Title: A functional role for dorsal cortex in coding the physical size of real familiar objects
In recent years, the notion that perceptual processes are situated within the ventral stream has come under increasing scrutiny following the discovery of object-selective representations in posterior parietal cortex, within the dorsal pathway. Previous attempts to isolate the functional role of dorsal cortex on perception have been hampered by possible contamination from ventral object areas. Here, we studied object recognition in a patient with profound visual form agnosia (D.F.) resulting from bilateral lesions of the lateral occipital and ventral-temporal cortices. D.F. is severely impaired in her ability to recognize images of objects and she shows no neural responses to images of objects as measured using fMRI. Like many other patients with agnosia, D.F. shows striking improvements in her ability to recognize objects when the stimuli are presented as real-world exemplars, compared to 2D images of objects –an intriguing phenomenon known as the ‘Real Object Advantage’ (ROA). Although the ROA has been attributed to the additional depth information that is present in real objects versus 2D images, it is also the case that depth cues specify to the perceiver the absolute distance of a real object, and therefore its physical size (whereas in the case of image vision, depth cues specify only the distance of the projection surface, and the physical size of the depicted object is unknown). We show that the ROA in patient D.F. emerges only when the physical size of an object matches its typical real-world size. Recognition of size-incongruent stimuli was comparable to that of 2D pictures. Recognition of matched 2D images of the same objects was not influenced by size congruency. Finally, we show that a ‘ROA’ for size-congruent real objects (but not 2D images) is also evident in neurologically healthy observers. Our results demonstrate a functional role for dorsal cortex in coding the physical size of familiar objects. Dorsal object areas might be especially tuned to physical size and distance in the service of planning and executing object-directed actions with objects.