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Grouping local elements of the visual environment together is crucial for meaningful perception. While our attentional system facilitates perception, it is limited in that we are unaware of some aspects of our environment that can still influence how we experience it. In this study, the neural mechanisms underlying the Ponzo illusion were examined under inattention and divided-attention conditions using functional magnetic resonance imaging to investigate the brain regions responsible for accessing visual stimuli. A line discrimination task was performed in which two horizontal lines were superimposed on a background of black and white dots that, on occasion, induced the Ponzo illusion if perceptually grouped together. Our findings revealed activation for perceptual grouping in the frontal, parietal, and occipital regions of the brain and activation in the bilateral frontal, temporal, and cingulate gyrus in response to divided attention compared with inattention trials. A direct comparison between grouping and attention showed involvement of the right supramarginal gyrus in grouping specifically under conditions of inattention, suggesting that even during implicit grouping complex visual processing occurs. Given that much of the visual world is not represented in conscious perception, these findings provide crucial information about how we make sense of visual scenes in the world.

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Attentional selection is constrained by object representations (object-based attention) that consist of low-level (e.g., boundaries signaled by closure) and high-level (e.g., semantic category) properties. Whereas low-level information has repeatedly been shown to constrain object-based attention with the use of simple geometric figures, high-level information (such as meaning) has only recently been shown to be an important factor in object-based guidance of attention. Here, we characterize the relative contributions of object boundaries (low-level) and object semantic identity (high-level) to attentional allocation by systematically reducing the contribution from both levels of description. We directly measure the degree to which attentional allocation is flexibly influenced by a combination of these factors. Object-based attentional guidance was observed only when either boundaries or semantic category was preserved, with a larger contribution for preserved semantic category. When both boundary and semantic category were disturbed, object-based influence was reduced. Object-based attentional guidance was therefore more reliant on high-level than low-level properties, suggesting that object-based attention efficiently guides behavior even in naturalistic conditions with real-world objects and environmental fluctuations (e.g., dim lighting, fog, blurry vision).

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