A primary function of the medial temporal lobe (MTL) is to signal prior encounter with behaviorally relevant stimuli. MTL match enhancement—increased activation when viewing previously encountered stimuli—has been observed for goal-relevant stimuli in nonhuman primates during delayed-match-to-sample tasks and in humans during more complex relational memory tasks. Match enhancement may alternatively reflect (a) an attentional response to familiar relative to novel stimuli or (b) the retrieval of contextual details surrounding the past encounter with familiar stimuli. To gain leverage on the functional significance of match enhancement in the hippocampus, high-resolution fMRI of human MTL was conducted while participants attended, ignored, or passively viewed face and scene stimuli in the context of a modified delayed-match-to-sample task. On each “attended” trial, two goal-relevant stimuli were encountered before a probe that either matched or mismatched one of the attended stimuli, enabling examination of the consequences of encountering one of the goal-relevant stimuli as a match probe on later memory for the other (nonprobed) goal-relevant stimulus. fMRI revealed that the hippocampus was insensitive to the attentional manipulation, whereas parahippocampal cortex was modulated by scene-directed attention, and perirhinal cortex showed more subtle and general effects of attention. By contrast, all hippocampal subfields demonstrated match enhancement to the probe, and a postscan test revealed more accurate recognition memory for the nonprobed goal-relevant stimulus on match relative to mismatch trials. These data suggest that match enhancement in human hippocampus reflects retrieval of other goal-relevant contextual details surrounding a stimulus's prior encounter.

The essential role of the medial temporal lobe (MTL) in long-term memory for individual events is well established, yet important questions remain regarding the mnemonic functions of the component structures that constitute the region. Within the hippocampus, recent functional neuroimaging findings suggest that formation of new memories depends on the dentate gyrus and the CA 3 field, whereas the contribution of the subiculum may be limited to retrieval. During encoding, it has been further hypothesized that structures within MTL cortex contribute to encoding in a content-sensitive manner, whereas hippocampal structures may contribute to encoding in a more domain-general manner. In the current experiment, high-resolution fMRI techniques were utilized to assess novelty and subsequent memory effects in MTL subregions for two classes of stimuli—faces and scenes. During scanning, participants performed an incidental encoding (target detection) task with novel and repeated faces and scenes. Subsequent recognition memory was indexed for the novel stimuli encountered during scanning. Analyses revealed voxels sensitive to both novel faces and novel scenes in all MTL regions. However, similar percentages of voxels were sensitive to novel faces and scenes in perirhinal cortex, entorhinal cortex, and a combined region comprising the dentate gyrus, CA 2 , and CA 3 , whereas parahippocampal cortex, CA 1 , and subiculum demonstrated greater sensitivity to novel scene stimuli. Paralleling these findings, subsequent memory effects in perirhinal cortex were observed for both faces and scenes, with the magnitude of encoding activation being related to later memory strength, as indexed by a graded response tracking recognition confidence, whereas subsequent memory effects were scene-selective in parahippocampal cortex. Within the hippocampus, encoding activation in the subiculum correlated with subsequent memory for both stimulus classes, with the magnitude of encoding activation varying in a graded manner with later memory strength. Collectively, these findings suggest a gradient of content sensitivity from posterior (parahippocampal) to anterior (perirhinal) MTL cortex, with MTL cortical regions differentially contributing to successful encoding based on event content. In contrast to recent suggestions, the present data further indicate that the subiculum may contribute to successful encoding irrespective of event content.

A core aspect of working memory (WM) is the capacity to maintain goal-relevant information in mind, but little is known about how this capacity develops in the human brain. We compared brain activation, via fMRI, between children (ages 7–12 years) and adults (ages 20–29 years) performing tests of verbal and spatial WM with varying amounts (loads) of information to be maintained in WM. Children made disproportionately more errors than adults as WM load increased. Children and adults exhibited similar hemispheric asymmetry in activation, greater on the right for spatial WM and on the left for verbal WM. Children, however, failed to exhibit the same degree of increasing activation across WM loads as was exhibited by adults in multiple frontal and parietal cortical regions. Thus, children exhibited adult-like hemispheric specialization, but appeared immature in their ability to marshal the neural resources necessary to maintain large amounts of verbal or spatial information in WM.

Functional neuroimaging studies of single-word processing have demonstrated decreased activation in left inferior prefrontal cortex (LIPC) during repeated semantic processing relative to initial semantic processing. This item-specific memory effect occurs under implicit test instructions and represents word-toword semantic repetition priming. The present study examined the stimulus generality of LIPC function by measuring prefrontal cortical activation during repeated relative to initial semantic processing of words (word-to-word semantic repetition priming) and of pictures (picture-to-picture semantic repetition priming). For both words and pictures, LIPC activation decreased with repetition, suggesting that this area subserves semantic analysis of stimuli regardless of perceptual form. Decreased activation was greater in extent for words than for pictures. The LIPC area may act as a semantic executive system that mediates on-line retrieval of long-term conceptual knowledge necessary for guiding task performance.