Imaging visual recognition: PET and fMRI studies of the functional anatomy of human visual recognition.

Until recently, the neural bases of visual object recognition in humans could be studied only by the use of brain-damaged subjects with naturally occurring lesions. Functional neuroimaging has given us the capability of studying visual recognition in the normal human brain. In the past ten years a number of PET and fMRI studies have attempted to isolate the neural substrates of human visual recognition. We have reviewed these studies and compared their conclusions regarding the anatomical locations of visual recognition processing in the human brain. The outcome was disappointing, revealing a wide range of locations. Our attempts to reduce the scatter by subgrouping the studies according to different task and stimulus properties were not successful. We discuss possible reasons for the lack of agreement among studies, including differences in the kinds of information yielded by lesion and imaging studies, and issues in the design and analysis of functional neuroimaging experiments. We conclude with a review of a more recent approach to the neuroimaging of human visual recognition, in which the effects of recognizing different types of visual stimuli are compared directly. With these experimental designs neuroimaging yields more replicable results, which also accord better with the known effects of lesions.