Attention may select objects or perceptual groups as fundamental units. Previous studies with event-related potentials (ERPs) have found that obligatory attention-spreading over spatial regions within stable objects is associated with intermediate feedforward visual processing, as reflected by the posterior N1 component of the ERP at a latency of 140-180 ms. The present study examined object-based spatial attention effects in response to individual objects, by recording lateralized ERP attention effects over the posterior scalp (i.e., contralateral versus ipsilateral to the attended visual fields). The stumuli were bilateral unfilled line objects with minimal figural enhancement, and their connectedness and the difficulty of the perceptual task were manipulated. The effects of spatial attention on successive ERP components (N1, P2, and N2) provided information on the timing of the different stages of processing that underlie the formation of perceptual objects.