Visuo-haptic integration process during object size discrimination: an fMRI study

When exploring objects during every day activities, visual and haptic cues provide information about their properties such as size, shape and texture. How these two streams of sensory information are integrated by the brain to form a single percept is still not fully understood. To better understand this process, the study investigated the cortical activation patterns that underlie object size perception based on unimodal visual or haptic information, and also when both forms of information were available. Functional MRI (fMRI) data were obtained from 12 healthy participants in a size perception task using visual and haptic stimuli inside a 3T MRI scanner. In each trial, a rectangular object of varying heights was compared against a reference object and participants verbally indicated which object was perceived as taller. Unimodal and bimodal stimulus presentations were tested. Correct response rates and size discrimination thresholds (DT) of 75% perceived as taller were then calculated. Activation volumes were derived for contrast analysis. Cortical regions with superadditive blood oxygen level dependent signal responses were interpreted as multisensory integration areas for visuo-haptic size perception. No significant differences were found in both correct response rates and DTs between unimodal and bimodal conditions. However, fMRI data showed superadditive responses in dorsolateral prefrontal cortex, supplementary motor area and inferior parietal cortices, which were associated with bimodal visuo-haptic size discrimination. To our knowledge, this is the first direct comparison of unimodal and bimodal visuo-haptic size perception that combined fMRI and psychophysical data to characterize sites of multisensory integration. Our finding showing involvement of the fronto-parietal network in visuo-haptic processing challenges a previous notion of the lateral occipital complex as the predominant locus of visuo-haptic integration. Our results also suggest that the underlying mechanism related to visuo-haptic perceptual processing seems to be task dependent.