Theory of a subjective vertical-horizontal conflict physiological motion sickness model for contemporary ships

Subjective vertical (SV) conflict theory postulates that motion sickness is elicited in all situations that lead to a difference between the sensed and subjective verticals. The sensed vertical is Earth's gravity as perceived by human sense modalities; the subjective vertical is also Earth's gravity, but in accordance with the expectations of the central nervous system, based on past interaction with the spatial environment. The SV conflict models have been successfully used to predict motion sickness on board high speed passenger ferries. However, a recent EU project, COMPASS, indicated that the role of horizontal accelerations in the elicitation of motion sickness aboard contemporary vessels is as important as that of vertical accelerations. Consequently, this paper, using an extended statement of the SV conflict theory, proposes that SV conflict models can be further elaborated by explicitly incorporating the effects of horizontal accelerations (normal to gravity) experienced aboard contemporary vessels. It is hypothesized that the explanation of motion sickness variability can be improved by considering the combined effects of subjective vertical conflict and subjective horizontal conflict (the difference between the sensed and expected horizontal accelerations). After presenting the theoretical aspects of subjective vertical-horizontal (SVH) conflict model, this paper demonstrates its application to 7 field trials of 3 different vessels. The proportion of commuters getting seasick (i.e., motion sickness incidences, MSI) during each field trial, has been statistically compared with the values predicted by the physiological (SVH and SV) and descriptive sickness prediction models. In general, SVH conflict model is outperforming the descriptive models and displaying approximately 20% improvement over the SV conflict model.