The Impact of Density and Ratio on Object-Ensemble Representation in Anterior-Medial Ventral Visual Cortex

摘要

Behavioral research has demonstrated that observers can extract summary statistics from large collections, or ensembles, of objects. We recently demonstrated that anterior-medial ventral visual cortex, along the collateral sulcus and parahippocampal gyrus and overlapping with the scene-sensitive parahippocampal place area, exhibited fMRI-adaptation whenever object-ensemble statistics repeated (i.e., adaptation across different photographs depicting the same ensemble). To understand the nature of this neural ensemble representation, here we investigated the encoding of object density in an ensemble. Observers viewed sequences of different photographs depicting the same ensemble with either fixed or varying density (achieved by changing the spacing between objects). For comparison, we presented sequences of identical photographs and sequences of photographs depicting different ensembles. In anterior-medial ventral visual cortex, compared to the different ensemble condition, we observed fMRI-adaptation whenever ensemble features repeated, even when density varied. This suggests that object density is not part of the neural ensemble representation in this brain region, possibly because such density manipulations may be seen as accidental, rather than reflecting an intrinsic property of ensembles. But perhaps the relative density of different types of objects in an ensemble holds greater ecological significance in everyday visual perception. To test this idea, we conducted a second experiment, and instead of varying density by varying spacing, we varied relative density by varying the ratio of two types of objects comprising an ensemble. Interestingly, we observed a release from adaptation with this ratio change. Taken together, these results indicate that while anterior-medial ventral visual cortex is insensitive to density changes resulting from changes in spacing, it does code the proportion, or relative density, of different objects comprising an ensemble. Thus object-ensemble processing in anterior-medial ventral visual cortex seems to depend on high-level information, such as the ratio of ensemble elements, rather than low-level information, such as spacing/spatial frequency.