Functional imaging of visual semantic processing in the human brain

Previous neuroimaging studies have identified a large network of cortical a reas involved in semantic processing in the human brain, which includes lef t occipito-temporal and infero-frontal areas. Most studies, however, invest igated exclusively the associative/functional semantic knowledge by using m ainly words and/or language related tasks, and this factor may have contrib uted to the large left hemisphere superiority found in semantic processing and to the controversial involvement of left prefrontal structures. The pre sent study investigates the neural basis of visual objects knowledge, acces sed exclusively through pictorial information. Regional cerebral blood flow (rCBF) was assessed using positron emission tomography (PET) during 3 cond itions in right-handed normal volunteers: resting with eyes closed, retriev al of semantic information related to visual properties of objects (real si ze), and visual categorization based on physical properties of the image. C onfirming previous experiments and neuropsychological findings, most activa tions were found in left occipito-temporal areas during retrieval of visual semantic knowledge. The absence of any activation in the left prefrontal i nferior cortex for visual semantic processing confirms recent observations which suggest that this region would not be involved in retrieval of visual semantic knowledge from living entities. Rather, such knowledge about visu al properties of objects, situated closely to cortical regions mediating pe rception of the visual attributes, can be retrieved directly from these reg ions when visual images are used as entry level stimuli.