Hands, not eyes, hold clue to illusion

Seeing is believing, but acting is more accurate. That’s what two Canadian psychologists conclude after  investigating the size-weight illusion, an error that arises when people try to estimate the weights of two objects of different sizes but the same mass.

Two boxes of the same weight, one shown with sensors to measure lifting force. Flanagan

J. Randall Flanagan and Michael A. Beltzner of Queen’s University in Kingston, Ontario, presented 40 people with two square boxes topped by handles, each unit weighing 0.39 kilograms. One box measured 5.2 centimeters on each edge; the other, 10.9 cm.

As expected, all volunteers guessed that the larger box was heavier before they touched the cubes. After lifting each box in turn 20 times, using the tips of their thumb and index finger, all reported that the smaller box was heavier. Another group fell prey to the illusion even when told before lifting the boxes that they were the same weight.

The usual explanation for the illusion, which has appeared in psychology journals since the 1890s, is that it’s due to a mismatch between expectations and sensory feedback. “You might expect a large object to be heavy,” says David Milner, a psychologist at the University of Durham in England, “and when it isn’t, it seems startlingly light.” It’s similar, he suggests, to lifting a half-empty suitcase.

Flanagan found this mismatch hypothesis unsatisfying, however, because it doesn’t specify how a discrepancy in feedback could become a perception. He used sensors on the boxes’ handles to measure the lifting force and grip strength that the volunteers applied—learning not just what they saw but what they did.

Initially, the participants used more power to lift the larger box. After hoisting the boxes 5 to 10 times, however, they applied equal force to the boxes. Yet despite this correct sensorimotor adaptation, the cognitive side of the illusion persisted. The volunteers still said that the smaller box was heavier, the researchers report in the July Nature Neuroscience.

“I’m happy that this happens,” says Melvyn A. Goodale of the University of Western Ontario in London. In the early 1990s, he and Milner advanced some of the theories of visual processing underlying Flanagan and Beltzner’s study.

Milner and Goodale reinterpreted work on visual processing that identified a “what” pathway concerned with labeling objects and a “where” pathway concerned with locating them.

Instead, Milner and Goodale suggested a division between “vision for cognition” and “vision for action.”

Adjustments to grip and force, says Milner, are examples of vision for action. They deal with information conveyed by the immediate scene. Judgments about object weight, in contrast, draw more heavily on an integration of the visual scene with cognitive comparisons among weights of familiar objects, he says.

From the primary visual cortex, these information streams diverge. Vision for action uses a dedicated path to the brain’s posterior parietal cortex, where sensory information is processed.

Vision for cognition takes a route to the inferotemporal region, which supports more-abstract cognitive functions.

Even though there’s interaction between the two streams, says Flanagan, information in one isn’t necessarily available to the other. This prevents the cognitive system from accessing sensorimotor learning. Researchers have reported similar disjunctures in other optical illusions (SN: 2/14/98, p. 108).

Outside of experimenter-contrived worlds, Goodale says, the assumptions underlying size-weight and optical illusions “are correct 99.9 percent of the time.”

Moreover, these judgments’ enduring utility has given them staying power. Only temporal lobe damage has, so far, made the cognitive side of the illusion disappear.

“I’ve sort of fantasized about experiments,” says Flanagan, hastening to add that he hasn’t actually done them, “where you would put people in a room where everything was inverted.

So, the chesterfield would be the lightest thing, and your cutlery would be the heaviest.”

Maybe after a week in there, he says, you could make the illusion go away.

From the Nature Index

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