Perceptual differentiation and category effects in normal object recognition - A PET study

The purpose of the present PET study was (i) to investigate the neural corr elates of object recognition, i.e, the matching of visual forms to memory, and (ii) to test the hypothesis that this process is more difficult for nat ural objects than for artefacts. This was done by using object decision tas ks where subjects decided whether pictures represented real objects or non- objects. The object decision tasks differed in their difficulty (the degree of perceptual differentiation needed to perform them) and in the category of the real objects used (natural objects versus artefacts). A clear effect of task difficulty was found in both the behavioural and in the PET data. In the PET data, the increase in task difficulty was associated with increa sed regional cerebral blood flow in the posterior part of the right inferio r temporal gyrus and in the anterior part of the right fusiform gyrus, This may be the neural correlate of matching visual forms to memory, and the am ount of activation in these regions may correspond to the degree of percept ual differentiation required for recognition to occur. With respect to beha viour, it took significantly longer to make object decisions on natural obj ects than on artefacts in the difficult object decision tasks. Natural obje cts also recruited larger parts of the right inferior temporal and anterior fusiform gyri compared with artefacts as task difficultly increased. Diffe rences in the amount of activation in these regions may reflect the greater perceptual differentiation required for recognizing natural objects. These findings are discussed in relation to category-specific impairments after neural damage.