Introduction
On the left side of the screen, there is a black square filled with dots of many colors. There are four arrows outside the black square with dots, pointing up, down, left, and right. Below, we describe the activity as if you are clicking through the various parts.
Click on one of the arrows next to the square containing multi-colored dots. You will see the colored dots begin moving in the direction of the arrow. Do not follow the dots with your eyes. Instead, keep your eyes stationary and fixed on the white cross that appears in the center of the moving dots. After watching the movement for 10 seconds or more, click the arrow again to stop the dots. You should experience a motion aftereffect (MAE) in which the dots seem to be “drifting” backward, in the direction opposite to their previous motion.
In addition to providing a strange visual experience, scientific study of the MAE has led to some interesting insights about how the visual system processes and encodes motion. Follow the links at left to learn more about this fascinating effect.
Activity Description
Click an arrow to start the colored dots moving in the direction of the arrow.
Click the arrow again to stop the motion (note that to stop the dots, you must click on the arrow pointing in the direction of current motion; if you click another arrow, the motion will continue in a different direction).
If you fixate for at least 10 seconds on the white cross that appears when the dots are in motion, you should perceive the dots drifting backwards once the motion stops.
Click the SPEED button to toggle between slow, medium, and fast motion.
Why Does the MAE Occur?
As detailed in other activities in this chapter, certain neurons in the brain are specialized for motion detection. Moreover, each motion-sensitive neuron is most responsive to a particular motion direction (some detect rightward motion, some detect upward motion, etc.). Researchers believe that all of these direction-specific neurons are constantly firing in a sort of dynamic equilibrium. When you are viewing a stationary scene, the various neurons’ responses cancel each other out, so that no motion is perceived. If elements in the scene start moving, for example, to the right, neurons sensitive to rightward motion will increase their firing rate, leading to the perception of rightward motion.
When you click on the right arrow in this activity, your rightward-motion-detecting neurons go crazy with excitement because of the rightward motion of all the dots in the display. If you watch the motion long enough (10 seconds or more), these neurons will become fatigued. When you then stop the dots, the rightward-motion-detecting neurons will then be firing at a slower rate than their complementary leftward-motion-detecting neurons. This pattern of neural activity is interpreted as leftward motion, and this is the source of the drifting that we call the MAE.
Interocular Transfer
Cover your left eye with your left hand, start the dots moving, watch for 10 seconds with your right eye, and then stop the dots and note how strong the MAE is. Keeping your left eye covered, start the dots moving in the same direction for 10 more seconds, and as soon as you stop them, close your right eye and uncover your left, so that you are viewing the stationary dots with your left eye. You should experience a noticeable MAE even with the eye that was not viewing the moving dots.
This result tells us something important about where our motion detection neurons are located. If the motion detectors were in the retina, we would not expect interocular transfer of the MAE. Motion-sensitive neurons in the right retina would have no effect on neurons in the left retina.
Therefore, we know that motion detection neurons must be located at some point “upstream” in the visual system, after information from the two eyes has been combined. Motion-sensitive cells are found as early as primary visual cortex (where information from the two eyes is only partially integrated—this is why the interocular MAE is not as strong as the MAE you experience if you view the stationary dots with the same eye that saw the moving dots). Information from striate cortex neurons is sent on to other areas of the brain, most notably the medial temporal (MT) lobe, for further processing.