***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: Brain temperature appears to be an important factor affecting motor activity , but it is not known to what extent brain temperature increases during prolonged exercise in humans .  Cerebral heat exchange was therefore evaluated in seven males during exercise with and without hyperthermia .  Middle cerebral artery mean blood velocity ( MCA V(mean ) ) was continuously monitored while global cerebral blood flow ( CBF ) and cerebral energy turnover were determined at the end of the two exercise trials in three subjects . The arterial to venous temperature difference across the brain ( v-aD(temp ) ) was determined via thermocouples placed in the internal jugular vein and in the aorta . The jugular venous blood temperature was always higher than that of the arterial blood , demonstrating that heat was released via the CBF during the normothermic as well as the hyperthermic exercise condition . However , heat removal via the jugular venous blood was 30 + /- 6 % lower during hyperthermia compared to the control trial . The reduced heat removal from the brain was mainly a result of a 20 + /- 6 % lower CBF ( 22 + /- 9 % reduction in MCA V(mean ) ) , because the v-aD(temp ) was not significantly different in the hyperthermic ( 0.20 + /- 0.05 degrees C ) compared to the control trial ( 0.22 + /- 0.05 degrees C ) . During hyperthermia , the impaired heat removal via the blood was combined with a 7 + /- 2 % higher heat production in the brain and heat was consequently stored in the brain at a rate of 0.20 + /- 0.06 J g(-1 ) min(-1 ) . The present results indicate that the average brain temperature is at least 0.2 degrees C higher than that of the body core during exercise with or without hyperthermia The effects of naltrexone-precipitated withdrawal from buprenorphine on behavior and regional cerebral blood flow ( rCBF ) were studied in 11 opiate-dependent patients . Patients initially received buprenorphine , 2 mg sublingually , every day for 7 days . They were then challenged sequentially with placebo and naltrexone , 25 mg orally , before single photon emission computed tomography with technetium-99m-d , l-hexamethyl-propylene amine oxime as tracer .  Behavioral ratings of withdrawal severity were made before and after naltrexone/placebo administration . Naltrexone produced significantly greater signs and symptoms of opiate withdrawal than placebo . Analysis of variance revealed no significant regionally specific effect of naltrexone on rCBF ratios . Severity of withdrawal , however , showed a significant negative correlation with rCBF in the anterior cingulate cortex following naltrexone . These results are interesting as the anterior cingulate region has been implicated in the emotional component of pain and in opiate-induced analgesia In a prospect i ve study of the natural development of total cerebral blood flow volume ( CBFV ) , the common , external and internal carotid and vertebral arteries were examined in 94 healthy children and adolescents between 3 and 18 years of age ( sex and age evenly distributed ) using a 7.0-MHz transducer of a computed sonography system .  Intravascular flow volumes were calculated with the product of angle-corrected time-averaged flow velocity and the cross-sectional area of the vessel . CBFV was determined as the sum of flow volumes in the internal carotid and vertebral arteries of both sides . CBFV increased significantly between 3 and 6.5 years of age ( from 687 ± 85 to 896 ± 110 ml/min ; age correlation , p ≤ 0.01 ) and declined thereafter ( p ≤ 0.001 ) to a constant level of ∼700 ml/min at 15 years of age . There was no difference in CBFV between sexes . The proportion of bilateral vertebral artery flow volume in total CBFV decreased significantly between the ages of 3 and 18 years ( p ≤ 0.001 ) . As the flow volumes of the external carotid arteries increased markedly from childhood to adulthood , flow volumes of the common carotid arteries were not representative of CBFV . Intrasession test-retest correlation of CBFV was high ( r = 0.89 , p ≤ 0.0001 ) . Reference data for the childhood years presented here and previously described results from healthy adults allow us to outline the natural evolution of CBFV in humans . The reliability of the method has already been demonstrated . Thus , it may now be introduced into clinical application The effects of intravenous amphetamine infusion ( 0.3 mg/kg ) on cerebral blood flow ( CBF ) and measures of autonomic and behavioral arousal were studied in 12 normal male volunteers in a placebo-controlled crossover design . Nonsignificant decreases were seen in CBF ( measured by 133Xe inhalation ) , despite significant increases in autonomic and behavioral arousal . The apparent dissociation of CBF and arousal appears to be compatible with other human experiments suggesting that amphetamine decreases CBF and metabolism , as well as with neurobiological findings on the effects of catecholamines on resting cortical activity and mechanisms of increased attention . The results differ substantially , however , from findings of increased CBF and metabolism in animals . Although the larger doses used in animals most likely explain the discrepancy , technical limitations in human brain imaging can not be excluded Reduced cerebral perfusion may contribute to the development of cerebrovascular and neurodegenerative diseases . Little is known on cerebral perfusion in the general population , as most measurement techniques are too invasive for application in large groups of healthy individuals .  Total cerebral blood flow ( tCBF ) can be noninvasively measured by magnetic resonance imaging ( MRI ) but is highly correlated with brain volume . We calculated total brain perfusion by dividing tCBF by brain volume , and we investigated determinants of total brain perfusion in comparison with tCBF . Secondly , we studied whether persons with a low tCBF or low total brain perfusion have a larger volume of white matter lesions ( WML ) . This study is based on 892 persons aged 60 to 91 years from the Rotterdam Study , a population -based cohort study . We performed two-dimensional ( 2D ) phase-contrast MRI for tCBF measurement .  Brain volume and WML volume were quantitatively assessed . Cardiovascular determinants were assessed by interview and physical examination . We assessed associations between cardiovascular determinants and flow measures with linear regression models , adjusted for age and sex . Associations between tCBF or total brain perfusion and WML volume were assessed using general linear models . We found that determinants of tCBF and total brain perfusion differed largely due to the large influence of brain volume on tCBF values . Persons with low total brain perfusion had a significantly larger WML volume compared with those with high total brain perfusion . Prospect i ve studies are required to unravel whether hypoperfusion contributes to WML formation or that tissue damage , manifested by WML , leads to brain hypoperfusion Regional cerebral blood flow ( rCBF ) was studied in 60 elderly persons ( aged 65 to 84 years ) recruited from a population -based study , with single photon emission computed tomography using technetium 99m-labeled hexamethylpropylene amine oxime . We investigated whether it is only age that affects rCBF or whether other factors can be indentified that explain this relationship . Using multiple linear regression analysis , increasing age was significantly associated with rCBF decrease in parietal , temporo-parietal , and temporal cortex , but not in frontal cortex . Adjustment with several risk factors for cerebrovascular disease , including hypertension , history of myocardial infa rct ion , factor VIIc , factor VIIIc , cholesterol and HDL cholesterol , smoking , and diabetes mellitus had no influence on these relations . Conversely , the association between age and rCBF was no longer statistically significant after adjustment with fibrinogen and indicators of carotid atherosclerosis , including intima-media wall thickness of the carotid artery and plaques in the carotid artery . Correction with local ratings of cortical atrophy did not affect the relations between age and rCBF . The results suggest that in the elderly population  rCBF declines with age in posterior cortical areas and that these changes may well be explained by the presence of atherosclerosis . Reduced contractility of the vascular muscle wall with increasing age result ing from atherosclerosis may be the underlying mechanism Objectives : To explore relationships between  scuba diving  activity , brain , and behaviour , and more specifically between global cerebral blood flow ( CBF ) or cognitive performance and total , annual , or last 6 months ’ frequencies , for st and ard dives or dives performed below 40 m , in cold water or warm sea geographical environments . Methods : A prospect i ve cohort study was used to examine divers from diving clubs around Lac Léman and Geneva University Hospital . The subjects were 215 healthy recreational divers ( diving with self-contained underwater breathing apparatus ) . Main outcome measures were : measurement of global CBF by 133Xe SPECT ( single photon emission computed tomography ) ; psychometric and neuropsychological tests to assess perceptual-motor abilities , spatial discrimination , attentional re sources , executive functioning , and memory ; evaluation of scuba diving activity  by question naire focusing on number and maximum depth of dives and geographical site of the diving activity ( cold water v warm water ) ; and  body composition analyses ( BMI ) . Results : ( 1 ) A negative influence of depth of dives on CBF and its combined effect with BMI and age was found . ( 2 ) A specific diving environment ( more than 80 % of dives in lakes ) had a negative effect on CBF . ( 3 ) Depth and number of dives had a negative influence on cognitive performance ( speed , flexibility and inhibition processing in attentional tasks ) . ( 4 ) A negative effect of a specific diving environment on cognitive performance ( flexibility and inhibition components ) was found . Conclusions : Scuba diving may have long-term negative neurofunctional effects when performed in extreme conditions , namely cold water , with more than 100 dives per year , and maximal depth below 40 Mobile phone use has increased worldwide but its possible effects on the brain remain unclear . The aim of the present study was to investigate the effect of acute exposure to a radio frequency electromagnetic field ( RF EMF ) generated by a mobile phone operating in the Global System for Mobile Communication ( GSM ) 900 MHz on cerebral blood flow .  Twenty-nine volunteers attended two experimental sessions : a sham exposure session and a real exposure session in a cross-over double-blind study in which a mobile phone was positioned on the left side of the head . In one session , the mobile phone was operated without RF radiation ( sham phone ) and in the other study it was operated with RF radiation ( real phone ) for 20 min . Thus , each subject served as its own control . Middle cerebral artery blood flow was monitored noninvasively by transcranial Doppler sonography to measure middle cerebral artery blood flow velocity . Pulsatility index and resistance index were also evaluated . A voluntary breath holding physiological test was carried out as a positive control for testing cerebral vasoreactivity . Hemodynamic variables were recorded and analyzed before , during and after mobile phone exposure . No significant changes were detected in studied variables in middle cerebral arteries during sham or real exposure . In the exposed side the cerebral blood flow velocity , the pulsatility index and the resistance index during sham and real exposure were respectively : [ 61.9 ± 1.3 , 61.7 ± 1.3 cm/s ( P = 0.89 ) ] ; [ 0.93 ± 0.03 , 0.90 ± 0.02 ( P = 0.84 ) ] and [ 0.58 ± 0.01 , 0.58 ± 0.01 ( P = 0.96 ) ] at baseline ; and [ 60.6 ± 1.3 , 62 ± 1.6 cm/s ( P = 0.40 ) ] ; [ 0.91 ± 0.03 , 0.87 ± 0.03 ( P = 0.97 ) ] ; [ 0.57 ± 0.01 , 0.56 ± 0.01 ( P = 0.82 ) ] after 20 min of exposure . Twenty minutes of RF exposure to a mobile phone does not seem to affect the cerebral circulation Psychedelic drugs have a long history of use in healing ceremonies , but despite renewed interest in their therapeutic potential , we continue to know very little about how they work in the brain . Here we used psilocybin , a classic psychedelic found in magic mushrooms , and a task-free functional MRI ( fMRI ) protocol design ed to capture the transition from normal waking consciousness to the psychedelic state . Arterial spin labeling perfusion and blood-oxygen level-dependent ( BOLD ) fMRI were used to map cerebral blood flow and changes in venous oxygenation before and after intravenous infusions of placebo and psilocybin .  Fifteen healthy volunteers were scanned with arterial spin labeling and a separate 15 with BOLD . As predicted , profound changes in consciousness were observed after psilocybin , but surprisingly , only decreases in cerebral blood flow and BOLD signal were seen , and these were maximal in hub regions , such as the thalamus and anterior and posterior cingulate cortex ( ACC and PCC ) . Decreased activity in the ACC/medial prefrontal cortex ( mPFC ) was a consistent finding and the magnitude of this decrease predicted the intensity of the subjective effects . Based on these results , a seed-based pharmaco-physiological interaction/functional connectivity 
    Output:
    
Several factors ( e.g. , caffeine , aging , and blood gases ) were found to induce a considerable effect on brain perfusion that was consistent across different studies ; for other factors , the modifying effect was found to be debatable , due to contradictory results or lack of evidence .