The effect of hypercapnia on the neural and hemodynamic responses to somatosensory stimulation (original) (raw)
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Hypercapnic normalization of BOLD fMRI: comparison across field strengths and pulse sequences
Neuroimage, 2004
The blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signal response to neural stimulation is influenced by many factors that are unrelated to the stimulus. These factors are physiological, such as the resting venous cerebral blood volume (CBV v ) and vessel size, as well as experimental, such as pulse sequence and static magnetic field strength (B 0 ). Thus, it is difficult to compare task-induced fMRI signals across subjects, field strengths, and pulse sequences. This problem can be overcome by normalizing the neural activity-induced BOLD fMRI response by a global hypercapniainduced BOLD signal. To demonstrate the effectiveness of the BOLD normalization approach, gradient-echo BOLD fMRI at 1.5, 4, and 7 T and spin-echo BOLD fMRI at 4 T were performed in human subjects. For neural stimulation, subjects performed sequential finger movements at 2 Hz, while for global stimulation, subjects breathed a 5% CO 2 gas mixture. Under all conditions, voxels containing primarily large veins and those containing primarily active tissue (i.e., capillaries and small veins) showed distinguishable behavior after hypercapnic normalization. This allowed functional activity to be more accurately localized and quantified based on changes in venous blood oxygenation alone. The normalized BOLD signal induced by the motor task was consistent across different magnetic fields and pulse sequences, and corresponded well with cerebral blood flow measurements. Our data suggest that the hypercapnic normalization approach can improve the spatial specificity and interpretation of BOLD signals, allowing comparison of BOLD signals across subjects, field strengths, and pulse sequences. A theoretical framework for this method is provided. D
Hemodynamic correlates of stimulus repetition in the visual and auditory cortices: An fMRI study
2004
We examined the effects of stimulus repetition upon the evoked hemodynamic response (HDR) in auditory and visual cortices measured by magnetic resonance imaging in two experiments. Experiment 1 focused on the effects of the interval duration between two identical stimuli on HDR. Pure auditory tones (1000 Hz) of 100-ms duration were presented singly or in pairs with intrapair intervals (IPIs: onset-to-onset) of 1, 4, and 6 s. In Experiment 2, using a withinsubject design, we aimed to compare the HDR refractory period in both sensory cortices as well as the HDRs to auditory and visual stimuli. Identical auditory tone as described above and visual stimuli of 500-ms high-contrast checkerboard patterns were presented singly or in identical pairs with an IPI of 1 s. Images were acquired at 1.5 T using a gradient-echo echo-planar imaging sequence sensitive to blood oxygenation level-dependent (BOLD) contrast. Experiment 1 revealed that the HDR evoked by an auditory stimulus is followed by a refractory period of 4 -6 s in the auditory cortex, as indicated by smaller HDR amplitudes to the second of each pair of stimuli. Furthermore, peak latency was dependent upon IPI, with longer latencies observed for shorter IPIs. Experiment 2 revealed that the HDR evoked in both sensory cortices by paired stimulus presentations is suppressed and delayed similarly by the refractory effects imposed by the preceding stimulus, suggesting similar refractory properties of the HDR at this specific IPI. We also provide evidence for additional neural resource allocation in response to repeated stimuli. D
Low frequency vibrations can be detected by both tactile and auditory systems. The aim of the present study is to find out, by means of whole-scalp magnetoencephalography (MEG), whether vibrotactile stimulation alone would activate human auditory cortical areas. We recorded MEG signals from eleven normal-hearing adults to 200-Hz vibrations (on average 19.5 dB above the individual tactile detection threshold), delivered to right-hand fingertips. All subjects reported a perception of a sound when they touched the vibrating tube, and they reported to perceive nothing when not touching the tube. The vibrotactile stimuli elicited clear and reproducible vibrotactile evoked fields (VTEFs) in ten subjects, whereas no MEG responses were observed when the tube was not touched. First responses to the vibrotactile stimuli, peaking around 60 ms, originated in the primary somatosensory cortex in all subjects. They were followed by activations in the auditory cortices, either bilaterally (N = 5) or unilaterally (N = 5), and by activations in the secondary somatosensory (SII) cortex, either contralaterally (N = 3) or ipsilaterally (N = 4). Both the SII and auditory activations consisted of transient responses at 100 -200 ms. Additional auditory sustained activation was identified in nine subjects, either bilaterally (N = 2) or ipsilaterally (N = 7), at 200 -700 ms. Our results suggest convergence of vibrotactile input to the auditory cortex in normal-hearing adults, in agreement with results previously obtained in a congenitally deaf adult. D
NeuroImage, 2006
Disruption of the early stages of information processing in limbic brain circuits may underlie symptoms of severe neuropsychiatric disorders. Prepulse inhibition of acoustic startle (PPI) is diminished in many of these disorders and may reflect the disruption of this CNS function. PPI is associated with brain activity in many of the same regions in humans as it is in laboratory animals, suggesting that neuroimaging studies in humans may help localize deficits that can then be elucidated in animal models. In this article, we employed a rapid presentation event-related design during continuous EPI BOLD scanning to examine hemodynamic response functions (HRFs) associated with PPI. Fourteen healthy participants listened to 100 pulse alone and 100 prepulse combined with pulse (prepulse -pulse) trials. PPI is the normalized difference in the startle response to the two trial types. Following the prepulse -pulse trials, the amplitudes of the HRFs in auditory cortices and in the anterior insula were increased, while in the cerebellum, thalamus and anterior cingulate, they were decreased, relative to the pulse alone trials. In addition, the timing of the prepulse -pulse responses was delayed in the auditory cortices, anterior insula and cerebellum. Finally, PPI measured outside the scanner was predicted by the difference in BOLD responses between trial types in the anterior insula and in the cerebellum. The results suggest that prepulse inhibition, and by extension early stages of information processing, modulate both the amplitude as well as timing of neural activity. D
NeuroImage, 2001
Optical imaging spectroscopy (OIS) and laser Doppler flowmetry (LDF) data sequences from anesthetized rats were used to determine the relationship between changes in oxy-and deoxygenated hemoglobin concentration and changes in blood volume and flow in the presence and absence of stimulation. The data from Jones et al. (accompanying paper) were used to explore the differences between two theoretical models of flow activation coupling. The essential difference between the two models is the extension of the model of Buxton and Frank by Hyder et al. (1998, J. Appl. Physiol. 85: 554 -564) to incorporate change in capillary diffusivity coupled to flow. In both models activation-increased flow changes increase oxygen transport from the capillary; however, in Hyder et al.'s model the diffusivity of the capillary itself is increased. Hyder et al. proposed a parameter (⍀), a scaling "constant" linking increased blood flow and oxygen "diffusivity" in the capillary bed. Thus, in Buxton and Frank's theory, ⍀ ؍ 0; i.e., there are no changes in diffusivity. In Hyder et al.'s theory, 0 < ⍀ < 1, and changes in diffusivity are assumed to be linearly related to flow changes. We elaborate the theoretical position of both models to show that, in principle, the different predictions from the two theories can be evaluated using optical imaging spectroscopy data. We find that both theoretical positions have limitations when applied to data from brief stimulation and when applied to data from mild hypercapnia. In summary, the analysis showed that although Hyder et al.'s proposal that diffusivity increased during activation did occur; it was shown to arise from an implementation of Buxton and Frank's theory under episodes of brief stimulation. The results also showed that the scaling parameter ⍀ is not a constant as the Hyder et al. model entails but in fact varies over the time course of the flow changes. Data from experiments in which mild hypercapnia was administered also indicated changes in the diffusivity of the capillary bed, but in this case the changes were negative; i.e., oxygen transport from the capillary decreased relative to baseline under hypercapnia. Neither of the models could account for the differences between the hypercapnia and activation data when matched for equivalent flow changes. A modification to the models to allow non-null tissue oxygen concentrations that can be moderated by changes due to increased metabolic demand following increased neural activity is proposed. This modification would allow modulation of oxygen transport from the capillary bed (e.g., changes in diffusivity) by tissue oxygen tension and would allow a degree of decoupling of flow and oxygen delivery, which can encompass both the data from stimulation and from hypercapnia.
Cerebral hemodynamic response to mental activation in normo- and hypercapnia
Stroke, 1980
Changes of regional cerebral blood flow from rest to mental activation by a visually presented spatial reasoning test were measured during normo-and bypercapnia in 10 healthy subjects. Hypercapnia, elicited by inhalation of 6% CO,, resulted in similar flow increases in all 32 cortical regions measured. Increases of flow during testing were seen in post-central regions of the brain whether the resting level was augmented by hypercapnia or not. The results show that an elevated local functional level in the cortex causes an automatic local vasodilatory response which is totally independent of the basal level of perfusion and availability of metabolic substrates.
NeuroImage, 2008
The brain vascular system has an autoregulatory mechanism that maintains blood perfusion within normal limits at the capillary level. Partially due to its clinical importance, it is of interest to better understand the mechanisms involved in vascular regulation. Therefore, using functional magnetic resonance imaging (fMRI), we quantitatively investigated hemodynamic response characteristics of regions supplied by the main cerebral arteries, during two breath holding tests (BHT): after inspiration and after expiration. We used an auto-regressive method capable of estimating four signal parameters: onset delay, full width at half maximum (FWHM), time-to-peak and amplitude. The onset delay was significantly longer for the posterior cerebral artery (PCA) than for middle cerebral artery (MCA) and anterior arteries (ACA). FWHM and time-to-peak were larger in the ACA territory, indicating a slower blood flow in this region. Differences were also observed in the amplitude among the three areas, where MCA and PCA territories showed the smallest and the highest amplitudes, respectively. Moreover, differences were found in amplitude and onset when BHT was performed after inspiration as compared to BHT after expiration. Time-to-peak and FWHM showed no statistical differences between these two challenges. Such results are related to regional anatomical specificities and biochemical mechanisms responsible for vasodilation, such as those related to vascularity and vessel sizes.
Journal of Cerebral Blood Flow & Metabolism, 2003
Anesthetics, widely used in magnetic resonance imaging (MRI) studies to avoid movement artifacts, could have profound effects on cerebral blood flow (CBF) and cerebrovascular coupling relative to the awake condition. Quantitative CBF and tissue oxygenation (blood oxygen level-dependent [BOLD]) were measured, using the continuous arterial-spin-labeling technique with echo-planarimaging acquisition, in awake and anesthetized (2% isoflurane) rats under basal and hypercapnic conditions. All basal blood gases were within physiologic ranges. Blood pressure, respiration, and heart rates were within physiologic ranges in the awake condition but were depressed under anesthesia (P < 0.05). Regional CBF was heterogeneous with whole-brain CBF values of 0.86 ± 0.25 and 1.27 ± 0.29 mL · g −1 · min −1 under awake and anesthetized conditions, respectively. Surprisingly, CBF was markedly higher (20% to 70% across different brain conditions) under isofluraneanesthetized condition compared with the awake state (P < 0.01). Hypercapnia decreased pH, and increased PCO 2 and PO 2 . During 5% CO 2 challenge, under awake and anesthetized conditions, respectively, CBF increased 51 ± 11% and 25 ± 4%, and BOLD increased 7.3 ± 0.7% and 5.4 ± 0.4%. During 10% CO 2 challenge, CBF increased 158 ± 28% and 47 ± 11%, and BOLD increased 12.5 ± 0.9% and 7.2 ± 0.5%. Since CBF and BOLD responses were substantially higher under awake condition whereas blood gases were not statistically different, it was concluded that cerebrovascular reactivity was suppressed by anesthetics. This study also shows that perfusion and perfusion-based functional MRI can be performed in awake animals.