Biomedical subjects

Michael D Rugg

Publications and source records attributed to Michael D Rugg.

12 recordsLinked to original sources

PubMed2003

Repetition effects elicited by objects and their contexts: an fMRI study.

Event-related fMRI responses were recorded during a recognition memory test for previously studied visual objects. Some studied objects were superimposed on the same context (landscape scenes) as at study, some were superimposed on a different studied context, and some were paired with new contexts. Unstudied objects were paired with either a studied or a new context. Relative to all other stimulus classes, test stimuli where both components were unstudied elicited enhanced responses in lateral and ventral extrastriate visual cortex. This effect, which is analogous to a previously described electrophysiological result obtained with the same experimental procedure, had the same magnitude regardless of whether a test item was composed of one or two studied components, or whether a single studied component was task relevant or task irrelevant. The findings point to the existence of repetition-sensitive neural mechanisms that operate in a non-linear manner.

Adult↗

PubMed2003

Neural correlates of retrieval processing in the prefrontal cortex during recognition and exclusion tasks.

Event-related fMRI was employed to contrast the neural activity elicited in prefrontal cortex during recognition memory and exclusion tests. The study phases preceding each memory test were identical, involving the presentation of study items (visually presented words) in one of two study contexts. For the recognition test subjects were required to respond positively to all old items regardless of study context, and to respond negatively to new items. For the exclusion task, positive responses were required to old items presented in one of the study contexts only; negative responses were required both to unstudied items and studied items from the alternative context (non-targets). No prefrontal region demonstrated greater activity for new items in the exclusion task. Thus, there was no evidence that retrieval cues were processed differently according to the specificity of the sought-for information. In several regions, most notably bilateral anterior prefrontal cortex, activity was greater for old than for new items regardless of task. Activity in right dorsolateral prefrontal cortex was also greater for old than for new items; these effects however were larger in the exclusion task. The findings are consistent with previous reports that activity in anterior prefrontal cortex elicited by recognition retrieval cues is sensitive to retrieval success, and extend these findings to the exclusion task. The findings for the right dorsolateral cortex add further weight to the proposal that this region supports post-retrieval monitoring of retrieved information.

Adult↗

PubMed2003

Probability effects on event-related potential correlates of recognition memory.

A common finding in event-related potential (ERP) studies of recognition memory is that recognised items elicit greater positivity at parietal electrode sites than new items (the 'left parietal old/new effect'). Parietal positivity (the P300 or P3b) is also elicited in detection tasks with no memory demands by items of low probability of occurrence and high 'target value'. It has been argued that correctly recognised items are typically associated with lower probability and higher target value than new items, raising the question of the extent to which the old/new effect receives a contribution from, or interacts with, P300 activity. The present study explored this issue by comparing ERPs associated with correctly classified old and new items across three different ratios of old to new items: 25:75, 50:50 and 75:25. The left parietal old/new effect was not influenced by this manipulation in the latency range in which it is conventionally measured. Probability did influence the parietal ERPs to correctly recognised items post-800 ms; the scalp distribution of this probability effect was however qualitatively distinct from that of the preceding old/new effect. The left parietal old/new effect appears to be a relatively pure reflection of episodic retrieval, uncontaminated by the non-mnemonic factors of probability and target value.

Adolescent↗

PubMed2003

Age effects on the neural correlates of successful memory encoding.

Event-related functional MRI (fMRI) was used to investigate the neural correlates of memory encoding as a function of age. While fMRI data were obtained, 14 younger (mean age 21 years) and 14 older subjects (mean age 68 years) made animacy decisions about words. Recognition memory for these words was tested at two delays such that older subjects' performance at the short delay was comparable to that of the young subjects at the long delay. This allowed age-associated changes in the neural correlates of encoding to be dissociated from the correlates of differential recognition performance. Activity in left inferior prefrontal cortex and the left hippocampal formation was greater for subsequently recognized words in both age groups, consistent with the findings of previous studies in young adults. In the prefrontal cortex, these 'subsequent memory effects' were, however, left-lateralized in the younger group but bilateral in the older subjects. In addition, for the younger group only, greater activity for remembered words was observed in anterior inferior temporal cortex, as were reversed effects ('subsequent forgetting' effects) in anterior prefrontal regions. The data indicate that older subjects engage much of the same neural circuitry as younger subjects when encoding new memories. However, the findings also point to age-related differences in both prefrontal and temporal activity during successful episodic encoding.

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↗