Activation time course of responses to illusory contours and salient region: a high-density electrical mapping comparison.

Until recently, early visual areas (V1/V2) were believed to respond mainly to illusory contours (ICs). At present, however, functional neuroimaging suggests that the human lateral occipital complex (LOC), a higher tier than V2, responds strongly to ICs and that IC-related activation in V1/V2 in fact might be driven by feedback input from the LOC. When Kanizsa-type ICs are modified by rounding the corners of the inducers and misaligning them slightly, the impression of an enclosed salient region (SR) remains, although ICs no longer are perceived. Stanley and Rubin (Stanley, D.A., Rubin, N., 2003. fMRI activation in response to illusory contours and salient regions in the human lateral occipital complex. Neuron, 37, 323-331) found that the LOC responded to SR, suggesting that the LOC subserves a rapid but crude region-based segmentation process preceding boundary completion in V1/V2. The present study compared the time course of cortical responses to ICs with those to SR using high-density (74-channel) event-related potentials (ERPs). Scalp mapping and statistical analysis indicated that shared negative modulation for ICs and SR was distributed bilaterally over the lateral occipital scalp at a latency of 70 to 180 ms. Slightly later, a weak negative modulation occurred with ICs but not SR at the occipital pole scalp from 170 to 180 ms. Dipoles for early and late modulations were fitted optimally in the LOC and occipital pole, respectively. The present results suggested that IC-related cortical activation could be separated into region-based segmentation and subsequent boundary completion.