fMRI was used to investigate the neural substrates supporting implicit and explicit sequence learning, focusing especially upon the role of the medial temporal lobe. Participants performed a serial reaction time task (SRTT). For implicit learning, they were naive about a repeating pattern, whereas for explicit learning, participants memorized another repeating sequence. fMRI analyses comparing repeating versus random sequence blocks demonstrated activation of frontal, parietal, cingulate, and striatal regions implicated in previous SRTT studies. Importantly, mediotemporal lobe regions were active in both explicit and implicit SRTT learning. Moreover, the results provide evidence of a role for the hippocampus and related cortices in the formation of higher order associations under both implicit and explicit learning conditions, regardless of conscious awareness of sequence knowledge.

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Event-related potentials (ERPs) were used to delineate the time course of activation of the processes and representations supporting visual object identification and memory. Following K. Srinivas (1993), 66 young people named objects in canonical or unusual views during study and an indirect memory test. Test views were the same or different from those at study. The first ERP repetition effect and earliest ERP format effect started at approximately 150 msec. Multiple ERP repetition effects appeared over time. All but the latest ones were largest for same views, although other aspects of their form specificity varied. Initial ERP format effects support multiple-views-plus-transformation accounts of identification and indicate the timing of processes of object model selection (frontal N350 from 148-250 to 500-700 msec) and view transformation via mental rotation (posterior N400/P600 from 250-356 to 700 msec). Thereafter, a late slow wave reflects a memory process more strongly recruited by different than same views. Overall, the ERP data demonstrate the activation of multiple memory processes over time during an indirect test, with earlier ones (within 148-400 msec) characterized by a pattern of form specificity consistent with the specific identification-related neural process or representational system supporting each memory function.

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Abstract& Categorization of visual objects entails matching a percept to long-term representations of structural knowledge. This object model selection is central to theories of human visual cognition, but the representational format(s) is largely unknown. To characterize these neural representations, eventrelated brain potentials (ERPs) to fragmented objects during an indirect memory test were compared when only local contour features, but not global shapes of the object and its parts, differed between encoding and retrieval experiences. The ERP effects revealed that the format of object representations varies across time according to the particular neural processing and memory system currently engaged. An occipitotemporal P2(00) showed implicit memory modulation to items that repeatedly engaged similar perceptual grouping processes but not items that merely reinstantiated visual features. After 500 msec, memory modulation of a late positive complex, indexing secondary categorization and/or explicit recollection processes, was sensitive to local contour changes. In between, a frontocentral N350, indexing the model selection and an implicit perceptual memory system, showed reactivation of object representations whenever the same global shapes were reactivated, despite local feature differences. These and prior N350 findings provide direct neurophysiological evidence that the neural representations supporting object categorization include knowledge beyond local contours and about higher-order perceptual structures, such as the global shapes of the object and its parts, that can differ between object views. The N350 is proposed to index a second state of interactive, recurrent, and feedback processing in occipital and ventral temporal neocortex supporting higherorder cognitive abilities and phenomenological awareness with objects. &