Visual Inertia Tn Apparent Motion

Four dots in an imaginary diamond were flashed in succession to give ambiguous apparent motion (AM). The top and bottom dots were flashed at time t, and replaced by the left and right dots at time t,. If two priming dots were gashed beforehand at time r0 in line with two parallel sides of the diamond, AM was seen along those sides. We call this “visual inertia”. The amount of visual inertia (measured by a null method) fell off with increasing angle between the priming dot and the side of the diamond. Visuai inertia was stih seen when the priming dots were presented to one eye and the test dots to the other, so the effect must be partly central. The timing and length of the priming path made little difference to visual inertia. However, static priming dots were ineffective. We conclude that the visual system was examining at least three successive time frames in deciding which items in one frame correspond with which items in succeeding frames. Movement perception Apparent motion Psychophysics Correspondence problem A dot which jumps back and forth between two positions at suitable temporal and spatial intervals will be seen in apparent motion (AM). Figure l(a, b) show four dots forming an imaginary diamond. The north and south dots are flashed on at time 1 and are replaced by the east and west dots at time 2, and this cycle repeats continuously. The apparent motion produced by this display is ambiguous, since it is equally likely to occur in a northeast-southwest direction [Fig. l(a)] as in a northwest-southeast direction [Fig. l(b)]. We did not use dots arranged in a square with horizontal and vertical sides. Gengerelli (1948) noted that with this configuration subjects favoured vertical motion; probably because of a reluctance to see AM across the vertical midline of the retina. We have used this four-dot stimulus to explore the factors which influence AM (Ramachandran and Anstis, 1983a). In particular we have described an effect which we called “visual inertia” (Rama~handran and Anstis, 1983b). If the quartet of dots was embedded in two straight parallel rows of dots which were flashed on in succession, there was a strong tendency to see AM in the direction of the embedding motion. Rows of stutic dots had no such effect, Eggleston (1984) has also investigated similar phenomena. We have used the expression “priming dots” to refer to the dots flashed before or after the dots in the quartet and which potentially alter their AM, and the expression “test dots” to refer to the quartet of dots.