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The hippocampus is critical for encoding and retrieving semantic and episodic memories. Animal studies indicate that the hippocampus is also required for relational learning tasks. A prototypical relational learning task, and the one investigated in this experiment, using event-related functional magnetic resonance imaging, is the transitive inference (TI) task. In the TI task, participants were to choose between A and B (A?B) and learned by trial and error to choose A (A > B). There were four such premise pairs during a training (A > B, B > C, C > D, D > E). These can be acquired distinctly or can be organized into a superordinate hierarchy (A > B > C > D > E), which would efficiently represent all the learned relations and allow inferences (e.g., B > D). At test there was no reinforcement: In addition to premise pairs, untrained pairings were introduced (e.g., A?E, B?D). Correctly inferring that B > D is taken as evidence for the formation of a superordinate hierarchy; several alternatives to the superordinate hierarchy hypothesis are considered. Awareness of the formation of this hierarchy was measured by a postscan questionnaire. Four main findings are reported: (1) Inferential performance and task awareness dissociated behaviorally and at the level of hemodynamic response; (2) As expected, performance on the inferred relation, B > D, corresponded to the ability to simultaneously acquire B > C and C > D premise pairs during training; (3) Interestingly, acquiring these "inner pairs" corresponded to greater hippocampal activation than the "outer pairs" (A > B, D > E) for all participants. However, a distinct pattern of hippocampal activity for these inner pairs differentiated those able to perform the inferential discrimination, B > D, at test. Because these inner premise pairs require contextual discrimination (e.g., C is incorrect in the context of B but correct in the context of D), we argue that the TI task is hippocampal-dependent because the premise pair acquisition necessary for inference is hippocampal-dependent; (4) We found B > D related hippocampal activity at test that is anatomically consistent with pre-consolidation recall effects shown in other studies.

A central role of the hippocampus is to consolidate conscious forms of learning and memory, while performance on implicit tasks appears to depend upon other structures. Recently, considerable debate has emerged about whether hippocampal-dependent tasks necessarily entail task awareness. In the contextual cueing task, repetition facilitation is implicit, but impaired in patients with amnesia. Whether the hippocampus alone or other MTL structures are required is unclear. Event-related functional magnetic resonance imaging revealed hippocampal activity that differentiates novel from repeated arrays. This pattern of results was observed without recognition of the repeating arrays. This finding provides support for the claim that the hippocampus is involved in processes outside the domain of conscious learning and memory. Hippocampal differentiation without recognition: An fMRI analysis of the contextual cueing taskIt has long been understood that damage to the hippocampus and medial temporal lobe structures (MTL) in humans diminishes or eliminates the capacity to form new long-term episodic (autobiographical) and semantic (knowledge) memory (Milner 1972;Takashima et al. 2006). Together, episodic and semantic memory are termed declarative memory (Cohen et al. 1985) because they both require conscious or deliberative access. Conversely, numerous studies demonstrate implicit forms of learning and memory that are not substantially affected by damage to the hippocampus or MTL (Keane et al. 1995;Stark and Squire 2000). Implicit forms of learning and memory are demonstrated by experience-dependent changes in task performance and do not require conscious recollection. Examples of implicit forms of learning and memory include perceptual priming or facilitation (repetition leads to greater accuracy and shorter response latency), procedural or skill learning, and simple forms of classical conditioning. The most common interpretation of this evidence is that distinct systems mediate declarative and implicit forms of learning and memory (Squire and Zola 1996;Cohen et al. 1997). Accordingly, the hippocampus would be critically involved in learning and memory if, and only if, conscious awareness of the contingencies occurs (Clark and Squire 1998;Reed and Squire 1999;Manns and Squire 2001;Smith et al. 2006).While there is no meaningful dispute that the hippocampus is required for conscious learning and memory formation, it may serve a broader function. Several studies have been recently published suggesting that the hippocampus is also implicated in certain implicit tasks (Chun and Phelps 1999;Ryan et al. 2000;Greene et al. 2006). However, for some of these tasks there is controversy about whether they are indeed implicit tasks (Smith and Squire 2005;Smith et al. 2006;Greene 2007), while for others there is controversy as to whether the task depends upon the hippocampus (Manns and Squire 2001).In the contextual cueing task (Chun and Jiang 1998), context-dependent target search is impaired in MTL amnesics but does not depend upon re...

This study assessed whether stationary auditory information could affect body and head sway (as does visual and haptic information) in sighted and congenitally blind people. Two speakers, one placed adjacent to each ear, significantly stabilized center-of-foot-pressure sway in a tandem Romberg stance, while neither a single speaker in front of subjects nor a head-mounted sonar device reduced center-of-pressure sway. Center-of-pressure sway was reduced to the same level in the two-speaker condition for sighted and blind subjects. Both groups also evidenced reduced head sway in the two-speaker condition, although blind subjects' head sway was significantly larger than that of sighted subjects. The advantage of the two-speaker condition was probably attributable to the nature of distance compared with directional auditory information. The results rule out a deficit model of spatial hearing in blind people and are consistent with one version of a compensation model. Analysis of maximum cross-correlations between center-of-pressure and head sway, and associated time lags suggest that blind and sighted people may use different sensorimotor strategies to achieve stability.

Classic findings from the neuropsychological literature invariably indicated that performances on tests of memory that can be accomplished without conscious awareness were largely spared in amnesia, while those that required conscious retrieval (e.g., via recognition or recall) of information learned in the very same sessions was devastatingly impaired. Based on reports of such dissociations, it was proposed that one of the fundamental distinctions between memory systems is whether or not they support conscious access to remembered content. Only recently have we come to realize that the putative systemic division of labor between conscious and unconscious memory is not so clean. A primary goal of this review is to examine recent evidence that has been advanced against the view that the hippocampus is selectively critical for conscious memory. Along the way, consideration is given to criticisms that have been levied against these findings, potential explanations for differences in the reported results are proposed, and methodological pitfalls in investigations of unconscious memory are discussed. Ultimately, it is concluded that a tipping point has been reached, and that while conscious recollection depends critically on hippocampal integrity, the reach of the hippocampus extends to unconscious aspects of memory performance when relational memory processing and representation are required.