It was observed that visual filters differing slightly from the identity (e.g. rotation by a few degrees) had a more lasting `relative-aftereffect' (after removal of the apparatus) than visual filters that were far from the identity (e.g. rotation by 180 degrees = ``upside-down''). By `relative-aftereffect', It is meant the degree to which one is incapacitated upon removal of the apparatus, compared to the severity of the mapping. Obviously a rotation by a degree or two does not have much of an effect, while rotating the image 180 degrees has a much more profound effect on one's ability to perform tasks. Furthermore, the visual filters close to the identity tended to leave a more pronounced (relative) opposite aftereffect. (For example, one would consistently reach too high after taking off the apparatus where the images had been translated down slightly, or reach too far `clockwise' after removing the apparatus that had been rotating images a few degrees counterclockwise.)
The `visual memory prosthetic' was based on a partially mediated reality, that is, only part of the visual field of view was mediated, in this case, with the computer-induced (and sometimes annotated) flashbacks.
The reason for using the `rot90' (rotate 90 degrees) arrangement was twofold: firstly this matched the aspect ratio of the face (which is generally taller than it is wide, in fact it is generally about 4 units high and 3 wide which exactly matches `rot90' video), and this created a distinct dual adaptation space.
When two (or more) adaptation spaces were distinct, for example, in the case of the identity map (unmediated zones of the glasses) and the rotation operation (`rot 90'), It was possible to sustain a dual adaptation space and switch back and forth between the `portrait' orientation of the identity operator and and `landscape' orientation of the `rot 90' operator without one causing lasting aftereffects in the other.