Optimal integration of shape information from vision and touch

Many tasks can be carried out by using several sources of information. For example, an object's size and shape can be judged based on visual as well as haptic cues. It has been shown recently that human observers integrate visual and haptic size information in a statistically optimal fashion, in the sense that the integrated estimate is most reliable (Ernst and Banks in Nature 415:429-433, 2002). In the present study, we tested whether this holds also for visual and haptic shape information. In previous studies virtual stimuli were used to test for optimality in integration. Virtual displays may, however, contain additional inappropriate cues that provide conflicting information and thus affect cue integration. Therefore, we studied optimal integration using real objects. Furthermore, we presented visual information via mirrors to create a spatial separation between visual and haptic cues while observers saw their hand touching the object and thus, knew that they were seeing and feeling the same object. Does this knowledge promote integration even though signals are spatially discrepant which has been shown to lead to a breakdown of integration (Gepshtein et al. in J Vis 5:1013-1023, 2005)? Consistent with the model predictions, observers weighted visual and haptic cues to shape according to their reliability: progressively more weight was given to haptics when visual information became less reliable. Moreover, the integrated visual-haptic estimate was more reliable than either unimodal estimate. These findings suggest that observers integrate visual and haptic shape information of real 3D objects. Thereby, knowledge that multisensory signals arise from the same object seems to promote integration.