Processing shape, motion and three-dimensional shape-from-motion in the human cortex.

Shape and motion are complementary visual features and each appears to be processed in unique cortical areas. However, object motion is a powerful cue for the perception of three-dimensional (3-D) shape, implying that the two types of information--motion and form--are well integrated. We conducted a series of fMRI experiments aimed at identifying the brain regions involved in inferring 3-D shape from motion cues. For each subject, we identified regions in occipital-temporal cortex that were activated when perceiving: (i) motion in unstructured random-dot patterns, (ii) 2-D and 3-D line drawing shapes, and (iii) 3-D shapes defined by motion cues (shape-from-motion, SFM). We found closely adjacent areas in the lateral occipital region activated by random motion and line-drawing shapes. In addition, we found that the SFM stimuli produced a greater MRI signal in only one of the areas identified with the random motion and line-drawing stimuli: the superior lateral occipital (SLO) region. High-resolution analysis showed that SFM objects and line drawings were processed in separate but adjacent sub-regions in SLO, suggesting that SLO codes object shape but retains topographic segregation based on shape cues. Expanding the analysis to the entire cortex identified a parietal area that had overlapping activation for both SFM and line drawings and increased MRI signal for 3-D versus 2-D shapes, suggesting this area is important for processing shape information.