Semantic context-dependent neural representations of odors in the human piriform cortex revealed by 7T MRI.

Olfactory perception depends not only on olfactory inputs but also on semantic context. Although multi-voxel activity patterns of the piriform cortex, a part of the primary olfactory cortex, have been shown to represent odor perception, it remains unclear whether semantic contexts modulate odor representation in this region. Here, we investigated whether multi-voxel activity patterns in the piriform cortex change when semantic context modulates odor perception and, if so, whether the modulated areas communicate with brain regions involved in semantic and memory processing beyond the piriform cortex. We also explored regional differences within the piriform cortex, which are influenced by olfactory input and semantic context. We used 2 × 2 combinations of word labels and odorants that were perceived as congruent and measured piriform activity with a 1-mm isotropic resolution using 7T MRI. We found that identical odorants labeled with different words were perceived differently. This labeling effect was observed in multi-voxel activity patterns in the piriform cortex, as the searchlight decoding analysis distinguished identical odors with different labels for half of the examined stimulus pairs. Significant functional connectivity was observed between parts of the piriform cortex that were modulated by labels and regions associated with semantic and memory processing. While the piriform multi-voxel patterns evoked by different olfactory inputs were also distinguishable, the decoding accuracy was significant for only one stimulus pair, preventing definitive conclusions regarding the locational differences between areas influenced by word labels and olfactory inputs. These results suggest that multi-voxel patterns of piriform activity can be modulated by semantic context, possibly due to communication between the piriform cortex and the semantic and memory regions.

Evaluation of simultaneous multi-slice readout-segmented diffusion-weighted MRI acquisition in human optic nerve measurements.

Diffusion-weighted magnetic resonance imaging (dMRI) is the only available method to measure the tissue properties of white matter tracts in living human brains and has opened avenues for neuroscientific and clinical studies on human white matter. However, dMRI using conventional simultaneous multi-slice (SMS) single-shot echo planar imaging (ssEPI) still presents challenges in the analyses of some specific white matter tracts, such as the optic nerve, which are heavily affected by susceptibility-induced artifacts. In this study, we evaluated dMRI data acquired by using SMS readout-segmented EPI (rsEPI), which aims to reduce susceptibility-induced artifacts by dividing the acquisition space into multiple segments along the readout direction to reduce echo spacing. To this end, we acquired dMRI data from 11 healthy volunteers by using SMS ssEPI and SMS rsEPI, and then compared the dMRI data of the human optic nerve between the SMS ssEPI and SMS rsEPI datasets by visual inspection of the datasets and statistical comparisons of fractional anisotropy (FA) values. In comparison with the SMS ssEPI data, the SMS rsEPI data showed smaller susceptibility-induced distortion and exhibited a significantly higher FA along the optic nerve. In summary, this study demonstrates that despite its prolonged acquisition time, SMS rsEPI is a promising approach for measuring the tissue properties of the optic nerve in living humans and will be useful for future neuroscientific and clinical investigations of this pathway.

The initial decrease in 7T-BOLD signals detected by hyperalignment contains information to decode facial expressions.

The initial decrease in the blood oxygenation level-dependent (BOLD) signal reflects primary neuronal activity more than the later hemodynamic positive peak responses. Moreover, ultra-high field BOLD has high sensitivity for the initial de-oxygenation signal. However, it is not fully understood how much information about task events and cognitive processes the initial decrease in the BOLD signal contains. Multivoxel pattern analysis (MVPA) of the BOLD signal has enabled the quantification of information contained in the activity patterns, but it has mainly relied on the positive peak responses. Here, we applied a signal-based functional inter-individual alignment algorithm (i.e., hyper-alignment) to a 7T-BOLD timeseries scanned while participants conducted a facial expression discrimination task. We found that the MVPA decoding accuracy in the bilateral amygdala 2 s after the face onset was significantly beyond chance. Furthermore, we confirmed that the voxels contributing to the decoding accuracy at 2 s displayed a decreasing hemodynamics response. These results demonstrated that the initial decrease in 7T-BOLD signals contains finer information about task events and cognitive processes than thought previously.

Assessment of olfactory information in the human brain using 7-Tesla functional magnetic resonance imaging.

Olfaction could prove to be an early marker of neurodegenerative diseases, including Alzheimer's and Parkinson's diseases. To use olfaction for disease diagnosis, elucidating the standard olfactory functions in healthy humans is necessary. However, the olfactory function in the human brain is less frequently assessed because of methodological difficulties associated with olfactory-related cerebral areas. Using ultra-high fields (UHF), functional magnetic resonance imaging (fMRI) with high spatial resolution and sensitivity may allow for the measurement of activation in the cerebral areas. This study aimed to apply 7-Tesla fMRI to assess olfactory function in the human brain by exposing individuals to four different odorants for 8 s. We found that olfactory stimulation mainly activated the piriform and orbitofrontal cortex in addition to the amygdala. Among these regions, univariate fMRI analysis indicated that subjective odor intensity significantly correlated with the averaged fMRI signals in the piriform cortex but not with subjective hedonic tone in any region. In contrast, multivariate fMRI analysis showed that subjective hedonic tone could be discriminated from the fMRI response patterns in the posterior orbitofrontal cortex. Thus, the piriform cortex is mainly associated with subjective odor intensity, whereas the posterior orbitofrontal cortex are involved in the discrimination of the subjective hedonic tone of the odorant. UHF-fMRI may be useful for assessing olfactory function in the human brain.

Repetitive transcranial magnetic stimulation restores altered functional connectivity of central poststroke pain model monkeys.

Central poststroke pain (CPSP) develops after a stroke around the somatosensory pathway. CPSP is hypothesized to be caused by maladaptive reorganization between various brain regions. The treatment for CPSP has not been established; however, repetitive transcranial magnetic stimulation (rTMS) to the primary motor cortex has a clinical effect. To verify the functional reorganization hypothesis for CPSP development and rTMS therapeutic mechanism, we longitudinally pursued the structural and functional changes of the brain by using two male CPSP model monkeys (Macaca fuscata) developed by unilateral hemorrhage in the ventral posterolateral nucleus of the thalamus. Application of rTMS to the ipsilesional primary motor cortex relieved the induced pain of the model monkeys. A tractography analysis revealed a decrease in the structural connectivity in the ipsilesional thalamocortical tract, and rTMS had no effect on the structural connectivity. A region of interest analysis using resting-state functional magnetic resonance imaging revealed inappropriately strengthened functional connectivity between the ipsilesional mediodorsal nucleus of the thalamus and the amygdala, which are regions associated with emotion and memory, suggesting that this may be the cause of CPSP development. Moreover, rTMS normalizes this strengthened connectivity, which may be a possible therapeutic mechanism of rTMS for CPSP.

Cerebral artery segmentation based on magnetization-prepared two rapid acquisition gradient echo multi-contrast images in 7 Tesla magnetic resonance imaging.

Cerebral artery segmentation plays an important role in the direct visualization of the human brain to obtain vascular system information. On ultra-high field magnetic resonance imaging, cerebral arteries appearing hyperintense on T1 weighted (T1w) images could be segmented from brain tissues such as gray and white matter. In this study, we propose an automated method to segment the cerebral arteries using multi-contrast images including T1w images of a magnetization-prepared two rapid acquisition gradient echo (MP2RAGE) sequence at 7 T. The proposed method, termed MP2rase-CA (MP2rage based RApid SEgmentation Cerebral Artery), employed a seed-based region-growing strategy and Frangi filtering as well as our brain tissue segmentation (MP2rase Brain Tissue). Time-of-flight (TOF) magnetic resonance angiography (MRA) images were obtained as a reference to evaluate the MP2rase-CA. We successfully performed vessel segmentations, from T1w MP2RAGE images, which mostly overlapped with the segmentations of large cerebral arteries from the TOF-MRA. We also investigated the effect of the large cerebral arteries on spatial transformation of anatomical images to standard coordinate space using vessel segmentation by MP2rase-CA. As a result, the T1w image without the cerebral arteries by MP2rase-CA showed better agreement with the standard atlas compared with the T1w image containing the arteries. In addition, voxel-based morphology showed significant differences between T1w images with and without cerebral arteries in brain areas nearby large arteries. Thus, because MP2rase-CA using MP2RAGE images can obtain brain tissue anatomical information as well as relatively large cerebral artery information without need for additional structure acquisition, it is useful and time saving for functional and structural studies.

Threat Anticipation in Pulvinar and in Superficial Layers of Primary Visual Cortex (V1). Evidence from Layer-Specific Ultra-High Field 7T fMRI.

The perceptual system gives priority to threat-relevant signals with survival value. In addition to the processing initiated by sensory inputs of threat signals, prioritization of threat signals may also include processes related to threat anticipation. These neural mechanisms remain largely unknown. Using ultra-high-field 7 tesla (7T) fMRI, we show that anticipatory processing takes place in the early stages of visual processing, specifically in the pulvinar and V1. When anticipation of a threat-relevant fearful face target triggered false perception of not-presented target, there was enhanced activity in the pulvinar as well as in the V1 superficial-cortical-depth (layers 1-3). The anticipatory activity was absent in the LGN or higher visual cortical areas (V2-V4). The effect in V1 was specific to the perception of fearful face targets and did not generalize to happy face targets. A preliminary analysis showed that the connectivity between the pulvinar and V1 superficial-cortical-depth was enhanced during false perception of threat, indicating that the pulvinar and V1 may interact in preparation of anticipated threat. The anticipatory processing supported by the pulvinar and V1 may play an important role in non-sensory-input-driven anxiety states.

Brain tissue segmentation based on MP2RAGE multi-contrast images in 7 T MRI.

We proposed a method for segmentation of brain tissues-gray matter, white matter, and cerebrospinal fluid-using multi-contrast images, including a T1 map and a uniform T1-weighted image, from a magnetization-prepared 2 rapid acquisition gradient echoes (MP2RAGE) sequence at 7 Tesla. The proposed method was evaluated with respect to the processing time and the similarity of the segmented masks of brain tissues with those obtained using FSL, FreeSurfer, and SPM12. The processing time of the proposed method (28 ± 0 s) was significantly shorter than those of FSL and SPM12 (444 ± 4 s and 159 ± 2 s for FSL and SPM12, respectively). In the similarity assessment, the tissue mask of the brain obtained by the proposed method showed higher consistency with those obtained using FSL than with those obtained using SPM12. The proposed method misclassified the subcortical structures and large vessels since it is based on the intensities of multi-contrast images obtained using MP2RAGE, which uses a similar segmentation approach as FSL but is not based on a template image or a parcellated brain atlas, which are used for FreeSurfer and SPM12, respectively. However, the proposed method showed good segmentation in the cerebellum and white matter in the medial part of the brain in comparison with the other methods. Thus, because the proposed method using different contrast images of MP2RAGE sequence showed the shortest processing time and similar segmentation ability as the other methods, it may be useful for both neuroimaging research and clinical diagnosis.

Comparison of Diffusion-Weighted Imaging in the Human Brain Using Readout-Segmented EPI and PROPELLER Turbo Spin Echo With Single-Shot EPI at 7 T MRI.

OBJECTIVES: The purpose of the present study was to compare periodically rotated overlapping parallel lines with enhanced reconstruction-type turbo spin echo diffusion-weighted imaging (pTSE-DWI) and readout-segmented echo planar imaging (rsEPI-DWI) with single-shot echo planar imaging (ssEPI-DWI) in a 7 T human MR system. We evaluated the signal-to-noise ratio (SNR), image distortion, and apparent diffusion coefficient values in the human brain. MATERIALS AND METHODS: Six healthy volunteers were included in this study. The study protocol was approved by our institutional review board. All measurements were performed at 7 T using pTSE-DWI, rsEPI-DWI, and ssEPI-DWI sequences. The spatial resolution was 1.2 × 1.2 mm in-plane with a 3-mm slice thickness. Signal-to-noise ratio was measured using 2 scans. RESULTS: The ssEPI-DWI sequence showed significant image blurring, whereas pTSE-DWI and rsEPI-DWI sequences demonstrated high image quality with low geometrical distortion compared with reference T2-weighted, turbo spin echo images. Signal loss in ventral regions near the air-filled paranasal sinus/nasal cavity was found in ssEPI-DWI and rsEPI-DWI but not pTSE-DWI. The apparent diffusion coefficient values for ssEPI-DWI were 824 ± 17 × 10 and 749 ± 25 × 10 mm/s in the gray matter and white matter, respectively; the values obtained for pTSE-DWI were 798 ± 21 × 10 and 865 ± 40 × 10 mm/s; and the values obtained for rsEPI-DWI were 730 ± 12 × 10 and 722 ± 25 × 10 mm/s. The pTSE-DWI images showed no additional distortion comparison to the T2-weighted images, but had a lower SNR than ssEPI-DWI and rsEPI-DWI. The rsEPI-DWI sequence provided high-quality images with minor distortion and a similar SNR to ssEPI-DWI. CONCLUSIONS: Our results suggest that the benefits of the rsEPI-DWI and pTSE-DWI sequences, in terms of SNR, image quality, and image distortion, appear to outweigh those of ssEPI-DWI. Thus, pTSE-DWI and rsEPI-DWI at 7 T have great potential use for clinical diagnoses. However, it is noteworthy that both sequences are limited by the scan time required. In addition, pTSE-DWI has limitations on the number of slices due to specific absorption rate. Overall, rsEPI-DWI is a favorable imaging sequence, taking into account the SNR and image quality at 7 T.

Right ventrolateral prefrontal cortex involvement in the integration of emotional processing: Parametric mediation analysis of fMRI.

Emotion plays an important role in goal-directed behavior. Although numerous neuroimaging studies have been conducted, the neural mechanisms behind emotion generation and regulation are still elusive. This is partly explained by large individual variations in emotional responses in addition to the fact that emotion is a complex process. The present functional magnetic resonance imaging study aimed firstly to specify valence-dependent brain activation, and secondly to clarify the interactions between the regions underlying emotional processing. We measured brain activation in 12 healthy adults while passively viewing affective pictures. The individual valence ratings were negatively correlated with activation in the frontal, parietal, occipital lobes, and in the subcortical regions including the amygdalae. The parametric mediation analysis performed on these regions showed that the right ventrolateral prefrontal cortex (Brodmann area 47) was main mediator of the valence ratings with other regions. Interestingly, there was no mediator region for the ventrolateral prefrontal cortex. These results indicated the possibility that the right ventrolateral prefrontal cortex mainly participated in the integration of visually induced emotional processes in the valence dimension.

Asymmetrical intersection between the middle cerebral artery and rhinal vein suggests asymmetrical gustatory cortex location in rodent hemispheres.

The rodent gustatory cortex is located in the anterior part of the insular cortex, which is near the dorsal part of the rhinal vein (RHV) and the intersection of the anterior and posterior regions of the middle cerebral artery (MCA). Thus, the intersection between the RHV and MCA is used as a landmark for the rodent gustatory cortex. In our previous study, we employed functional magnetic resonance imaging (MRI) to demonstrate that tastants evoked bilateral responses in the rodent insular cortices, but that these representations were asymmetrical between the hemispheres. In the present study, to clarify the observed asymmetrical responses, we performed magnetic resonance angiography in a 7.0-Tesla MRI scanner to determine the anatomical position of the rodent gustatory cortex, which was identified using the intersection of the MCA and RHV. We successfully observed the intersection while administering carbogen as an inhaled gas and found that the intersection in the left hemisphere is more anterior compared to that in the right hemisphere. Taken together with the previous functional MRI results, this result indicates that the gustatory representation in relation to the intersection may be identically conserved in the insular cortex of both hemispheres; therefore, the rodent gustatory cortex may be asymmetrically located between the left and right hemispheres. The result also suggests that this landmark location needs to be verified when investigating gustatory representations and responses.

Phantom and mouse experiments of time-domain fluorescence tomography using total light approach.

Phantom and mouse experiments of time-domain fluorescence tomography were conducted to demonstrate the total light approach which was previously proposed by authors. The total light approach reduces the computation time to solve the forward model for light propagation. Time-resolved temporal profiles were acquired for cylindrical phantoms having single or double targets containing indocyanine green (ICG) solutions. The reconstructed images of ICG concentration reflected the true distributions of ICG concentration with a spatial resolution of about 10 mm. In vivo experiments were conducted using a mouse in which an ICG capsule was embedded beneath the skin in the abdomen. The reconstructed image of the ICG concentration again reflected the true distribution of ICG although artifacts due to autofluorescence appeared in the vicinity of the skin. The effectiveness of the total light approach was demonstrated by the phantom and mouse experiments.

Near-infrared spectroscopic study on the effects of chewing on short-term memory.

Using near-infrared spectroscopy, we examined whether chewing gum improves performance in a short-term memory task - immediate recall of random eight-digit numbers - by assessing cerebral hemodynamic response in the prefrontal cortex. We found that the oxyhemoglobin concentration during and after chewing gum was higher than that before chewing; further, the concentration increased during the task, and this increase was reduced with chewing, although non-significantly. Chewing did not improve task performance. Therefore, chewing-induced hemodynamic responses were unrelated to the performance in short-term memory tasks.

Blood oxygenation level-dependent functional magnetic resonance imaging of bilateral but asymmetrical responses to gustatory stimulation in the rat insular cortex.

Evidence has suggested asymmetrical processing of taste in the human insular cortex, but this phenomenon has not been demonstrated in the rodent brain. Functional magnetic resonance imaging (fMRI) is a powerful tool for studying the functional organization of the brain. In this study, we established a blood oxygenation level-dependent (BOLD) fMRI method at 7 T to investigate the responses to gustatory stimulation in the insular cortex of anesthetized rats (220-310 g, n=15). BOLD signals were observed in the insular cortex in response to 0.5 M sucrose solution as the tastant but not observed in response to distilled water as the control. The reproducibility of the BOLD signals in response to the tastant was confirmed between fMRI runs in the same animal and across animals. The signals were mainly located between 2.3 mm and 0.0 mm anterior to the bregma in the insular cortex. Interestingly, the signals were observed in the insular cortex of both hemispheres, but they were asymmetrical: the anterior and posterior regions to the intersection of the middle cerebral artery and the rhinal fissure as the landmark of the gustatory cortex were dominant in the left and right hemispheres of the insular cortex, respectively. These results suggest that activity in both hemispheres of the insular cortex should be considered to analyze taste processing. We think that BOLD fMRI of taste function in rodents will improve our understanding of taste information processing.

Olfactory responses to natal stream water in sockeye salmon by BOLD fMRI.

Many studies have shown that juvenile salmon imprint olfactory memory of natal stream odors during downstream migration, and adults recall this stream-specific odor information to discriminate their natal stream during upstream migration for spawning. The odor information processing of the natal stream in the salmon brain, however, has not been clarified. We applied blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging to investigate the odor information processing of the natal stream in the olfactory bulb and telencephalon of lacustrine sockeye salmon (Oncorhynchus nerka). The strong responses to the natal stream water were mainly observed in the lateral area of dorsal telencephalon (Dl), which are homologous to the medial pallium (hippocampus) in terrestrial vertebrates. Although the concentration of L-serine (1 mM) in the control water was 20,000-times higher than that of total amino acid in the natal stream water (47.5 nM), the BOLD signals resulting from the natal stream water were stronger than those by L-serine in the Dl. We concluded that sockeye salmon could process the odor information of the natal stream by integrating information in the Dl area of the telencephalon.

A volumetric analysis of the brain and hippocampus of rats rendered perinatal hypothyroid.

The thyroid hormone is essential for the proper development of the central nervous system (CNS). Hormone deficiency during CNS development causes neurological abnormalities in the brain. The hippocampus is one of the brain regions vulnerable to hormone deficiency, and the volume of dentate gyrus (DG) and cornu ammonis (CA) are reduced by transient hypothyroidism during CNS development. However, it remains unclear whether transient hypothyroidism specifically reduces the whole hippocampal volume. In the present study, we used magnetic resonance imaging (MRI) to examine the effects of perinatal hypothyroidism on the ratio of hippocampal volume to brain volume as well as brain and hippocampal volumes overall. Perinatal hypothyroidism was induced by adding the anti-thyroid drug, methimazole, to the drinking water of pregnant dams from gestational day 15 to postnatal day 21. The MRI experiment was conducted when the rats were between 7 and 11 months old. The results showed reductions of the hippocampal and brain volume of the treated group. However, the ratio of hippocampal volume to brain volume was comparable between the control and treated groups. These results indicate that perinatal hypothyroidism minimizes the brain as a whole, but does not minimize the hippocampus in particular.

Comprehensive correlation between neuronal activity and spin-echo blood oxygenation level-dependent signals in the rat somatosensory cortex evoked by short electrical stimulations at various frequencies and currents.

It is essential to elucidate the relationship between blood oxygenation level-dependent (BOLD) signals and neuronal activity for the interpretation of the functional magnetic resonance imaging (fMRI) signals; this relationship has been quantitatively investigated by animal studies measuring evoked potentials as indices of neuronal activity. Although most human fMRI studies employ the event-related task design, in which the stimulus duration is short, few studies have investigated the relationship between BOLD signals and evoked potentials at short stimulus durations. The present study investigated this relationship in the somatosensory cortex of anesthetized rats by using electrical forepaw stimulation at a short duration of 4 s and comprehensively analyzed it at different frequencies (1-10 Hz) and currents (0.5-2.0 mA). Somatosensory evoked potential (SEP) responses were measured at the scalp using silver ball electrodes. The sum of the peak-to-peak amplitude (SigmaSEP) and average SEP (avg. SEP) responses were calculated. BOLD signals were obtained using a spin-echo echo-planar imaging sequence at 7 T. The relationship between the avg. SEP and BOLD signals varied with frequency, whereas that between SigmaSEP and BOLD signals showed a significant correlation at varying frequencies and currents. In particular, the relationship between SigmaSEP and SigmaBOLD, which is the sum of the BOLD signals obtained at each time point reflecting the area under the BOLD response curves, mostly converged, irrespective of the frequency. Our results suggest that SigmaBOLD obtained using a spin-echo sequence reflects the neural activity as quantified by SigmaSEP, which was determined at different frequencies and currents.

Biodistribution imaging of magnetic particles in mice: X-ray scanning analytical microscopy and magnetic resonance imaging.

Nano-sized particles have received much attention in view of their varied application in a wide range of fields. For example, magnetite (Fe(3)O(4)) nanoparticles have been investigated for various medical applications. In this study, we visualized the distribution of administered magnetic nanoparticles in mice using both X-ray scanning analytical microscopy (XSAM) and magnetic resonance imaging (MRI). After administration, the nanoparticles were rapidly dispersed via the blood circulation, and reached the liver, kidney and spleen. Using the XSAM and MRI methods in a complementary fashion, the biodistribution of nano-sized magnetite particles was successfully visualized.

Stimulus frequency dependence of blood oxygenation level-dependent functional magnetic resonance imaging signals in the somatosensory cortex of rats.

Understanding the mechanism of coupling between neuronal events and hemodynamic responses is important in non-invasive functional imaging of the brain. The stimulus frequency dependence of hemodynamic responses has been studied using a rat somatosensory cortex model; most results for short stimulus durations reveal peak frequencies at which the hemodynamic response is maximized. However, such peak frequencies have not been observed in studies using blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signals with long stimulus durations. To clarify whether the stimulus frequency dependence of BOLD signals depends on the stimulus duration, we measured BOLD signals at 7 T with short- and long-stimulus durations for stimulating rat forepaw at 1-10 Hz using spin-echo echo-planar imaging to enhance changes in activation focus. For both these durations, BOLD signals were significantly higher at stimulus frequencies of 3 or 5 Hz in agreement with the results of previous studies using optical techniques. Our results show that stimulus duration has little influence on the stimulus frequency dependence of BOLD signals in the rat somatosensory model. The discrepant results of most previous fMRI studies using gradient-echo sequence may be ascribed to the difference of imaging to enhance activation focus or draining vein.

Dynamics of changes in blood flow, volume, and oxygenation: implications for dynamic functional magnetic resonance imaging calibration.

Changes in cerebral blood flow (CBF), volume (CBV), and oxygenation (blood-oxygenation level dependent (BOLD)) during functional activation are important for calculating changes in cerebral metabolic rate of oxygen consumption (CMRo2) from calibrated functional MRI (fMRI). An important part of this process is the CBF/CBV relationship, which is signified by a power-law parameter: gamma=ln (1+DeltaCBV/CBV)/ln (1+DeltaCBF/CBF). Because of difficulty in measuring CBF and CBV with MRI, the value of gamma is therefore assumed to be approximately 0.4 from a prior primate study under hypercapnia. For dynamic fMRI calibration, it is important to know if the value of gamma varies after stimulation onset. We measured transient relationships between DeltaCBF, DeltaCBV, and DeltaBOLD by multimodal MRI with temporal resolution of 500 ms (at 7.0 T) from the rat somatosensory cortex during forepaw stimulation, where the stimulus duration ranged from 4 to 32 secs. Changes in CBF and BOLD were measured before the administration of the contrast agent for CBV measurements in the same subjects. We observed that the relationship between DeltaCBF and DeltaCBV varied dynamically from stimulation onset for all stimulus durations. Typically after stimulation onset and at the peak or plateau of the DeltaCBF, the value of gamma ranged between 0.1 and 0.2. However, after stimulation offset, the value of gamma increased to 0.4 primarily because of rapid and slow decays in DeltaCBF and DeltaCBV, respectively. These results suggest caution in using dynamic measurements of DeltaCBF and DeltaBOLD required for calculating DeltaCMRo2 for functional stimulation, when either DeltaCBV has not been accurately measured or a fixed value of gamma during hypercapnia perturbation is used.