WHY IS THE STUDY of perception so appealing? One reason is that you can gain deep insights into the inner workings of your own brain by doing relatively simple experiments that any schoolchild could have done 100 years ago. More on those in a moment.
Your sensory experience of the world does not involve faithfully transmitting the retinal image to a screen in the brain so that it can be “seen” by some inner eye. One piece of evidence for this fact is that your perception of an object (in a, do you see two faces or a goblet?) can change radically even if the image on the retina is held constant, which implies that even the simplest act of observation involves judgment by the brain.
Less obvious, but equally important, is the converse. Your perception of the world—or an object in it—can also remain stable if the image is changing rapidly on the retina. One example is how you take in a scene when you move your eyes around. Every time you glance around a room, the image dances around the retina at warp speed, hundreds of feet per second. Yet all appears rock steady. Why?
Now, at first you might think the world does not appear to lurch because all motion is relative. The clouds glide in the twilight sky, but we assume they are stable and attribute the motion to the smaller object, the moon.
A simple experiment demolishes this idea. Close one eye—let us say the left. Then, keeping the right eye open, use the right index finger to displace the right eyeball, rocking it side to side slightly in its socket. (Gently!) You will see the world jump as if in an earthquake, even though there is no relative motion on the retina.
Why do we see a stable world when we swivel our eyes naturally but not when we jiggle an orb manually? The answer came from the great 19th-century physician, physicist and ophthalmologist Hermann von Helmholtz. He suggested that when the command to move the eyes is sent from the frontal lobes to the muscles of the eyeballs, a faithful copy of the command (like a “CC” for an e-mail) also goes to visual motion–detecting centers in the back of the brain. As a result, they are tipped off ahead of time: “You are going to get some motion signals, but they are not caused by real movement of the world, so ignore them.”
We can speak of two independent systems in the brain, either of which can signal a sensation of motion. Neuropsychologist Richard L. Gregory of the University of Bristol in England calls these the image/retina system (caused by image movement on the retina) and the eye/head system (generated by sensing the movement of the eyes). Ordinarily, the brain subtracts one signal from the other. When you move your eyes around, these two motion signals cancel each other out and the world remains stable.
We know that the image/retina system exists because of the experiment in which you jiggled your eye with your finger. But how do we know the eye/head system can independently evoke a motion sensation? Think about what happens when your eyes track a glowing cigarette tip moving across a completely dark room. You correctly see it moving several feet, even though the cigarette image does not move much at all on your retina. Instead your eyes are making a big excursion. So the brain “concludes” that the cigarette must have moved an amount equivalent to the eye movement. Again, we can speak of the final movement perceived as resulting from the subtraction of image/retina signals (close to zero because you are tracking it) from eye/head signals (large, because the eyes move a large distance to keep the cigarette’s image on the fovea, the area of the retina responsible for acute vision). The net result is that you see the glowing orange spot moving several feet.
You can produce a more striking version of this effect by having a friend take a photograph of you while you look directly at the flash. The result is a persistent afterimage of the bulb caused by continued activity of the receptors long after the light burst is gone. This flash image is “glued” to your retina; it cannot move even a tiny bit. Yet if you go to a dark room and move your eyes around, you see the afterimage moving vividly with the eyes. The eye/head system is signaling a large value, but the image/retina signal is zero—so as a result of the subtraction, you see the afterimage moving even though it is fixed and stationary on the retina.
You can create a similar fixed afterimage without a flash by staring for 30 seconds at the central X in the image in b; you will see the afterimage when you shift your gaze to a blank sheet of paper. (Blink your eyes to refresh the image if necessary.)