Sizing Things Up

When you hoist two items of equal weight, your brain may be doing some heavy lifting

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 column 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, von 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. He 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 at a), which you can easily construct and use to mesmerize your friends. This perceptual trick was one of von 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.

This article was originally published with the title "Illusions: Sizing Things Up."

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