We exposed blindfolded participants to two conditions on a parallel swing, between which we differently manipulated expectations regarding the motion of this swing (within-participant design). In reality, we only let the swing move with a transient oscillation at the start of each condition. However, in a “Focus on motion” condition, we aimed at letting participants believe that the swing was moving for the entire condition. We therefore told participants before the start of this condition that the swing would be oscillating with varying peak-to-peak displacements, and asked them about this motion at regular intervals during the condition. We moreover demonstrated to participants that the swing could move back and forth. In a “Distraction from motion” condition, we aimed at letting participants believe that the swing was only oscillating at the beginning of the condition. We therefore told participants before the start of this condition that the swing would come to a stop after an initial perturbation, and distracted them from the swing’s possible motion by asking motion irrelevant questions about pitch differences of a tune during the condition. In summary, the cognitive (non-sensory) cues consisted of (1) instructions about the swing’s motion, (2) a discrimination task with different attentional allocation performed during the conditions, and (3) a demonstration of swing motion.
We recruited 24 participants (16 females) from the Vrije Universiteit Amsterdam in The Netherlands, where the experiment was performed. Participants were allowed to participate if they were 18 years or older, had experienced motion sickness in the last 5 years, were free of (self-known) vestibular and auditory complaints, were not pregnant, did not suffer from claustrophobia, and never participated in an experiment on our setup before. Our participants were aged between 18 and 24 years. We have obtained ethical approval from the faculty’s review board (reference number: VCWE-2020-180R1).
In both conditions, participants were seated on a parallel swing (Oosterveld 1970). The swing consisted of a 250 × 245 cm platform attached to the ceiling with four 6.65 m ropes (Fig. 2). Given this length, the swing oscillated with a natural frequency of 0.19 Hz when perturbed, close to the peak frequency of motion sickness incidence (Golding et al. 2001; ISO 1997).
To support the perception of oscillatory motion in the Focus condition, the experimenter unleashed the swing from a 10 cm forward displacement at the beginning of both conditions. This resulted in a transient oscillation returning the swing to a standstill within 1–2 min (see Fig. 3). To check the swing’s motion, we recorded its acceleration in the longitudinal direction using the accelerometer of a mobile phone, measuring at 20 Hz using MATLAB Mobile for iOS (version 8.4). We detrended the signal and removed the measurement noise using a bidirectional first order low-pass Butterworth filter with a cutoff frequency of 2 Hz. The resulting root mean square acceleration excluding the first 2 min was 0.003 ± 0.001 m/s2 (mean ± SD) on average in both conditions. This average is considered well below the threshold for motion perception, assumed between 0.1 and 0.01 m/s2 (Griffin 1990). Any percept of oscillatory self-motion can thus not be explained for by physical motion stimulation. We additionally minimized visual motion cues by blindfolding participants for the entire duration of the condition; somatosensory motion cues by airflow generated by a swiveling fan rotating at a frequency of 0.05 Hz, thus uncorrelated to the natural frequency of the swing; and auditory motion cues by a noise cancelling headphone (see also Fig. 2).
Tasks and measurements
Both conditions contained seven blocks, a break, and a set of three exploratory questions (see Fig. 4). Each block consisted of a discrimination task that was repeated seven times, followed by a sickness rating using the Motion Illness Symptoms Classification (MISC, Table 1; Bos et al. 2005; Reuten et al. 2021) and two additional questions on the perceived swing motion.
In the Focus condition, the discrimination task consisted of seven repetitions of 15 s focusing on the swing’s motion, each followed by the question whether the swing had moved farther or less far as compared to the previous time asked. After participants completed this task and rated their sickness, we asked them to indicate when the swing reversed direction. This question was added to strengthen the participants’ cognitive involvement with the swing’s oscillations. After this, we asked them to indicate the peak-to-peak displacement of the swing’s motion about that moment (further referred to as ‘displacement’). Four participants expressed their doubts on whether the swing was indeed moving. In these cases, the experimenter once used the encouragement “the swing is moving, but the movements may be very small, thus try to pay close attention to them”.
In the Distraction condition, the discrimination task consisted of seven repetitions of 15 s listening to a music clip (Jerry Martin’s “Under Construction”), each followed by the question whether the sample was played higher or lower in pitch as compared to the previous time asked. Pitch height was truly increased or decreased by 4.8 or 9.6% relative to the previous sample (adapted using Audacity 126.96.36.199) aiming to achieve a comparable level of mental workload and task difficulty compared to the task in the Focus condition. After completion of the discrimination task and sickness rating, we asked participants to indicate whether they thought the swing was still moving, and if they did, the second question then was which (peak-to-peak) displacement the swing had about that moment.
The blocks succeeded each other without additional manipulation. To offer participants a break from intensely concentrating, we asked them to perform an alternative task between block four and five. They had to list as many words as possible starting with a certain letter of the alphabet within 1 min. After participants had completed the seven blocks, we asked them three additional questions whilst still being seated on the swing. The first question was which percentage of time they thought the swing had moved (0% = never moved to 100% = always moved). The second question was on their ability to concentrate on the discrimination task (0 = poor to 10 = good). The last question was on the difficulty of the discrimination task (0 = very easy to 10 = very difficult).
After arrival, we instructed participants on the experimental procedure and asked them to sign an informed consent form. Participants filled out the Motion Sickness Susceptibility Questionnaire (MSSQ-Short; Golding 2006) from which we observed that the sample’s susceptibility to motion sickness fell within the 60th percentile. Following completion of the MSSQ-Short, participants performed the two conditions. Because of individual differences in response time and the additional question about the moment the swing reversed direction in the Focus condition, the conditions lasted between 25 and 35 min. We presented the conditions in counterbalanced order with a 45-min break in between, to allow for recovery of motion sickness. To minimize an observer–expectancy effect (see Rosenthal 1963; Rosenthal and Fode 1963), we provided all instructions via pre-recorded audio files, both before and during the conditions. Although we were interested in the effect of motion expectations on self-motion perception and motion sickness, we told participants that we were interested whether their ability to discriminate small differences in displacement and pitch were related. We introduced the MISC as a measure to monitor their level of well-being as it could influence their task performance.
We stopped a condition when a participant rated a MISC class of ≥ 6, which occurred once in the Focus condition and three times in the Distraction condition. After completing the experiment, participants were thanked for their participation and received study credits.
Our primary dependent variables were the displacement and MISC class participants rated at the end of each block in both conditions. Missing data as the result of the exerted MISC stop-criterium were substituted with the last rated displacement and MISC class. To answer our two questions, we averaged the seven displacements and MISC classes given by each participant in the Focus and Distraction condition and analyzed the within-participant differences using Wilcoxon signed-rank tests (with α = 0.05). To explore the data further, we report the averaged within-participant difference between the conditions for various measures, together with the between-participant standard deviation (mean difference ± SD, Focus minus Distraction).