Troxler—and YOU!

Today's reading delves deep into the visual system, so hold your breath and get ready to dive in. It's "Sound-aided Recover from and Persistence Against Visual Filling-in" by Bhavin Sheth and Shinsuke Shimojo of Caltech (Vision Research, 2004). I even found a PDF link for this one.

Ignaz Paul Vital Troxler (1780--1866) was a Swiss physician and philosopher who discovered a simple visual effect that continues to be studied today. Because his work generates so much research interest, there is even a Web site devoted to his life and work. The effect he discovered is aptly named "Troxler fading" and can be observed in a number of different circumstances. The basic effect is simple: stare at a figure where the center is a contrasting color from the background color. Within a minute or so, the center will appear to fade into the background. It eventually completely disappears. Try it yourself here. There is no trick to the site; it's a simple .gif file.

Are you sufficiently amazed yet? The effect is pervasive, and can be generated in a number of different ways, but what's truly important about it is what it tells us about the visual system. The eye is a lot like a camera, but the way we "see" the images our "camera" produces is very different. The eye has a lens, and even "film" (the retina), which detects light focused by the lens. The signal from the retina is then sent to the brain via a complex process, and it is only after the information has been processed in a number of ways that we actually "see" anything. What we see may have been initiated by the chemical response of cells inside our eye, but the processes going on inbetween that and our consciousness of "seeing" something have an important effect: what we "see" is not always what our retina "sees."

Troxler fading is a clear example of a disparity between the retinal image and what we see, so it provides a critical opportunity to understand how vision works. The same effect can be generated by presenting a flickering object. Watch a contrasting image that flashes randomly on the screen in the same spot for a minute or so, and it will eventually fade away.

What our mind appears to be doing is maximizing efficiency. If we keep getting the same uninteresting signal, over and over again, eventually we just ignore it—those brain cycles can be used for something else. But what can cause us to pay attention again? That's what Sheth and Shimoju wanted to explore, so they devised an experiment to find out. They asked people to stare at a white cross on the middle of their green computer screen, then flashed two red dots—one on each side of the cross—randomly until they both faded from view. They then began to play a clicking sound, synchronized with the appearance of either the left-hand or right-hand dot. Remarkably, when the click was played, the participants could again see the corresponding dot, but the other dot remained invisible.

So sound can affect the way we see objects before we are even conscious that the object is there. The effect is so strong that even when the opposite dot's brightness is doubled, viewers see the two dots equally well. If the sound is played from the beginning of the experiment, only the object with no accompanying sound "disappears" from view. Even though the sound does not come "from" the object, sound causes us direct our attention to the thing that is changing. Ventriloquists rely on this phenomenon as well. We hear speech, and we see the dummy's mouth move, so we assume the dummy is the one doing the talking (when we all know the real dummy is the one with his hand up a doll's tuckus!).

Perhaps the larger point is an incisive window into the way the mind works. The brain is a collection of expert systems: for vision, for language, for touch, smell, emotion, and a host of other skills. These systems can work independently, but they are also connected in remarkable ways. This evidence showing the connection of the auditory system with the vision system is just one of them.