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. Specifically, there is still a debate which regions of the brain are activated
during visuo-haptic perception and whether one can identify areas that are uniquely
activated during visuo-haptic integration, but not during unimodal perception. Previous
research indicated that the lateral occipital complex (LOC) might be such a region. In an
attempt to fill this knowledge gap, this study investigated the cortical activation patterns
that underlie object size perception based unimodal visual or haptic information, and
when both forms of information were available (bimodal).
Brain imaging data were obtained from 12 healthy participants in a size perception
task using visual and haptic stimuli. In each trial, a 6 cm reference object was judged
against a comparison object (heights: 5.2-6.8 cm) and participants verbally indicated
which object was perceived as taller. Unimodal and bimodal stimulus presentations were
tested. Based on their responses size discrimination thresholds (DT) at the 75% correct
response rate were obtained. In addition, functional brain activation volumes were
derived during each condition for contrast analysis.
Thresholds and associated correct response rates were not significantly different
between unimodal and bimodal conditions. Bimodal visuo-haptic size discrimination was
associated with super-additive activation in the dorsolateral prefrontal cortex (DLPFC),
supplementary motor area (SMA) and inferior parietal cortices, while the LOC revealed
no significant activation. In addition, during crossmodal stimulation the left SMA and
primary sensorimotor cortex revealed significant activations when the visual cue was
presented first. That is, during crossmodal visuo-haptic object perception having a prior
visual experience activates different areas than having a prior haptic experience of the object. To our knowledge, this is the first direct comparison of unimodal and bimodal visuohaptic
size perception that combined brain imaging and psychophysical data to identify sites of multisensory integration. The finding of a fronto-parietal network involved in
visuo-haptic processing challenges a previous notion of the LOC as the sole locus of
visuo-haptic integration. Our results suggest that the underlying visuo-haptic perceptual
process seems to be task dependent.
University of Minnesota Ph.D. dissertation. March 2013. Major: Kinesiology. Advisor: Dr. Jürgen Konczak. 1 computer file (PDF); v, 41 pages.
Visuo-haptic integration process during object size discrimination: an fMRI study.
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