Extensive research suggests that autistic individuals have deficits in global visual processing that may cause an attentional bias toward local details. This tendency has also been noted in nonclinical samples with high autistic-like traits, as measured using the autism-spectrum quotient (AQ). However, as top-down attention as an executive control can modulate early visual processing, it is still unclear whether this local processing bias is due to atypicality in bottom-up processing or a top-down attentional set. The present study explored this issue by examining event-related potentials (ERPs) in a sustained focal-attention task that involved bilateral stimulus arrays. In this task, a P1 spatial attention effect (at approximately 100–150 ms post-stimulus) reflects top-down attentional modulation of incoming sensory processing, and an N1 attention effect (150–200 ms) reflects obligatory attention-spreading based on perceptual grouping. The results showed that AQ scores were negatively correlated with the N1 attention effects for conditions in which bilateral stimuli were grouped with feature similarity and amodal completion. This finding supports the view that bottom-up processing in perceptual organization varies with autism spectrum.