Neural gain changes subserving perceptual acuity.

When small and large objects of equal weight are lifted, the small object feels heavier than the large one (the size-weight illusion) and requires more effort to lift (the size-effort illusion). It has been suggested that these illusions result from neural gain changes designed to maintain acuity under different working conditions. If this suggestion is correct, a given mass should produce a larger increase in perceived weight or effort when added to the small object. This was found to be the case.