Factors influencing thresholds for monocular movement parallax.

Abstract

This expt. has been concerned with certain quantitative aspects of monocular movement parallax. The effects of the following variables on monocular parallax measures have been detd.: (a) differences in size of moving objects, (b) intensity of illumination, (c) rate of movement of objects, and (d) axis of movement of objects. Ten practiced subjects were used. The apparatus used in the expt. consists of equipment whereby measurement may be made of the precision of distance settings ([omega] t) of 2 needles, one above the other, moving at constant and equal speeds back and forth across an illuminated field. The results of an expt. on the effect of differences in size of the stimulus needles indicate that the settings are made on the basis of distance cues other than visual angle. Log [omega]t_ decreases with increase in the logarithm of the intensity of illumination over the cone range. The decrease is rapid at low intensities and then becomes more gradual. A final low constant level of cj t is reached at high intensities (about 100 millilamberts). Hecht''s intensity discrimination curve is applied to the data. A 3-fold increase in rate of movement of the needles was seen to increase in wt by a factor of 3. Control expts. indicate that the increase in wt at the higher rates is not due to the greater frequency of exposures at these rates, but that either or both duration of exposure and length of excursion are important variables, wt is about twice as large for movements along the vertical axTs as for movements along the horizontal axis. [omega] t assumes intermediate values for axes lying between the horizontal and the vertical.