THE GREAT German physicist Hermann von Helmholtz not only discovered the first law of thermodynamics (the conservation of energy) but also invented the ophthalmoscope and was first to measure nerve impulse velocity. He is, in addition, widely regarded as the founding father of the science of human visual perception—and is, to both of us, an inspiration.

We have often emphasized in our writings that even the simplest act of perception involves active interpretation, or “intelligent” guesswork, by the brain about events in the world; it involves more than merely reading out the sensory inputs sent from receptors. In fact, perception often seems to mimic aspects of inductive thought processes. To emphasize perception’s thoughtlike nature, Helmholtz used the phrase “unconscious inference.” Sensory input (for example, an image on the retina at the back of the eye) is interpreted based on its context and on the observer’s experience with, and knowledge of, the world. Helmholtz used the word “unconscious” because, unlike for many aspects of thinking, no conscious cogitation is typically required for perception. By and large it is on autopilot.

Weighing the Evidence

A powerful demonstration of the predictive power of perception is seen with the size-weight, or Charpentier-Koseleff, illusion (conceptual representation in a), which you can easily construct and use to mesmerize your friends. This perceptual trick was one of Helmholtz’s favorites, and we shall soon see why.

To set up, take two objects that are similar in shape, color and texture but different in size—such as hollow metal or plastic cylinders. Hide enough weight inside the smaller one so that its weight is identical to that of the larger object. Because the two containers appear similar, except for size, observers will naturally assume the larger one is proportionally heavier than the smaller one. Now ask a friend to pick them up and compare their weight.

She will surprise you by reporting that the objects are not equal in physical weight. She will insist the larger object feels much lighter than the smaller one. She will continue to assert this incorrect fact even if you tell her that you want her to report absolute weight, not density (weight per unit volume).

Try it yourself. Remarkably, even though you know the objects weigh the same (after all, you constructed them), you will experience the larger object as feeling considerably lighter than the smaller one. As with many illusions, knowledge of reality is insufficient to correct or override the misperception. We neuroscientists say that perception is immune to intellectual correction—that it is “cognitively impenetrable.”

Impervious Illusion

Furthermore, the visual information continuously overrides the feedback from muscle signals telling you that the weights are physically identical. The illusion is impervious not only to high-level conceptual knowledge that the objects weigh the same but also to “bottom up” signals from other sources, such as feedback from muscle receptors, telling you they weigh the same. You can repeat this experiment many times, but you will still experience the illusion.

Why does the effect occur? When you reach out for the bigger object, you expect it to weigh more (given the assumption that it is made of the same stuff) and you exert greater lifting force. Because it weighs the same as the smaller object (which you expected to weigh less), however, you actually experience it as being lighter, relative to the smaller object.

As an analogy, imagine you run into someone who looks unintelligent and you initially expect him to be so. If he then starts talking normally, he seems even brighter than average! It is as if you calibrate your judgment of a person’s capabilities by the way he looks, and therefore your final “reading” of his true skills—based on his verbal output—is an overestimate.

Insight from a Visual Trick

The size-weight illusion may be easier to understand if we couch it in terms of a more familiar visual illusion, the Ponzo, or railroad track, illusion (b, on preceding page). Two horizontal yellow bars are shown lying between two longer converging lines. Although the bars are identical, they are not seen as such: the top bar appears longer than the bottom bar. We can explain the illusion in terms of a visual effect called size constancy; if two objects of identical physical size are at different distances from a viewer, they are correctly perceived as being the same physical size, even though the images cast by them on the retina are different sizes. Quite simply, the brain “understands” there is a trade-off between retinal-image size and distance and, in effect, says, “That object’s image is small because it is far; its actual size must be much bigger.” To evaluate distance, the visual system uses various sources of information called cues, such as perspective, motion parallax, texture gradients and stereopsis. It then applies the appropriate correction for distance in order to judge true size.

But with the Ponzo illusion, the two horizontal bars are the same physical size on the retina. The converging lines provide a powerful trigger to read them—falsely in this case—as lying at different distances away (as though you are peering down a railroad track and see the railroad ties at increasing distance). Because your visual system “believes” the top bar is farther away, it infers that the top bar must really be larger than its size on the retina would indicate (relative to the other bar). You therefore perceive it as being larger.

To put it differently, size-constancy scaling enables you to perceive accurately the size of objects when you correctly perceive distance to those objects. In the Ponzo illusion, however, the misleading depth cue from the converging lines causes you to misapply the size-constancy algorithm so that the top bar is seen as being larger. Remarkably, the illusion overrides the visual signals from the retina informing the visual-size judgment centers in the brain that the two bars are exactly the same length. And because these mechanisms are all on autopilot, knowing that they are identical in size does not correct the illusion.

Brain Expectations

The situation with size and weight is analogous. (Read “actual weight signaled by muscles” for “actual retinal-image size.”) Your brain says, “For the big object, I expect the muscle tension to be much greater in order to lift it.” But because the muscle tension required is much lower than expected, the object is felt as unexpectedly light. This experience overrides your judgment of actual weight signaled by your muscles.

Remember that we said the size-weight judgment system is on autopilot. So we can ask how dumb or smart it is on its own. What if we now use as test objects a disk and a ring of the same outer diameter (c), and, as with the standard size-weight illusion, we adjust each of them so that they have the identical physical weight? Of course, as before, anyone picking up the ring will expect it to weigh much less because it looks as if it has less total volume. But you (the experimenter, aware of the size-weight illusion) would predict the reverse—that the hollow ring would be felt as being much heavier than the solid disk. In fact, in collaboration with Edward M. Hubbard, now at INSERM in France, we have found that a subject will experience no size-weight illusion; she will correctly judge the objects to be the same weight. The brain seems to merely utilize the outer diameter in making the judgment, rather than the overall volume. This experiment shows that the visual system is not sophisticated enough to understand that what is relevant is the total mass, not the outer diameter alone.

In addition to size, the brain takes other factors into account for gauging anticipated weight. For example, if you pick up a plastic beer mug, it will feel unusually light. Again, this effect occurs because you expect it to be made of glass and, therefore, to be heavy. The original size-weight illusion may turn out to be largely hardwired (we do not know), but surely the beer mug–weight illusion must be learned. Our hominid ancestors were not exposed to mugs.

Felt vs. Real

What other insights can we gain from this illusion? Perhaps there is a practical application. Our house (which is very tall) has many stairs, and we expect to fatigue more quickly running up and down while carrying heavy loads than we would carrying light ones. Physical exertion increases when you are carrying greater weight; your heart beats faster, your blood pressure rises and you sweat. One typically assumes that this extra effort is because the muscles consume more glucose, and this information is fed back into the brain to generate the adaptive response of increased heart rate, blood pressure and sweating to allow for, and to anticipate, increased oxygen consumption resulting from hard work.

But is it conceivable that part of this preparation may also involve the felt weight of the object sending direct brain signals to the body? Imagine you run up and down a staircase with a large object and then compare the degree of tiredness you feel with that produced when carrying a much smaller object whose physical weight is the same as the larger item (and therefore feels heavier because of the illusion). Does the additional felt weight, as opposed to real weight, increase your sense of exertion or tiredness? In other words, is the fatigue determined by actual physical exertion? And would such imagined work actually increase your heart rate, blood pressure and sweating?

If so, the implication would be that merely feeling excess exertion causes the brain to send more signals to the heart to raise blood pressure, heart rate and tissue oxygenation. There have been sporadic reports that repeated imagined exercise can increase muscle strength, but precious little evidence. (We have started to explore this area in collaboration with neuroscientist Paul McGeoch of the University of California, San Diego.)

If it turns out that the felt weight determines how tired you feel, then next time you buy a suitcase for travel you should buy a large one; it will feel much lighter even if you stuff it with exactly the same amount of material! Quirks of perception have profound theoretical implications—but they can have practical consequences, too.