Biomedical subjects

Peter A Bandettini

Publications and source records attributed to Peter A Bandettini.

13 recordsLinked to original sources

PubMed2006

How long to scan? The relationship between fMRI temporal signal to noise ratio and necessary scan duration.

Recent advances in MRI receiver and coil technologies have significantly improved image signal-to-noise ratios (SNR) and thus temporal SNR (TSNR). These gains in SNR and TSNR have allowed the detection of fMRI signal changes at higher spatial resolution and therefore have increased the potential to localize small brain structures such as cortical layers and columns. The majority of current fMRI processing strategies employ multi-subject averaging and therefore require spatial smoothing and normalization, effectively negating these gains in spatial resolution higher than about 10 mm3. Reliable detection of activation in single subjects at high resolution is becoming a more common desire among fMRI researchers who are interested in comparing individuals rather than populations. Since TSNR decreases with voxel volume, detection of activation at higher resolutions requires longer scan durations. The relationship between TSNR, voxel volume and detectability is highly non-linear. In this study, the relationship between TSNR and the necessary fMRI scan duration required to obtain significant results at varying P values is determined both experimentally and theoretically. The results demonstrate that, with a TSNR of 50, detection of activation of above 2% requires at most 350 scan volumes (when steps are taken to remove the influence of physiological noise from the data). Importantly, these results also demonstrate that, for activation magnitude on the order of 1%, the scan duration required is more sensitive to the TSNR level than at 2%. This study showed that with voxel volumes of approximately 10 mm3 at 3 T, and a corresponding TSNR of approximately 50, the required number of time points that guarantees detection of signal changes of 1% is about 860, but if TSNR increases by only 20%, the time for detection decreases by more than 30%. More than just being an exercise in numbers, these results imply that imaging of columnar resolution (effect size=1% and assuming a TR of 1 s) at 3 T will require either 10 min for a TSNR of 60 or 40 min for a TSNR of 30. The implication is that at these resolutions, TSNR is likely to be critical for determining success or failure of an experiment.

Algorithms↗

PubMed2006

Direct magnetic resonance detection of neuronal electrical activity.

Present noninvasive neuroimaging methods measure neuronal activity indirectly, via either cerebrovascular changes or extracranial measurements of electrical/magnetic signals. Recent studies have shown evidence that MRI may be used to directly and noninvasively map electrical activity associated with human brain activation, but results are inconclusive. Here, we show that MRI can detect cortical electrical activity directly. We use organotypic rat-brain cultures in vitro that are spontaneously active in the absence of a cerebrovascular system. Single-voxel magnetic resonance (MR) measurements obtained at 7 T were highly correlated with multisite extracellular local field potential recordings of the same cultures before and after blockade of neuronal activity with tetrodotoxin. Similarly, for MR images obtained at 3 T, the MR signal changed solely in voxels containing the culture, thus allowing the spatial localization of the active neuronal tissue.

Animals↗

PubMed2006

Separating respiratory-variation-related fluctuations from neuronal-activity-related fluctuations in fMRI.

Subtle changes in a subject's breathing rate or depth, which occur naturally during rest at low frequencies (<0.1 Hz), have been shown to be significantly correlated with fMRI signal changes throughout gray matter and near large vessels. The goal of this study was to investigate the impact of these low-frequency respiration variations on both task activation fMRI studies and resting-state functional connectivity analysis. Unlike MR signal changes correlated with the breathing motion ( approximately 0.3 Hz), BOLD signal changes correlated with across-breath variations in respiratory volume ( approximately 0.03 Hz) appear localized to blood vessels and regions with high blood volume, such as gray matter, similar to changes seen in response to a breath-hold challenge. In addition, the respiration-variation-induced signal changes were found to coincide with many of the areas identified as part of the 'default mode' network, a set of brain regions hypothesized to be more active at rest. Regions could therefore be classified as being part of a resting network based on their similar respiration-induced changes rather than their synchronized neuronal activity. Monitoring and removing these respiration variations led to a significant improvement in the identification of task-related activation and deactivation and only slight differences in regions correlated with the posterior cingulate at rest. Regressing out global signal changes or cueing the subject to breathe at a constant rate and depth resulted in an improved spatial overlap between deactivations and resting-state correlations among areas that showed deactivation.

Adult↗

PubMed2006

The role of awareness in delay and trace fear conditioning in humans.

Expression of conditional fear without awareness has been previously demonstrated during delay conditioning, a procedure in which the conditioned stimulus (CS) and unconditioned stimulus (UCS) overlap. However, less is known about the role of awareness in trace fear conditioning, where an interval of time separates the CS and UCS. The present study assessed skin conductance response (SCR) and UCS expectancy during delay and trace conditioning. UCS predictability was varied on a trial-by-trial basis by presenting perithreshold auditory CSs. Differential UCS expectancies were demonstrated only on perceived delay and trace trials. Learning-related SCRs were observed during both perceived and unperceived delay CSs. In contrast, differential SCRs were demonstrated only for perceived trace CSs. These data suggest that awareness is necessary for conditional responding during trace, but not delay, fear conditioning.

Adult↗

PubMed2005

Improved BOLD detection in the medial temporal region using parallel imaging and voxel volume reduction.

Using gradient-echo EPI, signal dropout due to macroscopic off resonance effects can prevent blood-oxygenation-level-dependent (BOLD) signal change detection. The anterior medial temporal lobe (MTL) is located near these susceptibility gradients and therefore shows considerable signal dropout with GE-EPI. Reducing the volume of the image voxel reduces susceptibility-related signal dropout. However, this is accompanied by a prohibitive reduction in signal-to-noise ratio (SNR). To compensate for SNR loss with smaller voxels, we used a multi-channel MRI receiver with an array of receive-only 16-element surface coils at 3 T. We demonstrate that the reduction of susceptibility artifacts, through use of high resolution images, coupled with the gains in image SNR from the array coil improves the temporal signal-to-noise ratio (TSNR) and enhances the contrast-to-noise ratio (CNR). Furthermore, a comparison of 2 mm with 4-mm-thick axial images both with the same in-plane resolution showed that thinner slices enhanced TSNR and CNR throughout the ventral-medial regions of the temporal lobes, with the greatest improvement in the most anterior regions of the MTL. Further improvements were seen when adjacent 2 mm slices were combined to match overall voxel volume. These results demonstrate that BOLD investigation of anterior MTL function can be enhanced by decreasing voxel size but only in combination with the SNR gained by using the 16-channel head coil system.

Adult↗

PubMed2005

The effect of stimulus duty cycle and "off" duration on BOLD response linearity.

An ongoing question in functional MRI is precisely how measured signal changes relate to neuronal activity. While this question has been probed using animal models and electrophysiologic measures of neuronal activity, it has also been probed by examining, in humans, the spatial location, magnitude, and temporal dynamics of signal changes to well understood stimuli. With regard to dynamics, several earlier studies have revealed a larger than expected response to brief stimuli, hypothesized to result from nonlinearities in either the hemodynamics or the neuronal activity. In this study, we investigate the linearity of the increase in blood oxygenation level dependent (BOLD) contrast as a function of stimulus duty cycle, as well as the linearity of the decrease in BOLD as a function stimulus "off" duration. These findings not only shed further light on the mechanisms behind BOLD contrast but also give practical information as to what to keep in mind when performing and interpreting event related fMRI experiments. These experiments demonstrated: a) the BOLD signal decrease, on stimulus cessation, was smaller than predicted by a linear system--opposite to what has been reported in the literature associated with a signal increase, and b) the deconvolved event-related BOLD signal is highly dependent on duty cycle (the fraction of time activated vs. non-activated), Several potential mechanisms explaining these dynamics are discussed and modeled. We find that the experimental results are most consistent with a nonlinear neuronal response, but do not rule out significant effects of nonlinear hemodynamic factors, in particular the nonlinear relationship between oxygen extraction fraction and blood flow.

Adaptation, Physiological↗

PubMed2005

The role of the human amygdala in the production of conditioned fear responses.

The amygdala plays a central role in the acquisition and expression of fear memories. Laboratory animal studies indicate that the amygdala both receives sensory information and produces learned behavioral and autonomic fear responses. However, prior functional imaging research in humans has largely focused on amygdala activity elicited by fearful stimuli, giving less attention to this region's role in the production of fear responses. In contrast, the present study used functional magnetic resonance imaging to investigate the amygdala's influence on the generation of conditional fear responses. Significant increases in amygdala activity were observed during the production of conditioned (learning-related), but not orienting, nonspecific, and unconditioned (nonlearning-related) skin conductance responses. Further, greater amygdala activity was demonstrated during conditioned response production than during conditioned stimulus presentation. These results suggest the amygdala not only responds to fearful stimuli, but also generates learning-related changes in human autonomic fear expression.

Acoustic Stimulation↗

PubMed2004

Experimental designs and processing strategies for fMRI studies involving overt verbal responses.

Event-related paradigms have been used increasingly in the past few years for the localization of function in tasks involving overt speech. These designs exploit the differences in the temporal characteristics between the rapid motion-induced and the slower hemodynamic signal changes. The optimization of these designs and the best way to analyze the acquired data has not yet been fully explored. The purpose of this study is to investigate various design and analysis strategies for maximizing the detection of function while minimizing task-induced motion artifacts. Both event-related and blocked paradigms can be specifically designed to meet these goals. Various event-related and blocked designs were compared both in simulation and in experiments involving overt word reading in their ability to detect function and to avoid speech-induced motion artifact. A blocked design with task and control durations of 10 s and an event-related design with a minimum stimulus duration (SD) of 5 s and an average interstimulus interval (ISI) of 10 s were found to optimally detect blood oxygenation level-dependent signal changes without significant motion artifact. Ignoring images acquired during the speech can help recover function in areas particularly affected by motion but substantially reduces the detection power in other regions. Using the stimulus timing as an additional regressor to model the motion offers little benefit in practice due to the variability of the motion-induced signal change.

Algorithms↗

PubMed2003

Expression of conditional fear with and without awareness.

Conditional responding during simple Pavlovian conditioning is often characterized as a form of implicit memory. The extent to which this type of associative learning is independent of awareness is an issue of continuing debate. Previous studies have demonstrated conditioning in the absence of awareness. However, their results have been questioned based on methodological concerns with postexperimental questionnaires. In the present study, skin conductance response (SCR) and unconditioned stimulus (UCS) expectancy were measured concurrently as participants were exposed to a differential delay fear conditioning procedure in which one tone (CS+) predicted a loud white noise, whereas a second tone (CS-) was presented alone. UCS predictability was varied on a trial-by-trial basis by presenting conditioned stimuli (CSs) at volumes just above or below the perceptual threshold. Differential UCS expectancy (awareness) was observed only on perceived trials, whereas differential SCR developed on both perceived and unperceived trials. Although perceived stimuli elicited larger SCRs, the magnitude of conditioning, indexed by differential conditioned response expression (conditioned SCR to CS+ minus the SCR to CS-), was not influenced by stimulus perception. These data indicate that conditional fear can be expressed when individuals are unaware of fear-eliciting stimuli and suggest that the degree of conditioning is independent of awareness during differential Pavlovian fear conditioning.

Adult↗

PubMed2003

The spatial extent of the BOLD response.

Functional magnetic resonance imaging is routinely used to localize brain function, with multiple brain scans averaged together to reveal activation volumes. In this study, we examine the seldom-studied effect of multiple scan averaging on the extent of activation volume. Using restricted visual field stimulation, we obtained a large number of scan repetitions and analyzed changes in activation volume with progressively increased averaging and across single scans. Activation volume increased monotonically with averaging and failed to asymptote when as many as 22 scans were averaged together. Expansions in the spatial extent of activation were not random; rather, they were centered about activation loci that appear with little or no averaging. Using empirical and simulated data, changes with averaging in activation volumes and cross correlation coefficient distributions revealed the presence of considerably more activated voxels than commonly surmised. Many voxels have low SNR and remain undetected without extensive averaging. The primary source of such voxels was not downstream venous drainage since there was no significant and consistent delay difference between voxels activated at different averaging levels. Voxels with low SNR may reflect a diffuse subthreshold activity centered about spiking neurons, dephasing gradients from distal veins, or simply a blood flow response extending beyond the locus of neuronal firing. Across single scans, as much as twofold changes in activation volume were observed. These changes were not correlated with the order of scan acquisition, subject task performance, or signal and noise properties of activated voxels. Instead, they may reflect subtle changes between overlapping noise and signal frequency components.

Brain↗

PubMed2002

Toward direct mapping of neuronal activity: MRI detection of ultraweak, transient magnetic field changes.

A novel method based on selective detection of rapidly changing DeltaB(0) magnetic fields and suppression of slowly changing DeltaB(0) fields is presented. The ultimate goal of this work is to present a method that may allow detection of transient and subtle changes in B(0) in cortical tissue associated with electrical currents produced by neuronal activity. The method involves the detection of NMR phase changes that occur during a single-shot spin-echo (SE) echo-planar sequence (EPI) echo time. SE EPI effectively rephases all changes in B(0) that occur on a time scale longer than the echo time (TE) and amplifies all DeltaB(0) changes that occur during TE/2. The method was tested on a phantom that contains wires in which current can be modulated. The sensitivity and flexibility of the technique was demonstrated by modulation of the temporal position and duration of the stimuli-evoked transient magnetic field relative to the 180 RF pulse in the imaging sequence-requiring precise stimulus timing. Currently, with this method magnetic field changes as small as 2 x 10(-10) T (200 pT) and lasting for 40 msec can be detected. Implications for direct mapping of brain neuronal activity with MRI are discussed.

Brain Mapping↗

PubMed2002

Detection versus estimation in event-related fMRI: choosing the optimal stimulus timing.

With the advent of event-related paradigms in functional MRI, there has been interest in finding the optimal stimulus timing, especially when the interstimulus interval is varied during the imaging run. Previous works have proposed stimulus timings to optimize either the estimation of the impulse response function (IRF) or the detection of signal changes. The purpose of this paper is to clarify that estimation and detection are fundamentally different goals and to determine the optimal stimulus timing and distribution with respect to both the accuracy of estimating the IRF and the power of detection assuming a particular hemodynamic model. Simulated stimulus distributions are varied systematically, from traditional blocked designs to rapidly varying event related designs. These simulations indicate that estimation of the hemodynamic impulse response function is optimized when stimuli are frequently alternated between task and control states, with shorter interstimulus intervals and stimulus durations, whereas the detection of activated areas is optimized by blocked designs. The stimulus timing for a given experiment should therefore be generated with the required detectability and estimation accuracy.

Cerebral Cortex↗

PubMed2002

Task-independent functional brain activity correlation with skin conductance changes: an fMRI study.

Lesions of the ventromedial prefrontal cortex cause a loss of skin conductance response (SCR) to stimuli with affective content and an inability to integrate information with social consequences into decisions. Previous behavioral studies using a gambling task were able to differentiate patients with lesions in this region from normal subjects. In the present imaging study, this region, among others, was shown to be "spontaneously" activated during three different cognitive states: a gambling task, a working memory task, and resting state. SCR data were simultaneously collected during the scanning process. Six subjects were scanned at 1.5 T during all three states, and one subject was scanned at 3 T during the resting state only. SCR data were used as a reference function for correlation analysis with the fMRI time series during each of the three tasks. SCR changes were evident during the gambling and two-back tasks as well as during rest. SCR activity was not observed to be specifically related to reward-based decisions in the gambling task. Correlation of the fMRI time series directly with the SCR data revealed a consistent set of activated regions. The activity of these regions showing correlation with the SCR appeared independent of the cognitive state. Further, the subject scanned only at rest (without the possible confound of task-related carryover activity) replicated the findings in the original six subjects. From these data, SCR appears to be a marker of a network that is active during, but independent of, the task being studied.

Adult↗