Neuroimaging of memory. Introduction.
Explore the source record for details and available documents.
Brain↗
Biomedical subjects
D J Madden
Publications and source records attributed to D J Madden.
Functional neuroimaging of memory: implications for cognitive aging.
Our understanding of the ways in which changes in specific neural systems mediate adult age differences in memory is rapidly increasing, due in no small part to the advent of functional neuroimaging techniques. This article reviews age-related changes in memory performance obtained with behavioral measures, describes models of the neural mechanisms of memory, and derives predictions from these models regarding age-related changes in brain activation patterns. The neuroimaging findings obtained to date support models emphasizing the role of prefrontal cortex in age-related changes in memory functioning, especially for episodic memory retrieval. In general, neural activation associated with episodic memory encoding is regionally similar for younger and older adults but relatively lower in magnitude for older adults. During retrieval, activation that is restricted to the right prefrontal cortex for younger adults is more likely to be bilateral for older adults. Prefrontal activation exhibits an age-related increase when working memory tasks require simple storage and an age-related decrease when working memory requires higher-level executive processes. Although the evidence is limited, behavioral performance and activation patterns appear to be similar among younger and older adults on tests of semantic (context-independent) and implicit memory. We conclude that several methodological issues, such as defining the relation between brain structure and function, and determining the relationship between performance and activation, are particularly important for understanding age-related changes. Future directions for aging research include further investigation of the relation between encoding and retrieval and the identification of both spared and impaired neural systems.
Adult↗
Adult age differences in the functional neuroanatomy of verbal recognition memory.
Adult age differences are frequently observed in the performance of memory tasks, but the changes in neural function mediating these differences are largely unknown. We used (H2)15O positron emission tomography (PET) to measure changes in regional cerebral blood flow (rCBF) during Encoding, Baseline, and Retrieval conditions of a recognition memory task. Twelve young adults (20-29 years) and 12 older adults (62-79 years) participated. During each task condition, participants made a two-choice manual response to each of 64 words. Analyses of the performance data yielded evidence of age-related slowing of encoding and retrieval processes, and an age-related decline in the accuracy of yes/no recognition (d'). The rCBF activation associated with both encoding and retrieval was greater for older adults than for young adults, but this pattern was more clearly evident for memory retrieval. For young adults, rCBF activation during retrieval occurred primarily in right prefrontal cortex, whereas older adults exhibited a more bilateral pattern of prefrontal activation. Regression analyses predicting reaction time in the memory task from regional PET counts confirmed that the neural system mediating memory retrieval is more widely distributed for older adults than for young adults. Both age groups exhibited some decrease in rCBF activation in the second half of the test session, relative to the first half. The practice-related decrease in rCBF activation was more prominent for young adults, suggesting that the older adults' recruitment of additional neural systems reflects a more continual allocation of attention to support task performance.
Adult↗
Adult age differences in visual search accuracy: attentional guidance and target detectability.
Previous research, relying primarily on reaction time measures of highly accurate performance, suggests that both younger and older adults can increase the efficiency of visual search by guiding attention to a candidate subset of items. The authors investigated attentional guidance when accuracy was well below ceiling to focus more specifically on the role of perceptual processes. In the most difficult condition (conjunction search), the likelihood of missing a target was greater for older adults than for younger adults, and this effect was not attributable entirely to generalized slowing. Both age groups were able to improve search efficiency by attending to a distinct subset of display items, indicating that attentional guidance to perceptual features does not exhibit age-related decline. A signal-detection model of the conjunction search data demonstrated that the age difference represented an age-related decline in target detectability.
Adult↗
Age differences in the strategic allocation of visual attention.
The allocation of visual spatial attention was investigated in two groups of adults, younger (n = 24; M = 19 yrs) and older (n = 24; M = 68 yrs). Two sequential target displays were presented on a computer screen. If a target letter appeared in Display 1, then observers were to identify a target letter in Display 2. Based on accuracy of Display 1 target detection, the older adults had a more restricted range of visual processing than the younger adults. Based on reaction times for Display 2 target identification, older adults appeared to use a spotlight (serial) scanning mechanism, whereas younger adults appeared to use an activity-distribution (parallel) mechanism. Results are consistent with age-related cognitive slowing, but also suggest a difference in strategy according to the availability of visual information.
Adolescent↗
Aging and recognition memory: changes in regional cerebral blood flow associated with components of reaction time distributions.
We used H(2)15O positron emission tomography (PET) to measure age-related changes in regional cerebral blood flow (rCBF) during a verbal recognition memory task. Twelve young adults (20 to 29 years) and 12 older adults (62 to 79 years) participated. Separate PET scans were conducted during Encoding, Baseline, and Retrieval conditions. Each of the conditions involved viewing a series of 64 words and making a two-choice response manually. The complete reaction time (RT) distributions in each task condition were characterized in terms of an ex-Gaussian model (convolution of exponential and Gaussian functions). Parameter estimates were obtained for the mean of the exponential component (tau), representing a task-specific decision process and the mean of the Gaussian component (mu) representing residual sensory coding and response processes. Independently of age group, both tau and mu were higher in the Encoding and Retrieval conditions than in the Baseline condition, and tau was higher during Retrieval than during Encoding. Age-related slowing in task performance was evident primarily in mu. For young adults, rCBF activation in the right prefrontal cortex, in the Retrieval condition, was correlated positively with mu but not with tau. For older adults, rCBF changes (both increases and decreases) in several cortical regions were correlated with both mu and tau. The data suggest that the attentional demands of this task are relatively greater for older adults and consequently lead to the recruitment of additional neural systems during task performance.
Adult↗
Age similarities in the inertial properties of attention.
Adult age differences in the mode of allocation of visual attention were investigated, using a visual search task with a circular display containing one target letter and seven distractor letters. In two experiments, a total of 56 younger adults (M = 20 years) and 56 older adults (M = 66 years) searched for a target appearing with equal probability at one of two cued locations. The first cue appeared 115 msec before display onset, and the second cue appeared with display onset; distance between the two cued locations was varied. Target identification performance indicated that attention was inertial, in that reaction time for second-cued targets was related either to the area of the portion of the visual field containing possible target locations or to the mean path length of a serial self-terminating search. There were no age-related decrements in the allocation of visual attention.
Adolescent↗
Interaction of hypertension and age in visual selective attention performance.
Previous research suggests that some aspects of cognitive performance decline as a joint function of age and hypertension. In this experiment, 51 unmedicated individuals with mild essential hypertension and 48 normotensive individuals, 18-78 years of age, performed a visual search task. The estimated time required to identify a display character and shift attention between display positions increased with age. This attention shift time did not differ significantly between hypertensive and normotensive participants, but regression analyses indicated some mediation of the age effect by blood pressure. For individuals less than 60 years of age, the error rate was greater for hypertensive than for normotensive participants. Although the present design could detect effects of only moderate to large size, the results suggest that effects of hypertension may be more evident in a relatively general measure of performance (mean error rate) than in the speed of shifting visual attention.
Adult↗
Time course of allocation of visual attention after equating for sensory differences: an age-related perspective.
Adult age differences in the time course of the allocation of visual attention were investigated, in 2 experiments that both included the same 10 younger adults (M = 22 years) and 10 older adults (M = 68 years). In Experiment 1, older adults accumulated information about target identity at a slower rate than younger adults, as represented by the rise in accuracy as a function of target duration. To equate performance in a baseline condition in a spatial-cuing paradigm (Experiment 2), target duration was set for each observer on the basis of the data in Experiment 1. Performance for the 2 age groups was comparable, both in the baseline condition and in the time course of attention, as indexed by the function relating accuracy to cue-target onset asynchrony. The authors conclude that, in this spatial-cuing paradigm, an age-related change is evident in sensory processing but not in attentional allocation.
Adult↗
Word frequency effects at brief exposure durations: comment on Paap and Johansen (1994).
K. R. Paap and L. S. Johansen (1994) proposed that word frequency effects do not occur on a lexical decision task (LDT) when postmasked target exposure duration is sufficiently brief because such a task prevents verification--their hypothesized locus of the word frequency effect. In making this assertion, they proposed that the activation interpretation of A. R. Dobbs, A. Friedman, and J. Lloyd (1985) and of P. A. Allen, M. McNeal, and D. Kvak (1992) was flawed. However, evidence that Paap and Johansen's conclusions were wrong and that their experimental design contained flaws is provided here. In Experiment 1 of the present study, word frequency effects were evident on an LDT at the 75% accuracy level proposed by Paap and Johansen as being sufficiently low to prevent verification. In Experiment 2 the mental lexica of participants from the same population as that used for Experiment 1 contained very-low-frequency words. Thus, the present results are consistent with an activation locus.
Analysis of Variance↗
Adult age differences in regional cerebral blood flow during visual world identification: evidence from H215O PET.
We used H215O PET to investigate adult age differences in regional cerebral blood flow (rCBF) during the performance of a visual word identification task. The study participants were 20 healthy, right-handed men: 10 young adults between 18 and 27 years of age, and 10 older adults between 63 and 75 years of age. The word identification task comprised six blocks of test trials representing four task conditions; subjects responded manually. The task conditions varied with regard to whether semantic retrieval was required (e.g., word/nonword discrimination vs simple response to each stimulus) and with regard to the difficulty of visual encoding (e.g., words presented normally vs words with asterisks inserted between adjacent letters). Each subject performed all six trial blocks, concurrently with each of six H215O PET scans. Analyses of quantitative CBF data obtained from the arterial time-activity curve demonstrated a significant age-related decline in global CBF rate. Analyses of the changes in rCBF between task conditions indicated that retrieval of semantic information sufficient to distinguish words from nonwords is mediated by a ventral occipitotemporal cortical pathway. Specific areas within this pathway were also associated with visual encoding processes. Several rCBF activations were significantly greater for young adults than for older adults, indicating an age-related decline in processing efficiency within this ventral occipitotemporal pathway. Although the performance data demonstrated a greater age-related slowing for visual encoding than for semantic retrieval, these age-related performance changes were not associated with corresponding changes in rCBF activation.
Adolescent↗
Adult age differences in the use of distractor homogeneity during visual search.
Previous research has suggested that an age-related decline may exist in the ability to inhibit distracting information during visual search. The present experiments used a conjunction search task in which the within-item features of the target (an upright L) and the distractors (rotated Ls) were identical. In each of 2 experiments, both young and older adults searched the display significantly more rapidly when the distractors were all rotated in the same direction (homogeneous) than when the distractors were rotated in different directions (heterogeneous). The concept of a generalized, age-related slowing was able to account for many aspects of the data, although the degree of relative improvement associated with distractor homogeneity was greater for young adults than for older adults.
Adolescent↗
Methodological issues in the assessment of neuropsychologic function after cardiac surgery.
This report reviews critical issues facing investigators interested in neuropsychologic sequelae after cardiac operations: (1) experimental design; (2) selective attrition; (3) selection of instruments; (4) moderating factors; (5) definitions of cognitive decline; (6) statistical analysis; and (7) clinical significance. Implications for further research in the area are discussed.
Cognition Disorders↗
Aging and the speed/accuracy relation in visual search: evidence for an accumulator model.
Two models of performance in visual classification tasks, the fast-guess model and the accumulator model, offer contrasting accounts of the relation between speed and accuracy. We attempted to distinguish these models in the context of age-related changes in visual search performance. Twenty-four young adults (mean age 21.50 years) and 24 older adults (mean age 66.17 years) performed a visual search task requiring the detection of an upright L among rotated L's. The results supported the accumulator model, in that mean reaction time (RT) was higher for error responses than for correct responses, and there was a positive relation between RT and error rate. Both of these effects were more pronounced for older adults than for young adults, even when visual acuity was covaried statistically. We conclude that age-related changes in visual search performance involve a decline in the efficiency of sampling the amount of evidence necessary to exceed a decision criterion.
Adolescent↗
Adult age differences in shifting focused attention.
Young and older adults performed a choice response task in which 1 of 2 target letters was presented visually at 1 of 4 display locations. In 2 experiments, the validity of a target location cue and the presence of nontarget characters (distractors) were varied. With target-only displays and 40% cue validity (Experiment 1), the estimated time to shift attention between display locations was essentially 0 ms for both age groups. With 70% cue validity, Experiment 2 demonstrated significant increases in the attention shift time as a function of both increased age and the presence of distractors (asterisks). The results suggest that age-related changes in the shifting of focused attention are minimal except when the processing of nontarget information is required.
Adolescent↗
Adult age differences in attention: filtering or selection?
We examined the effect of target letter redundancy for target-only (TO) and target-plus-noise (TPN) trials on a visual search, divided attention task where target letters were presented in one or two corners of a two-corner display. Half of the two-letter displays also included a noise letter. In both Experiment 1 (two-choice vs go/no-go) and Experiment 2 (all go/no-go), older adults showed larger redundancy gains than did young adults, and this effect did not interact with task type or visual similarity. However, for the "no-go" trials in both experiments, there were no age differences in overall errors. These results suggest that there are age differences in the activation of selective attention rather than age differences in inhibitory control. In Experiment 2, young adults under lower-luminance presentation conditions (18 cd/m2) showed a smaller redundancy gain than did older adults under higher-luminance presentation conditions (40 cd/m2). These results provided further support of the age differences in activation interpretation, as well as indicating that older adults' larger redundancy gain was not due to an age decrement in retinal illuminance.
Adolescent↗
Effects of aerobic exercise on cognitive and psychosocial functioning in patients with mild hypertension.
The effects of 16 weeks of physical exercise training on the psychological functioning of 90 patients with mild hypertension were examined. At baseline and after 16 weeks of training, patients completed a psychometric test battery that included objective measures of neuropsychological performance and standardized self-report measures of psychosocial functioning. Patients were randomly assigned to one of three groups: aerobic exercise, strength training and flexibility exercise, or a waiting list control group. After training, there were no group differences on any of the psychological measures, even though patients who engaged in exercise perceived themselves as functioning better in a number of psychological domains.
Adult↗
Influence of age and processing stage on visual word recognition.
The authors used a lexical-decision task in 3 different experiments to examine whether age differences in word recognition were consistent across processing stage. In all experiments, word frequency and length were manipulated. In Experiments 1 and 2, encoding difficulty was varied, and in Experiment 3, response selection difficulty was varied. In all 3 experiments, there were no age differences for word frequency. However, in Experiments 1 and 2, older adults showed a larger decrement for encoding. In Experiment 3, age differences were larger when response selection load increased. These results suggest that age differences in word recognition occur because older adults exhibit primarily peripheral-rather than central-processing decrements. The implications of these data for generalized and localized slowing models are discussed.
Adolescent↗