The use of cerebral blood flow as an index of neuronal activity in functional neuroimaging: experimental and pathophysiological considerations

Covariance PET patterns in early Alzheimer's disease and subjects with cognitive impairment but no dementia: utility in group discrimination and correlations with functional performance

Although multivariate analytic techniques might identify diagnostic patterns that are not captured by univariate methods, they have rarely been used to study the neural correlates of Alzheimer's disease (AD) or cognitive impairment. Nonquantitative H2(15)O PET scans were acquired during rest in 17 probable AD subjects selected for mild severity [mean-modified Mini Mental Status Examination (mMMS) 46/57; SD 5.1], 16 control subjects (mMMS 54; SD 2.5) and 23 subjects with minimal to mild cognitive impairment but no dementia (mMMS 53; SD 2.8). Expert clinical reading had low success in discriminating AD and controls. There were no significant mean flow differences among groups in traditional univariate SPM Noxel-wise analyses or region of interest (ROI) analyses. A covariance pattern was identified whose mean expression was significantly higher in the AD as compared to controls (P = 0.03; sensitivity 76-94%; specificity 63-81%). Sites of increased concomitant flow included insula, cuneus, pulvinar, lingual, fusiform, superior occipital and parahippocampal gyri, whereas decreased concomitant flow was found in cingulate, inferior parietal lobule, middle and inferior frontal, supramarginal and precentral gyri. The covariance analysis-derived pattern was then prospectively applied to the cognitively impaired subjects: as compared to subjects with Clinical Dementia Rating (CDR) = 0, subjects with CDR = 0.5 had significantly higher mean covariance pattern expression (P = 0.009). Expression of this pattern correlated inversely with Selective Reminding Test total recall (r = -0.401, P = 0.002), delayed recall (r = -0.351, P = 0.008) and mMMS scores (r = -0.401, P = 0.002) in all three groups combined. We conclude that patients with AD may differentially express resting cerebral blood flow covariance patterns even at very early disease stages. Significant alterations in expression of resting flow covariance patterns occur even for subjects with cognitive impairment. Expression of covariance patterns correlates with cognitive and functional performance measures, holding promise for meaningful associations with underlying biopathological processes.

Brain function in coma, vegetative state, and related disorders

We review the nosological criteria and functional neuroanatomical basis for brain death, coma, vegetative state, minimally conscious state, and the locked-in state. Functional neuroimaging is providing new insights into cerebral activity in patients with severe brain damage. Measurements of cerebral metabolism and brain activations in response to sensory stimuli with PET, fMRI, and electrophysiological methods can provide information on the presence, degree, and location of any residual brain function. However, use of these techniques in people with severe brain damage is methodologically complex and needs careful quantitative analysis and interpretation. In addition, ethical frameworks to guide research in these patients must be further developed. At present, clinical examinations identify nosological distinctions needed for accurate diagnosis and prognosis. Neuroimaging techniques remain important tools for clinical research that will extend our understanding of the underlying mechanisms of these disorders.

Flicker observation light induces diameter response in retinal arterioles: a clinical methodological study

AIM

To investigate diameter changes in retinal arterioles in response to flicker variations of the examination light.

METHODS

One randomised eye of five healthy subjects (mean age 33.8 (SD 1.6) years) was examined. The arterial diameter response to flicker light (12.5 Hz, 530-600 nm, duration 20 seconds) was automatically and continuously measured online three times by retinal vessel analyser (RVA) and once offline from flash images using the VesselMap program.

RESULTS

An arterial diameter response to flicker light was found both by RVA and by analysis of flash images (p<0.001). The maximum induced dilation reached at the end of the flicker period was +7.4% (SD 2.4%) in the RVA measurements and +3.5% (0.8%) in the photographs (p = 0.01). In both techniques the vascular diameter overshot the baseline approximately 10 seconds. In the RVA measurements a minimum of -4.6% (1.9%) (p = 0.01) was measured 22 (4.7) seconds after the end of flicker exposure.

CONCLUSION

Flicker evoked response for retinal arterioles was found both by RVA and by analysis of flash images. The authors believe that the method is suitable for the quantitative investigation of retinal vasosclerosis, especially in association with arteriosclerotic and hypertensive systemic disease.

A review of functional neuroimaging studies of vagus nerve stimulation (VNS)

Vagus nerve stimulation (VNS) is a new method for preventing and treating seizures, and shows promise as a potential new antidepressant. The mechanisms of action of VNS are still unknown, although the afferent direct and secondary connections of the vagus nerve are well established and are the most likely route of VNS brain effects. Over the past several years, many groups have used functional brain imaging to better understand VNS effects on the brain. Since these studies differ somewhat in their methodologies, findings and conclusions, at first glance, this literature may appear inconsistent. Although disagreement exists regarding the specific locations and the direction of brain activation, the differences across studies are largely due to different methods, and the results are not entirely inconsistent. We provide an overview of these functional imaging studies of VNS. PET (positron emission tomography) and SPECT (single photon emission computed tomography) studies have implicated several brain areas affected by VNS, without being able to define the key structures consistently and immediately activated by VNS. BOLD (blood oxygen level dependent) fMRI (functional magnetic resonance imaging), with its relatively high spatio-temporal resolution, performed during VNS, can reveal the location and level of the brain's immediate response to VNS. As a whole, these studies demonstrate that VNS causes immediate and longer-term changes in brain regions with vagus innervations and which have been implicated in neuropsychiatric disorders. These include the thalamus, cerebellum, orbitofrontal cortex, limbic system, hypothalamus, and medulla. Functional neuroimaging studies have the potential to provide greater insight into the brain circuitry behind the activity of VNS.

Hypoactivation of the prefrontal cortex during verbal fluency test in PTSD: a near-infrared spectroscopy study

Several studies have suggested that there is frontal dysfunction in subjects with posttraumatic stress disorder (PTSD). We investigated the relationship between alterations of the hemodynamic response of the prefrontal cortex during a cognitive task (verbal fluency task; VFT) and memory function measured using the Wechsler Memory Scale-Revised (WMS-R). The subjects were victims of the Tokyo Subway Sarin attack with (n = 8) or without (n = 26) PTSD. Hemodynamic response in the prefrontal cortex was measured using a 24-channel near-infrared spectroscopy (NIRS) system. Subjects with PTSD had a significantly smaller response of oxygenated hemoglobin and total hemoglobin during the VFT compared with those without PTSD, although there was no significant difference in performance on the VFT. Subjects with PTSD had significantly lower scores on attention and concentration in the WMS-R, which was positively correlated with the increase of total hemoglobin during the VFT. The 'frontal dysfunction' observed in subjects with PTSD may be a secondary phenomenon to reduced attentional capacity.

Activation of the prefrontal cortex to trauma-related stimuli measured by near-infrared spectroscopy in posttraumatic stress disorder due to terrorism

To develop a noninvasive method for psychophysiological assessment of posttraumatic stress disorder (PTSD), 34 victims of the Tokyo Subway Sarin Attack in 1995 including 8 diagnosed as PTSD and 12 controls were examined by a multichannel near-infrared spectroscopy (NIRS) system. Hemodynamic response in the prefrontal cortex was monitored during the presentation of trauma-related and control stimuli by video images. Skin conductance response (SCR) was also examined. Oxygenated hemoglobin significantly increased during the trauma-related image in the victims with or without PTSD. Deoxygenated hemoglobin significantly decreased only in victims with PTSD. No significant alteration was found in controls. Significantly enhanced SCR was also observed in the victims with PTSD during trauma-related stimuli. The findings suggest that measurement of cerebral hemodynamic response by NIRS is useful for psychophysiological assessment of PTSD.

Contributions of magnetic source imaging to the understanding of dyslexia

The problem in dyslexia is difficulty learning how the printed word maps onto spoken language. Magnetic Source Imaging protocols were used in three studies. The first study with dyslexic children showed greater activity in the right temporoparietal region. The second study showed the aberrant neural circuit was present in initial stages of reading acquisition. The third study demonstrated that the aberrant profile could be normalized following intensive instruction. These findings suggest that dyslexia represents a functional deficit in the neural circuit that mediates the conversion of print to sound, which is amenable to change given adequate instruction.

Glucose transport and utilization are altered in the brain of rats deficient in n-3 polyunsaturated fatty acids

Long-chain polyunsaturated (n-3) fatty acids have been reported to influence the efficiency of membrane receptors, transporters and enzymes. Because the brain is particularly rich in docosahexaenoic acid (DHA, 22:6 n-3), the present study addresses the question of whether the 22:6 n-3 fatty acid deficiency induces disorder in regulation of energy metabolism in the CNS. Three brain regions that share a high rate of energy metabolism were studied: fronto-parietal cortex, hippocampus and suprachiasmatic nucleus. The effect of the diet deficient in n-3 fatty acids resulted in a 30-50% decrease in DHA in membrane phospholipids. Moreover, a 30% decrease in glucose uptake and a 20-40% decrease in cytochrome oxidase activity were observed in the three brain regions. The n-3 deficient diet also altered the immunoreactivity of glucose transporters, namely GLUT1 in endothelial cells and GLUT3 in neurones. In n-3 fatty acid deficient rats, GLUT1-immunoreactivity readily detectable in microvessels became sparse, whereas the number of GLUT3 immunoreactive neurones was increased. However, western blot analysis showed no significant difference in GLUT1 and GLUT3 protein levels between rats deficient in n-3 fatty acids and control rats. The present results suggest that changes in energy metabolism induced by n-3 deficiency could result from functional alteration in glucose transporters.

Differential metabolic activity in the striosome and matrix compartments of the rat striatum during natural behaviors

The striosome and matrix compartments of the striatum are clearly identified by their neurochemical expression patterns and anatomical connections. To determine whether these compartments are distinguishable functionally, we used [14C]deoxyglucose metabolic mapping in the rat and tested whether neutral behavioral states (free movement, gentle restraint, and focal tactile stimulation under gentle restraint) were associated with regions of high metabolic activity in the matrix, in striosomes, or in both. We identified metabolic peaks in the striatum by means of image analysis, striosome-matrix boundaries by [3H]naloxone binding, and primary somatosensory corticostriatal input clusters by injections of anterograde tracer into electrophysiologically identified sites in SI. Peak metabolic activity was primarily confined to the matrix compartment under each behavioral condition. These findings show that during relatively neutral behavioral conditions the balance of activity between the two compartments favors the matrix and suggest that this balance is present in the striatum as part of normal behavior and processing of afferent activity.