Georges Monette

Generalized collinearity diagnostics

Abstract Working in the context of the linear model y = Xβ + e, we generalize the concept of variance inflation as a measure of collinearity to a subset of parameters in β (denoted by β 1, with the associated columns of X given by X 1). The essential idea underlying this generalization is to examine the impact on the precision of estimation—in particular, the size of an ellipsoidal joint confidence region for β 1—of less-than-optimal selection of other columns of the design matrix (X 2), treating still other columns (X 0) as unalterable, even hypothetically. In typical applications, X 1 contains a set of dummy regressors coding categories of a qualitative variable or a set of polynomial regressors in a quantitative variable; X 2 contains all other regressors in the model, save the constant, which is in X 0. If σ 2 V denotes the realized variance of , and σ 2 U is the variance associated with an optimal selection of X 2, then the corresponding scaled dispersion ellipsoids to be compared are ℰ v = {x : x′V ...

Measuring premorbid IQ in traumatic brain injury: An examination of the validity of the Wechsler Test of Adult Reading (WTAR)

Estimation of premorbid IQ in traumatic brain injury (TBI) is clinically and scientifically valuable because it permits the quantification of the cognitive impact of injury. This is achieved by comparing performances on tests of current ability to estimates of premorbid IQ, thereby enabling current capacity to be interpreted in light of preinjury ability. However, the validity of premorbid IQ tests that are commonly used for TBI has been questioned. In the present study, we examined the psychometric properties of a recently developed test, the Wechsler Test of Adult Reading (WTAR), which has yet to be examined for TBI. The cognitive performance of a group of 24 patients recovering from TBI (with a mean Glasgow Coma Scale score in the severely impaired range) was measured at 2 and 5 months postinjury. On both occasions, patients were administered three tests that have been used to measure premorbid IQ (the WTAR and the Vocabulary and Matrix Reasoning subtests of the Wechsler Adult Intelligence Scale 3rd Edition, WAIS-III) and three tests of current ability (Symbol Digit Modalities Test–Oral and Similarities and Block Design subtests of the WAIS-III). We found that performance significantly improved on tests of current cognitive ability, confirming recovery. In contrast, stable performance was observed on the WTAR from Assessment 1 (M = 34.25/50) to Assessment 2 (M = 34.21/50; r = .970, p < .001). Mean improvement across assessments was negligible (t = −0.086, p = .47; Cohens d = −0.005), and minimal individual participant change was observed (modal scaled score change = 0). WTAR scores were also highly similar to scores on a demographic estimate of premorbid IQ. Thus, converging evidence—high stability during recovery from TBI and similar IQ estimates to those of a demographic equation—suggests that the WTAR is a valid measure of premorbid IQ for TBI. Where word pronunciation tests are indicated (i.e., in patients for whom English is spoken and read fluently), these results endorse the use of the WTAR for patients with TBI.

Prediction of Return to Productivity After Severe Traumatic Brain Injury: Investigations of Optimal Neuropsychological Tests and Timing of Assessment

OBJECTIVES (1) To examine predictive validity of global neuropsychological performance, and performance on timed tests (controlling for manual motor function) and untimed tests, including attention, memory, executive function, on return to productivity at 1 year after traumatic brain injury (TBI). (2) To compare predictive validity at 8 weeks versus 5 months postinjury. (3) To examine predictive validity of early degree of recovery (8wk-5mo postinjury) for return to productivity. DESIGN Longitudinal, within subjects. SETTING Inpatient neurorehabilitation and community. PARTICIPANTS Patients (N=63) with moderate to severe TBI. INTERVENTIONS Not applicable. MAIN OUTCOME MEASURES PRIMARY OUTCOME return to productivity at 1 year postinjury. Primary predictors: neuropsychological composite scores. Control variables: posttraumatic amnesia, acute care length of stay (LOS), Glasgow Coma Scale score, age, and estimated premorbid intelligence quotient. RESULTS Return to productivity was significantly correlated with global neuropsychological performance at 5 months postinjury (P<.05) and showed a trend toward significance at 8 weeks. Performance on the untimed composite score, and more specifically executive and memory functions, mirrored this pattern. Logical Memory performance significantly predicted return to productivity, but not other memory tests. Timed tests showed no significance or trend at either time point. Early degree of recovery did not predict return to productivity. Among control variables, only acute care LOS was predictive of return to productivity. CONCLUSIONS Findings validate utility of early neuropsychological assessment for predicting later return to productivity. They also provide more precise information regarding the optimal timing and test type: results support testing at 5 months postinjury on untimed tests (memory and executive function), but not simple attention or speed of mental processing. Findings are discussed with reference to previous literature.

Examining Moderators of Cognitive Recovery Trajectories After Moderate to Severe Traumatic Brain Injury

OBJECTIVES To examine the influence of cognitive reserve-related moderator variables on recovery trajectories during the first year after traumatic brain injury (TBI). Using mixed effects models, we measured (1) the level of cognitive function at 2 and 12 months postinjury and (2) the trajectories of cognitive recovery during the first 12 months postinjury. DESIGN Repeated-measures design with neuropsychological testing at 2, 5, and 12 months postinjury. SETTING Large, urban inpatient neurorehabilitation program. PARTICIPANTS Patients (N=75) with moderate-to-severe TBI. INTERVENTIONS Not applicable. MAIN OUTCOME MEASURES PRIMARY OUTCOMES neuropsychological composite scores including simple speed of processing, complex speed of processing, memory, untimed executive functions, and attention span. Primary predictors: age, estimated premorbid intelligence quotient (IQ), and years of education. RESULTS Only age significantly moderated trajectories. Decreasing age significantly enhanced recovery of speed of processing, both simple (2-12mo postinjury, P<.001) and complex (2-12mo postinjury, P<.05; 5-12mo postinjury, P<.005). Decreasing age and increasing estimated premorbid IQ were associated with higher performance at 2 and 12mo postinjury for simple speed of processing (premorbid IQ, 2 and 12mo), complex speed of processing (age, 2 and 12mo), untimed executive functions (premorbid IQ, 2 and 12mo), and memory (premorbid IQ, 2 and 12mo). CONCLUSIONS Recovery of speed of processing (both simple and complex) was favorably moderated by younger age. Older age is associated with more neuronal loss and less integrity of white matter, and speed of processing is associated with white matter networks. The recuperative effects of younger age may therefore be attributable to greater reserve capacity (as indexed by white matter integrity). Lower age and higher estimated premorbid IQ were associated with higher functioning on a variety of cognitive outcomes. This may reflect the buffering effects of reserve capacity or premorbid differences in age and IQ-related cognitive functioning. Implications for rehabilitation and recovery mechanisms are discussed.

Elliptical Insights: Understanding Statistical Methods through Elliptical Geometry

Visual insights into a wide variety of statistical methods, for both didactic and data analytic purposes, can often be achieved through geometric diagrams and geometrically based statistical graphs. This paper extols and illustrates the virtues of the ellipse and her higher-dimensional cousins for both these purposes in a variety of contexts, including linear models, multivariate linear models and mixed-effect models. We emphasize the strong relationships among statistical methods, matrix-algebraic solutions and geometry that can often be easily understood in terms of ellipses.

Narrative and emotion integration in psychotherapy: Investigating the relationship between autobiographical memory specificity and expressed emotional arousal in brief emotion-focused and client-centred treatments of depression

Abstract Clinically depressed individuals have consistently been shown to demonstrate a bias for overgeneral autobiographical memory (ABM) disclosure, a strategy used to protect against the access of intense, primary emotions that may accompany specific memories. The present study examined how ABM specificity in client narratives was related to expressed emotional arousal in brief emotion-focused and client-centred psychotherapy for depression. Emotion episodes identified in two early-, two middle-, and two late-therapy transcripts drawn from 34 clients from the York I Depression Study were rated for degree of ABM specificity and expressed emotional arousal. A hierarchical linear modelling analysis demonstrated that greater ABM specificity was associated with higher expressed emotional arousal for clients who were no longer depressed at therapy termination.

Mathematical models of cognitive recovery

Longitudinal psychological test results are used as dependent variables to explore the complex relationship between length of coma, time of testing on the recovery curve, and corresponding cognitive status after traumatic brain injury (TBI). A database containing 319 TBI patients with a broad spectrum of coma duration was used. Statistical analysis of mixed effects modelling was applied to longitudinal WAIS-R (Wechsler Adult Intelligence Scale-Revised) scores to construct two mathematical models (verbal IQ and performance IQ). The models predict the course of recovery (initial cognitive level post-coma, eventual recovery level, and level of cognitive functioning at any point on the recovery curve) when the duration of coma is known. Performance IQ was found to recover at a rate that is almost four times slower than verbal IQ. The results have important clinical rehabilitation implications. This statistical modelling technique also enables the medical researcher to investigate disease progression or recovery using structured assessments, which would normally be part of the routine medical monitoring.

Developmental and gender influences on executive function following concussion in youth hockey players

Abstract Background: Concussion is the most common athletic injury in youth who are simultaneously undergoing rapid developmental changes in the brain, specifically the development of executive functions (EF). The developing brain is more vulnerable to concussive injury with a protracted and different trajectory of recovery than that of adults. Thus, there is a critical need to enhance understanding of how concussion affects EF in youth. Objective: To investigate the effects of age, gender and concussion history (i.e. concussion incidence, recency, severity) on EF in youth hockey players. Methods: This 3-year cross-sectional and longitudinal multiple cohort study examined data from 211 hockey players of 8–15 years of age. Mixed-effects modelling was used to examine the influence of age, gender and concussion on EF in youth athletes. Findings: Baseline analyses revealed significant age and gender effects on measures of EF. Multiple effects of concussion history on measures of cognitive flexibility (F = 2.48, p = 0.03) and psychomotor speed (F = 2.59, p = 0.04) were found. Implications: This study highlights the impact of age, gender and concussion on EF in youth. These findings provide foundational knowledge to better manage cognitive sequelae following sports-related concussion.

Nutrient Preferences of Brazilian Hummingbirds

Abstract The Stripe-tailed Hummingbird (Eupherusa eximia) prefers nectar that is highly supplemented in both vitamins and minerals, when offered the choice of no, low, or high supplemented nectar. We tested the responses of hummingbirds in southern Brazil to four solutions: nectar, nectar supplemented with vitamins, nectar supplemented with minerals, and water with both vitamins and minerals added. Hummingbirds spent less time at and made fewer visits to the supplemented water than to the different nectar solutions, suggesting that nutrients alone are not enough to attract birds. One of eight species, the Violet-capped Woodnymph (Thalurania glaucopis), visited the mineral supplemented nectar more often than both nectars and one unidentified hermit species (Phaethornis sp.) visited the mineral treatment more than straight nectar. The Black-throated Mango (Anthracothorax nigricollis) made more visits to the vitamin supplemented nectar than to mineral-rich nectar whereas the Glittering-bellied Emerald (Chlorostilbon aureoventris) avoided the vitamin treatment, preferring straight nectar. A general pattern of preference was not found among species.

A Preliminary Exploration of Concussion and Strength Performance in Youth Ice Hockey Players

The objective of this study was to describe the effect of concussion on upper and lower body strength in children and youth athletes. The participant group was made up of 178 unique male and female ice hockey players (ages 8-14 years). Using a 3-year prospective longitudinal research design, baseline and post-concussion data on hand grip strength, jump tests, and leg maximal voluntary contraction were collected. Using a linear mixed-effects model, no significant differences were found when comparing the baseline strength performance of individuals who went on to experience a concussion and those who did not. When accounting for sex, multiple concussions, and ongoing changes in strength associated with age, weaker hand grip scores were found following concussion while participants were still symptomatic. Lower squat jump heights were achieved while participants were symptomatic as well as when they were no longer self-reporting symptoms associated with concussion. This study represents an initial step towards better understanding strength performance following concussion that may limit the on and off ice performance of youth ice hockey players, as well as predispose youth to subsequent injuries.