The size of the visual size cue used for programming manipulative forces during precision grip

We used a perturbation technique to quantify the contribution of visual siz e cues to the programming of target force when lifting an object. Our resul ts indicate that the nervous system attaches a reasonable weight to visual size cues when programming the target grip force for a novel object. In a s ubsequent lift of the same object, however, the confidence attached to the visual size cue fell dramatically. It is not clear whether the decrease in the use of size information was accelerated by the presence of a cue confli ct or whether the fall represents the normal shift towards the use of a mem ory-based representation for programming grip force. In a second experiment , we used the "size-weight illusion" to explore the relationship between th e verbal report of an object's weight and the programming of the grip and l oad force. We found that erroneous motor programming (as indexed by a numbe r of measures) was neither necessary nor sufficient for the size-weight ill usion to occur. These findings call for a re-evaluation of a previous expla nation for the size-weight illusion. We suggest that the illusion arises be cause the cognitive system attempts to rationalise the fact that objects of apparently equal density but different size feel as if they have the same weight.