We sought to identify the neural substrates underlying perceptual grouping, and examined whether the grouping-related neural activities are modulated by task relevance and attention by recording high-density event-related potentials (ERPs). Participants were presented with stimulus arrays, in which local elements were either evenly distributed or grouped into rows or columns by proximity or similarity, and had to discriminate orientations of the perceptual groups, or to identify the colors of dots around stimulus displays or of the fixation cross. We found that proximity grouping was indexed by a positive activity over the medial occipital cortex with a peak latency of about 100 ms after stimulus onset (Pd100), whereas grouping by similarity of shape was reflected in a negative activity with longer latency over the occipito-temporal areas. Dipole modeling based on a realistic boundary element head model localized the Pd100 to the right calcarine cortex. Moreover, we showed that the grouping-related activities were weakened when stimulus arrays were of low task relevance and fell outside an attended area of field. The results suggest that human calcarine cortex is engaged in early grouping operations defined by proximity, reinforcing the previous fMRI findings. Moreover, our ERP results indicate that the neural bases underlying perceptual grouping in human visual cortex can be modulated by task relevance and attention as early as 100 ms after sensory stimulation. Hum Brain Mapp, 2005. (c) 2005 Wiley-Liss, Inc.