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