IMPLICIT AND EXPLICIT CATEGORIZATION OF NATURAL SCENES

Previous studies have shown that pronounced posterior selection negativity and enlarged late positive potential amplitude are indices of selective attentional processing in explicit categorization tasks. More recently several studies have revealed that in a passive viewing condition emotional stimuli elicit less early posterior positivity and larger late positive potential (LPP) then neutral images, possibly reflecting an implicit categorization process. The aim of present study was to examine if an explicit categorization task affects emotional modulation of the ERPs. Participants were asked to performa categorization task, where in one condition the target was human and in the other condition the target was animal. The stimuli involved three categories: animal, distractor (objects), and human (erotic couples, neutral people, mutilated bodies). Each picture was presented centrally for 30 ms (ITI: 2 – 3 s). Dense sensor event-related brain potentials were measured. Results showed that the early occipito-temporal ERP component was modulated by picture content but was not affected by the task. On the other hand, the LPP showed that emotionally arousing pictures determined larger LPP than neutral stimuli regardless of target instructions but in the human target condition, pleasant and neutral pictures elicited larger LPP than when the same pictures were distractors. To summarize, our data suggest that visual categorization of emotional stimuli involves different mechanisms with different time courses: an early task-independent mechanism, followed by a task related process.