The illusion of a transparent three-dimensional rotating sphere is elicited by a two-dimensional cluster of moving dots. The direction of motion is ambiguous, producing alternating perceptual states. To disambiguate the rotation we presented a haptic rotating sphere additionally to the visual one. The axial orientations of the visual and the haptic sphere were congruent for disambiguation and incongruent for synchronous bimodal stimulation with incongruent direction information. Thereby physical incongruence and perceptual incongruence– the haptic direction of rotation is perceived as being opposite to the visual also the rotation axis is the same –could be examined.
Congruent visual-haptic stimulation resulted in an increased activation of primary sensory areas compared to incongruent visual-haptic stimulation. Activation was also found in higher heteromodal cortical areas such as the superior temporal sulcus (STS), the temporoparietal area (Tpt) and the human motion complex (hMT+). In conclusion, congruent crossmodal stimulation enhanced brain activity in areas attributed to multisensory integration, even though this effect did not emerge as clear in the behavioral data.