Corinne Pettigrew

Relationship of medial temporal lobe atrophy, APOE genotype, and cognitive reserve in preclinical Alzheimer's disease

This study evaluated the utility of baseline and longitudinal magnetic resonance imaging (MRI) measures of medial temporal lobe brain regions collected when participants were cognitively normal and largely in middle age (mean age 57 years) to predict the time to onset of clinical symptoms associated with mild cognitive impairment (MCI). Furthermore, we examined whether the relationship between MRI measures and clinical symptom onset was modified by apolipoprotein E (ApoE) genotype and level of cognitive reserve (CR). MRI scans and measures of CR were obtained at baseline from 245 participants who had been followed for up to 18 years (mean follow‐up 11 years). A composite score based on reading, vocabulary, and years of education was used as an index of CR. Cox regression models showed that lower baseline volume of the right hippocampus and smaller baseline thickness of the right entorhinal cortex predicted the time to symptom onset independently of CR and ApoE‐ɛ4 genotype, which also predicted the onset of symptoms. The atrophy rates of bilateral entorhinal cortex and amygdala volumes were also associated with time to symptom onset, independent of CR, ApoE genotype, and baseline volume. Only one measure, the left entorhinal cortex baseline volume, interacted with CR, such that smaller volumes predicted symptom onset only in individuals with lower CR. These results suggest that MRI measures of medial temporal atrophy, ApoE‐ɛ4 genotype, and the protective effects of higher CR all predict the time to onset of symptoms associated with MCI in a largely independent, additive manner during the preclinical phase of Alzheimers disease. Hum Brain Mapp 36:2826–2841, 2015.

Hypothetical Preclinical Alzheimer Disease Groups and Longitudinal Cognitive Change

IMPORTANCE Clinical trials testing treatments for Alzheimer disease (AD) are increasingly focused on cognitively normal individuals in the preclinical phase of the disease. To optimize observing a treatment effect, such trials need to enroll cognitively normal individuals likely to show cognitive decline over the duration of the trial. OBJECTIVE To identify which group of cognitively normal individuals shows the greatest cognitive decline over time based on their cerebrospinal fluid biomarker profile. DESIGN, SETTING, AND PARTICIPANTS In this cohort study, cognitively normal participants were classified into 1 of the following 4 hypothetical preclinical AD groups using baseline cerebrospinal fluid levels of Aβ and tau or Aβ and phosphorylated tau (p-tau): stage 0 (high Aβ and low tau), stage 1 (low Aβ and low tau), stage 2 (low Aβ and high tau), and suspected non-AD pathology (SNAP) (high Aβ and high tau). The data presented herein were collected between August 1995 and August 2014. MAIN OUTCOMES AND MEASURES An a priori cognitive composite score based on the following 4 tests previously shown to predict progression from normal cognition to symptom onset of mild cognitive impairment or dementia: Paired Associates immediate recall, Logical Memory delayed recall, Boston Naming, and Digit-Symbol Substitution. Linear mixed-effects models were used to compare the cognitive composite scores across the 4 groups over time, adjusting for baseline age, sex, education, and their interactions with time. RESULTS Two hundred twenty-two cognitively normal participants were included in the analyses (mean follow-up, 11.0 years [range, 0-18.3 years] and mean baseline age, 56.9 years [range, 22.1-85.8 years]). Of these, 102 were stage 0, 46 were stage 1, 28 were stage 2, and 46 were SNAP. Individuals in stage 2 (low Aβ and high tau [or p-tau]) had lower baseline cognitive scores and a greater decline in the cognitive composite score relative to the other 3 groups (β ≤ -0.06 for all and P ≤ .001 for the rate of decline for all). Individuals in stage 0, stage 1, and SNAP did not differ from one another in cognitive performance at baseline or over time (11.0 years) and showed practice-related improvement in performance. The APOE ε4 genotype was not associated with baseline cognitive composite score or the rate of change in the cognitive composite score. CONCLUSIONS AND RELEVANCE These results suggest that, to optimize observing a treatment effect, clinical trials enrolling cognitively normal individuals should selectively recruit participants with abnormal levels of both amyloid and tau (ie, stage 2) because this group would be expected to show the greatest cognitive decline over time if untreated.

Relationship of cognitive reserve and cerebrospinal fluid biomarkers to the emergence of clinical symptoms in preclinical Alzheimer's disease

The levels of β-amyloid (Aβ) and phosphorylated tau (p-tau), as measured in cerebrospinal fluid, have been associated with the risk of progressing from normal cognition to onset of clinical symptoms during preclinical Alzheimers disease. We examined whether cognitive reserve (CR) modifies this association. Cerebrospinal fluid was obtained at baseline from 239 participants (mean age, 57.2 years) who had been followed for up to 17 years with clinical and cognitive assessments (mean follow-up, 8 years). A composite score based on the National Adult Reading Test, vocabulary, and years of education at baseline was used as an index of CR. Cox regression models showed that the increased risk of progressing from normal cognition to symptom onset was associated with lower CR, lower baseline Aβ, and higher baseline p-tau. There was no interaction between CR and Aβ, suggesting that the protective effects of higher CR are equivalent across the observed range of amyloid levels. In contrast, both tau and p-tau interacted with CR, indicating that CR was more protective at lower levels of tau and p-tau.

Relationship of cognitive reserve and APOE status to the emergence of clinical symptoms in preclinical Alzheimer's disease

The APOE ε4 allele increases the risk of developing Alzheimer’s disease, whereas the APOE ε2 allele reduces risk. We examined whether cognitive reserve (CR), as measured by an index consisting of education, reading, and vocabulary, modifies these associations. CR was measured at baseline in 257 cognitively normal individuals (mean age 57.2 years) who have been followed for up to 17 years (mean follow-up = 9.2 years). Cox regression models showed that CR and APOE ε4 independently affected the risk of progressing from normal cognition to onset of clinical symptoms: CR reduced risk by about 50% in both ε4 carriers and non-carriers, while ε4 increased risk by about 150%. In contrast, APOE ε2 interacted with CR, such that CR was more protective in ε2 carriers than non-carriers. This suggests that individuals with an ε2 genotype may disproportionately benefit from lifetime experiences that enhance cognition.

Cognitive declines in healthy aging: evidence from multiple aspects of interference resolution.

The present study tested the hypothesis that older adults show age-related deficits in interference resolution, also referred to as inhibitory control. Although oftentimes considered as a unitary aspect of executive function, various lines of work support the notion that interference resolution may be better understood as multiple constructs, including resistance to proactive interference (PI) and response-distractor inhibition (e.g., Friedman & Miyake, 2004). Using this dichotomy, the present study assessed whether older adults (relative to younger adults) show impaired performance across both, 1, or neither of these interference resolution constructs. To do so, we used multiple tasks to tap each construct and examined age effects at both the single task and latent variable levels. Older adults consistently demonstrated exaggerated interference effects across resistance to PI tasks. Although the results for the response-distractor inhibition tasks were less consistent at the individual task level analyses, age effects were evident on multiple tasks, as well as at the latent variable level. However, results of the latent variable modeling suggested declines in interference resolution are best explained by variance that is common to the 2 interference resolution constructs measured herein. Furthermore, the effect of age on interference resolution was found to be both distinct from declines in working memory, and independent of processing speed. These findings suggest multiple cognitive domains are independently sensitive to age, but that declines in the interference resolution constructs measured herein may originate from a common cause.

Cortical thickness in relation to clinical symptom onset in preclinical AD.

Mild cognitive impairment (MCI) and Alzheimers disease (AD) dementia are preceded by a phase of disease, referred to as ‘preclinical AD’, during which cognitively normal individuals have evidence of AD pathology in the absence of clinical impairment. This study examined whether a magnetic resonance imaging (MRI) measure of cortical thickness in brain regions, collectively known as ‘AD vulnerable’ regions, predicted the time to onset of clinical symptoms associated with MCI and whether cortical thickness was similarly predictive of clinical symptom onset within 7 years post baseline versus progression at a later point in time. These analyses included 240 participants from the BIOCARD study, a cohort of longitudinally followed individuals who were cognitively normal at the time of their MRI (mean age = 56 years). Participants have been followed for up to 18 years (M follow-up = 11.8 years) and 50 participants with MRIs at baseline have developed MCI or dementia over time (mean time to clinical symptom onset = 7 years). Cortical thickness in AD vulnerable regions was based on the mean thickness of eight cortical regions. Using Cox regression models, we found that lower mean cortical thickness was associated with an increased risk of progression from normal cognition to clinical symptom onset within 7 years of baseline (p = 0.03), but not with progression > 7 years from baseline (p = 0.30). Lower cortical thickness was also associated with higher levels of phosphorylated tau, measured in cerebrospinal fluid at baseline. These results suggest that cortical thinning in AD vulnerable regions is detectable in cognitively normal individuals several years prior to the onset of clinical symptoms that are a harbinger of a diagnosis of MCI, and that the changes are more likely to be evident in the years proximal to clinical symptom onset, consistent with hypothetical AD biomarker models.

Relationship between cerebrospinal fluid biomarkers of Alzheimer's disease and cognition in cognitively normal older adults

The pathophysiological processes underlying Alzheimers disease (AD) are hypothesized to begin years to decades before clinical symptom onset, while individuals are still cognitively normal. Although many studies have examined the effect of biomarkers of amyloid pathology on measures of cognitive performance, less is known about the effect of tau pathology on cognitive performance. The present study examined the association between cerebrospinal fluid (CSF) biomarkers of AD pathology (amyloid, total tau (t-tau), and phosphorylated tau (p-tau)) and cognition in a large sample of cognitively normal middle-aged and older adults. Associations were examined with multivariate regressions, in which either amyloid and t-tau or amyloid and p-tau were included as simultaneous predictors of cognitive performance. Cognitive performance was measured with three composite scores assessing working memory, verbal episodic memory, and visuospatial episodic memory. In their respective models, CSF measures of both t-tau and p-tau were associated with the visuospatial episodic memory composite score (p<.001 and p=.02, respectively), but not with the other measures of cognition. In contrast, CSF amyloid was not significantly associated with cognitive performance, raising the possibility that measures of tau pathology have a more direct relationship with cognition in cognitively normal individuals. These results also suggest that tau pathology may have effects on visuospatial episodic memory during preclinical AD that precede alterations in other cognitive domains.

African Americans and Clinical Research: Evidence Concerning Barriers and Facilitators to Participation and Recruitment Recommendations

Purpose of the Study The goal of the study was to examine barriers and facilitators to clinical research participation among African Americans, as well as recommendations for overcoming these. Design and Methods Eight focus groups were conducted consisting of 64 individuals. These focus groups targeted 2 groups of individuals: (a) community members, including both individuals involved in research and individuals not involved in research, and (b) community leaders, including clergy, community health care providers and service providers who may influence decisions to participate in research. Results Among participants in both groups, the most common barriers to participation included fear and mistrust of research due to multiple factors, such as a lack of information about research and prevailing knowledge of historical occurrences. Facilitators to research participation included intrinsic factors, such as a desire to help others, and extrinsic factors, such as familiarity with the research recruiter. The focus groups also directly engaged participants in discussions of strategies that might improve recruitment, such as the importance of providing personal stories that enable community members to understand the potential benefits of research. Implications Findings from these focus groups address the mandate from funding agencies that emphasize the importance of including racially diverse populations in clinical research studies, and offer potential solutions for increasing the recruitment and retention of minority participants.

Role for memory capacity in sentence comprehension: Evidence from acute stroke

Background: Previous research has suggested that short-term and working memory (WM) resources play a critical role in sentence comprehension, especially when comprehension mechanisms cannot rely on semantics alone. However, few studies have examined this association in participants in acute stroke, before the opportunity for therapy and reorganisation of cognitive functions. Aims: The present study examined the hypothesis that severity of short-term memory (STM) deficit due to acute stroke predicts the severity of impairment in the comprehension of syntactically complex sentences. Furthermore, we examined the association between damage to the short-term and WM network and impaired sentence comprehension, as an association would be predicted by the previous hypothesis. Methods & Procedures: Forty-seven participants with acute stroke and 14 participants with a transient ischemic attack (TIA; the control group) were included in the present study. Participants received a language battery and clinical or research scans within 48 hrs of hospital admittance. The present study focused on the behavioural data from the STM and WM span tasks and a sentence-picture matching comprehension task included in this battery. Using regression analyses, we examined whether short-term and WM measures explained significant variance in sentence comprehension performance. Outcomes & Results: Consistent with prior research, STM explained significant variance in sentence comprehension performance in acute stroke; in contrast, WM accounted for little variance beyond that which was already explained by STM. Furthermore, ischemia that included the short-term/WM network was sufficient to cause sentence comprehension impairments for syntactically complex sentences. Conclusions: The present study suggests that STM resources are an important source of sentence comprehension impairments.

The role of working memory capacity and interference resolution mechanisms in task switching

Theories of task switching have emphasized a number of control mechanisms that may support the ability to flexibly switch between tasks. The present study examined the extent to which individual differences in working memory (WM) capacity and two measures of interference resolution, response–distractor inhibition and resistance to proactive interference (PI), account for variability in task switching, including global costs, local costs, and N-2 repetition costs. A total of 102 young and 60 older adults were tested on a battery of tasks. Composite scores were created for WM capacity, response–distractor inhibition, and resistance to PI; shifting was indexed by rate residual scores, which combine response time and accuracy and account for individual differences in processing speed. Composite scores served as predictors of task switching. WM was significantly related to global switch costs. While resistance to PI and WM explained some variance in local costs, these effects did not reach significance. In contrast, none of the control measures explained variance in N-2 repetition costs. Furthermore, age effects were only evident for N-2 repetition costs, with older adults demonstrating larger costs than young adults. Results are discussed within the context of theoretical models of task switching.