The material from which an object is made can determine how heavy it feels (Seashore, 1899). Interestingly, a metal block that has been adjusted to have the same size and mass as a polystyrene block will feel lighter than the polystyrene block. We recently showed that participants experiencing this material-weight illusion’ (MWI) do not apply forces that match their perceptual experience of heaviness ‐ just like in the size‐weight illusion ( Flanagan & Beltzner, 2000).
Our previous study showed that forces on early trials were scaled to each participant’s expectations of how much a particular block should weigh ‐ excessive force was applied to the metal block and insufficient force was applied to the polystyrene block. Forces on later trials scaled to the real weight of each block ‐ identical levels of force were applied to all the blocks. MWI persisted throughout – the polystyrene block felt the heaviest and the metal block felt the lightest.We followed this finding up with two experiments:
Experiment 1 – different weight, different material: We adjusted the weight of each block slightly in the opposite direction to the illusion, predicting that we would find opposing perceptual and motor responses (e.g., Grandy & Westwood, 2006).
Experiment 2 – different weight, same material: We also removed the visual differences between the blocks, keeping the slight difference in weight, predicting that the dissociation between perception and action would disappear.