Stimulus repetition often leads to facilitated processing, resulting in neural decreases (repetition suppression) and faster RTs (repetition priming). Such repetition-related effects have been attributed to the facilitation of repeated cognitive processes and/or the retrieval of previously encoded stimulus–response (S-R) bindings. Although previous research has dissociated these two forms of learning, their interaction in the brain is not fully understood. Utilizing the spatial and temporal resolutions of fMRI and EEG, respectively, we examined a long-lag classification priming paradigm that required response repetitions or reversals at multiple levels of response representation. We found a repetition effect in occipital/temporal cortex (fMRI) that was time-locked to stimulus onset (EEG) and robust to switches in response, together with a repetition effect in inferior pFC (fMRI) that was time-locked to response onset (EEG) and sensitive to switches in response. The response-sensitive effect occurred even when changing from object names (words) to object pictures between repetitions, suggesting that S-R bindings can code abstract representations of stimuli. Most importantly, we found evidence for interference effects when incongruent S-R bindings were retrieved, with increased neural activity in inferior pFC, demonstrating that retrieval of S-R bindings can result in facilitation or interference, depending on the congruency of response between repetitions.

Although functional neuroimaging studies have supported the distinction between explicit and implicit forms of memory, few have matched explicit and implicit tests closely, and most of these tested perceptual rather than conceptual implicit memory. We compared event-related fMRI responses during an intentional test, in which a group of participants used a cue word to recall its associate from a prior study phase, with those in an incidental test, in which a different group of participants used the same cue to produce the first associate that came to mind. Both semantic relative to phonemic processing at study, and emotional relative to neutral word pairs, increased target completions in the intentional test, but not in the incidental test, suggesting that behavioral performance in the incidental test was not contaminated by voluntary explicit retrieval. We isolated the neural correlates of successful retrieval by contrasting fMRI responses to studied versus unstudied cues for which the equivalent “target” associate was produced. By comparing the difference in this repetition-related contrast across the intentional and incidental tests, we could identify the correlates of voluntary explicit retrieval. This contrast revealed increased bilateral hippocampal responses in the intentional test, but decreased hippocampal responses in the incidental test. A similar pattern in the bilateral amygdale was further modulated by the emotionality of the word pairs, although surprisingly only in the incidental test. Parietal regions, however, showed increased repetition-related responses in both tests. These results suggest that the neural correlates of successful voluntary explicit memory differ in directionality, even if not in location, from the neural correlates of successful involuntary implicit (or explicit) memory, even when the incidental test taps conceptual processes.

Lesion and neuroimaging studies suggest that orbito-frontal cortex (OFC) supports temporal aspects of episodic memory. However, it is unclear whether OFC contributes to the encoding and/or retrieval of temporal context and whether it is selective for temporal relative to nontemporal (spatial) context memory. We addressed this issue with two complimentary studies: functional magnetic resonance imaging to measure OFC activity associated with successful temporal and spatial context memory during encoding and retrieval in healthy young participants, and a neuropsychological investigation to measure changes in spatial and temporal context memory in OFC lesion patients. Imaging results revealed that OFC contributed to encoding and retrieval of associations between objects and their temporal but not their spatial contexts. Consistent with this, OFC patients exhibited impairments in temporal but not spatial source memory accuracy. These results suggest that OFC plays a critical role in the formation and subsequent retrieval of temporal context.

The relationship between recognition memory and repetition priming remains unclear. Priming is believed to reflect increased processing fluency for previously studied items relative to new items. Manipulations that affect fluency can also affect the likelihood that participants will judge items as studied in recognition tasks. This attribution of fluency to memory has been related to the familiarity process, as distinct from the recollection process, that is assumed by dual-process models of recognition memory. To investigate the time courses and neural sources of fluency, familiarity, and recollection, we conducted an event-related potential (ERP) study of recognition memory using masked priming of test cues and a remember/know paradigm. During the recognition test, studied and unstudied words were preceded by a brief, masked word that was either the same or different. Participants decided quickly whether each item had been studied (“old” or “new”), and for items called old, indicated whether they “remembered” (R) the encoding event, or simply “knew” (K) the item had been studied. Masked priming increased the proportion of K, but not R, judgments. Priming also decreased response times for hits but not correct rejections (CRs). Four distinct ERP effects were found. A medial-frontal FN400 (300–500 msec) was associated with familiarity (R, K Hits > CRs) and a centro-parietal late positivity (500–800 msec) with recollection (R Hits > K Hits, CRs). A long-term repetition effect was found for studied items judged “new” (Misses > CRs) in the same time window as the FN400, but with a posterior distribution. Finally, a centrally distributed masked priming effect was visible between 150 and 250 msec and continued into the 300–500 msec time window, where it was topographically dissociable from the FN400. These results suggest that multiple neural signals are associated with repetition and potentially contribute to recognition memory.