TY - THES T1 - The neural bases of regularity learning A1 - Döller,Christian Y1 - 2005/02/16 N2 - The existence of two separate learning and memory systems has been proposed in experimental psychology and cognitive neuroscience: These systems seem to be specialized for complementary functions, namely (1) fast learning unique episodes and (2) gradual learning regularities across multiple episodes (regularity learning). In three experiments, the present thesis aimed at specifying the neural and cognitive bases of regularity learning. Functional magnetic resonance imaging (fMRI) was used as the main method. In all experiments, volunteers had to learn object-position conjunctions in several experimental trials and blocks. The design of the experiments included two conditions, a context-specific (CS) and an invariant learning (IL) condition. In the CS condition, objects and positions were variably mapped across trials. In the IL condition positions (Experiment [Exp] 1A and 3B) or objects (Exp 1B, 2, and 3A) were held constant within blocks, enabling subjects to extract regularities across trials, i.e. invariant positions or objects in object-position conjunctions. In Exp 1A and Exp 1B, performance increased within blocks in the IL condition, but not in the CS condition. In both experiments, hippocampal activity decreased as a function of learning in the IL condition. Conversely, the hippocampus was activated continuously in the CS condition. In contrast to a right-lateralized hippocampal activation decrease during learning spatial regularities (Exp 1A), learning object regularities (Exp 1B) was associated with a decrease of bilateral hippocampal activity. In both experiments, lateral prefrontal and striatal brain regions showed an increase of activity as a function of learning in the IL condition. In Exp 2, the cognitive processes underlying learning object regularities were investigated in more detail. In six behavioral studies, the influence of object distinctiveness (Exp 2A-2B), memory load and learning duration (Exp 2C-2E), and the effect of simultaneous object-position bindings (Exp 2F) on learning invariant objects in object-position conjunctions was examined. The behavioral data indicate that the increase of object distinctiveness mainly affects overall memory performance, whereas manipulation of the learning duration and the memory load affects the learning process. Moreover, a dissociation of two learning mechanisms could be observed: a process operating within learning blocks and a process operating across blocks. Finally, Exp 3 aimed at specifying whether separable brain regions mediate these two learning mechanisms. In contrast to Exp 1A and 1B, trials were blocked by condition to minimize the probability that subjects based their judgment on a common strategy for both conditions. On a behavioral level, subjects showed a performance increase within and across learning blocks in an object IL condition (Exp 3A) and in a spatial IL condition (Exp 3B), but not in the respective CS conditions. On a neural level, within-block learning was associated with a learning-related decrease of hippocampal and a learning-related increase of prefrontal-striatal activity in both experiments. In contrast, distinct prefrontal-striatal regions were selectively involved in across-block learning. To summarize, on a cognitive level regularity learning within the framework of the present thesis can be characterized by the following criteria: (1) the extraction of regularities across input pattern, (2) the gradual nature of the extraction process, (3) the robust maintenance of extracted regularities over time, and (4) the aggregated representational format of the extracted information. On a neural level, the reduced learning-related hippocampal activation presumably reflects lower binding requirements in the IL condition, since variable objects can be bound to invariant positions or vice versa. Furthermore, the imaging results suggest a domain-specific hemispheric specialization of the hippocampus during regularity learning, reflecting hippocampal sensitivity to perceptual stimulus attributes of invariant episodic features (bottom-up mechanism). The learning-related prefrontal modulation seems to reflect the requirement to extract and maintain regularities across trials and the adjustment of object-position conjunctions on the basis of the extracted knowledge, possibly mediated by rule-like prefrontal representations (top-down mechanism). Finally, the striatum might encode the increased predictability of invariant features as a function of learning and possibly provides an internal reinforcement signal to the PFC. In conclusion, the present results provide new insights into the neural bases of regularity learning and point to a transition of the relative roles of distinct neural systems during the time-course of regularity learning, i.e. learning is accompanied by a shift from a hippocampal to a prefrontal-striatal brain system. KW - Neocortex KW - Hippocampus KW - Lernen KW - Gedächtnis CY - Saarbrücken PB - Saarländische Universitäts- und Landesbibliothek AD - Postfach 151141, 66041 Saarbrücken UR - http://scidok.sulb.uni-saarland.de/volltexte/2005/428 ER -