Temporal and spatial MRI responses to subsecond visual activation

Deconvolution of the Functional Ultrasound Response in the Mouse Visual Pathway Using Block-Term Decomposition

Functional ultrasound (fUS) indirectly measures brain activity by detecting changes in cerebral blood volume following neural activation. Conventional approaches model such functional neuroimaging data as the convolution between an impulse response, known as the hemodynamic response function (HRF), and a binarized representation of the input signal based on the stimulus onsets, the so-called experimental paradigm (EP). However, the EP may not characterize the whole complexity of the activity-inducing signals that evoke the hemodynamic changes. Furthermore, the HRF is known to vary across brain areas and stimuli. To achieve an adaptable framework that can capture such dynamics of the brain function, we model the multivariate fUS time-series as convolutive mixtures and apply block-term decomposition on a set of lagged fUS autocorrelation matrices, revealing both the region-specific HRFs and the source signals that induce the hemodynamic responses. We test our approach on two mouse-based fUS experiments. In the first experiment, we present a single type of visual stimulus to the mouse, and deconvolve the fUS signal measured within the mouse brain's lateral geniculate nucleus, superior colliculus and visual cortex. We show that the proposed method is able to recover back the time instants at which the stimulus was displayed, and we validate the estimated region-specific HRFs based on prior studies. In the second experiment, we alter the location of the visual stimulus displayed to the mouse, and aim at differentiating the various stimulus locations over time by identifying them as separate sources.

Statistical power or more precise insights into neuro-temporal dynamics? Assessing the benefits of rapid temporal sampling in fMRI

Functional magnetic resonance imaging (fMRI), a non-invasive and widely used human neuroimaging method, is most known for its spatial precision. However, there is a growing interest in its temporal sensitivity. This is despite the temporal blurring of neuronal events by the blood oxygen level dependent (BOLD) signal, the peak of which lags neuronal firing by 4-6 seconds. Given this, the goal of this review is to answer a seemingly simple question - "What are the benefits of increased temporal sampling for fMRI?". To answer this, we have combined fMRI data collected at multiple temporal scales, from 323 to 1000 milliseconds, with a review of both historical and contemporary temporal literature. After a brief discussion of technological developments that have rekindled interest in temporal research, we next consider the potential statistical and methodological benefits. Most importantly, we explore how fast fMRI can uncover previously unobserved neuro-temporal dynamics - effects that are entirely missed when sampling at conventional 1 to 2 second rates. With the intrinsic link between space and time in fMRI, this temporal renaissance also delivers improvements in spatial precision. Far from producing only statistical gains, the array of benefits suggest that the continued temporal work is worth the effort.

Characterization of the hemodynamic response function across the majority of human cerebral cortex

A brief (<4 s) period of neural activation evokes a stereotypical sequence of vascular and metabolic events to create the hemodynamic response function (HRF) measured using functional magnetic resonance imaging (fMRI). Linear analysis of fMRI data requires that the HRF be treated as an impulse response, so the character and temporal stability of the HRF are critical issues. Here, a simple audiovisual stimulus combined with a fast-paced task was used to evoke a strong HRF across a majority, ∼77%, of cortex during a single scanning session. High spatiotemporal resolution (2-mm voxels, 1.25-s acquisition time) was used to focus HRF measurements specifically on the gray matter for whole brain. The majority of activated cortex responds with positive HRFs, while ∼27% responds with negative (inverted) HRFs. Spatial patterns of the HRF response amplitudes were found to be similar across subjects. Timing of the initial positive lobe of the HRF was relatively stable across the cortical surface with a mean of 6.1 ± 0.6 s across subjects, yet small but significant timing variations were also evident in specific regions of cortex. The results provide guidance for linear analysis of fMRI data. More importantly, this method provides a means to quantify neurovascular function across most of the brain, with potential clinical utility for the diagnosis of brain pathologies such as traumatic brain injury.

Statistical analysis of high density diffuse optical tomography

High density diffuse optical tomography (HD-DOT) is a noninvasive neuroimaging modality with moderate spatial resolution and localization accuracy. Due to portability and wear-ability advantages, HD-DOT has the potential to be used in populations that are not amenable to functional magnetic resonance imaging (fMRI), such as hospitalized patients and young children. However, whereas the use of event-related stimuli designs, general linear model (GLM) analysis, and imaging statistics are standardized and routine with fMRI, such tools are not yet common practice in HD-DOT. In this paper we adapt and optimize fundamental elements of fMRI analysis for application to HD-DOT. We show the use of event-related protocols and GLM de-convolution analysis in un-mixing multi-stimuli event-related HD-DOT data. Statistical parametric mapping (SPM) in the framework of a general linear model is developed considering the temporal and spatial characteristics of HD-DOT data. The statistical analysis utilizes a random field noise model that incorporates estimates of the local temporal and spatial correlations of the GLM residuals. The multiple-comparison problem is addressed using a cluster analysis based on non-stationary Gaussian random field theory. These analysis tools provide access to a wide range of experimental designs necessary for the study of the complex brain functions. In addition, they provide a foundation for understanding and interpreting HD-DOT results with quantitative estimates for the statistical significance of detected activation foci.

Prior MDMA (Ecstasy) use is associated with increased basal ganglia-thalamocortical circuit activation during motor task performance in humans: an fMRI study

MDMA (3,4-methylenedioxymethamphetamine; Ecstasy) is a popular recreational drug that produces long-lasting serotonin (5-HT) neurotoxicity consisting of reductions in markers for 5-HT axons. 5-HT innervates cortical and subcortical brain regions mediating motor function, predicting that MDMA users will have altered motor system neurophysiology. We used functional magnetic resonance imaging (fMRI) to assay motor task performance-associated brain activation changes in MDMA and non-MDMA users. 24 subjects (14 MDMA users and 10 controls) performed an event-related motor tapping task (1, 2 or 4 taps) during fMRI at 3 T. Motor regions of interest were used to measure percent signal change (PSC) and percent activated voxels (PAV) in bilateral motor cortex, sensory cortex, supplementary motor area (SMA), caudate, putamen, pallidum and thalamus. We used SPM5 to measure brain activation via three methods: T-maps, PSC and PAV. There was no statistically significant difference in reaction time between the two groups. For the Tap 4 condition, MDMA users had more activation than controls in the right SMA for T-score (p=0.02), PSC (p=0.04) and PAV (p=0.03). Lifetime episodes of MDMA use were positively correlated with PSC for the Tap 4 condition on the right for putamen and pallidum; with PAV in the right motor and sensory cortex and bilateral thalamus. In conclusion, we found a group difference in the right SMA and positive dose-response association between lifetime exposure to MDMA and signal magnitude and extent in several brain regions. This evidence is consistent with MDMA-induced alterations in basal ganglia-thalamocortical circuit neurophysiology and is potentially secondary to neurotoxic effects on 5-HT signaling. Further studies examining behavioral correlates and the specific neurophysiological basis of the observed findings are warranted.

Blood oxygenation level-dependent visualization of synaptic relay stations of sensory pathways along the neuroaxis in response to graded sensory stimulation of a limb

Blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) was used to test at which levels of the neuroaxis signals are elicited when different modalities of sensory information from the limbs ascend to cortex cerebri. We applied graded electric stimuli to the rat hindlimbs and used echo-planar imaging to monitor activity changes in the lumbar spinal cord and medulla oblongata, where primary afferents of painful and nonpainful sensation synapse, respectively. BOLD signals were detected in ipsilateral lumbar spinal cord gray matter using sufficiently strong stimuli. Using stimuli well below the threshold needed for signals to be elicited in the spinal cord, we found BOLD responses in dorsal medulla oblongata. The distribution of these signals is compatible with the neuroanatomy of the respective synaptic relay stations of the corresponding sensory pathways. Hence, the sensory pathways conducting painful and nonpainful information were successfully distinguished. The fMRI signals in the spinal cord were markedly decreased by morphine, and these effects were counteracted by naloxone. We conclude that fMRI can be used as a reliable and valid method to monitor neuronal activity in the rat spinal cord and medulla oblongata in response to sensory stimuli. Previously, we also documented BOLD signals from thalamus and cortex. Thus, BOLD responses can be elicited at all principal synaptic relay stations along the neuroaxis from lumbar spinal cord to sensory cortex. Rat spinal cord fMRI should become a useful tool in experimental spinal cord injury and pain research.

Effects of mild hypoxic hypoxia on poststimulus undershoot of blood-oxygenation-level-dependent fMRI signal in the human visual cortex

Characteristics of the blood-oxygenation-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signal poststimulus undershoot in the visual cortex were studied at varying levels of arterial blood oxygen saturation (Ysat). Undershoot with an amplitude of -0.6+/-0.2% appeared after positive BOLD response (+1.7+/-0.5%) under control conditions. Cerebral blood volume (CBV), as determined with vascular-space-occupancy-dependent fMRI, increased by 26-43% during the positive BOLD peak, but the CBV proceeded at baseline level during the BOLD poststimulus undershoot. Mild hypoxic hypoxia (Ysat ranging from 0.82 to 0.89) had no effect on the amplitude or duration of poststimulus undershoot in activated BOLD pixels. Hypoxia did not influence CBV during the BOLD poststimulus undershoot. In contrast, the positive BOLD signal at the level of all activated pixels was smaller in hypoxia than in normoxia. The present results show that the BOLD poststimulus undershoot is not influenced by curtailed oxygen availability and that, during the undershoot, CBV is not different from resting state.

fMRI localizer technique: Efficient acquisition and functional properties of single retinotopic positions in the human visual cortex

Current fMRI retinotopic mapping procedures often use checkerboard stimuli consisting of expanding rings and rotating wedges to measure the topography within human visual areas. Efficient procedures are well described in the literature. For many experimental paradigms, e.g., visuo-spatial attention paradigms, the identification of task-relevant positions is the only mandatory prerequisite. To define these specific "regions-of-interest" (ROIs), spatially defined localizers are used. A precise evaluation of localizer techniques in regard to efficient scanning time, optimal BOLD (blood oxygenic level dependent) response, as well as quantification of the resulting ROIs within each visual area (size, overlap, surround effects) has not been studied to date. Here, we suggest a mapping procedure designed to quantify spatial and functional properties of single positions at close proximity in multiple human visual areas. During a passive viewing task, various stimuli (e.g., checkerboards or colored objects) subtending 1.4 degrees of visual angle were presented at one out of four positions in a randomized block design. We measured the degree of overlap between positions at different hierarchical levels of the visual system (V1-V4v) and quantified modulatory effects on a specific position by stimulation at neighboring (1.7 degrees spacing) or distant positions (5.1 degrees or 8.5 degrees spacing). Within each visual area, "mexican-hat" distributions of local signal intensity changes, which describe a particular combination of facilitatory and suppressive effects, were found. Cubic fitting revealed the most localized tuning effect in V1, which gradually decreased throughout the higher visual areas. Colored objects were most efficient in localizing circumscribed retinotopic positions in both early and higher areas.

Effects of EPI readout bandwidth on measured activation map and BOLD response in fMRI experiments

The purpose of this study was to evaluate the effects of echo planar imaging (EPI) readout bandwidth and its interaction with data processing procedures on the measured blood oxygenation level dependent (BOLD) response and activation in fMRI experiments. Seventeen healthy subjects were scanned during a brief visual stimulation paradigm with two EPI pulse sequences having 'high' (1953 Hz/pixel) and 'low' (780 Hz/pixel) readout bandwidth. Functional data were analyzed with a general linear model including temporal filtering and a basic correlation model following (1) no preprocessing, (2) realignment, or (3) realignment and spatial smoothing. A range of statistical thresholds were used to generate activation maps. Despite slightly higher BOLD signal detected with the high bandwidth sequence from matched ROIs in the primary visual cortex, results showed that the low bandwidth pulse sequence was more sensitive under most conditions evaluated. That is, the low bandwidth sequence detected greater numbers of activated voxels with lower cluster average BOLD signal (e.g., low bandwidth detected 1.4 times more voxels, with average BOLD signal 30% lower compared to high bandwidth for P = 0.05 (corrected) with the 3rd preprocessing procedure using the general linear model). However, there was significant interaction between bandwidth and data preprocessing procedures. Of particular interest, the sensitivity advantage of the low bandwidth pulse sequence decreased for the smoothed data as the activation threshold became less conservative. For the frequently used threshold of P = 0.001 (uncorrected) and cluster size of at least 5 voxels, the bandwidth advantage became insignificant. These findings demonstrate that the effects of bandwidth should be considered carefully in the design, analysis, and interpretation of BOLD fMRI studies.

BOLD responses to visual stimulation in survivors of childhood cancer

Children surviving certain cancers have a high incidence of cognitive deficits caused by central nervous system (CNS) disease or treatments directed at the CNS. To establish the feasibility of using blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) to study cognitive deficits in survivors of childhood cancer, we tested the hypothesis that this population has the same BOLD response to visual stimulation as healthy subjects. We used BOLD fMRI to measure spatial and temporal patterns of brain activity after brief visual stimulation in 16 survivors of childhood cancer, 11 age-similar healthy siblings of survivors, and 16 healthy adults. Functional data for the survivors were analyzed with two general linear models, one used a canonical hemodynamic response function (HRF) and the other used a Fourier set as basis functions. The measured BOLD signal and brain activation patterns were similar in the survivors with both models. The BOLD signal for survivors was qualitatively similar in timing and shape, but there were significant quantitative differences as compared with healthy subjects. The activation was normally located in the primary visual cortex in 13 survivors, but the activation volume was significantly smaller in brain tumor survivors than in other groups. These findings demonstrate the feasibility of using BOLD fMRI to investigate brain function in survivors of childhood cancer. However, fMRI studies in this population must take into account effects of quantitative differences in their BOLD responses as compared to healthy subjects.

A comparison of methods for characterizing the event-related BOLD timeseries in rapid fMRI

Information about the shape and temporal duration of the blood oxygenation level dependent (BOLD) response can inform both functional neuroanatomy and psychological theory. However, the BOLD response evolves over 20 s or more, making it difficult to distinguish the unique characteristics of the response evoked by temporally adjacent stimuli. Fortunately, event-related BOLD signals can be extracted given that there is adequate variance in the distribution of inter-stimulus intervals (ISI). Unfortunately, the ISI distribution that yields the highest statistical efficiency is not always optimal from a psychological perspective; variability in the stimulus timing may complicate the interpretation of neuroimaging data in terms of underlying cognitive operations. In the present paper, Monte Carlo simulations are used to evaluate two techniques for estimating the event-related BOLD timeseries-event-related averaging and deconvolution using the Ordinary Least Squares estimate -with respect to maintaining acceptable levels of statistical power and experimental validity. While the unbiased deconvolution technique more robustly estimates the shape of the BOLD response functions, both methods succeed in accurately re-producing known differences between evoked BOLD responses when the stimulus ordering is randomized. However, the deconvolution method is more effective at preserving differences when there are sequential dependencies in the stimulus presentation order and restricted ISI distributions are used; particularly if the second of two sequentially dependent stimuli is omitted on some portion of the trials. Importantly, the successful re-production of the evoked BOLD response using restricted ISI distributions often maximizes the ability to make psychologically valid experimental conclusions.

An integrative MEG-fMRI study of the primary somatosensory cortex using cross-modal correspondence analysis

We develop a novel approach of cross-modal correspondence analysis (CMCA) to address whether brain activities observed in magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) represent a common neuronal subpopulation, and if so, which frequency band obtained by MEG best fits the common brain areas. Fourteen adults were investigated by whole-head MEG using a single equivalent current dipole (ECD) and synthetic aperture magnetometry (SAM) approaches and by fMRI at 1.5 T using linear time-invariant modeling to generate statistical maps. The same somatosensory stimulus sequences consisting of tactile impulses to the right sided: digit 1, digit 4 and lower lip were used in both neuroimaging modalities. To evaluate the reproducibility of MEG and fMRI results, one subject was measured repeatedly. Despite different MEG dipole locations and locations of maximum activation in SAM and fMRI, CMCA revealed a common subpopulation of the primary somatosensory cortex, which displays a clear homuncular organization. MEG activity in the frequency range between 30 and 60 Hz, followed by the ranges of 20-30 and 60-100 Hz, explained best the defined subrepresentation given by both MEG and fMRI. These findings have important implications for improving and understanding of the biophysics underlying both neuroimaging techniques, and for determining the best strategy to combine MEG and fMRI data to study the spatiotemporal nature of brain activity.

Relation between regional functional MRI activation and vascular reactivity to carbon dioxide during normal aging

Recent blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging studies have shown a reduction of cerebral activation during aging, which may be associated with age-related changes of the cerebral vascular system. The authors used a global hypercapnic breath-holding challenge to define nonneuronal contributions to a significantly reduced activation in the primary sensorimotor cortex during finger tapping in a group of old (n = 6; mean age 65 years) compared with a group of young (n = 6; mean age 27 years) subjects. Within significantly activated voxels in both groups during finger tapping, the mean BOLD signal amplitudes were significantly smaller in the group of older subjects for both tasks. In those voxels showing significant activation only in young subjects during finger tapping, the response to hypercapnia was also greatly diminished in older subjects. The attenuated hypercapnic BOLD signal response in older subjects within this region suggests that age-dependent changes of the cerebral vasculature may alter the neuronal-vascular coupling. In older subjects, cerebral vessels may not react as effectively in response to a vasodilating stimulus, which will lead to differences in the number of voxels that pass a criterion threshold despite similar neuronal activation.

Acoustic noise and functional magnetic resonance imaging: current strategies and future prospects

Functional magnetic resonance imaging (fMRI) has become the method of choice for studying the neural correlates of cognitive tasks. Nevertheless, the scanner produces acoustic noise during the image acquisition process, which is a problem in the study of auditory pathway and language generally. The scanner acoustic noise not only produces activation in brain regions involved in auditory processing, but also interferes with the stimulus presentation. Several strategies can be used to address this problem, including modifications of hardware and software. Although reduction of the source of the acoustic noise would be ideal, substantial hardware modifications to the current base of installed MRI systems would be required. Therefore, the most common strategy employed to minimize the problem involves software modifications. In this work we consider three main types of acquisitions: compressed, partially silent, and silent. For each implementation, paradigms using block and event-related designs are assessed. We also provide new data, using a silent event-related (SER) design, which demonstrate higher blood oxygen level-dependent (BOLD) response to a simple auditory cue when compared to a conventional image acquisition.

Functional MRI of self-controlled stereoscopic depth perception

Stereoscopic depth perception was studied in healthy young adults using fMRI imaging at 2.0 T. In a novel paradigm we compared the cortical activation elicited by single-image stereograms which create alternating 2D and 3D percepts (event-related analysis triggered on the self-controlled switches between the two percepts) with the activation caused by a more conventional approach contrasting pairs of stereoscopic images with pairs of identical images (block design). The data show a distributed network of cortical areas embedded within the visual pathways that included about one-quarter of the cortical surface activated by 2D visual stimulation and about one-half of the area activated by 3D percepts based on stereoscopic image pair. 3D perception recruited mostly neuronal populations in higher order visual areas: whereas about 40% of the visually activated locations along the intraparietal sulcus were also activated by 3D perception based on single-image stereograms (resp. 90% stereoscopic images), only 10% such overlap was found in striate cortex. The study revealed no sup-port for a right-hemispheric lateralization of depth perception.

Functional MRI of cortical activations induced by transcranial magnetic stimulation (TMS)

The effects of repetitive transcranial magnetic stimulation (rTMS) on human brain activity and associated hemodynamics were investigated by blood-oxygenation-level-dependent (BOLD) MRI using echo-planar imaging at 2.0 T. Apart from bilateral activation of the auditory cortex by the audible rTMS discharges (23 bursts, 1 s duration, 10 Hz, 10-20 s interstimulus intervals), BOLD responses were restricted to cortical representations of actual finger movements performed either voluntarily or evoked by suprathreshold rTMS of the motor cortex. Neither subthreshold rTMS of the motor cortex nor suprathreshold rTMS of the lateral premotor cortex induced a detectable BOLD response. These findings suggest that neuronal depolarization as induced by rTMS modulates the spiking output of a brain area but does not automatically alter cerebral blood flow and oxygenation. The observation of BOLD MRI activations probably reflects the afferent intracortical processing of real movements.

Task-related signal decrease on functional magnetic resonance imaging

An atypical pattern of signal change was identified on functional magnetic resonance (fMR) imaging in pathologic patients. Three normal volunteers and 34 patients with pathologic lesions near the primary motor cortex underwent fMR imaging with echo-planar imaging while performing a hand motor task. Signal intensities were evaluated with the z-score method, and the time course and changes of the signal intensity were calculated. Nine of the 34 patients with pathologic lesions displayed a significant task-related signal reduction in motor-related areas. They also presented a conventional task-related signal increase in other motor-related areas. The time courses of the increase and decrease were the inverse of each other. There was no significant difference between rates of signal increase and decrease. Our findings suggest that this atypical signal decrease is clinically significant, and that impaired vascular reactivity and altered oxygen metabolism could contribute to the task-related signal reduction. Brain areas showing such task-related signal decrease should be preserved at surgery.

Event-related functional imaging and episodic memory

Electrical and haemodynamic measures of neural activity can be time-locked to an event-of-interest, such as the presentation of a stimulus or a behavioural response. Both of these measures can be employed in studies where the aim is to elucidate the relationship between neural activity and cognitive processes. This review highlights a number of considerations that arise when these techniques are employed in pursuit of this goal, with a particular emphasis on functional imaging studies of retrieval from episodic memory. The review includes: a discussion of some limitations that each technique imposes at the stage of experimental design, consideration of the relative strengths and weaknesses of each technique, a commentary on assumptions that are common to both, and a brief review of the ways in which these techniques can be extended in order to index two distinct classes of cognitive operations that have correspondingly distinct neural signatures.

Assessment of cerebrovascular reactivity with functional magnetic resonance imaging: comparison of CO(2) and breath holding

Cerebral blood flow (CBF) and oxygenation changes following both a simple breath holding test (BHT) and a CO(2) challenge can be detected with functional magnetic resonance imaging techniques. The BHT has the advantage of not requiring a source of CO(2) and acetazolamide and therefore it can easily be performed during a routine MR examination. In this study we compared global hemodynamic changes induced by breath holding and CO(2) inhalation with blood oxygenation level dependent (BOLD) and CBF sensitized fMRI techniques. During each vascular challenge BOLD and CBF signals were determined simultaneously with a combined BOLD and flow-sensitive alternating inversion recovery (FAIR) pulse sequence. There was a good correlation between the global BOLD signal intensity changes during breath holding and CO(2) inhalation supporting the notion that the BHT is equivalent to CO(2) inhalation in evaluating the hemodynamic reserve capacity with BOLD fMRI. In contrast, there was no correlation between relative CBF changes during both vascular challenges, which was probably due to the reduced temporal resolution of the combined BOLD and FAIR pulse sequence.

A weighted least-squares algorithm for estimation and visualization of relative latencies in event-related functional MRI

The properties of the hemodynamic latencies in functional maps have been relatively unexplored. Accurate methods of estimating hemodynamic latencies are needed to take advantage of this feature of fMRI. A fully automated, weighted least-squares (WLS) method for estimating temporal latencies is reported. Using a weighted linear model, the optimal latency and amplitude of the fMRI response can be determined for those voxels that pass a detection threshold. There is evidence from previous studies that the hemodynamic response may be time-locked to the stimulus within certain limits, less variable earlier in its evolution, and able to resolve information about relative hemodynamic timing. This information can be used to test hypotheses about the sequence and spatial distribution of neural activity. The method can be used to weight the earliest evolution of the hemodynamic response more heavily and decrease bias resulting from the hemodynamic response function. Additionally, the WLS method can control for varying response shapes across the brain and improve latency comparisons between brain regions. The WLS method was developed to study the properties of hemodynamic latencies, which may be increasingly important as event-related fMRI continues to be advanced.

Correlations in low-frequency BOLD fluctuations reflect cortico-cortical connections

Cross-correlation of low-frequency temporal fluctuations (<0.08 Hz) was used to correlate widely separated anatomic regions during continuous performance of a spatial working memory task. The regions of highest correlation to right-hemisphere dorsolateral prefrontal cortex correspond to the regions of largest baseline signal change in a conventional block-style functional MRI paradigm. Additionally, it is shown that the correlations between elements of the functional network increase during performance of a task that activates the network when compared to a task that does not directly stimulate the functionally connected network.