SWITCHING ATTENTION WITHOUT SHIFTING THE SPOTLIGHT - OBJECT-BASED ATTENTIONAL MODULATION OF BRAIN POTENTIALS

Although psychophysical evidence for object-based attention has been r eported, corresponding studies with event-related potentials (ERPs) ar e scarce. Here subjects were presented with perceptual fields containi ng two superimposed objects (transparent surfaces generated by two set s of dots in rigid rotation around fixation, each set of a different c olor and direction of motion) or only one object (the same dots but ei ther at rest or all rotating in the same direction). Brief (150-msec) rectilinear displacements affected either of the sets at random ISIs o f 350 to 550 msec. Attention was directed to one set of dots, guided b y color, in order to discriminate the direction of their displacement. Motion-onset ERPs elicited by these displacements were compared for a ttended and unattended dots. When the perceptual field consisted of tw o objects, strong suppression of P1 and N1 was obtained in the ERPs as sociated with the unattended object. No suppression was found with the field containing a single object, although an enhanced selection nega tivity was found in ERPs associated with attended dots (selected by co lor). Since the two objects occupied the same region of visual space, the suppression of P1/N1 cannot be explained by the space-based mechan isms but is consistent with object-based attentional selection at earl y stages of vision. The results highlight the role of perceptual organ ization in enabling alternative attentional mechanisms.