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The reason for pseudotorsion

Pseudotorsion is caused by the fact that, in tertiary positions of gaze, the vertical retina meridian does not coincide with a vertical line in space, nor does the horizontal retina meridian coincide with a horizontal line in space. The reason for this discrepancy is that horizontal and vertical are defined according to the coordinate system used. To illustrate this point: If the reader is sitting in front of one of the four walls of the room he is in, let him for a moment look at the left upper hand corner of this wall. The reader will perceive the left upper hand 90-degree corner of a wall in front of him as being larger than 90 deg. What, in this case, is horizontal and what is vertical ? It is clear that the angle between the horizontal and the vertical retinal meridians is perceived as 90 degrees at all times, no matter what the direction of gaze is. Hence, the afterimage cross cannot coincide with the left upperhand corner of the wall in front of us. The rays of the left upper hand corner are horizontal and vertical in space but, as the angle is perceived as larger than 90 degrees, the rays of the angle cannot possibly coincide with horizontal and vertical retina meridians. In fact, both rays of the angle, projected on the retina, deviate with a small angle from the horizontal and vertical retinal meridian, one clockwise and one anti-clockwise. This small angle is called pseudotorsion.
Summarizing, pseudotorsion results from the coordinate system employed or, in other words the reference that one chooses to be vertical or horizontal (Roelofs, 1934, 1954).