Contribution of the lateral occipital and parahippocampal cortices to pattern separation of objects and contexts.

Contextual features are integral to episodic memories; yet, we know little about context effects on pattern separation, a hippocampal function promoting orthogonalization of overlapping memory representations. Recent studies suggested that various extrahippocampal brain regions support pattern separation; however, the specific role of the parahippocampal cortex-a region involved in context representation-in pattern separation has not yet been studied. Here, we investigated the contribution of the parahippocampal cortex (specifically, the parahippocampal place area) to context reinstatement effects on mnemonic discrimination, using functional magnetic resonance imaging. During scanning, participants saw object images on unique context scenes, followed by a recognition task involving the repetitions of encoded objects or visually similar lures on either their original context or a lure context. Context reinstatement at retrieval improved item recognition but hindered mnemonic discrimination. Crucially, our region of interest analyses of the parahippocampal place area and an object-selective visual area, the lateral occipital cortex indicated that while during successful mnemonic decisions parahippocampal place area activity decreased for old contexts compared to lure contexts irrespective of object novelty, lateral occipital cortex activity differentiated between old and lure objects exclusively. These results imply that pattern separation of contextual and item-specific memory features may be differentially aided by scene and object-selective cortical areas.

Learning to distinguish: shared perceptual features and discrimination practice tune behavioural pattern separation.

Pattern separation is a computational mechanism performed by the hippocampus allowing the reduction of overlap between sensory inputs with similar perceptual features. Our first aim was to develop a new paradigm sensitive to the behavioural consequences of pattern separation (mnemonic discrimination). For this purpose, we constructed morphed face stimuli with parametrically changing levels of similarity. After encoding participants saw studied items and similar lure faces. Perceptual similarity affected false recognition and there was a gradual reduction in discrimination accuracy with the increment of similarity between the stimuli. However, confidence ratings were sensitive to smaller changes (Experiment 1) than the other test type with "old"/"similar"/"new" response options (Experiment 2). Mnemonic discrimination relies strongly on retrieving details of the original stimulus. Therefore, we investigated whether pattern separation can be tuned by retrieval in the form of a discrimination task (Experiment 3). Our findings suggest that repeatedly encountering the stimuli within a two-alternative forced-choice task (in comparison with the repeated presentation of the material) increased both the correct identification and the false recognition of similar stimuli two days after encoding. We conclude that basic computational mechanisms of the hippocampus can be tuned by a task that requires discrimination between studied and new stimuli.

Behavioural pattern separation is strongly associated with familiarity-based decisions.

Pattern separation is the process that minimises interference between memory representations with similar features and is suggested to be associated with hippocampus-related recollection. We tested this hypothesis using the incidental Mnemonic Similarity Task with old (target), similar (lure), and new (foil) items presented on a recognition test, which is widely used for detecting individual differences in behavioural pattern separation performance. In Experiment 1, participants made old/similar/new decisions and rated decision confidence on a scale ranging from "not at all sure" to "very sure". In Experiment 2, participants made recognition confidence judgments on a scale ranging between "sure it was new" and "sure it was old". In Experiment 3, subjects gave old/similar/new decisions and made a secondary Remember/Know/Guess judgment. In Experiment 1, confidence ratings were higher for targets compared to lures when we analysed correct responses (old for targets and similar for lures). Additionally, we found a symmetrical ROC curve and a linear zROC curve for target-lure discrimination in Experiment 2. Finally, we found a bias toward Know responses when we analysed behavioural pattern separation performance (i.e., the rate of similar responses given to the lures). These findings suggest that familiarity, rather than recollection, contributes to behavioural pattern separation performance.

Litmus test of rich episodic representations: Context-induced false recognition.

Context-dependent episodic memory is typically investigated using tasks in which retrieval occurs either in the reinstated context of encoding or in a completely new context. A fundamental question of episodic memory models is the level of detail in episodic memory representations containing contextual information about the encoded event. The present study examined whether memory is affected when the contexts of encoding and retrieval are highly similar but not exactly the same. At encoding, participants saw unique object images presented on the background of unique context scene images. On a surprise recognition test, the objects were either old or visually similar to ones seen at encoding (lure stimuli). The objects were presented on either the old or a lure context image; the lure context image was visually similar to the corresponding object's encoding context. Context reinstatement increased the hit rate for the old objects, but also increased the false alarm for the lure objects. This latter finding indicates that the presence of the encoding context at test does not always aid recognition memory decisions. These results suggest that slight visual differences between the contexts of encoding and retrieval matter, as context reinstatement leads to a tendency to respond Old even in case of small differences in the old and lure contexts.

An event-related potential study of the testing effect: Electrophysiological evidence for context-dependent processes changing throughout repeated practice.

The testing effect refers to a special form of performance improvement following practice. Specifically, repeated retrieval attempts improve long-term memory. In the present study we examined the underlying mechanisms of the testing effect as a function of time by investigating the electrophysiological correlates of repeated retrieval practice. We additionally investigated the ERP waveforms of the repeated practice phase as a function of the accuracy on the final test in a "difference due to memory" (Dm) analysis. We found a parietally distributed, increased positive amplitude between 500 and 700 ms, and a more positive parietal wave between 700 and 1000 ms in the later relative to the early phases of retrieval practice. We found parietal Dm effects in the same two time windows in the retrieval practice condition with a more positive amplitude predicting retrieval success on the final test. We interpret the earlier waveform as a component associated with episodic recollection and the later ERP as a component related to post-retrieval evaluation processes. Our results demonstrate the important role of these retrieval-related processes in the facilitating effect of retrieval practice on later retrieval, and show that the involvement of these processes changes throughout practice.

Irrelevant background context decreases mnemonic discrimination and increases false memory.

One of the greatest commonplaces in memory research is that context improves recall and enhances or leaves recognition intact. Here we present results which draw attention to the fact that the reappearance of irrelevant and unattended background contexts of encoding significantly impairs memory discrimination functions. This manuscript presents the results of two experiments in which participants made indoor/outdoor judgements for a large number of object images presented together with individual, irrelevant and presumably unattended background scenes. On a subsequent unexpected recognition test participants saw the incidentally encoded target objects, visually similar lures or new foil objects on the same or new background scenes. Our results showed that although the reappearance of the background scene raised the hit rate for target objects, it decreased mnemonic discrimination, a behavioral score for pattern separation, a hippocampal function that is affected in early dementia. Furthermore, the presence of the encoded background scene at the recognition test increased the false recognition of lure objects, even when participants were explicitly instructed to neglect the context scene. Altogether these results gave evidence that if context increases recognition hits for target memories, it does so at the cost of increasing false recognition and diminished discriminability for similar information.

Retrieval practice makes procedure from remembering: An automatization account of the testing effect.

The "testing effect" refers to the striking phenomenon that repeated retrieval practice is one of the most effective learning strategies, and certainly more advantageous for long-term learning, than additional restudying of the same information. How retrieval can boost the retention of memories is still without unanimous explanation. In 3 experiments, focusing on the reaction time (RT) of retrieval, we showed that RT of retrieval during retrieval practice followed a power function speed up that typically characterizes automaticity and skill learning. More important, it was found that the measure of goodness of fit to this power function was associated with long-term recall success. Here we suggest that the automatization of retrieval is an explanatory component of the testing effect. As a consequence, retrieval-based learning has the properties characteristic of skill learning: diminishing involvement of attentional processes, faster processing, resistance to interference effects, and lower forgetting rate. (PsycINFO Database Record