Procedure

Participants first provided their written consent and filled a form to ensure the respect of inclusion criteria and to collect demographic data. The studies using CO2 consisted in 4 different phases: (1) an individual determination of CO2 concentration sensitivity; (2) a lateralization task; (3) the EOG calibration; and (4) the cross modal spatial cueing task. Studies using pure eucalyptol were composed of phases 2, 3 and 4 only. The entire experimental session took approximatively 70 minutes with the CO2 and 60 minutes with the eucalyptol (Figure 18C).

Figure 18. The experimental set up (a), its nasal metallic ending pieces (b) and the experimental protocol.

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4.1.4.1 Determination of CO2 concentration perceptivity

This procedure allowed participants to become familiar with the set up and to choose the concentration they find clearly perceptible but not painful. In parallel, participants were instructed and trained on how to proceed with the cued sniffing procedure, which was going to be used during the entire study in order to avoid any interference in the nasal perception due to the natural flow (Scheibe, van Thriel, & Hummel, 2008). For each trial (manually triggered), a countdown of 4 seconds was presented on the computer screen, followed by the trigeminal stimulation delivered at zero during approximately 500 ms, except in the study C1870 in which the puffs lasted approximately 1 second. Participants were requested to hold their breath from the “1” of the countdown until the end of the trial when they could resume normal breathing.

The CO2 concentration was set first at the weakest concentration (3% v/v)⁵. Depending on participant’s perception, the CO2 concentration was adjusted to reach a reported comfortable, not painful but clear perception.

4.1.4.2 Lateralization task

Participants performed a lateralization performance task to assess their ability to lateralize the trigeminal stimulations (CO2 or eucalyptol depending on the study) delivered at the chosen concentration. Twenty trigeminal and 20 control (cleaned air) stimulations embedded in a constant 1 l/min airflow were randomly but equally delivered in the right or left nostril for 0.5 s (except in study C1870 in which it was 1 s). For each trial (every 15 s), participants were requested to indicate if they felt any change in the left or right nostril by pressing the left or right arrow of a keyboard, accordingly. The success criterion was determined using the binomial distribution (one-tailed, α < .05, probability of guessing p = .5, n = 10) to ensure that participants lateralize the trigeminal stimulation in a statistical significant manner. Participants

5 The mix is first created in a mixing carboy by manually adjusting the input flow of pure CO2 into the cleaned airflow.

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obtaining less than 7 correct judgments out of 10 possible correct answers for CO2 trials were not included in the main experiment (n = 12 participants across all studies).

4.1.4.3 The electrooculographic calibration task

Since foveal processing is faster than a non-foveal (peripheral) processing of stimuli, the gaze must be tracked in order to avoid effect confound (Chica et al., 2014). Knowing when participants did not maintain their gaze at the center of the screen allowed excluding those with too many eyes saccades. This ensured that the effect observed could only be the results of the lateral trigeminal cues on the spatial attention. The electrooculography (EOG) allows for the tracking of the gaze. The EOG measure was calibrated through a 5-minutes protocol using the same fixation cross and the same target visual stimuli used in the spatial cueing task.

Participants were requested to look at a white fixation cross in the middle of the black screen for 5 seconds and orient their gaze at the white targets randomly appearing for 1 second on the right (10 trials) or left side (10 trials) at the same location as in the main orienting task. The distance between the fixation cross and the center of each target was 15 cm, resulting in an angle of 10.62° for the participant.

4.1.4.4 The cross-modal spatial cueing task

In this cross modal adaptation of a classical spatial cueing task (Posner, 1980), an chemosensory cue was delivered either in the right or left nostril prior to a visual target presented during 120 ms in either the right or the left visual field (Figure 19).

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Figure 19. Scheme of the experimental sequence, here a valid trial. Each trial started with a countdown, followed by the presentation of a fixation cross. Following the offset of the fixation cross, the lateralized olfactory stimulation was administered in the right or left nostril for a specific period of time depending of the study (see Table 3). Afterwards, the target, a white horizontal or vertical line, appeared for 120 ms in the right or the left visual hemi field. In a valid trial, the line was in the same side of the CO2 or air stimulation; in a trial defined as invalid, the line was in the opposite side of the olfactory stimulation. Participants were required to indicate as fast and as accurately as possible the

orientation (horizontal or vertical) of the line following the lateralized cue induced by left or right nostril CO2 stimulation.

In valid trials, cues and targets were delivered on the same side. In invalid trials, they were delivered on opposite sides. We chose different SOA windows depending on the study (Table 3).

Table 3

Average of the stimulus onset asynchrony (SOA) in ms (±SD)

Study C580 Study C680 Study C1160 Study C1870

SOA = 580 ± 26 SOA = 680 ± 18 SOA = 1160 ± 48 SOA = 1870 ± 17

Study E610 Study E830 Study E1120

SOA = 610 ± 15 SOA = 830 ± 31 SOA = 1120 ± 18

A natural SOA jitter (variation of time) due to the loading of the olfactometer’s control libraries by E-prime software prevented the participant from anticipating the appearance of the target.

The SOA also determined the duration of the olfactory cue. Both nasal and visual stimulations were randomly delivered, respectively, either in the left (50%) or the right nostril (50%) and in Figure 7

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the right (50%) and left (50%) visual field. Thus, nasal cues were non-informative of the target location (i.e., exogenous cueing). Using their right hand, participants were requested to indicate as fast and accurately as possible the orientation of the target using the left arrow key (i.e., horizontal) or the down arrow key (i.e., vertical), both orientations being presented in equal proportions. A total of 64 trials were delivered randomly to each participant (16 for each combination of CO2, air, valid and invalid conditions).