The weight of expectation: Implicit, rather than explicit, prior expectations drive the size-weight illusion.

In the size-weight illusion, small objects feel heavier than identically weighted larger objects. This illusion is thought to be a consequence of how one's prior expectations can influence conscious perception--lifters expect the large object to outweigh the small object and subsequently experience it as feeling lighter than they expected it to be. Here, we directly examined how a familiar object's identity can affect how heavy someone expects it to be, and how these expectations will influence subsequent perceptions of heaviness. We describe two novel weight illusions induced with familiar objects. In one condition, participants judged the weight of a set of similar-size objects with very different natural weights (a polystyrene sphere, a tennis ball, and a cricket ball), which had all been adjusted to weigh the same amount as one another. In this condition, participants experienced a small, but reliable, weight illusion, with the lightest looking ball feeling heavier than the heaviest looking ball. In the other condition, participants judged the weights of a different set of balls, which were different sizes, but similar natural weights, to one another (a golf ball, a foam soccer ball, and an inflated beach ball). Again, participants experienced a perceptual illusion, but in the opposite direction. Surprisingly, participant's perceptions matched, rather than contrasted with, their explicit expectations such that, even though they expected the golf ball to outweigh the beach ball they perceived the golf ball as feeling heavier than the beach ball. The effect of object mass appeared to dominate the effect of conscious expectations, suggesting that contrasting expectations of heaviness are not necessary to experience weight illusions and that current models of this robust perceptual effect must be revised.