Stimuli were generated with the Vpixx© software (
http://www.vpixx.com) and driven by a Macintosh G3 computer. Stimuli were rear-projected by an LCD projector (InFocus LP725) onto a diffused translucent screen (Da-Lite© screen), subtending 104 × 79 deg of visual angle. The image had a resolution of 800 × 600 pixels, and the refresh rate was 60 Hz. A camera was set above the center of the screen and was connected to a video recorder for on-line viewing and recordings of the participant during the experimental sessions.
During each trial, a pair of stimuli display was presented on the screen.
Figure 1 demonstrates dimensions of the two stimuli displays (79 × 45 deg). Each display contained 150 bright dots (58 cd/m
2) with an apparent uniform dot density (evenly distributed) on a dark rectangle region (2 cd/m
2; for a Michelson contrast of 93%) on a gray background (30 cd/m
2;
u′ = 0,1910 and
v′ = 0,4942).
The uniform dot density was achieved by reusing the dots that were falling off the display (“dead” dots) to fill the regions with the lowest time-averaged dot density. Therefore, the number of dots over space–time was maintained constant (see
Movie). The luminance calibration was done using a Minolta Chroma Meter (CS-100). Each dot subtended 0.66° × 0.66° at the viewing distance of 57 cm. The moving dots had continuous trajectories, and each frame lasted 16.67 ms where the position of each element was updated on every video frame of the stimulus animation (60 times/sec). The incoherent dots or noise of both displays had a behavior where the dots were randomly “jittering” back and forth along its radial path. That is, each incoherent dot had a 50% chance (with replacement) of changing direction on each frame. See the
Movie for alternating examples of expanding and contracting motion and the jitter noise.
One side contained coherent optic flow motion, referred to as the target display, which has a signal-to-noise ratio of varying levels. This optic flow stimulus simulated an observer traveling in translation through a circular tunnel, where the dots were randomly distributed along the walls. On the screen, the dots were moving in a radial trajectory, in expansion from the origin of motion, which was set in the center of each stimulus. The other side contained the neutral stimulus, which was equivalent to the target display in all respects (i.e., dot speed, density, radial trajectory, etc.) except for the coherence of motion, which was always set at 0% (incoherent motion or non-directional noise). This non-directional radial motion led to a planar (no impression depth or self motion) and erratic (non-directional) motion impression. The side that contained the target display was randomly selected. Non-directional radial motion has been chosen as neutral stimuli because it differed from the target stimuli only in the global directionality of the information. Furthermore, we observed in pilot experiments that a random directional noise pattern was much more salient than the target display and the infants always looked at the control stimulus. Therefore, we used this random directional motion as the central fixation target before each trial. In order to simulate the optic flow field seen when traveling down a tunnel, dot speed accelerated with the square of the distance from the origin of expansion, or by a factor of four as the distance doubled. In this study, the dots' speed were of 1.25°/sec at an eccentricity of 5° and would travel at a speed of 5°/sec at 10°, to 20°/sec at 20° of his origin.