Introduction Complex hierarchical structures play a central role in many aspects of human cognition, prominently including both language and music [1]. A promising method to represent complex hierarchical structures - realized in nature and attractive for experimental research - is recursion. Recursion is a self-referential process by which infnite use can be made of fnite means [2]. In this study, we addressed hierarchy in the visual domain, using a novel paradigm based on fractal images [2]. We compared brain activity across three different tasks: a similarity task, and two hierarchical tasks in which subjects were asked to recognize the repetition of a rule operating transformations either within an existing hierarchical le- vel (non-recursive), or generating new hierarchical levels (recursive). Similar hierarchical images were generated by both rules (Fig.1). Methods Fourty subjects (19 male, aged 20-32y) participated in this study. Functional images were acquired on a 3 Tesla TIM Trio system (Siemens, Erlangen, Germany) using single-shot gradient-echo EPI. 36 slices (2.7 mm thickness and 20% gap) were aligned AC-PC with a matrix size of 128x128, a FOV of 220 and TE/TR of 32/2200 ms. To in- vestigate the neural circuits associated with the representation of non-recursive and recursive rules, we adapted the Visual Recursion Task and the Embedded Iteration Task as described in [4]. Image processing performed using SPM8 including slice-time correction, correction for motion, spatial normalization and smoothing. Task related activations were individually modeled and then submitted to a repeated measure GLM. Results Our main fndings were the following: 1) Both hierarchical tasks (recursive and non-recursive) activated a bilateral network (the dorsal stream) which includes visual association areas and fronto-parietal circuits associated with spatial reasoning (Fig.2); 2) Compared to non-recursive processes, the representation of self-similarity rules generating new hierarchical levels recruited regions within the parieto-medial temporal pathway - including the posterior cingulate cortex and retrosplenial cortex - and their projections to the medial temporal cortex, which have been associated with the integration of spatial and categorical information [3]. We also found activations in the anterior portions of superi - or and middle temporal gyri (Fig.3); 3) In contrast, within-level non-recursive rules activated more strongly the dorsal stream, and the dorsal fronto-parietal network (Fig.3). Conclusions In this study [4] we investigated the brain circuits active during the discrimination of hierarchies generated by rule-based processes. We contrasted a rule that adds elements to fxed hierarchical levels with a rule that generates new sel f-similar levels. We found that within-level hierarchical transformations activate to a greater extent regions involved in visuo-spa - tial processing [3]. On the other hand, processes that generate new self-similar hierarchical levels activate neural circuits involved in the integration of objects in contexts, and in semantic processing [3, 5, 6]. The generation of self-similar hierar - chies in the visuo-spatial domain seems to require the integration of the ‘where’ information with the ’what’ information. References [1] Fitch, W. T., & Friederici, A. D. (2012). Artifcial grammar learning meets formal language theory: An overview. Philo- sophical Transactions of the Royal Society B: Biological Sciences, 367(1598), 1933-1955. [2] Martins, M. D. (2012). Distinctive signatures of recursion. Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 367(1598), 2055–64. [3] Kravitz, D. J., Saleem, K. S., Baker, C. I., & Mishkin, M. (2011). A new neural framework for visuospatial processing. Nature reviews. Neuroscience, 12(4), 217–30. [4] Martins M. J., Fishmeister, F., Puig Waldmüller, E., Oh, J., Geissler, A., Fitch W. T., Beisteiner R (2014). Fractal Image Perception provides Novel Insights into Hierarchical Cognition. NeuroImage, 96, 300-308. [5] Ranganath, C., & Ritchey, M. (2012). Two cortical systems for memory-guided behaviour. Nature Reviews Neuro- science, 13(10), 713-726. [6] Binder, J. R., Desai, R. H., Graves, W. W., & Conant, L. L. (2009). Where is the semantic system? A critical review and meta-analysis of 120 functional neuroimaging studies. Cerebral Cortex, 19(12), 2767-2796. 1 Department of Cognitive Biology, University of Vienna, Vienna, Austria; 2 Study Group Clinical fMRI, Department of Neurology, Medical University Vienna, Vienna, Austria; 3 High Field MR Centre of Excellence, Medical University of Vienna, Vienna, Austria Mauricio Martins 1 , Florian Ph.S Fischmeister 2,3 , Estella Puig-Waldmueller 1 , Jinook Oh 1 , Alexander Geißler 3 , Roland Beisteiner 2,3 , Tecumseh Fitch 1 Discrimination of self-similar visual hierarchies activates the parieto-medial temporal pathway Contact: Mauricio Martins, mauricio.martins@univie.ac.at Department of Cognitive Biology, University of Vienna, Vienna, Austria Figure 1: Examples of Visual Recursion Task (VRT) stimuli (showing cross-level transformations), Embedded Iteration Task (EIT) stimuli (showing within-level transformations), and PSVT (Positional Similarity Visual Task) stimuli (a simple matching to sample baseline). Figure 1: Brain activations specifc for both recursive (cross-level) and non-recursive (within-level) hierarchical tasks, in comparison with a simple similarity baseline. Results are presented at P < 0.05 with FDR correction. Figure 3: Brain activation contrast between recursive (cross-level; shown in red) and non-recursive (within-level; shown in blue) hierarchical tasks. Results are presented at P < 0.05 with FDR correction. This study was supported by the Austrian Science Fund (P23611) This work was supported by the research cluster grant “Shared Neural Resources for Music and Language” (University of Vienna and Medical University of Vienna). ense. F1000 Posters: Use Permi Commons License. F1000 Posters: Use Permitted under Cre der Creative Commons License. F1000 Posters: Use Permitted under Creative Commo Permitted under Creative Commons License. F1000 Posters: Use Permitted under Creative Commons Licen Posters: Use Permitted under Creative Commons License. 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