The stimulus to the research reported was the finding that in complete darkness - the conditions of Space - afterimages can change systematically when their apparent distance alters due to movement of the observer. If the apparent size changes of the afterimage during movement of the observer could be measured, the constancy scaling process could be quantified. Since afterimages cannot be physically controlled, a new method was devised. The logic of the method is that the observer views a luminous object whose size may be varied in a controlled manner related to his movement. The display is made to shrink in size as he approaches it, the rate of shrinkage being adjusted until it is reported as appearing unchanging. A verbal report of an amount of expansion or contraction is unsatisfactory since it cannot reliably be quantified, but an observer can without difficulty state whether or not he can detect a change. This is a null technique which uses the observer as a detector rather than as a calibrated instrument. Author
Prepared in cooperation with Edinburgh Univ., (Scotland), Bionics Research Lab.