Biomedical subjects

Bharat B Biswal

Publications and source records attributed to Bharat B Biswal.

14 recordsLinked to original sources

PubMed2006

Evidence for multiple manipulation processes in prefrontal cortex.

The prefrontal cortex (PFC) is known to subserve working memory (WM) processes. Brain imaging studies of WM using delayed response tasks (DRTs) have shown memory-load-dependent activation increases in dorsal prefrontal cortex (PFC) regions. These activation increases are believed to reflect manipulation of to-be-remembered information in the service of memory-consolidation. This speculation has been based on observations of similar activation increases in tasks that overtly require manipulation by instructing participants to reorder to-be-remembered list items. In this study, we tested the assumption of functional equivalence between these two types of WM tasks. Participants performed a DRT under two conditions with memory loads ranging from 3 to 6 letters. In an "item-order" condition, participants were required to remember letters in the order in which they were presented. In a "reordering" condition, participants were required to remember the letters in alphabetical order. Load-related activation increases were observed during the encoding and maintenance periods of the order maintenance condition, whereas load-related activation decreases were observed in the same periods of the reordering condition. These results suggest that (1) the neural substrates associated with long-list retention and those associated with reordering are not equivalent, (2) cognitive processes associated with long-list retention may be more closely approximated by item-order maintenance than by reordering, and (3) multiple forms of WM manipulation are dissociable on the basis of fMRI data.

Adolescent↗

PubMed2006

Neural correlates of cognitive efficiency.

Since its inception, experimental psychology has sought to account for individual differences in human performance. Some neuroimaging research, involving complex behavioral paradigms, has suggested that faster-performing individuals show greater neural activity than slower performers. Other research has suggested that faster-performing individuals show less neural activity than slower performers. To examine the neural basis of individual performance differences, we had participants perform a simple speeded-processing task during fMRI scanning. In some prefrontal cortical (PFC) brain regions, faster performers showed less cortical activity than slower performers while in other PFC and parietal regions they showed greater activity. Regional-causality analysis indicated that PFC exerted more influence over other brain regions for slower than for faster individuals. These results suggest that a critical determinant of individual performance differences is the efficiency of interactions between brain regions and that slower individuals may require more prefrontal executive control than faster individuals to perform successfully.

Adolescent↗

PubMed2006

Measurements of tissue T1 spin-lattice relaxation time and discrimination of large draining veins using transient EPI data sets in BOLD-weighted fMRI acquisitions.

The signal intensity during the dynamic approach to the equilibrium state of longitudinal magnetization is a function of sequence parameters, such as repetition time and flip angle, and depends on tissue characteristics, including longitudinal relaxation time of stationary tissue and the rate of blood inflow. A method is presented to extract information from data acquired during the transient state prior to T1 equilibrium using echo-planar acquisitions in T2*-weighted functional magnetic resonance imaging (fMRI) experiments. A voxel in a single slice acquisition is assumed to contain either stationary tissue or large vessels with flowing blood. Models are presented to characterize longitudinal magnetization relaxation of heterogeneous stationary tissue and blood inflow. The data were fitted to theoretical models for longitudinal relaxation of stationary tissue and inflowing blood assuming no residual signal prior to each RF excitation. Parameters were estimated at 3 T for each model using least squares estimation. A goodness-of-fit criterion was applied to exclude voxels that have transient data that does not fit the selected (best fit) model. Voxels that best fit the inflow model, measured at various TR and flip angles, were assumed to contain large draining veins and were excluded from functional maps. Histogram analysis of T1 distributions for activated voxels in a visual paradigm demonstrated the distributions are centered at T1 values of gray matter with tails at both sides of the center due to partial voluming of gray matter with white matter and CSF respectively. The mean gray matter volume fraction in activated voxels was about 0.9. The results indicate that transient data sets can provide additional information that is useful for both localization and characterization of the functionally relevant BOLD response.

Adult↗

PubMed2006

Spatial extent of CBF response during whisker stimulation using trial averaged laser Doppler imaging.

The spatial pattern of activation in response to multiple whisker stimulation was studied using high-resolution laser Doppler (LD) imaging in urethane-anesthetized rats. LD flux change representing cerebral blood flow (CBF) responses were analyzed from a single trial or after averaging a number of similar trials. CBF change in a single trial was observed predominantly over pixels having low baseline flux values (microvessels), which included diffuse circular patterns of activation 400-800 microm in diameter similar to the histological dimensions of individual barrels established in the layer IV of the rat somatosensory cortex. The overall activation pattern varied considerably between each trial (only about 9-10% overlap); however, the diffuse circular pattern of activation was reproducible in every single trial within and across all rats. With trial averaging, no significant increase was observed in the outer boundary of activation, but the number of activated pixels increased within the diffuse circular patterns of activation. Emergence of further active pixels primarily within the diffuse circular regions of activity with trial averaging indicates distinct CBF responses in the septal and barrel regions, with a lesser LD signal to noise ratio in the barrel core.

Afferent Pathways↗

PubMed2005

Bootstrap resampling method to estimate confidence intervals of activation-induced CBF changes using laser Doppler imaging.

Laser Doppler imaging (LDI) signal and noise characteristics can vary significantly depending upon the underlying vascular caliber. Further, noise characteristics are not constant over time (non-stationary) and can vary during resting and activated conditions in a typical experiment. Since only a limited number of images can be acquired in a single run, concatenation of data from similar experimental trials becomes necessary which can induce further variation in temporal noise due to instrumental response. In conventional statistical analysis methods such as cross-correlation, a fixed significance threshold is generally used (for the entire image) to detect activation assuming constant noise over time and a normal distribution. As a consequence, statistical significance can become strong or weak due to temporal differences in baseline LD noise, which can possibly deviate from a normal distribution. The main emphasis of this study was the application of bootstrap resampling in conjunction with cross-correlation to estimate the confidence intervals on a pixel-by-pixel basis to avoid distributional specifications on the additive measurement error leading to reliable whisker activation-induced CBF changes. At a 95% confidence level, bootstrap resampling followed by confidence intervals for the correlation coefficient distribution increased the number of active pixels by almost 45% when compared to conventional cross-correlation. These pixels were mostly confined to areas with intermediate and large baseline LD flux with considerable deviation from normality. It is suggested that confidence intervals of the bootstrap estimates can lead to unbiased detection of CBF change in the cerebral cortex, particularly in regions with large temporal variation in noise and low CNR.

Afferent Pathways↗

PubMed2005

The influence of gender on auditory and language cortical activation patterns: preliminary data.

PURPOSE AND BACKGROUND: Intersex cortical and functional asymmetry is an ongoing topic of investigation. In this pilot study, we sought to determine the influence of acoustic scanner noise and sex on auditory and language cortical activation patterns of the dominant hemisphere. MATERIALS AND METHODS: Echoplanar functional MR imaging (fMRI; 1.5T) was performed on 12 healthy right-handed subjects (6 men and 6 women). Passive text listening tasks were employed in 2 different background acoustic scanner noise conditions (12 sections/2 seconds TR [6 Hz] and 4 sections/2 seconds TR [2 Hz]), with the first 4 sections in identical locations in the left hemisphere. Cross-correlation analysis was used to construct activation maps in subregions of auditory and language relevant cortex of the dominant (left) hemisphere, and activation areas were calculated by using coefficient thresholds of 0.5, 0.6, and 0.7. RESULTS: Text listening caused robust activation in anatomically defined auditory cortex, and weaker activation in language relevant cortex of all 12 individuals. As a whole, there was no significant difference in regional cortical activation between the 2 background acoustic scanner noise conditions. When sex was considered, men showed a significantly (P < .01) greater change in left hemisphere activation during the high scanner noise rate condition than did women. This effect was significant (P < .05) in the left superior temporal gyrus, the posterior aspect of the left middle temporal gyrus and superior temporal sulcus, and the left inferior frontal gyrus. CONCLUSION: Increase in the rate of background acoustic scanner noise caused increased activation in auditory and language relevant cortex of the dominant hemisphere in men compared with women where no such change in activation was observed. Our preliminary data suggest possible methodologic confounds of fMRI research and calls for larger investigations to substantiate our findings and further characterize sex-based influences on hemispheric activation patterns.

Adult↗

PubMed2004

Effect of anesthesia on CBF, MAP and fMRI-BOLD signal in response to apnea.

Systemic effects of anesthesia on the dynamics of the apnea-induced Blood Oxygen Level Dependent (BOLD) signal is still not clear. In the present study, the dynamics of the fMRI-BOLD signal and blood flow using laser Doppler flowmetry (LDF) was studied in rats in response to apnea. Two anesthetics namely pentobarbital and urethane, hypothesized to have distinct effects on the mean arterial blood pressure (MAP) were used. During normoxic baseline conditions, MAP decreased in response to apnea in rats anesthetized with pentobarbital but increased with urethane. However, MAP did not change significantly in response to apnea during hyperoxic or hypercapnic baseline conditions with both anesthetics. LDF increased in response to apnea during normoxia, hyperoxia or hypercapnia and was influenced by MAP during normoxia. Apnea-induced BOLD signal dynamics was similar with both anesthetics, dominated by an alteration in arterial blood oxygenation and independent of changes in MAP. Our results suggest that anesthesia-dependent MAP change modulates the apnea-induced cerebral blood flow (CBF) response but has a minimal effect on the fMRI-BOLD signal probably due to uncoupling of CBF and oxygen consumption.

Anesthesia↗

PubMed2004

Negative functional response to sensory stimulation and its origins.

Functional imaging studies typically give prominence to positive responses. However, negative changes that accompany activation-induced positive responses are not yet clearly understood. The authors investigated the origin of sustained negative CBF responses that accompanied positive CBF changes. Measurements were made in the rat somatosensory cortex in response to whisker stimulation using laser-Doppler imaging. Flux images indicative of CBF were obtained at rest and during whisker stimulation with a spatial resolution of 200 microm. Large and intermediate blood vessels in the cortical surface exhibiting high flux values were clearly resolved. This greatly reduced the contamination of the tissue pixel volume with macroscopic blood vessels. Regions that responded positively to whisker stimulation were from areas with intermediate to low baseline flux and distinctly away from high flux areas. Stimulation-induced change in signal intensity was the largest in pixels with low baseline flux, presumably from tissue and microvessels. Simultaneously, a sustained decrease in signal intensity was observed in regions with high baseline flux values. The temporal coherence, macrovascular origin, lesser trial-to trial variability, and complete absence of the negative CBF response in the microvascular regions suggest that it may be purely hemodynamic in nature.

Animals↗

PubMed2003

Regional dynamics of the fMRI-BOLD signal response to hypoxia-hypercapnia in the rat brain.

PURPOSE: To examine the regional blood oxygenation level-dependent (BOLD) signal response to rapid changes in arterial oxygen tension. MATERIALS AND METHODS: Functional MR imaging (fMRI) was carried out in five male Sprague-Dawley rats anesthetized with Sodium Pentobarbital. Rats were subjected to different durations of apnea as a rapid, graded, and reversible hypoxic-hypercapnic stimulus. Dynamics of the BOLD signal response were studied on a pixel-by-pixel basis in the cerebral cortex, hippocampus, third ventricle, and thalamus in the rat brain. RESULTS: Apnea induced a BOLD signal drop in all the brain regions studied, the magnitude of which increased with longer durations of the stimulus. The signal recovered to preapnic baseline levels after resumption of normal ventilation. Regional variation in the BOLD signal dynamics was observed with the magnitude of the BOLD signal change in the hippocampus being the least, followed by a relatively larger change in the thalamus, cerebral cortex, and third ventricle. The time (t(0)) for the signal change after the onset of the stimulus was estimated for every pixel. Time delay maps generated show the highest onset time values in the hippocampus followed by the thalamus, cerebral cortex, and third ventricle. CONCLUSION: The regional dynamics of the BOLD signal in the brain in response to apnea may vary depending on the rate of oxygen metabolism in addition to cerebral blood flow (CBF).

Animals↗

PubMed2003

Baseline physiological state and the fMRI-BOLD signal response to apnea in anesthetized rats.

To decipher the biophysical mechanism behind the fMRI-BOLD response to apnea and its dependence on the baseline cerebral blood flow and oxygenation, fMRI and laser Doppler flow (LDF) studies were carried out in anesthetized rats. Baseline cerebral blood flow (CBF) and PaO2 were modulated by ventilating with different gas mixtures namely, room air (21% O2), 100% O2, carbogen (95% O2+5% CO2), 2% CO2 in air or 5% CO2 in air, respectively. A decrease in BOLD signal intensity was observed after the onset of apnea with either room air, 2% CO2 or 5% CO2 ventilation. PaO2 and cerebral tissue PO2 decreased during apnea under these conditions. However, the apnea-induced BOLD signal intensity was unaffected with carbogen ventilation and increased with 100% O2 ventilation, during which PaO2 remained constant and cerebral tissue PO2 increased. When baseline CBF was high during hypercapnia, a faster decrease occurred in the apnea-induced BOLD signal. Apnea induced the largest increase in CBF of 85 +/- 25% when ventilated with 2% CO2 while a 44 +/- 8% increase was observed with room air. During the other ventilatory conditions, minimal or no significant change in CBF was observed during apnea. These results show a significant correlation between the BOLD signal change and tissue PO2 in response to apnea under different physiological conditions. Apnea-induced increase in CBF affects the magnitude of the BOLD signal response when PaO2 remains constant or changes minimally.

Anesthesia↗

PubMed2002

Differential fMRI-BOLD signal response to apnea in humans and anesthetized rats.

Blood oxygenation level dependent (BOLD) signal intensity (SI) and regional cerebral blood flow (CBF) during a 20-s apnea stimulus in awake humans and pentobarbital-anesthetized rats were measured to assess the usefulness of apnea in estimating cerebral vasodilatory capacity for functional MRI (fMRI) experiments. Rats were ventilated with either room air or 100% O(2.) While breathing room air, apnea for 20 s increased the BOLD SI in humans but decreased it in rats. However, in rats ventilated with 100% O(2), BOLD SI increased upon apnea for 20 s. CBF measurements in rats using laser Doppler flowmetry (LDF) showed a 45% +/- 8% increase during apnea with room air ventilation, and a 10% +/- 3% increase with 100% O(2). Arterial blood oxygen saturation fell from 96% +/- 1% to 29% +/- 5%, and cerebral tissue PO(2) decreased from 15 +/- 3 mmHg to 6 +/- 2 mmHg by the end of 20-s apnea in rats breathing room air. However, with 100% O(2) respiration, apnea produced no change in the arterial blood oxygen saturation, which remained at 99%, but increased tissue PO(2) from 35 +/- 9 mmHg to 39 +/- 10 mmHg. From the results obtained in rats ventilated with room air, it is concluded that apnea induces hypoxia that results in a decrease in fMRI-BOLD signal. The signal decrease occurred despite an increase in P(a)CO(2) and CBF. This BOLD response is the opposite of that observed in humans, who presumably do not develop hypoxia within the applied apnea period. These studies highlight the importance of the choice of ventilating gas mixture on the outcome of BOLD experiments during systemic perturbations.

Animals↗

PubMed2002

Functional magnetic resonance imaging technology and traumatic brain injury rehabilitation: guidelines for methodological and conceptual pitfalls.

OBJECTIVES: To illuminate the current methodological and conceptual pitfalls inherent in conducting functional magnetic resonance imaging (fMRI) research with individuals who have sustained traumatic brain injury (TBI) and to discuss appropriate remedies. The aim is describe fMRI research, its limitations, and how to best use this technology to examine TBI. DISCUSSION: The topics discussed in this article include issues regarding signal detection, brain activation measurement, head movement, and sources of signal artifact. Issues surrounding data interpretation and the importance of analyzing the brain as a connected neural network is also discussed. Finally, problems with spatial normalization when examining individuals with TBI are reviewed. CONCLUSIONS: To date, there is a scarcity of research applying fMRI technology to the study of TBI. However, because it is a noninvasive procedure with high availability in hospital settings across the country, the next decade of TBI research will likely include a proliferation of this form of investigation. At this time, much work is needed to better understand how to optimally use this technology to examine the effects of TBI on behavior. For fMRI to enhance TBI research it will be imperative to establish valid research protocols and reliable methods of data interpretation.

Brain Injuries↗

PubMedDate not supplied

Intensity-dependent activation of the primary auditory cortex in functional magnetic resonance imaging.

PURPOSE: The purpose of this study was to investigate the activation patterns of the primary auditory cortex in response to varying intensities of pure tone stimuli. METHOD: A 1,000-Hz pure tone stimulus was delivered monaurally to the right ear of 12 normal-hearing right-handed volunteers in 20-second on-off cycles. Stimuli were applied at 20 and 50 dB hearing level (HL) above threshold in 12 subjects and at 0, 20, 40, and 50 dB HL above threshold in 6 subjects. Functional magnetic resonance imaging (fMRI) data were obtained using a 1.5-T scanner and echoplanar imaging. Activated pixels were identified in the transverse temporal gyrus (TTG) of both hemispheres in response to pure tone stimuli at each intensity level using cross-correlation analysis (0.6; P < 0.0001). RESULTS: Of the 24 right and left TTGs imaged (n = 12), activation to pure tone stimuli at 20 and 50 dB HL above threshold was seen in 46% and 79% of TTGs, respectively, with bilateral hemispheric activation in 27% and 64% of subjects, respectively. The mean numbers of activated voxels were 4.0 and 13.0, respectively. Of the 12 right and left TTGs imaged at 0, 20, 40, and 50 dB HL above threshold, activation was seen in 33%, 42%, 58%, and 75% of TTGs, respectively. The mean numbers of activated voxels were 5.8, 3.2, 9.8, and 15.3, respectively. There was a nonsignificant trend toward contralateral (left) dominant TTG activation with increased tone intensity. CONCLUSION: Our results show an increased likelihood of TTG activation, increased TTG activation volume, and increased bilateral hemisphere TTG activation with increasing pure tone intensity. Our results suggest that the primary auditory cortex reflects or is directly involved in the central processing of sound intensity and that varying the intensity of even simple stimuli can alter the patterns of fMRI activation in auditory cortex.

Acoustic Stimulation↗

PubMedDate not supplied

Decoupling of the hemodynamic and activation-induced delays in functional magnetic resonance imaging.

PURPOSE: The objective of this study was to develop a technique to decouple the hemodynamic delay from the task-induced delay on a voxel-by-voxel basis in functional magnetic resonance imaging (fMRI) data sets. Prior fMRI studies have reported variability in temporal delays of up to several seconds among activated voxels. It is currently assumed that this variability primarily arises from differences in the onset of task-induced activation, although the precise source of these delays has not been well characterized. Here, we hypothesize that the total delay during task activation can be modeled as a combination of neuronal (caused by differences in onset of neuronal firing), vasomotor (caused by flow changes during activation), and transit (caused by differences in the red blood cell arrival time) delays. METHOD: Subjects were scanned using a sequential dynamic susceptibility contrast (DSC) protocol during rest and fMRI of the motor cortex using a bilateral finger-tapping task. The total delay was determined using correlation coefficient analysis, whereas the intrinsic delay was determined from the DSC MRI. Subtraction of the transit delay from the corresponding total delay for each voxel yielded the task-induced delay. RESULTS: In all subjects, a transit delay of 2.3 (+/-1.1) seconds and a task-induced delay of 0.7 (+/-0.6) second was observed between voxels, which is in good agreement with reports in the literature using other techniques. These results demonstrate the feasibility of the DSC MRI for separating the hemodynamic and task-induced delays in fMRI studies. CONCLUSION: This approach has the potential to elucidate the temporal characteristics of the blood oxygenation level-dependent signal during fMRI as well as to further our understanding of the dynamics of the activation-induced signal in neuroimaging.

Adult↗