No association between peripheral serotonin-gene-related DNA methylation and brain serotonin neurotransmission in the healthy and depressed state.

BACKGROUND: Methylation of serotonin-related genes has been proposed as a plausible gene-by-environment link which may mediate environmental stress, depressive and anxiety symptoms. DNA methylation is often measured in blood cells, but little is known about the association between this peripheral epigenetic modification and brain serotonergic architecture. Here, we evaluated the association between whole-blood-derived methylation of four CpG sites in the serotonin transporter (SLC6A4) and six CpG sites of the tryptophan hydroxylase 2 (TPH2) gene and in-vivo brain levels of serotonin transporter (5-HTT) and serotonin 4 receptor (5-HT4) in a cohort of healthy individuals (N = 254) and, for 5-HT4, in a cohort of unmedicated patients with depression (N = 90). To do so, we quantified SLC6A4/TPH2 methylation using bisulfite pyrosequencing and estimated brain 5-HT4 and 5-HTT levels using positron emission tomography. In addition, we explored the association between SLC6A4 and TPH2 methylation and measures of early life and recent stress, depressive and anxiety symptoms on 297 healthy individuals. RESULTS: We found no statistically significant association between peripheral DNA methylation and brain markers of serotonergic neurotransmission in patients with depression or in healthy individuals. In addition, although SLC6A4 CpG2 (chr17:30,236,083) methylation was marginally associated with the parental bonding inventory overprotection score in the healthy cohort, statistical significance did not remain after accounting for blood cell heterogeneity. CONCLUSIONS: We suggest that findings on peripheral DNA methylation in the context of brain serotonin-related features should be interpreted with caution. More studies are needed to rule out a role of SLC6A4 and TPH2 methylation as biomarkers for environmental stress, depressive or anxiety symptoms.

Oral contraceptives and the serotonin 4 receptor: a molecular brain imaging study in healthy women.

OBJECTIVE: Sex steroid hormones potently shape brain functions, including those critical to maintain mental health such as serotonin signaling. Use of oral contraceptives (OCs) profoundly changes endogenous sex steroid hormone levels and dynamics. Recent register-based studies show that starting an OC is associated with increased risk of developing depression. Here, we investigate whether use of OCs in healthy women is associated with a marker of the serotonin system in terms of serotonin 4 receptor (5-HT4R) brain imaging. METHODS: [11C]SB207145-PET imaging data on 53 healthy women, of whom 16 used OCs, were available from the Cimbi database. We evaluated global effects of OC use on 5-HT4R binding in a latent variable model based on 5-HT4R binding across cortical and subcortical regions. RESULTS: We demonstrate that OC users have 9-12% lower global brain 5-HT4R binding potential compared to non-users. Univariate region-based analyses (pallidostriatum, caudate, hippocampus, amygdala, anterior cingulate cortex, and neocortex) supported the global effect of OC use with the largest difference present in the hippocampus (-12.8% (95% CI [-21.0; -3.9], Pcorrected = 0.03). CONCLUSION: We show that women who use OCs have markedly lower brain 5-HT4R binding relative to non-users, which constitutes a plausible molecular link between OC use and increased risk of depressive episodes. We propose that this reflects a reduced 5-HT4R gene expression, possibly related to a blunted ovarian hormone state among OC users.

Molecular imaging of neuroinflammation in patients after mild traumatic brain injury: a longitudinal 123 I-CLINDE single photon emission computed tomography study.

BACKGROUND AND PURPOSE: Neuroinflammation has been proposed as part of the pathogenesis of post-concussion symptoms (PCS), but the inflammatory response of the human brain to mild traumatic brain injury (mTBI) remains unknown. We hypothesized that a neuroinflammatory response is present in mTBI at 1-2 weeks post-injury and persists in patients with PCS. METHODS: We scanned 14 patients with mTBI without signs of structural damage at 1-2 weeks and 3-4 months post-injury and 22 healthy controls once using the single photon emission computed tomography tracer 123 I-CLINDE, which visualizes translocator protein (TSPO), a protein upregulated in active immune cells. PCS was defined as three or more persisting symptoms from the Rivermead Post Concussion Symptoms Questionnaire at 3 months post-injury. RESULTS: Across brain regions, patients had significantly higher 123 I-CLINDE binding to TSPO than healthy controls, both at 1-2 weeks after the injury in all patients (P = 0.011) and at 3-4 months in the seven patients with PCS (P = 0.006) and in the six patients with good recovery (P = 0.018). When the nine brain regions were tested separately and results were corrected for multiple comparisons, no individual region differed significantly, but all estimated parameters indicated increased 123 I-CLINDE binding to TSPO, ranging from 2% to 19% in all patients at 1-2 weeks, 13% to 27% in patients with PCS at 3-4 months and -9% to 17% in patients with good recovery at 3-4 months. CONCLUSIONS: Neuroinflammation was present in mTBI at 1-2 weeks post-injury and persisted at 3-4 months post-injury with a tendency to be most pronounced in patients with PCS.

BDNF val66met association with serotonin transporter binding in healthy humans.

The serotonin transporter (5-HTT) is a key feature of the serotonin system, which is involved in behavior, cognition and personality and implicated in neuropsychiatric illnesses including depression. The brain-derived neurotrophic factor (BDNF) val66met and 5-HTTLPR polymorphisms have predicted differences in 5-HTT levels in humans but with equivocal results, possibly due to limited sample sizes. Within the current study we evaluated these genetic predictors of 5-HTT binding with [11C]DASB positron emission tomography (PET) in a comparatively large cohort of 144 healthy individuals. We used a latent variable model to determine genetic effects on a latent variable (5-HTTLV), reflecting shared correlation across regional 5-HTT binding (amygdala, caudate, hippocampus, midbrain, neocortex, putamen and thalamus). Our data supported a significant BDNF val66met effect on 5-HTTLV such that met-carriers showed 2-7% higher subcortical 5-HTT binding compared with val/val individuals (P=0.042). Our data did not support a BDNF val66met effect in neocortex and 5-HTTLPR did not significantly predict 5-HTTLV. We did not observe evidence for an interaction between genotypes. Our findings indicate that met-carriers have increased subcortical 5-HTT binding. The small difference suggests limited statistical power may explain previously reported null effects. Our finding adds to emerging evidence that BDNF val66met contributes to differences in the human brain serotonin system, informing how variability in the 5-HTT level emerges and may represent an important molecular mediator of BDNF val66met effects on behavior and related risk for neuropsychiatric illness.

Heat-washout measurements compared to distal blood pressure and perfusion in orthopaedic patients with foot ulcers.

Distal blood pressure and local skin perfusion pressure were compared to measurement of blood flow rate (BFR) measured by the heat-washout method in orthopaedic patients with and without diabetes, all with a foot ulcer in one foot, compared to healthy controls. The correlation was good between heat-washout and distal blood pressure in patients with diabetes with and without an ulcer (P = 0·024 and 0·059, respectively). The correlation was weak in patients without diabetes with and without an ulcer, most probably due to power problems (P = 0·118 and 0·116, respectively). The correlation in the healthy controls was poor (P = 0·333 and 0·685 for right and left 1. Toe, respectively) probably because not all measurements were performed under optimal conditions with maximally dilated arterioles and warm hands and feet. The patients already have maximally dilated arterioles to extract the maximal amount of oxygen from the surrounding tissue, and therefore, measurements are easier made in these subjects. BFR in the first toe increased significantly in all patients when the foot was moved from heart level to 50 cm below heart level (P = between 0·03 and 0·05) as previously seen in patients with claudication. There was no statistical difference in the healthy controls, consistent with previous findings. These results may indicate that the heat-washout method can be used as an alternative to strain gauge blood pressure in the evaluation of peripheral artery disease and wound healing potentials. Furthermore, the heat-washout measurements can be used bedside.

Regional brain volumes, diffusivity, and metabolite changes after electroconvulsive therapy for severe depression.

OBJECTIVE: To investigate the role of hippocampal plasticity in the antidepressant effect of electroconvulsive therapy (ECT). METHOD: We used magnetic resonance (MR) imaging including diffusion tensor imaging (DTI) and proton MR spectroscopy (1 H-MRS) to investigate hippocampal volume, diffusivity, and metabolite changes in 19 patients receiving ECT for severe depression. Other regions of interest included the amygdala, dorsolateral prefrontal cortex (DLPFC), orbitofrontal cortex, and hypothalamus. Patients received a 3T MR scan before ECT (TP1), 1 week (TP2), and 4 weeks (TP3) after ECT. RESULTS: Hippocampal and amygdala volume increased significantly at TP2 and continued to be increased at TP3. DLPFC exhibited a transient volume reduction at TP2. DTI revealed a reduced anisotropy and diffusivity of the hippocampus at TP2. We found no significant post-ECT changes in brain metabolite concentrations, and we were unable to identify a spectral signature at ≈1.30 ppm previously suggested to reflect neurogenesis induced by ECT. None of the brain imaging measures correlated to the clinical response. CONCLUSION: Our findings show that ECT causes a remodeling of brain structures involved in affective regulation, but due to their lack of correlation with the antidepressant effect, this remodeling does not appear to be directly underlying the antidepressant action of ECT.

Central 5-HT neurotransmission modulates weight loss following gastric bypass surgery in obese individuals.

The cerebral serotonin (5-HT) system shows distinct differences in obesity compared with the lean state. Here, it was investigated whether serotonergic neurotransmission in obesity is a stable trait or changes in association with weight loss induced by Roux-in-Y gastric bypass (RYGB) surgery. In vivo cerebral 5-HT2A receptor and 5-HT transporter binding was determined by positron emission tomography in 21 obese [four men; body mass index (BMI), 40.1 ± 4.1 kg/m(2)] and 10 lean (three men; BMI, 24.6 ± 1.5 kg/m(2)) individuals. Fourteen obese individuals were re-examined after RYGB surgery. First, it was confirmed that obese individuals have higher cerebral 5-HT2A receptor binding than lean individuals. Importantly, we found that higher presurgical 5-HT2A receptor binding predicted greater weight loss after RYGB and that the change in 5-HT2A receptor and 5-HT transporter binding correlated with weight loss after RYGB. The changes in the 5-HT neurotransmission before and after RYGB are in accordance with a model wherein the cerebral extracellular 5-HT level modulates the regulation of body weight. Our findings support that the cerebral 5-HT system contributes both to establish the obese condition and to regulate the body weight in response to RYGB.

Cerebral serotonin transporter binding is inversely related to body mass index.

Overweight and obesity is a health threat of increasing concern and understanding the neurobiology behind obesity is instrumental to the development of effective treatment regimes. Serotonergic neurotransmission is critically involved in eating behaviour; cerebral level of serotonin (5-HT) in animal models is inversely related to food intake and body weight and some effective anti-obesity agents involve blockade of the serotonin transporter (SERT). We investigated in 60 healthy volunteers body mass index (BMI) and regional cerebral SERT binding as measured with [(11)C]DASB PET. In a linear regression model with adjustment for relevant covariates, we found that cortical and subcortical SERT binding was negatively correlated to BMI (-0.003 to -0.012 BP(ND) unit per kg/m(2)). Tobacco smoking and alcohol consumption did not affect cerebral SERT binding. Several effective anti-obesity drugs encompass blockade of the SERT; yet, our study is the first to demonstrate an abnormally decreased cerebral SERT binding in obese individuals. Whether the SERT has a direct role in the regulation of appetite and eating behaviour or whether the finding is due to a compensatory downregulation of SERT secondary to other dysfunction(s) in the serotonergic transmitter system, such as low baseline serotonin levels, remains to be established.

Brain serotonin 2A receptor binding: relations to body mass index, tobacco and alcohol use.

Manipulations of the serotonin levels in the brain can affect impulsive behavior and influence our reactivity to conditioned reinforcers. Eating, tobacco smoking, and alcohol consumption are reinforcers that are influenced by serotonergic neurotransmission; serotonergic hypofunction leads to increased food and alcohol intake, and conversely, stimulation of the serotonergic system induces weight reduction and decreased food/alcohol intake as well as tobacco smoking. To investigate whether body weight, alcohol intake and tobacco smoking were related to the regulation of the cerebral serotonin 2A receptor (5-HT(2A)) in humans, we tested in 136 healthy human subjects if body mass index (BMI), degree of alcohol consumption and tobacco smoking was associated to the cerebral in vivo 5-HT(2A) receptor binding as measured with (18)F-altanserin PET. The subjects' BMI's ranged from 18.4 to 42.8 (25.2+/-4.3) kg/m(2). Cerebral cortex 5-HT(2A) binding was significantly positively correlated to BMI, whereas no association between cortical 5-HT(2A) receptor binding and alcohol or tobacco use was detected. We suggest that our observation is driven by a lower central 5-HT level in overweight people, leading both to increased food intake and to a compensatory upregulation of cerebral 5-HT(2A) receptor density.

Reduced 5-HT2A receptor binding in patients with mild cognitive impairment.

Previous studies of patients with Alzheimer's disease (AD) have described reduced brain serotonin 2A (5-HT(2A)) receptor density. It is unclear whether this abnormality sets in early in the course of the disease and whether it is related to early cognitive and neuropsychiatric symptoms. We assessed cerebral 5-HT(2A) receptor binding in patients with mild cognitive impairment (MCI) and related 5-HT(2A) receptor binding to clinical symptoms. Sixteen patients with MCI of the amnestic type (mean age 73, mean MMSE 26.1) and 17 age and sex matched control subjects were studied with MRI and [(18)F]altanserin PET in a bolus-infusion approach. A significant global reduction of 20-30% in 5-HT(2A) binding (atrophy corrected) was found in most neocortical areas. Reduced 5-HT(2A) binding in the striatum correlated significantly with Neuropsychiatric Inventory depression and anxiety scores. We conclude that widespread reductions in 5-HT(2A) receptor binding were found in amnestic MCI, pointing at the presence of serotonergic dysfunction in prodromal AD. This may provide some of the pathophysiological background for the neuropsychiatric symptoms found in early AD.

The (18)F-fluorodeoxyglucose lumped constant determined in human brain from extraction fractions of (18)F-fluorodeoxyglucose and glucose.

Quantification of regional cerebral glucose metabolism (CMRglc) using positron emission tomography and 18F-fluorodeoxyglucose (PET-FDG) requires knowledge of the correction factor between FDG and glucose net clearance, the FDG lumped constant (LC). Because diverging values for LC have been obtained, the authors reevaluated LC by measuring the ratio of the cerebral net extraction fractions of FDG (E*) and glucose (E) from arteriovenous cerebral measurements. Thirty subjects were studied (mean age = 25 +/- 4 years): 12 during a programed infusion of FDG and 18 after a bolus injection of FDG. In the infusion study, LC was calculated as the ratio E*/E. In the bolus study, E* was calculated from the slope of a Patlak-Gjedde plot. Lumped constant was significantly smaller in the infusion study as compared with the bolus study (0.48 +/- 0.16 vs. 0.81 +/- 0.27, P < 0.001). In 4 subjects studied during continuous FDG infusion for 2.5 hours, LC decreased to 0.36 +/- 0.11. These results suggest that the "steady-state" method underestimates LC because E* continues to decline because of significant labeled product. Further, the authors provide evidence for resetting of LC toward a greater value. The subsequent resetting of CMRglc provides a physiologically more meaningful estimate and allows for comparison of CMRglc values between different methodologies.

Correlations of brain MRI parameters to disability in multiple sclerosis.

OBJECTIVES: The objective was to correlate magnetic resonance imaging (MRI) T2-weighted lesion load and measures of white matter atrophy in the brain to disability in a population-based sample of patients with multiple sclerosis (MS). MATERIAL AND METHODS: A well defined cohort of patients was drawn at random from the general MS population by using the Danish Multiple Sclerosis Registry. A semi-automated local thresholding technique was used to quantify T2-weighted lesions on MRI; whereas manual tracing was applied to measure the corpus callosum brain ratio (CCR) and the ventricle brain ratio (VBR). RESULTS: A sample of 86 patients with a mean age of 43.3 years (SD 4.3), mean disease duration of 13.6 years (SD 4.4) and a median Expanded Disability Status Score (EDSS) of 6.0 was identified. The correlation between total lesion area of the brain (TLA) and disability (EDSS) for the whole sample was moderate (Spearman rank correlation coefficient r=0.48, P<0.001). Also correlations of CCR and VBR to disability (r=0.32-0.46) were significant. CONCLUSIONS: Correlations of TLA and disability in this study were rather strong. Hence, T2-weighted MRI lesion load in the brain still plays an important role as a surrogate marker of disease and as a secondary outcome measure in phase III treatment trials.

Quantitation of regional cerebral blood flow corrected for partial volume effect using O-15 water and PET: II. Normal values and gray matter blood flow response to visual activation.

One of the most limiting factors for the accurate quantification of physiologic parameters with positron emission tomography (PET) is the partial volume effect (PVE). To assess the magnitude of this contribution to the measurement of regional cerebral blood flow (rCBF), the authors have formulated four kinetic models each including a parameter defining the perfusable tissue fraction (PTF). The four kinetic models used were 2 one-tissue compartment models with (Model A) and without (Model B) a vascular term and 2 two-tissue compartment models with fixed (Model C) or variable (Model D) white matter flow. Furthermore, rCBF based on the autoradiographic method was measured. The goals of the study were to determine the following in normal humans: (1) the optimal model, (2) the optimal length of fit, (3) the model parameters and their reproducibility, and (4) the effects of data acquisition (2D or 3D). Furthermore, the authors wanted to measure the activation response in the occipital gray matter compartment, and in doing so test the stability of the PTF, during perturbations of rCBF induced by visual stimulation. Eight dynamic PET scans were acquired per subject (n = 8), each for a duration of 6 minutes after IV bolus injection of H2(15)O. Four of these scans were performed using 2D and four using 3D acquisition. Visual stimulation was presented in four scans, and four scans were during rest. Model C was found optimal based on Akaike's Information Criteria (AIC) and had the smallest coefficient of variance after a 6-minute length of fit. Using this model the average PVE corrected rCBF during rest in gray matter was 1.07 mL x min(-1) x g(-1) (0.11 SD), with an average coefficient of variance of 6%. Acquisition mode did not affect the estimated parameters, with the exception of a significant increase in the white matter rCBF using the autoradiographic method (2D: 0.17 mL x min(-1) x g(-1) (0.02 SD); 3D: 0.21 mL x min(-1) x g(-1) (0.02 SD)). At a 6-minute fit the average gray matter CBF using Models C and D were increased by 100% to 150% compared with Models A and B and the autoradiographic method. There were no significant changes in the perfusable tissue fraction by the activation induced rCBF increases. The largest activation response was found using Model C (median = 39.1%). The current study clearly demonstrates the importance of PVE correction in the quantitation of rCBF in normal humans. The potential use of this method is to cost-effectively deliver PVE corrected measures of rCBF and tissue volumes without reference to imaging modalities other than PET.

Quantitation of regional cerebral blood flow corrected for partial volume effect using O-15 water and PET: I. Theory, error analysis, and stereologic comparison.

Limited spatial resolution of positron emission tomography (PET) can cause significant underestimation in the observed regional radioactivity concentration (so-called partial volume effect or PVE) resulting in systematic errors in estimating quantitative physiologic parameters. The authors have formulated four mathematical models that describe the dynamic behavior of a freely diffusible tracer (H215O) in a region of interest (ROI) incorporating estimates of regional tissue flow that are independent of PVE. The current study was intended to evaluate the feasibility of these models and to establish a methodology to accurately quantify regional cerebral blood flow (CBF) corrected for PVE in cortical gray matter regions. Five monkeys were studied with PET after IV H2(15)O two times (n = 3) or three times (n = 2) in a row. Two ROIs were drawn on structural magnetic resonance imaging (MRI) scans and projected onto the PET images in which regional CBF values and the water perfusable tissue fraction for the cortical gray matter tissue (hence the volume of gray matter) were estimated. After the PET study, the animals were killed and stereologic analysis was performed to assess the gray matter mass in the corresponding ROIs. Reproducibility of the estimated parameters and sensitivity to various error sources were also evaluated. All models tested in the current study yielded PVE-corrected regional CBF values (approximately 0.8 mL x min(-1) x g(-1) for models with a term for gray matter tissue and 0.5 mL x min(-1) x g(-1) for models with a term for a mixture of gray matter and white matter tissues). These values were greater than those obtained from ROIs tracing the gray matter cortex using conventional H2(15)O autoradiography (approximately 0.40 mL x min(-1) x g(-1)). Among the four models, configurations that included two parallel tissue compartments demonstrated better results with regards to the agreement of tissue time-activity curve and the Akaike's Information Criteria. Error sensitivity analysis suggested the model that fits three parameters of the gray matter CBF, the gray matter fraction, and the white matter fraction with fixed white matter CBF as the most reliable and suitable for estimating the gray matter CBF. Reproducibility with this model was 11% for estimating the gray matter CBF. The volume of gray matter tissue can also be estimated using this model and was significantly correlated with the results from the stereologic analysis. However, values were significantly smaller compared with those measured by stereologic analysis by 40%, which can not be explained by the methodologic errors. In conclusion, the partial volume correction was essential in quantitation of regional CBF. The method presented in this article provided the PVE-corrected regional CBF in the cortical gray matter tissue. This study also suggests that further studies are required before using MRI derived anatomic information for PVE correction in PET.

Cerebral activation during micturition in normal men.

Specific cerebral lesions have shown the crucial role of the brain in the control of micturition. The precise identification of the anatomical cerebral structures involved in micturition can contribute to a better understanding of the control of micturition and the development of therapeutic models. Various neuropathological and animal studies have referred to the medulla oblongata, pons, limbic system, superior frontal lobe and premotor cortical regions as areas implicated in micturition control. The aim of this study was to investigate whether the activity of these areas during micturition can be confirmed by PET in normal men. The distribution of the regional cerebral blood flow after bolus injection of (15)O water was used as an indirect measure of cerebral activation. PET scans were performed during the following three conditions: (i) at rest with the bladder empty; (ii) during simulated micturition after instillation of isotonic saline into the urinary bladder; and (iii) the withholding of urine (saline). Normal micturition using this model was achieved in eight out of 12 right-handed normal subjects. The changes in bladder contraction, bladder pressure and intra-abdominal pressure were monitored on-line during the whole scanning session by a cystometry device. The images were analysed using statistical parametric mapping at a significance threshold of P < 0.05 with correction for multiple independent comparisons. Micturition versus rest was associated with bilateral activation of areas close to the postcentral gyrus, inferior frontal gyrus, globus pallidus, cortex cerebelli, vermis and midbrain. On the left side, activation of the middle frontal gyrus, superior frontal gyrus, superior precentral gyrus, thalamus and the caudal part of the anterior cingulate gyrus was seen, while on the right side we found activation in the supramarginal gyrus, mesencephalon and insula. When the threshold value was lowered to P < 0.001 (Z > 3.09) without correction for multiple comparisons, we found additional activation in the medial pontine tegmentum, mesencephalon, right thalamus, right middle frontal gyrus and left insula. When urine- withholding was compared with rest, the left insula showed a tendency to activate. We conclude from this study, in which urinary bladder contraction was verified cystometrically, that the onset and maintenance of micturition in normal men is associated with a vast network of cortical and subcortical regions, confirming observations from clinical and animal studies.

Generalizable patterns in neuroimaging: how many principal components?

Generalization can be defined quantitatively and can be used to assess the performance of principal component analysis (PCA). The generalizability of PCA depends on the number of principal components retained in the analysis. We provide analytic and test set estimates of generalization. We show how the generalization error can be used to select the number of principal components in two analyses of functional magnetic resonance imaging activation sets.

Metabolic and hemodynamic evaluation of brain metastases from small cell lung cancer with positron emission tomography.

Brain metastases from small cell lung cancer respond to chemotherapy, but response duration is short and the intracerebral concentration of chemotherapy may be too low because of the characteristics of the blood-brain barrier. Positron emission tomography has been applied in a variety of tumors for studies of metabolic and hemodynamic features. This study was performed to determine regional cerebral metabolic rate of glucose (rCMRglu), regional cerebral blood flow (rCBF), and regional cerebral blood volume (rCBV) in brain metastases from small cell lung cancer and the surrounding brain. Tumor rCMRglu, rCBF, and rCBV exerted a broad variability, but were higher than the corresponding values in white matter and higher than or similar to those of gray matter. Tumor rCMRglu and rCBF were highly correlated (P < 0.01, r = 0.79). No correlation between survival and metabolic or hemodynamic parameters could be demonstrated. After radiotherapy, mean tumor rCMRglu decreased from 0.40 to 0.31 micromol/g/min (not significant), and rCBF and rCBV remained unchanged. However, cortical rCBF demonstrated a trend of increased values after radiotherapy from 0.37 to 0.49 ml/g/min (P = 0.13). No change in rCMRglu was observed in gray or white matter after radiotherapy. Global CBF seems to be reversibly depressed by the metastases, but local hemodynamic changes in the tumor could not be detected with positron emission tomography in this study. An association between high tumor rCMRglu and rCBF as an indicator of hypoxia was not observed. Other methods for noninvasive in vivo analysis of tumor hemodynamics are needed, especially for discrimination between tumor necrosis and hypoxia.

Parieto-occipital cortex activation during self-generated eye movements in the dark.

A number of extrastriate visual areas in the parieto-occipital cortex are known from single-cell recordings of the macaque monkey to be involved in the coding of eye-position signals in the brain. These are important for the accurate location of visual objects in extrapersonal space. It can be predicted that these areas will show increased activation during the performance of eye movements at high frequency. In the present study PET and measurements of the regional distribution of cerebral blood flow (rCBF) were used as indirect measures of neural activity. Two independent groups of normal volunteers performed large-amplitude self-generated eye movements in complete darkness, thus removing the confounding effects of visual stimulation on parieto-occipital activation. The first group (group A; n = 5) served as a hypothesis-generating group and the second group (group B; n = 4) served as a hypothesis-testing group. The data were analysed using statistical parametric mapping at a significance level corrected for multiple comparisons (group A, Z > 4.08; group B, Z > 4.04). Significant rCBF increases were found for both groups in the frontal eye fields, supplementary eye fields, cerebellar vermis and putamina/thalami. Additionally, activation was found in the cunei in the posterior bank of the parieto-occipital sulcus. Also, the extraocular muscles were activated and, as a consequence of the partial volume effect, projected to the orbitofrontal cortices. At a less conservative threshold, activation was found close to the intraparietal sulci on the left side (Z = 3.91, P = 0.09) and right side (Z = 3.33, P = 0.42). The locations of these areas were confirmed in group B with reference to high-resolution structural MRI scans. The activation of the parieto-occipital cortex without overt visual stimuli is interpreted as the result of neural activity related to the reception of efferent copies of motor commands and/or the activation of neurons coding for eye position relative to the orbits. These are important constituents for the location and remapping of visual stimuli in space.

Brain activation during word identification and word recognition.

Previous memory research has suggested that the effects of prior study observed in priming tasks are functionally, and neurobiologically, distinct phenomena from the kind of memory expressed in conventional (explicit) memory tests. Evidence for this position comes from observed dissociations between memory scores obtained with the two kinds of tasks. However, there is continuing controversy about the meaning of these dissociations. In recent studies, Ostergaard (1998a, Memory Cognit, 26:40-60; 1998b, J. Int. Neuropsychol. Soc., in press) showed that simply degrading visual word stimuli can dramatically alter the degree to which word priming shows a dissociation from word recognition; i.e., effects of a number of factors on priming paralleled their effects on recognition memory tests when the words were degraded at test. In the present study, cerebral blood flow changes were measured while subjects performed the word identification (reading) and recognition memory tasks used previously by Ostergaard. The results are the direct comparisons of the two tasks and the effects of stimulus degradation on blood flow patterns during the tasks. Clear differences between word identification and word recognition were observed: the latter task evoked considerably more prefrontal activity and stronger cerebellar activation. Stimulus degradation was associated with focal increases in bilateral fusiform regions within the occipital lobe. No task, degradation, or item repetition effects were demonstrated in mesial temporal regions, no repetition effects were observed in any region, and there was no evidence for different effects of stimulus degradation in the priming and recognition memory conditions. Power limitations may have contributed to the null effects.

The activation pattern in normal humans during suppression, imagination and performance of saccadic eye movements.

The distribution of activated cerebral regions was examined in nine normal subjects during four different eye movement-related conditions: (1) fixation-fixation on a central light emitting diode; (2) saccadic suppression-fixation on a diode in the presence of flashing lateral targets; (3) reflexive/volitional saccades-performance of overt eye movements to two laterally lit targets and back to the centre; and (4) imagined saccades-imagining, but not performing, the same eye movements. The regional neural activity was measured indirectly using repetitive bolus injections of oxygen-15-labelled water and positron emission tomography (PET) to yield time-integrated images of the normalized count distribution. These were aligned and anatomically normalized to a standard stereotactic space and the averages of each condition were analysed categorically using statistical parametric mapping. Compared to central fixation, reflexive/volitional saccades significantly activated regions in the classically known cortical oculomotor regions. The most notable activation during the saccade suppression task, compared to central fixation alone, was a bilateral activation of the parietal cortex with a right-sided preponderance, activation of the supplementary eye field/caudal cingulate regions, and activation of frontal regions close to the frontal eye fields. Imagined performance of eye movements without overt eye movements activated the supplementary eye field and frontal eye fields identically to regions involved in overt eye movements, thus demonstrating that over eye movements are not a prerequisite of the activation of these regions in normal humans.