ALTHOUGH OUR PERCEPTION of the world seems effortless and instantaneous, it actually involves considerable image processing, as we have noted in many of our previous columns. Curiously enough, much of the current scientific understanding of that process is based on the study of visual illusions.
Analysis and resolution of an image into distinct features begin at the earliest stages of visual processing. This was discovered in cats and monkeys by a number of techniques, the most straightforward of which was to use tiny needles—microelectrodes—to pick up electrical signals from cells in the retina and the areas of the brain associated with vision (of which there are nearly 30). By presenting various visual targets to monitored animals, investigators learned that cells in early-processing brain areas are each sensitive mainly to changes in just one visual parameter, not to others. For instance, in the primary visual cortex (V1, also called area 17), the main feature extracted is the orientation of edges. In the area known as V4 in the temporal lobes, cells react to color (or, strictly speaking, to wavelengths of light, with different cells responding to different wavelengths). Cells in the area called MT are mainly interested in direction of movement.
One characteristic of these cells that may seem surprising is that their activity when stimulated is not constant. A neuron that responds to red, for instance, will initially fire vigorously but taper off over time as it adapts, or “fatigues,” from steady exposure. Although part of this adaptation may result from depletion of neurotransmitters, it also likely reflects the evolutionary logic that the goal of the cell is to signal change rather than a steady state (that is, if nothing changes, there is literally nothing for the cell to get excited about).
How do we know that such cells also exist in humans? Simply put, we descended from apelike ancestors, and there is no reason why we would have lost those cells during evolution. But we can also infer the existence (and properties) of feature-detecting cells in humans from the results of psychological experiments in which the short-term viewing of one pattern very specifically alters the perception of a subsequently viewed pattern.
For example, if you watch a waterfall for a minute and then transfer your gaze to the grass on the ground below, the grass will seem to move uphill. This illusion occurs because the brain normally interprets motion in a scene from the ratio of activity among cells responding to different directions of movement. (Similarly, the wide range of hues you see on the screen of your television set is based on the relative activity of tiny dots reflecting just three colors—red, green and blue.) By gazing at the waterfall, you fatigue the cells for downward movement; when you then look at a stationary image, the higher baseline of activity in the upward-motion cells results in a ratio that is interpreted as the grass going up. The illusion implies that the human brain must have such feature-detecting cells because of the general dictum that “if you can fatigue it, it must be there.” (This is only a rule of thumb. One of us “adapted” to the dreadful climate and food in England, but there are no “weather cells” or “food-quality cells” in his brain.)
The waterfall effect (or motion aftereffect, as it is also known) was first noted by Aristotle. Unfortunately, as pointed out by 20th-century philosopher Bertrand Russell, Aristotle was a good observer but a poor experimenter, allowing his preconceived notions to influence his observations. He believed, erroneously, that the motion aftereffect was a form of visual inertia, a tendency to continue seeing things move in the same direction because of the inertia of some physical movement stimulated in the brain. He assumed, therefore, that the grass would seem to move downward as well—as if to continue to mimic the movement of the waterfall! If only he had spent a few minutes observing and comparing the apparent movements of the waterfall and the grass, he would not have made the mistake—but experiments were not his forte. (He also proclaimed that women have fewer teeth than men, never having bothered to count Mrs. Aristotle’s teeth.)