Global shape integration and illusory form perception in the absence of awareness

Previous research on perceptual organization operations still provides contradictory evidence on whether the integration of sparse local elements into coherently unified shapes and the construction of the illusory form are accomplished without the need of awareness. In the present study, three experiments were conducted in which participants were presented with masked (Experiment 1, SOA = 27 ms; Experiment 2; SOA = 53 ms) and unmasked (Experiment 3) primes consisting of geometric shapes (a square or a diamond) that could be congruent or incongruent with subsequent probe stimuli (square vs. diamond). Furthermore, the primes were divided into: a grouping condition (where local elements may group together into global shapes), an illusory condition (where the arrangement of local elements produced illusory shapes) and a hybrid condition (where both operations were presented simultaneously). While no priming effects were found for the shortest SOA (27 ms), both grouping and illusory primes produced significant priming effects in the longer SOA (53 ms). On the other hand, results in Experiment 3 (unmasked) showed strong priming effects for the grouping of the inducers in both the grouping and the hybrid conditions, and also a significant but weaker priming effect for the illusory condition. Overall, our results support the possibility of the integration of local visual features into a global shape in the absence of awareness and, likewise, they suggest an early subliminal construction of the illusory shape, implying that feedback projections from higher to lower visual areas are not crucial in the construction of the illusory form.