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Mitsouko van Assche, Anne Giersch; Creating links in empty space: an fMRI study of perceptual organization. Journal of Vision 2010;10(7):1202. doi: https://doi.org/10.1167/10.7.1202.
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When faced to the need of comparing two objects at the same time, information selection can be modulated by the presence of grouping factors. Automatically processed grouping cues enable effortless selection of groups of objects. On the other hand, selecting non-automatically grouped objects implies the creation of mental links between them through top-down processes. To explore the neural basis of bottom-up grouping versus top-down grouping, 16 participants were tested in a variant of the Repetition Discrimination Task (Beck & Palmer, JEP:HPP 2002) in a fMRI experiment. Circles and squares were presented in spatial alternation except for two figures, the target pair (i.e., two contiguous figures that were identical, either two squares or two circles), around a central fixation point. Contiguous figures could be linked by a connector (within-group pair) or not (between-group pair), and located within the same or in separate hemifields. Participants had to determine the identity of the target pair (i.e., circles or squares). Two blocks incited subjects to prioritize either target type, by manipulating the proportion of within-group and between-group trials. Each block was followed by a series of trials with equivalent proportion of within- and between-group trials, with an event-related design. Continuous eye movements recording in the scanner ensured to check correct central fixation. The behavioural data show a cost of RT for between-group compared to within-group, and reproduce earlier results (Van Assche, Gos, Giersch, Vis Cogn 2008). Frontal and parieto-occipital areas were more recruited to identify between-group compared to within-group targets, especially when in separate hemifields. For between-group pairs, supplementary internal temporal activations were observed after inciting to prioritize between-group compared to within-group pairs. The results are discussed in terms of the building of a hierarchical representation superimposing between-group on within-group pairs.
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