Biomedical subjects

Alexa M Morcom

Publications and source records attributed to Alexa M Morcom.

7 recordsLinked to original sources

PubMed2007

Age effects on the neural correlates of episodic retrieval: increased cortical recruitment with matched performance.

Functional neuroimaging investigations have revealed a range of age-related differences in the neural correlates of episodic memory retrieval. Typically, whereas activity is reduced in older compared with younger adults in some regions, other regions are engaged exclusively, or to a greater extent, in older adults. It is unclear whether such differences merely represent the neural correlates of the lower levels of memory performance and impaired recollection typical of older adults. This issue was addressed in the present event-related functional magnetic resonance imaging study. The level of recollection was matched between groups of healthy younger and older adults for a subset of picture items in a source memory task by manipulating the number of study presentations. Contrasts of the activity elicited by old items attracting correct source judgments and correctly identified new items revealed that the 2 groups recruited many of the same brain regions. However, a striking pattern of age-related differences was also observed. In older adults, retrieval-related increases in activity were more widespread and of greater magnitude than in the young. Moreover, regions demonstrating retrieval-related decreases in activity were almost absent in the older participants. These findings suggest an age-related decline in the efficiency with which neural populations support cognitive function.

Adolescent↗

PubMed2006

Does the brain have a baseline? Why we should be resisting a rest.

In the last few years, the notion that the brain has a default or intrinsic mode of functioning has received increasing attention. The idea derives from observations that a consistent network of brain regions shows high levels of activity when no explicit task is performed and participants are asked simply to rest. The importance of this putative "default mode" is asserted on the basis of the substantial energy demand associated with such a resting state and of the suggestion that rest entails a finely tuned balance between metabolic demand and regionally regulated blood supply. These observations, together with the fact that the default network is more active at rest than it is in a range of explicit tasks, have led some to suggest that it reflects an absolute baseline, one that must be understood and used if we are to develop a comprehensive picture of brain functioning. Here, we examine the assumptions that are generally made in accepting the importance of the "default mode". We question the value, and indeed the interpretability, of the study of the resting state and suggest that observations made under resting conditions have no privileged status as a fundamental metric of brain functioning. In doing so, we challenge the utility of studies of the resting state in a number of important domains of research.

Journal Article↗

PubMed2004

Effects of age on retrieval cue processing as revealed by ERPs.

The electrophysiological correlates of retrieval cue processing were investigated in healthy young (18-30 years) and older (63-75 years) subjects (n = 16 per group). Retrieval orientation--the differential processing of cues according to the form of the sought-for information--and retrieval difficulty were manipulated in a factorial design. In separate study-test cycles, subjects studied either words or pictures, and performed a yes/no recognition memory task with words as the test items. ERPs elicited by correctly classified new words differed markedly according to study material in the young subjects, replicating previous findings. In the older subjects, this effect was smaller than in the young, and had a later onset and earlier offset. The scalp topography of the effect was however statistically indistinguishable in the two groups. These age-related ERP differences were unmodulated by task difficulty, and remained reliable when recognition performance was matched across the groups. By contrast, the magnitude and timing of ERP difficulty effects were unaffected by age. The findings suggest that older subjects are less able than young individuals to vary their processing of retrieval cues in response to different retrieval demands.

Adolescent↗

PubMed2004

Neural correlates of retrieval orientation: effects of study-test similarity.

ERPs elicited by correctly classified unstudied items in tests of yes/no recognition memory were used to investigate the neural correlates of retrieval cue processing. Items in Experiment 1 consisted of pictures and their corresponding names, allowing study and test material to be factorially crossed in four separate study-test cycles. The ERPs elicited by unstudied pictures and words were, in each case, more negative-going when the study material belonged to the alternative rather than the same class of items. These findings demonstrate that previously reported ERP "retrieval orientation effects" depend on differences in similarity between study and test items, and not on the form of the sought for material. In Experiments 2a and 2b, study materials were auditory words and pictures, and the test items were visual words. In both experiments, ERPs elicited by unstudied test words were more negative-going when pictures rather than auditory words were the study material. Thus, ERP retrieval orientation effects do not depend on the employment of a copy cue condition. It is proposed that the effects reflect differences in the processing necessary to maximize overlap between cue and memory representations.

Acoustic Stimulation↗

PubMed2004

The effects of age on the neural correlates of successful episodic retrieval: an ERP study.

The neural correlates of successful episodic retrieval (recollection), as reflected in event-related potentials (ERPs), were investigated in young (ca 20 years; n = 18) and older (ca 70 years; n = 16) healthy individuals. Subjects classified a series of pictures according to whether each item was new or had been encountered at study in the context of an animacy or a size judgment task. By manipulating the number of times items were presented for study, subsets of test items were formed for which source accuracy did not differ according to age. Relative to ERPs elicited by unstudied pictures, ERPs elicited by items attracting equivalent levels of source accuracy showed marked age-related differences. Those from younger subjects demonstrated the positive-going left parietal and right frontal old/new effects described in several previous studies of source memory. By contrast, analogous ERPs from older subjects contained a large left-lateralized negative effect that overshadowed the positive-going effects evident in the young. No age-related differences in either parietal or frontal ERP old/new effects were detected at electrode sites overlying the right hemisphere. It is possible that the age-related ERP differences observed in this task primarily reflect the use of different kinds of information as a basis for source judgments.

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

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↗