Abstract
Tools such as pliers systematically change the relationship between an object's size and the hand opening required to grasp it. Previous work suggests the brain takes this into account, integrating visual and haptic size information that refers to the same object, independent of the similarity of the ‘raw’ visual and haptic signals (Takahashi et al., VSS 2009). Variations in tool geometry also affect the reliability (precision) of haptic size estimates, however, because they alter the change in hand opening caused by a given change in object size. Here, we examine whether the brain appropriately adjusts the weights given to visual and haptic size signals when tool geometry changes. We first estimated each cue's reliability by measuring size-discrimination thresholds in vision-alone and haptics-alone conditions. We varied haptic reliability using tools with different object-size:hand-opening ratios (1:1, 0.7:1, and 1.4:1). We then measured the weights given to vision and haptics with each tool, using a cue-conflict paradigm. The weight given to haptics varied with tool type in a manner that was well predicted by the single-cue reliabilities (MLE model; Ernst and Banks, 2002). This suggests that the process of visual-haptic integration appropriately accounts for variations in haptic reliability introduced by different tool geometries.