Biomedical subjects

Michael D Rugg

Publications and source records attributed to Michael D Rugg.

46 records · Page 3Linked to original sources

PubMed2003

Strategic influences on recollection in the exclusion task: electrophysiological evidence.

One assumption underlying the use of the exclusion task as part of the process dissociation procedure is that studied items are successfully excluded only when they are recollected. The present study employed event-related potentials (ERPs) to demonstrate that successful exclusion does not necessarily require recollection. In two experiments, the study tasks for to-be-excluded items were identical, but the tasks employed with target items differed, giving better memory for these items in Experiment 1 than in Experiment 2. Successfully excluded items elicited the ERP signature for recollection--the left parietal old/new effect--in Experiment 2 only. These findings indicate that the subjects adopted different retrieval strategies in the two experiments. It is suggested that they made more use of source information about to-be-excluded items in the second experiment than in the first.

Adolescent↗

PubMed2002

The neural basis of episodic memory: evidence from functional neuroimaging.

We review some of our recent research using functional neuroimaging to investigate neural activity supporting the encoding and retrieval of episodic memories, that is, memories for unique events. Findings from studies of encoding indicate that, at the cortical level, the regions responsible for the effective encoding of a stimulus event as an episodic memory include some of the regions that are also engaged to process the event 'online'. Thus, it appears that there is no single cortical site or circuit responsible for episodic encoding. The results of retrieval studies indicate that successful recollection of episodic information is associated with activation of lateral parietal cortex, along with more variable patterns of activity in dorsolateral and anterior prefrontal cortex. Whereas parietal regions may play a part in the representation of retrieved information, prefrontal areas appear to support processes that act on the products of retrieval to align behaviour with the demands of the retrieval task.

Brain Mapping↗

PubMed2002

Getting ready to remember: the neural correlates of task set during recognition memory.

Event-related potentials (ERPs) were employed to investigate the neural correlates of episodic and semantic task sets. ERPs elicited by cues signalling an upcoming recognition memory test trial showed a sustained positivity relative to those signalling an upcoming semantic test trial, lasting from 500 ms post-cue until the arrival of the test item. However, this effect was present only on the second successive trial on which subjects performed the recognition task. Thus, when episodic vs semantic tasks vary trial-by-trial, the establishment of a recognition memory task-set is not achieved within a single trial. The findings are discussed in relation to the notion of episodic retrieval mode.

Adult↗

PubMed2002

Anatomically informed basis functions for EEG source localization: combining functional and anatomical constraints.

Distributed linear solutions have frequently been used to solve the source localization problem in EEG. Here we introduce an approach based on the weighted minimum norm (WMN) method that imposes constraints using anatomical and physiological information derived from other imaging modalities. The anatomical constraints are used to reduce the solution space a priori by modeling the spatial source distribution with a set of basis functions. These spatial basis functions are chosen in a principled way using information theory. The reduced problem is then solved with a classical WMN method. Further (functional) constraints can be introduced in the weighting of the solution using fMRI brain responses to augment spatial priors. We used simulated data to explore the behavior of the approach over a range of the model's hyperparameters. To assess the construct validity of our method we compared it with two established approaches to the source localization problem, a simple weighted minimum norm and a maximum smoothness (Loreta-like) solution. This involved simulations, using single and multiple sources that were analyzed under different levels of confidence in the priors.

Brain↗

PubMed2002

Systematic regularization of linear inverse solutions of the EEG source localization problem.

Distributed linear solutions of the EEG source localization problem are used routinely. Here we describe an approach based on the weighted minimum norm method that imposes constraints using anatomical and physiological information derived from other imaging modalities to regularize the solution. In this approach the hyperparameters controlling the degree of regularization are estimated using restricted maximum likelihood (ReML). EEG data are always contaminated by noise, e.g., exogenous noise and background brain activity. The conditional expectation of the source distribution, given the data, is attained by carefully balancing the minimization of the residuals induced by noise and the improbability of the estimates as determined by their priors. This balance is specified by hyperparameters that control the relative importance of fitting and conforming to prior constraints. Here we introduce a systematic approach to this regularization problem, in the context of a linear observation model we have described previously. In this model, basis functions are extracted to reduce the solution space a priori in the spatial and temporal domains. The basis sets are motivated by knowledge of the evoked EEG response and information theory. In this paper we focus on an iterative "expectation-maximization" procedure to jointly estimate the conditional expectation of the source distribution and the ReML hyperparameters on which this solution rests. We used simulated data mixed with real EEG noise to explore the behavior of the approach with various source locations, priors, and noise levels. The results enabled us to conclude: (i) Solutions in the space of informed basis functions have a high face and construct validity, in relation to conventional analyses. (ii) The hyperparameters controlling the degree of regularization vary largely with source geometry and noise. The second conclusion speaks to the usefulness of using adaptative ReML hyperparameter estimates.

Algorithms↗

PubMed2002

The birth of a memory.

Laying down new memories has long been thought to involve interactions between the hippocampus and multiple regions of the neocortex. Functional neuroimaging studies performed over the past four years provide evidence for this proposal. A recent electrophysiological study offers a possible mechanism by which interactions between brain regions take place during memory formation.

Cerebral Cortex↗

PubMed2002

State-related and item-related neural correlates of successful memory encoding.

Neuroimaging studies show that the efficacy of long-term memory encoding of a stimulus is indexed by transient neural activity elicited by that stimulus. Here, we show that successful memory encoding is also indexed by neural activity that is tonically maintained throughout a study task. Using functional magnetic resonance imaging (fMRI), transient and sustained neural activity were dissociated with a mixed event-related and blocked design. In a series of short task blocks, human subjects made semantic or phonological decisions about visually presented words. After statistically removing item-related activity, we found that the mean level of activity across a task block was correlated with the number of words subsequently remembered from that block. These correlations were found in inferior medial parietal and left prefrontal cortex for the semantic task, and in superior medial parietal cortex for the phonological task. Our findings suggest that state-related activity in these brain regions is involved in memory encoding.

Adolescent↗

PubMed2002

Brain activity underlying encoding and retrieval of source memory.

Neural activity elicited during the encoding and retrieval of source information was investigated with event-related functional magnetic resonance imaging (efMRI). During encoding, 17 subjects performed a natural/artificial judgement on pictures of common objects which were presented randomly in one of the four quadrants of the display. At retrieval, old pictures were mixed with new ones and subjects judged whether each picture was new or old and, if old, indicated in which quadrant it was presented at encoding. During encoding, study items that were later recognized and assigned a correct source judgement elicited greater activity than recognized items given incorrect judgements in a variety of regions, including right lateral occipital and left prefrontal cortex. At retrieval, regions showing greater activity for recognized items given correct versus incorrect source judgements included the right hippocampal formation and the left prefrontal cortex. These findings indicate a role for these regions in the encoding and retrieval of episodic information beyond that required for simple item recognition.

Adult↗

PubMed2002

Electrophysiological dissociation of retrieval orientation and retrieval effort.

The neural correlates of retrieval orientation-the differential processing of retrieval cues according to the form of the sought-for information-and retrieval effort were investigated in a factorial design. ERPs elicited by test words were recorded during four recognition memory tests. Orientation was manipulated by varying study material: The study phases preceding two of the tests employed pictures, whereas the study phases preceding the other two tests employed words. Effort was manipulated by varying difficulty, using a combination of the variables of length of study list and study-test interval. ERPs elicited by correctly classified new test words were sensitive to both the study material and, to a much lesser extent, the difficulty of manipulations. Whereas difficulty effects onset early and were short-lived, the effects of study material onset later, extended for several hundred milliseconds, and did not vary according to difficulty. It was concluded that retrieval orientation exerts a major influence on the processing of recognition memory test items.

Cues↗

PubMedDate not supplied

Modulation of retrieval processing reflects accuracy of emotional source memory.

There is considerable evidence that encoding and consolidation of memory are modulated by emotion, but the retrieval of emotional memories is not well characterized. Here we manipulated the emotional context with which affectively neutral stimuli were associated during encoding, allowing us to examine neural activity associated with retrieval of emotional memories without confounding the emotional attributes of cue items and the retrieved context. Using a source memory procedure we were also able to compare how retrieval processing was modulated when the emotional encoding context was recollected or not. An interaction between emotional encoding context and accuracy of source memory revealed that successful retrieval of emotional context was associated with activity in left amygdala, and a left frontotemporal network including anterior insula, prefrontal cortex and cingulate. In contrast, when contextual retrieval was unsuccessful, items encoded in emotional contexts elicited enhanced activity in right amygdala and a right-lateralized network that included extrastriate visual areas. These findings indicate distinct effects of emotion on successful and unsuccessful retrieval of source information, including lateralization of amygdala responses.

Adolescent↗