Malcolm J. Avison

Neural substrates of facial emotion processing using fMRI

We identified human brain regions involved in the perception of sad, frightened, happy, angry, and neutral facial expressions using functional magnetic resonance imaging (fMRI). Twenty-one healthy right-handed adult volunteers (11 men, 10 women; aged 18-45; mean age 21.6 years) participated in four separate runs, one for each of the four emotions. Participants viewed blocks of emotionally expressive faces alternating with blocks of neutral faces and scrambled images. In comparison with scrambled images, neutral faces activated the fusiform gyri, the right lateral occipital gyrus, the right superior temporal sulcus, the inferior frontal gyri, and the amygdala/entorhinal cortex. In comparisons of emotional and neutral faces, we found that (1) emotional faces elicit increased activation in a subset of cortical regions involved in neutral face processing and in areas not activated by neutral faces; (2) differences in activation as a function of emotion category were most evident in the frontal lobes; (3) men showed a differential neural response depending upon the emotion expressed but women did not.

Principal component analysis of the dynamic response measured by fMRI: a generalized linear systems framework.

Principal component analysis (PCA) is one of several structure-seeking multivariate statistical techniques, exploratory as well as inferential, that have been proposed recently for the characterization and detection of activation in both PET and fMRI time series data. In particular, PCA is data driven and does not assume that the neural or hemodynamic response reaches some steady state, nor does it involve correlation with any pre-defined or exogenous experimental design template. In this paper, we present a generalized linear systems framework for PCA based on the singular value decomposition (SVD) model for representation of spatio-temporal fMRI data sets. Statistical inference procedures for PCA, including point and interval estimation will be introduced without the constraint of explicit hypotheses about specific task-dependent effects. The principal eigenvectors capture both the spatial and temporal aspects of fMRI data in a progressive fashion; they are inherently matched to unique and uncorrelated features and are ranked in order of the amount of variance explained. PCA also acts as a variation reduction technique, relegating most of the random noise to the trailing components while collecting systematic structure into the leading ones. Features summarizing variability may not directly be those that are the most useful. Further analysis is facilitated through linear subspace methods involving PC rotation and strategies of projection pursuit utilizing a reduced, lower-dimensional natural basis representation that retains most of the information. These properties will be illustrated in the setting of dynamic time-series response data from fMRI experiments involving pharmacological stimulation of the dopaminergic nigro-striatal system in primates.

Altered brain activation in cognitively intact individuals at high risk for Alzheimer’s disease

OBJECTIVE To determine whether brain function is altered in cognitively normal individuals at high risk for AD several years before the typical age at onset for this illness. BACKGROUND Neuropathologic alterations in AD precede cognitive impairment by several years. It is unknown whether functional alterations in neural circuitry accompany these neuropathologic changes, and if so, whether they may be detectable before onset of symptoms. METHODS We used functional MRI to compare cortical activation between two groups of cognitively normal women differing only in their risk for developing AD. Visual naming and letter fluency tasks were used to activate brain areas subserving object and face recognition, previously described sites of hypometabolism and neuropathologic alteration in AD. The risk groups differed in family history of AD and apolipoprotein E allele status, but were matched in age, education, and measures of cognitive performance. Average age of the study participants was 52 years. RESULTS The regional patterns of brain activation were similar between groups. However, the high risk group showed areas of significantly reduced activation in the mid- and posterior inferotemporal regions bilaterally during both tasks despite identical naming and letter fluency performance. CONCLUSIONS Cognitively normal individuals at high risk for AD demonstrate decreased brain activation in key areas engaged during naming and fluency tasks. Decreased activation in the high risk group may be a consequence of the presence of subclinical neuropathology in the inferotemporal region or in the inputs to that region. If so, these findings provide evidence of a window of opportunity for disease-modifying treatment before the onset of symptomatic AD.

Acceleration of HIV dementia with methamphetamine and cocaine.

We report a patient with rapidly accelerating HIV dementia accompanied by seizures and an unusual movement disorder despite highly potent antiretroviral therapy. This clinical constellation was associated with the non-parenteral use of methamphetamine and cocaine. Fractional enhancement time on post contrast magnetic resonance imaging studies revealed a progressive breakdown of the blood brain barrier particularly in the basal ganglia. The movement disorder but not the dementia responded to a combination of dopamine replacement and anticholinergic therapy. While the movement disorder may have been unmasked by concomitant anticonvulsant therapy, we suggestin this instance, that prior drug abuse synergized with HIV to cause a domino effect on cerebral function. Careful attention and analysis to histories of remote non-injecting drug abuse may help substantiate our hypothesis.

Hypoinsulinemia regulates amphetamine-induced reverse transport of dopamine

The behavioral effects of psychomotor stimulants such as amphetamine (AMPH) arise from their ability to elicit increases in extracellular dopamine (DA). These AMPH-induced increases are achieved by DA transporter (DAT)-mediated transmitter efflux. Recently, we have shown that AMPH self-administration is reduced in rats that have been depleted of insulin with the diabetogenic agent streptozotocin (STZ). In vitro studies suggest that hypoinsulinemia may regulate the actions of AMPH by inhibiting the insulin downstream effectors phosphotidylinositol 3-kinase (PI3K) and protein kinase B (PKB, or Akt), which we have previously shown are able to fine-tune DAT cell-surface expression. Here, we demonstrate that striatal Akt function, as well as DAT cell-surface expression, are significantly reduced by STZ. In addition, our data show that the release of DA, determined by high-speed chronoamperometry (HSCA) in the striatum, in response to AMPH, is severely impaired in these insulin-deficient rats. Importantly, selective inhibition of PI3K with LY294002 within the striatum results in a profound reduction in the subsequent potential for AMPH to evoke DA efflux. Consistent with our biochemical and in vivo electrochemical data, findings from functional magnetic resonance imaging experiments reveal that the ability of AMPH to elicit positive blood oxygen level–dependent signal changes in the striatum is significantly blunted in STZ-treated rats. Finally, local infusion of insulin into the striatum of STZ-treated animals significantly recovers the ability of AMPH to stimulate DA release as measured by high-speed chronoamperometry. The present studies establish that PI3K signaling regulates the neurochemical actions of AMPH-like psychomotor stimulants. These data suggest that insulin signaling pathways may represent a novel mechanism for regulating DA transmission, one which may be targeted for the treatment of AMPH abuse and potentially other dopaminergic disorders.

Women at risk for AD show increased parietal activation during a fluency task

Background Imaging studies have shown disparities in resting metabolism and in functional activation between cognitively normal individuals at high and low risk for AD. A recent study has shown increased parietal activation in high-risk subjects during a paired associates recall task, which the authors postulated might overlap activation typically observed in verbal fluency. Objective To determine whether parietal activation is altered in a letter fluency task in cognitively normal individuals at high risk for AD. MethodsfMRI was used to compare cortical activation between two groups of cognitively normal women differing in their risk for developing AD. A letter fluency task was used, which activates left frontal and parietal regions. The risk groups differed in family history of AD and APOE allele status but were matched in age, education, and measures of cognitive performance. Average age of the study participants was 53 years. Results The regional patterns of brain activation were similar between groups and similar to patterns observed by other investigators. However, the high-risk group showed significantly increased activation in the left parietal region despite identical letter fluency performance between risk groups. Conclusions Cognitively normal individuals at high risk for AD show increased brain activation in the left parietal region with letter fluency, a region adjacent to that observed by others using a recall task. This convergence of results indicates disruption of functional circuits involving the left parietal lobe in asymptomatic individuals at increased risk for AD.

High-Resolution 7T MRI of the Human Hippocampus In Vivo

To describe an initial experience imaging the human hippocampus in vivo using a 7T magnetic resonance (MR) scanner and a protocol developed for very high field neuroimaging.

Functional MRI of apomorphine activation of the basal ganglia in awake rhesus monkeys

Functional magnetic resonance imaging (fMRI) was used to analyze blood oxygen level-dependent (BOLD) responses in the nigrostriatal system (caudate nucleus, putamen and substantia nigra) of awake rhesus monkeys to systemic apomorphine administration. The study (1) measured BOLD responses as an index of neuronal activity in the three structures following injections of the mixed D1/D2 agonist, and (2) assessed the effects of isoflurane anesthesia on the fMRI responses. Compared to control saline injections, 0.1 mg/kg apomorphine significantly activated the caudate nucleus (P < or = 0.005), putamen (P < or = 0.001) and substantia nigra (P < or = 0.005). The responses were consistent with activation of GABAergic neurons in these three structures seen in other animal models. Isoflurane gas measurably blunted the response to apomorphine, so that a significant apomorphine activation was only seen in the substantia nigra of anesthetized animals. Even there, the mean MR signal change was reduced from 9.8% in awake monkeys to 2.3% in anesthetized animals. The data support the hypothesis that fMRI can be used to study the effects of drugs that alter basal ganglia activity in awake rhesus monkeys.

Neuroimaging correlates of HIV-associated BBB compromise

The mechanisms underlying blood-brain barrier (BBB) compromise in human immunodeficiency virus (HIV) infection and the ways in which BBB compromise might impair neurocognitive function remain poorly understood. This study had two aims: (1) to examine the relationship between BBB breakdown, measured using contrast-enhanced magnetic resonance imaging (CE-MRI), plasma viral load, and neurological status; and (2) to examine the influence of highly active antiretroviral therapy (HAART) on the relationship between neuroinflammation using myoinositol/creatine (mI/Cr), a surrogate marker of glial activation as measured by magnetic resonance spectroscopy (MRS), and BBB compromise determined by CE-MRI. In 25 HIV-infected patients, we found that: (1) the severity of neurocognitive impairment correlated with the degree of BBB breakdown in the basal ganglia; (2) for any given degree of BBB compromise, patients with high plasma viral load were more severely impaired; (3) BBB compromise correlated with mI/Cr in the basal ganglia; and (4) for any given level of mI/Cr, the severity of BBB compromise and the severity of neurocognitive impairment were significantly less in patients on HAART than in those who were HAART-naive. These results confirm a role for BBB compromise in the pathogenesis of HIV-associated neurocognitive impairment and suggest that elevated plasma viral load in the presence of BBB compromise may increase the risk for development of HIV-associated dementia (HAD). Additionally, they suggest a salutary effect of HAART on the incidence and severity of HAD, which may, in part, be due to protection of BBB integrity.

Inflammatory changes and breakdown of microvascular integrity in early human immunodeficiency virus dementia.

Increased postcontrast enhancement in contrast-enhanced magnetic resonance imaging (CE-MRI) of the central nervous system (CNS) is a predictor of human immunodeficiency virus (HIV) dementia severity in HIV-infected subjects. The present study confirms this earlier finding in a mildly impaired patient cohort, and demonstrates that the increased postcontrast enhancement is correlated with increased cerebrospinal fluid (CSF) levels of monocyte chemoattractant protein (MCP)-1, an inflammatory chemokine, and increased CNS levels of mI, a microglial marker. These results suggest that early CNS inflammation may underlie the microvascular changes observed, and may be a factor in the development of HIV dementia.