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During the last war much research was done on determining those factors which influence our ability to judge vertically and our own bodily orientation in space. The importance of these factors to the airplane pilot is obvious. These, of course, involve the physiology of the vestibular and visual systems, particularly as they are affected by head and body orientations and by psychologic influences, such as optokineses.
This monograph, in Dutch, with English, German and French summaries, deals with specific researches in this field: a study of the apparent vertical in the frontoparallel plane (that is, so far as a tilting to the right or left is concerned—but not stereoscopic vertical), and the apparent median plane of the head when subjected to (1) surrounds consisting of grid pattern of lines that can also be tilted and (2) tilting of the head to one side. Mesker cites and discusses the researches of
De optische localisatie onder invloed van optische en houdingsfactoren (Optical Localization Under the Influence of Optical and Postural Factors).. AMA Arch Ophthalmol. 1953;50(6):795-796. doi:10.1001/archopht.1953.00920030806025