Representations of real objects and pictures in the dorsal and ventral visual streams differ based on physical stimulus distance

The human visual system is thought to be divided into two anatomically and functionally distinct pathways: a ventral pathway that subserves object perception and a dorsal pathway that subserves goal-directed action and spatial vision. Most fMRI studies of visual perception use stimuli in the form of two-dimensional images projected onto a mirror. But because such stimuli neither afford action nor occupy a definite egocentric location, they may drive cortical responses differently than physical stimuli placed within the observer’s grasp –especially in dorsal cortex. Here, we presented participants in the scanner with meaningful real objects (tools) and closely matched, printed 2-D pictures of those objects. The stimuli were placed either within reach of participants’ hands or beyond their reach. On each trial, participants performed a 1-back action-matching task in which they evaluated but did not manually interact with the stimuli. Univariate contrasts identified regions in both ventral and dorsal cortex that responded more strongly to real objects than to pictures, and more strongly to within-reach than out-of-reach stimuli. These areas overlapped extensively with regions that showed a significant interaction effect, where stimulus reachability modulated fMRI activation more strongly for real objects than it did for pictures –arguing against explanations based on retinal size. Notably, representational similarity analysis showed that whereas ventral regions represented object shape under both distance conditions, dorsal regions did so only when stimuli were within reach. These results suggest that stimuli that lack a definite egocentric position may fail to capture the dorsal stream’s involvement in object perception because of qualitative differences in these regions’ representations of nearby objects. Distance from the observer appears especially consequential in the context of real, naturalistic stimuli, perhaps because of their relevance for grasping and interaction.