short and long presentation durations of the grey square. Participants fixed their gaze on a fixation point for 500 ms and then observed the grey square appearing in the center of the screen, which could last for a short (250 ms) or long (750 ms) presentation period. Participants learned to categorize the duration as “short” or “long” by pressing the corresponding key (down arrow key for the “short” responses and up arrow key for the “long”). There were 12 trials for each duration with an inter stimulus interval (ISI) of one second. To ensure that the participants had clearly understood the task, they received feedback for the first 4 trials.

During the test phase, subjects had to categorize a series of stimuli as ‘rather short’ or ‘rather long’ based on the two templates used during the training phase. Thus, the grey square was presented during 250, 300, 350, 400, 450, 500, 550, 600, 650, 700 or 750 ms in random order, with ISI of one second and a fixation point between two consecutive stimuli. The participants immersed their hand in tepid water (control condition) or cold water (pain condition) while performing the time estimation task of visual targets. A two-condition (pain vs. control) within-subject design was used. The order of conditions was randomized, half of the participants starting with the control condition. Each condition was divided in two blocks: half of the participants begun with their right hand and continued with their left hand, and vice versa for the other half. Each target duration was presented eight times per block. To avoid the hand becoming numb, each condition was split into two blocks of 3–4 minutes each, using alternating hands. The first hand immersed in cold water was therefore retrieved at the end of the block, the subsequent block being performed with the other hand after a 2-minute break. Participants could remove their hand from the water but they were encouraged to place it back again as soon as possible. The pain condition and the control condition were separated by a 10-minute break (Fig. 1). A total of 352 trials were presented (11 durations × 8 presentations × 2 blocks × 2 conditions).

Figure 1 Illustration of the procedure with the two training phases and the two test phases. In the training phases, participants learned to discriminate the ‘short’ (250 ms) and ‘long’ (750 ms) durations. In the test phases, participants were instructed to categorize a series of grey squares as ‘rather short’ or ‘rather long’ based on the two templates previously learned. The test phases were divided in two blocks: one block with the right hand in the water and one block with the left hand in the water. A trial in the test phase corresponded to: a fixation point for 500 ms, a grey square presented during 250, 300, 350, 400, 450, 500, 550, 600, 650, 700 or 750 ms and a blank screen until subject’s response. The stimulus duration was random and equiprobable. The Inter Stimulus Interval was 1000 ms. The pain and control conditions were counterbalanced as well as the hand immersed in the water (representing by the curved arrows). Pain intensity was assessed using a Visual Analogue Scale (VAS) at the end of each block. Full size image

To ensure that the unpleasantness resulting from the water at room temperature in the control condition did not induce a time distortion, we ran a complementary experiment in which the participants had to put their hand in an empty container (see supplementary data).

Pain evaluation

Pain intensity was assessed using a visual analog scale (VAS) presented on the screen, formed by a 622 pixels horizontal line with six schematic faces16 from no pain in the left (“aucune douleur” in French) to excruciating pain in the right (“douleur intolérable” in French). Participants indicated their response by clicking on the line. The analog scores were converted to a 0- to 10-point scale. VAS were presented at the end of the training phases and at the end of each block of the test phases (two scores for the pain condition and two scores for the control condition). A pain score for each condition (pain and control) was calculated by averaging the scores obtained at the end of each block.

Statistical analysis

Initial data processing and subsequent analyses were performed using RStudio version 3.2.2 (R Foundation for Statistical Computing). Raw responses were converted into proportions of ‘long duration’ responses per participant and condition (i.e. the proportion of responses where the participant classified the target duration as being ‘long’, irrespective of its actual duration). The data were plotted against the actual duration of the stimulus, and fitted locally using the “model-free” statistical package17. This representation allows illustrating a systematic bias toward longer estimations by a leftward shift of the function (the subjects will more often classify the duration of the visual stimulus as ‘long’). The shifting of the function (i.e. the stimulation duration giving rise to 50% of “long” responses and 50% of “short” responses) was calculated for each subject. The difference between the bisection point in pain and control conditions were compared using a bilateral paired Student t-test (with Cohen’s d for the effect size). This analysis was also conducted on the just noticeable difference (JND) as a measure of sensitivity of the temporal bisection task. The pain scores were compared using bilateral independent or paired Student t-tests.

For all analyses, a bilateral p value of 0.05 was used as the criterion for statistical significance. Means and standard errors are given for each condition.