Transparency via temporal alternation

This research shows that temporal alternation constitutes a distinct route to transparency ( Holcombe, 2001; also pdf of VSS poster)

Below, for example, the rapid temporal alternation of two images results in transparency**

In other words, both the rightward-tilted bright bars and leftward-tilted darker bars seem to be available simultaneously, albeit accompanied by a percept of flicker. If your browser displays the animated GIF above correctly, it shows two perpendicular sinusoidal gratings in rapid alternation.
**Note: the transparency percept is sensitive to the speed and smoothness of the presentation and these depend on your particular computer and web browser configuration. If the animation is too slow, such that you experience first one pattern, then the other, or if the display seems to stutter or present one frame longer than another, then try quitting any other concurrently-running applications on your computer. For best results, it is recommended you download the movie files and view them with a quicktime movie player as the only application running. If the animation still appears to be going so slow that you experience the patterns one at a time, then try this.

square-wave plaid The transparency percept in the movie cannot be mediated by the mechanisms which create transparency in other situations, such as monocular static displays like the top one on the right. Many report that in this image they perceive bright tilted bars laying on top of dark bars. This is similar to what is perceived via temporal alternation above, but unfortunately there are limitations to the static technique, some of which can be overcome by the temporal alternation method. For example, using the conventional static method, one cannot present two colors in the same location without additional spatial context (i.e., an area where the colors do not overlap).

sinewave plaid An explanation based on the static mechanism for the rapidly alternating display would posit that the two patterns sum internally and static cues subsequently allow decomposition into separate surfaces. To the contrary, summing the two patterns results in the bottom pattern on the right, which does not appear transparent. A simple nonlinearity before summation also is not the explanation, as the transparency works with a simple flickering light.

This phenomenon, among others, suggests that central stages of the human visual system tend to integrate over 100 msec or more of stimulation.

This phenomenon also provides a new way for graphics designers, artists, and engineers to present two things in the same location. For some more examples, click here.