Visual fMRI Research Articles

Effects of Refractive Errors on Visual Functional Magnetic Resonance Imaging

Objective: The purpose of our study is to evaluate the effects of refractive errors on functional magnetic resonance imaging (fMRI) of visual cortex. Methods and Materials: We performed a prospective study. The study included 13 patients with refractive error (group 1) and 30 emetropic volunteers (group 2). Group 2 was also subgrouped as 20-32 years old (young) and over 45 years old (old) to analyse accommodation effect. fMRI data were acquired with a block design paradigm with 3 Tesla MR system. In both groups, images initially were acquired in normal refractive state. fMRI was performed again in both groups during refractive error. Activation areas on visual cortex were calculated as square centimeter. Total activated areas on visual cortex was compared between normal refractive state and induced/ uncorrected refractive error. Results: In group 1, activation areas of visual cortex during uncorrected refractive error revealed significantly decrease compared with activation areas during corrected refractive error (p=0.001). In group 2, induced myopia resulted significant decrease in activation areas compared with normal refractive state. Decrease in activation areas were significant both in 2 and 4 diopters (D) of myopia compared with normal refractive state (p=0.003, p<0.001 respectively). Both in young and old subgroup, activation areas were significantly decreased during induced myopia. We revealed no difference between young and old subgroups. Conclusion: The refractive errors have a clear effect on fMRI of visual cortex. Thus, to achieve accurate results they should be corrected in patients that included in the visual fMRI study.

Developmental synergy between thalamic structure and interhemispheric connectivity in the visual system of preterm infants.

Thalamic structural co-variation with cortical regions has been demonstrated in preterm infants, but its relationship to cortical function and severity of non-cystic white matter injury (non-cystic WMI) is unclear. The relationship between thalamic morphology and both cortical network synchronization and cortical structural connectivity has not been established. We tested the hypothesis that in preterm neonates, thalamic volume would correlate with primary cortical visual function and microstructural integrity of cortico-cortical visual association pathways. A total of 80 term-equivalent preterm and 44 term-born infants underwent high-resolution structural imaging coupled with visual functional magnetic resonance imaging or diffusion tensor imaging. There was a strong correlation between thalamic volume and primary visual cortical activation in preterms with non-cystic WMI (r = 0.81, p-value = 0.001). Thalamic volume also correlated strongly with interhemispheric cortico-cortical connectivity (splenium) in preterm neonates with a relatively higher severity of non-cystic WMI (p-value < 0.001). In contrast, there was lower correlation between thalamic volume and intrahemispheric cortico-cortical connectivity, including the inferior longitudinal fasciculus and inferior frontal orbital fasciculus. This study shows distinct temporal overlap in the disruption of thalamo-cortical and interhemispheric cortico-cortical connectivity in preterm infants suggesting developmental synergy between thalamic morphology and the emergence of cortical networks in the last trimester.

Visual fMRI under General Anaesthesia in Children with Therapy Refractory Epilepsies

Introduction: Assessment of the visual field is an integral part of the clinical neurological examination of children with therapy refractory epilepsies. Especially in the field of epilepsy surgery, the evaluation of any preoperatively existing visual field defects is important with regards to surgical planning and the risk estimation of additional postoperative deficits. However, performing a formal perimetry or a clinical visual field examination using confrontation techniques is often not possible in children with therapy refractory epilepsies due to young age of the patients, cognitive impairment, or lack of cooperation. We report first experience with visual functional magnetic resonance imaging (fMRI) examinations under general anesthesia.

R(∗)2 mapping for robust brain function detection in the presence of magnetic field inhomogeneity.

T(*)2 mapping or R(*)2 mapping for brain function offers advantages such as providing quantitative measurements independent of the MRI acquisition parameters (e.g. echo time TE). However, magnetic field susceptibility in the human brain can prevent an accurate estimation of R(*)2, which in turn impacts the ability to study brain function. The present work investigates the effects of in-plane magnetic susceptibility-induced magnetic field gradients on R(*)2 decay. An iterative method is developed for R(*)2 estimation with an increased robustness to field inhomogeneity. The new method is further tested in a visual fMRI experiment with and without magnetic field gradients and its performance is compared to a standard BOLD fMRI and a BOLD fMRI based on echo summation. Reduced sensitivity in fMRI to in-plane magnetic gradients is obtained with the present methodology.

Matched-filter acquisition for BOLD fMRI

We introduce matched-filter fMRI, which improves BOLD (blood oxygen level dependent) sensitivity by variable-density image acquisition tailored to subsequent image smoothing. Image smoothing is an established post-processing technique used in the vast majority of fMRI studies. Here we show that the signal-to-noise ratio of the resulting smoothed data can be substantially increased by acquisition weighting with a weighting function that matches the k-space filter imposed by the smoothing operation. We derive the theoretical SNR advantage of this strategy and propose a practical implementation of 2D echo-planar acquisition matched to common Gaussian smoothing. To reliably perform the involved variable-speed trajectories, concurrent magnetic field monitoring with NMR probes is used. Using this technique, phantom and in vivo measurements confirm reliable SNR improvement in the order of 30% in a “resting-state” condition and prove robust in different regimes of physiological noise. Furthermore, a preliminary task-based visual fMRI experiment equally suggests a consistent BOLD sensitivity increase in terms of statistical sensitivity (average t-value increase of about 35%). In summary, our study suggests that matched-filter acquisition is an effective means of improving BOLD SNR in studies that rely on image smoothing at the post-processing level.

Improving the spatial resolution of magnetic resonance inverse imaging via the blipped-CAIPI acquisition scheme

Using simultaneous acquisition from multiple channels of a radio-frequency (RF) coil array, magnetic resonance inverse imaging (InI) achieves functional MRI acquisitions at a rate of 100ms per whole-brain volume. InI accelerates the scan by leaving out partition encoding steps and reconstructs images by solving under-determined inverse problems using RF coil sensitivity information. Hence, the correlated spatial information available in the coil array causes spatial blurring in the InI reconstruction. Here, we propose a method that employs gradient blips in the partition encoding direction during the acquisition to provide extra spatial encoding in order to better differentiate signals from different partitions. According to our simulations, this blipped-InI (bInI) method can increase the average spatial resolution by 15.1% (1.3mm) across the whole brain and from 32.6% (4.2mm) in subcortical regions, as compared to the InI method. In a visual fMRI experiment, we demonstrate that, compared to InI, the spatial distribution of bInI BOLD response is more consistent with that of a conventional echo-planar imaging (EPI) at the level of individual subjects. With the improved spatial resolution, especially in subcortical regions, bInI can be a useful fMRI tool for obtaining high spatiotemporal information for clinical and cognitive neuroscience studies.

An Improved RV Based Fuzzy Cluster Analysis (IRVFCA) Method for Task-Related Functional MRI Data Analysis

Functional magnetic resonance imaging (fMRI) has become one of the important tools of functional connectivity studies of the human brain. Fuzzy clustering method (FCM) is a common method for analysis of FMRI data. Traditional FCA methods measure the similarity between the BOLD time course of a centroid and the ones of all other voxels in the brain on the basis of Pearson correlation coefficient. This article puts forward a multi-voxel-based similarity measure, an improved RV (IRV) measure, which takes the hypothesis into account that the function homogeneous voxels of brain volume are spatially clustered within a local region. Experimental validation is presented through four visual fMRI data sets which shows that the IRVFCA method not only has improved the speed of FCA, but has comparatively raised the accuracy of the method.

Parallel imaging acceleration of EPIK for reduced image distortions in fMRI

EPI with Keyhole (EPIK) is a hybrid imaging technique used to improve the performance of EPI in dynamic MRI applications. The method had been previously validated at 1.5 T with both phantom and in vivo images; EPIK was able to provide a higher temporal resolution and less image distortions than single-shot EPI. The data presented here demonstrate that the performance of EPIK can be further improved by accelerating it with the parallel imaging. For this work, this combination was tested at 3 T. After initial evaluation using phantom images, use of the method in functional MRI was verified with visual fMRI measurements as well as MRI simulation results. The results showed that accelerated EPIK had increased temporal resolution with favorable robustness against susceptibility artifacts when compared with EPI or non-accelerated EPIK.

Detailed functional and structural characterization of a macular lesion in a rhesus macaque

Animal models are powerful tools to broaden our understanding of disease mechanisms and to develop future treatment strategies. Here we present detailed structural and functional findings of a rhesus macaque suffering from a naturally occurring bilateral macular dystrophy (BMD), partial optic atrophy and corresponding reduction of central V1 signals in visual fMRI experiments when compared to data in a healthy macaque (CTRL) of similar age. Retinal imaging included infrared and autofluorescence recordings, fluorescein and indocyanine green angiography and spectral domain optical coherence tomography (OCT) on the Spectralis HRA+OCT platform. Electroretinography included multifocal and Ganzfeld-ERG recordings. Animals were killed and eyes analyzed by immunohistochemistry. Angiography showed reduced macular vascularization with significantly larger foveal avascular zones (FAZ) in the affected animal (FAZBMD=8.85mm(2) vs. FAZCTRL=0.32mm(2)). OCT showed bilateral thinning of the macula within the FAZ (total retinal thickness, TRTBMD=174±9µm) and partial optic nerve atrophy when compared to control (TRTCTRL=303±45µm). Segmentation analysis revealed that inner retinal layers were primarily affected (inner retinal thickness, IRTBMD=33±9µm vs. IRTCTRL=143±45µm), while the outer retina essentially maintained its thickness (ORTBMD=141±7µm vs. ORTCTRL=160±11µm). Altered macular morphology corresponded to a preferential reduction of central signals in the multifocal electroretinography and to a specific attenuation of cone-derived responses in the Ganzfeld electroretinography, while rod function remained normal. We provided detailed characterization of a primate macular disorder. This study aims to stimulate awareness and further investigation in primates with macular disorders eventually leading to the identification of a primate animal model and facilitating the preclinical development of therapeutic strategies.

Retinotopic mapping in a patient with optic chiasm compression: converging evidence from visual field testing and fMRI

Retinotopic mapping in a patient with optic chiasm compression: converging evidence from visual field testing and fMRI

High‐resolution functional MRI at 3 T: 3D/2D echo‐planar imaging with optimized physiological noise correction

High-resolution functional MRI (fMRI) offers unique possibilities for studying human functional neuroanatomy. Although high-resolution fMRI has proven its potential at 7 T, most fMRI studies are still performed at rather low spatial resolution at 3 T. We optimized and compared single-shot two-dimensional echo-planar imaging (EPI) and multishot three-dimensional EPI high-resolution fMRI protocols. We extended image-based physiological noise correction from two-dimensional EPI to multishot three-dimensional EPI. The functional sensitivity of both acquisition schemes was assessed in a visual fMRI experiment. The physiological noise correction increased the sensitivity significantly, can be easily applied, and requires simple recordings of pulse and respiration only. The combination of three-dimensional EPI with physiological noise correction provides exceptional sensitivity for 1.5 mm high-resolution fMRI at 3 T, increasing the temporal signal-to-noise ratio by more than 25% compared to two-dimensional EPI. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.

Visual functional magnetic resonance imaging of preterm infants

The aim of this study was to determine the feasibility of undertaking visual functional magnetic resonance imaging (fMRI) in very preterm children. Forty-seven infants born at less than 32 weeks gestational age (25 males, 22 females; mean (SD) age at birth 28.8 wks [1.9]) were scanned using 1.5 T MRI as part of a longitudinal neuroimaging study. These infants were scanned at preterm age (within 2 wks of birth) and at term-equivalent age. Quantitative T2* data and fMRI in response to visual stimuli (flashing strobe) were acquired in this population. T2* values were compared at preterm age and at term-equivalent age using a two-tailed t-test. A general linear model was used to evaluate occipital lobe response to visual stimuli. T2* values were significantly higher at preterm age than at term-equivalent age in both the medial and lateral occipital lobes (preterm infants: 187.2 ms and 198.4 ms respectively; term infants: 110.9 ms and 133.2 ms respectively; p<0.002). Significant positive occipital lobe activation (q<0.01) was found in 3 out of 65 (5%) fMRIs carried out at preterm age and in 19 out of 26 (73%) scans carried out at term-equivalent age. Visual stimuli do not elicit a reliable blood oxygen level-dependent (BOLD) response in very preterm infants during the preterm period. This suggests that BOLD fMRI may not be the appropriate modality for investigating occipital lobe function in very preterm infants.

Reduction in high‐calorie food reward after a lifestyle intervention in obese endometrial cancer survivors

Obesity is associated with an increased risk of endometrial cancer (EC) and, EC patients have the highest risk of death among all obesity‐associated cancers. Yet, only two lifestyle interventions targeting EC survivors have been completed. EC patients lost weight in these trials but food disinhibition, as determined by the Three‐Factor Eating Questionnaire, decreased, suggesting an increase in emotional eating and, possibly, food reward. We evaluated food behavior using a visual functional magnetic resonance imaging (fMRI) task involving paired stimulus presentation of high‐calorie, low‐calorie and non‐food images in 11 obese, Stage I/II EC patients enrolled in Survivors in Uterine Cancer Empowered by Exercise and a Healthy Diet (SUCCEED), a lifestyle intervention aimed at improving nutritional and exercise behaviors in 16 group sessions over 6 months using social cognitive theory. At 6 months post‐intervention compared to baseline, we observed a decrease in activation in brain regions involved in food reward and motivation (posterior cingulate, globus pallidus, thalamus; cluster corrected p&lt;0.005) in response to high‐calorie vs. non‐food contrasts in the fed state in the Tx group (n=8). These changes were not found in the UC group (n=3). To our knowledge, this is the first study to demonstrate that a behavioral lifestyle intervention helps to reduce high‐calorie food reward in obese EC survivors.

Brain network involved in visual processing of movement stimuli used in upper limb robotic training: an fMRI study

BackgroundThe potential of robot-mediated therapy and virtual reality in neurorehabilitation is becoming of increasing importance. However, there is limited information, using neuroimaging, on the neural networks involved in training with these technologies. This study was intended to detect the brain network involved in the visual processing of movement during robotic training. The main aim was to investigate the existence of a common cerebral network able to assimilate biological (human upper limb) and non-biological (abstract object) movements, hence testing the suitability of the visual non-biological feedback provided by the InMotion2 Robot.MethodsA visual functional Magnetic Resonance Imaging (fMRI) task was administered to 22 healthy subjects. The task required observation and retrieval of motor gestures and of the visual feedback used in robotic training. Functional activations of both biological and non-biological movements were examined to identify areas activated in both conditions, along with differential activity in upper limb vs. abstract object trials. Control of response was also tested by administering trials with congruent and incongruent reaching movements.ResultsThe observation of upper limb and abstract object movements elicited similar patterns of activations according to a caudo-rostral pathway for the visual processing of movements (including specific areas of the occipital, temporal, parietal, and frontal lobes). Similarly, overlapping activations were found for the subsequent retrieval of the observed movement. Furthermore, activations of frontal cortical areas were associated with congruent trials more than with the incongruent ones.ConclusionsThis study identified the neural pathway associated with visual processing of movement stimuli used in upper limb robot-mediated training and investigated the brain’s ability to assimilate abstract object movements with human motor gestures. In both conditions, activations were elicited in cerebral areas involved in visual perception, sensory integration, recognition of movement, re-mapping on the somatosensory and motor cortex, storage in memory, and response control. Results from the congruent vs. incongruent trials revealed greater activity for the former condition than the latter in a network including cingulate cortex, right inferior and middle frontal gyrus that are involved in the go-signal and in decision control. Results on healthy subjects would suggest the appropriateness of an abstract visual feedback provided during motor training. The task contributes to highlight the potential of fMRI in improving the understanding of visual motor processes and may also be useful in detecting brain reorganisation during training.

Investigating the neural correlates of personally familiar face recognition using dynamic visual stimulation-an fMRI study

Investigating the neural correlates of personally familiar face recognition using dynamic visual stimulation-an fMRI study

Search for patterns of functional specificity in the brain: A nonparametric hierarchical Bayesian model for group fMRI data

Functional MRI studies have uncovered a number of brain areas that demonstrate highly specific functional patterns. In the case of visual object recognition, small, focal regions have been characterized with selectivity for visual categories such as human faces. In this paper, we develop an algorithm that automatically learns patterns of functional specificity from fMRI data in a group of subjects. The method does not require spatial alignment of functional images from different subjects. The algorithm is based on a generative model that comprises two main layers. At the lower level, we express the functional brain response to each stimulus as a binary activation variable. At the next level, we define a prior over sets of activation variables in all subjects. We use a Hierarchical Dirichlet Process as the prior in order to learn the patterns of functional specificity shared across the group, which we call functional systems, and estimate the number of these systems. Inference based on our model enables automatic discovery and characterization of dominant and consistent functional systems. We apply the method to data from a visual fMRI study comprised of 69 distinct stimulus images. The discovered system activation profiles correspond to selectivity for a number of image categories such as faces, bodies, and scenes. Among systems found by our method, we identify new areas that are deactivated by face stimuli. In empirical comparisons with previously proposed exploratory methods, our results appear superior in capturing the structure in the space of visual categories of stimuli.

Identification and characterisation of midbrain nuclei using optimised functional magnetic resonance imaging

Localising activity in the human midbrain with conventional functional MRI (fMRI) is challenging because the midbrain nuclei are small and located in an area that is prone to physiological artefacts. Here we present a replicable and automated method to improve the detection and localisation of midbrain fMRI signals. We designed a visual fMRI task that was predicted would activate the superior colliculi (SC) bilaterally. A limited number of coronal slices were scanned, orientated along the long axis of the brainstem, whilst simultaneously recording cardiac and respiratory traces. A novel anatomical registration pathway was used to optimise the localisation of the small midbrain nuclei in stereotactic space. Two additional structural scans were used to improve registration between functional and structural T1-weighted images: an echo-planar image (EPI) that matched the functional data but had whole-brain coverage, and a whole-brain T2-weighted image. This pathway was compared to conventional registration pathways, and was shown to significantly improve midbrain registration. To reduce the physiological artefacts in the functional data, we estimated and removed structured noise using a modified version of a previously described physiological noise model (PNM). Whereas a conventional analysis revealed only unilateral SC activity, the PNM analysis revealed the predicted bilateral activity. We demonstrate that these methods improve the measurement of a biologically plausible fMRI signal. Moreover they could be used to investigate the function of other midbrain nuclei.

Effect of Lens-Induced Myopia on Visual Cortex Activity: A Functional MR Imaging Study

Myopia is a type of refractive error that blurs retinal image and in turn can change neural signals transferred from retina to visual cortex. The purpose of this study was to evaluate the effect of induced myopia on occipital visual cortex activity by fMRI results. BOLD fMRI was performed in 13 emmetropic volunteers (refractive error, <±0.50D) with normal visual acuity, good binocular vision, and no history of neurologic illness. Visual stimulus was counterphasing vertical luminance sinusoidal grating with spatiotemporal frequency of 1.84 cycles per degree/8 Hz and contrast of 60%. The functional images were acquired in block design, during normal refractive state and induced myopia produced by convex noncoating plastic lenses of +1D, +3D, +5D, by using an EPI gradient-echo sequence in a 1.5T MR imaging scanner. fMRI data were processed by using FSL software. fMRI responses to visual stimuli demonstrated that percentage of BOLD signal intensity change and number of activated voxels within occipital visual cortex were reduced remarkably in induced myopic states of 1D, 3D, and 5D in comparison with normal refractive state; the results did not show a significant and regular decreasing trend in number of activated voxels and BOLD signal intensity change in these 3 different values of induced myopia. The findings suggest that induced myopia has a considerable effect on visual cortex activity, because myopia induced by lens of +1D is sufficient to change fMRI results significantly. Accordingly, it is essential to correct myopia before visual fMRI studies, even if it is at low levels.

Abnormal visual activation in Parkinson's disease patients

Among nonmotor symptoms observed in Parkinson's disease (PD) dysfunction in the visual system, including hallucinations, has a significant impact in their quality of life. To further explore the visual system in PD patients we designed two fMRI experiments comparing 18 healthy volunteers with 16 PD patients without visual complaints in two visual fMRI paradigms: the flickering checkerboard task and a facial perception paradigm. PD patients displayed a decreased activity in the primary visual cortex (Broadmann area 17) bilaterally as compared to healthy volunteers during flickering checkerboard task and increased activity in fusiform gyrus (Broadmann area 37) during facial perception paradigm. Our findings confirm the notion that PD patients show significant changes in the visual cortex system even before the visual symptoms are clinically evident. Further studies are necessary to evaluate the contribution of these abnormalities to the development visual symptoms in PD.

Detecting stable distributed patterns of brain activation using Gini contrast

The relationship between spatially distributed fMRI patterns and experimental stimuli or tasks offers insights into cognitive processes beyond those traceable from individual local activations. The multivariate properties of the fMRI signals allow us to infer interactions among individual regions and to detect distributed activations of multiple areas. Detection of task-specific multivariate activity in fMRI data is an important open problem that has drawn much interest recently. In this paper, we study and demonstrate the benefits of random forest classifiers and the associated Gini importance measure for selecting voxel subsets that form a multivariate neural response. The Gini importance measure quantifies the predictive power of a particular feature when considered as part of the entire pattern. The measure is based on a random sampling of fMRI time points and voxels. As a consequence the resulting voxel score, or Gini contrast, is highly reproducible and reliably includes all informative features. The method does not rely on a priori assumptions about the signal distribution, a specific statistical or functional model or regularization. Instead, it uses the predictive power of features to characterize their relevance for encoding task information. The Gini contrast offers an additional advantage of directly quantifying the task-relevant information in a multiclass setting, rather than reducing the problem to several binary classification subproblems. In a multicategory visual fMRI study, the proposed method identified informative regions not detected by the univariate criteria, such as the t-test or the F-test. Including these additional regions in the feature set improves the accuracy of multicategory classification. Moreover, we demonstrate higher classification accuracy and stability of the detected spatial patterns across runs than the traditional methods such as the recursive feature elimination used in conjunction with support vector machines.