title: Sparse regularization techniques provide novel insights into outcome integration processes.
abstract: By exploiting information that is contained in the spatial arrangement of neural activations, multivariate pattern analysis (MVPA) can detect distributed brain activations which are not accessible by standard univariate analysis. Recent methodological advances in MVPA regularization techniques have made it feasible to produce sparse discriminative whole-brain maps with highly specific patterns. Furthermore, the most recent refinement, the Graph Net, explicitly takes the 3D-structure of fMRI data into account. Here, these advanced classification methods were applied to a large fMRI sample (N=70) in order to gain novel insights into the functional localization of outcome integration processes. While the beneficial effect of differential outcomes is well-studied in trial-and-error learning, outcome integration in the context of instruction-based learning has remained largely unexplored. In order to examine neural processes associated with outcome integration in the context of instruction-based learning, two groups of subjects underwent functional imaging while being presented with either differential or ambiguous outcomes following the execution of varying stimulus-response instructions. While no significant univariate group differences were found in the resulting fMRI dataset, L1-regularized (sparse) classifiers performed significantly above chance and also clearly outperformed the standard L2-regularized (dense) Support Vector Machine on this whole-brain between-subject classification task. Moreover, additional L2-regularization via the Elastic Net and spatial regularization by the Graph Net improved interpretability of discriminative weight maps but were accompanied by reduced classification accuracies. Most importantly, classification based on sparse regularization facilitated the identification of highly specific regions differentially engaged under ambiguous and differential outcome conditions, comprising several prefrontal regions previously associated with probabilistic learning, rule integration and reward processing. Additionally, a detailed post-hoc analysis of these regions revealed that distinct activation dynamics underlay the processing of ambiguous relative to differential outcomes. Together, these results show that L1-regularization can improve classification performance while simultaneously providing highly specific and interpretable discriminative activation patterns.
authors.affiliation: Department of Psychology, Technische Universität Dresden, Germany. Electronic address: mohr@psychologie.tu-dresden.de.
authors.forename: Holger
authors.initials: H
authors.lastname: Mohr
authors.affiliation: Department of Psychology, Technische Universität Dresden, Germany.
authors.forename: Uta
authors.initials: U
authors.lastname: Wolfensteller
authors.affiliation: Department of Psychology, Technische Universität Dresden, Germany.
authors.forename: Steffi
authors.initials: S
authors.lastname: Frimmel
authors.affiliation: Department of Psychology, Technische Universität Dresden, Germany.
authors.forename: Hannes
authors.initials: H
authors.lastname: Ruge
mesh_terms: D000328:Adult; D000465:Algorithms; D001921:Brain / Q000502:physiology*; D001931:Brain Mapping / Q000379:methods*; D005260:Female; D006801:Humans; D007090:Image Interpretation, Computer-Assisted; D007091:Image Processing, Computer-Assisted; D007858:Learning / Q000502:physiology; D016014:Linear Models; D008279:Magnetic Resonance Imaging; D008297:Male; D015999:Multivariate Analysis; D059906:Neuroimaging / Q000379:methods; D015203:Reproducibility of Results; D060388:Support Vector Machine; D055815:Young Adult
keywords: Graph Net; Instruction-based learning; MVPA; Outcome integration; Regularization; Structured sparsity