A novel functional magnetic resonance imaging compatible search-coil eye-tracking system

Measuring eye states in functional MRI

Background

In many functional magnetic resonance imaging (fMRI) studies, experimental design often depends on the eye state (i.e., whether the participants had their eyes open or closed). Closed eyes during an fMRI is the general convention, particularly when patients are in a resting-state, but the eye state is difficult to verify. Although knowledge of the impact of the eye state on brain activity is steadily growing, only a few research groups have implemented standardized procedures to monitor eye movements and eye state. These procedures involve advanced methods that are costly (e.g., fMRI-compatible cameras) and often time-consuming (e.g., EEG/EOG).

Results

We present a simple method that distinguishes open from closed eyes utilizing functional MR images alone. The utility of this method was demonstrated on fMRI data from 14 healthy subjects who had to open and close their eyes according to a predetermined protocol (3.0 T MRI scanner, EPI sequence with 3 × 3 × 3 mm voxels, TR 2.52 s).

Conclusion

The method presented herein is capable of extracting the movement direction of the eyes. All described methods are applicable for pre- and post-normalized MR images and are freely available through a MATLAB toolbox.

Towards improved animal models for evaluating social cognition and its disruption in schizophrenia: the CNTRICS initiative

Social cognition refers to processes used to monitor and interpret social signals from others, to decipher their state of mind, emotional status and intentions, and select appropriate social behaviour. Social cognition is sophisticated in humans, being embedded with verbal language and enacted in a complex cultural environment. Its disruption characterises the entire course of schizophrenia and is correlated with poor functional outcome. Further, deficits in social cognition are related to impairment in other cognitive domains, positive symptoms (paranoia and delusions) and negative symptoms (social withdrawal and reduced motivation). In light of the significance and inadequate management of social cognition deficits, there is a need for translatable experimental procedures for their study, and identification of effective pharmacotherapy. No single paradigm captures the multi-dimensional nature of social cognition, and procedures for assessing ability to infer mental states are not well-developed for experimental therapeutic settings. Accordingly, a recent CNTRICS meeting prioritised procedures for measuring a specific construct: "acquisition and recognition of affective (emotional) states", coupled to individual recognition. Two complementary paradigms for refinement were identified: social recognition/preference in rodents, and visual tracking of social scenes in non-human primates (NHPs). Social recognition is disrupted in genetic, developmental or pharmacological disease models for schizophrenia, and performance in both procedures is improved by the neuropeptide oxytocin. The present article surveys a broad range of procedures for studying social cognition in rodents and NHPs, discusses advantages and drawbacks, and focuses on development of social recognition/preference and gaze-following paradigms for improved study of social cognition deficits in schizophrenia and their potential treatment.

Intracortical recordings and fMRI: an attempt to study operational modules and networks simultaneously

The brain can be envisaged as a complex adaptive system. It is characterized by a very high structural complexity and by massive connectivity, both of which change and evolve in response to experience. Information related to sensors and effectors is processed in both a parallel and a hierarchical fashion; the connectivity between different hierarchical levels is bidirectional, and its effectiveness is continuously controlled by specific associational and neuromodulatory centers. When questions are addressed at the level of a distributed, large-scale whole system such as that underlying perception and cognition, it is not clear what should be considered as an elementary operational unit because the behavior of integral, aggregate systems is always emergent and most often remains unpredicted by the behaviors of single cells. To localize and comprehend the neural mechanisms underlying our perceptual or cognitive capacities, concurrent studies of microcircuits, of local and long-range interconnectivity between small assemblies, and of the synergistic activity of larger neuronal populations are called for. In other words, multimodal methodologies that include invasive neuroscientific methods as well as global neuroimaging techniques are required, such as the various functional aspects of magnetic resonance imaging. These facts were the driving force behind the decision to begin animal-MRI in my lab. The wonderful idea of the editors of NeuroImage to publish a Special Issue commemorating 20years of functional fMRI provides me with the opportunity of sharing not only our first moments of frustration with the readers, but also our successful results.

Magnetic eye tracking: a new approach employing a planar transmitter

A new scleral search coil (SSC) tracking approach employing a planar transmitter has been developed theoretically and tested experimentally. A thin and flat transmitter is much more convenient in installation, operation, and maintenance than the conventional large cubic one. A planar transmitter also increases the mobility of SSC systems, simplifies their accommodation in a limited clinical space, enables bedside testing, and causes no visual distractions and no discomfort to the users. Moreover, it allows tracking not only the SSC orientation, but also its location, which is very important for many medical and scientific applications. The suggested approach provides the speed and precision that are required in SSC applications. The experimental results show that it can be used for the diagnosis of vestibular disorders. The tracking precision is in good agreement with its theoretical estimation.

Pictures of a thousand words: investigating the neural mechanisms of reading with extremely rapid event-related fMRI

Reading is one of the most important skills human beings can acquire, but has proven difficult to study naturalistically using functional magnetic resonance imaging (fMRI). We introduce a novel Event-Related Reading (ERR) fMRI approach that enables reliable estimation of the neural correlates of single-word processing during reading of rapidly presented narrative text (200-300 ms/word). Application to an fMRI experiment in which subjects read coherent narratives and made no overt responses revealed widespread effects of orthographic, phonological, contextual, and semantic variables on brain activation. Word-level variables predicted activity in classical language areas as well as the inferotemporal visual word form area, specifically supporting a role for the latter in mapping visual forms onto articulatory or acoustic representations. Additional analyses demonstrated that ERR results replicate across experiments and predict reading comprehension. The ERR approach represents a powerful and extremely flexible new approach for studying reading and language behavior with fMRI.

Neurophysiology of the BOLD fMRI signal in awake monkeys

BACKGROUND

Simultaneous intracortical recordings of neural activity and blood-oxygen-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) in primary visual cortex of anesthetized monkeys demonstrated varying degrees of correlation between fMRI signals and the different types of neural activity, such as local field potentials (LFPs), multiple-unit activity (MUA), and single-unit activity (SUA). One important question raised by the aforementioned investigation is whether the reported correlations also apply to alert subjects.

RESULTS

Monkeys were trained to perform a fixation task while stimuli within the receptive field of each recording site were used to elicit neural responses followed by a BOLD response. We show -- also in alert behaving monkeys -- that although both LFP and MUA make significant contributions to the BOLD response, LFPs are better and more reliable predictors of the BOLD signal. Moreover, when MUA responses adapt but LFP remains unaffected, the BOLD signal remains unaltered.

CONCLUSIONS

The persistent coupling of the BOLD signal to the field potential when LFP and MUA have different time evolutions suggests that BOLD is primarily determined by the local processing of inputs in a given cortical area. In the alert animal the largest portion of the BOLD signal's variance is explained by an LFP range (20-60 Hz) that is most likely related to neuromodulation. Finally, the similarity of the results in alert and anesthetized subjects indicates that at least in V1 anesthesia is not a confounding factor. This enables the comparison of human fMRI results with a plethora of electrophysiological results obtained in alert or anesthetized animals.

fMRI of the temporal lobe of the awake monkey at 7 T

Increasingly 7 T scanners are used for fMRI of humans and non-human primates, promising improvements in signal-to-noise, spatial resolution and specificity. A disadvantage of fMRI at 7 T, but already at 3 T, is that susceptibility artifacts from air-filled cavities like the ear canal and nasal cavity cause signal loss and distortion. This limits the applicability of fMRI in these areas, thereby limiting study of these areas, but it also limits study of processes that span large-scale cortical networks or the entire brain. Our goal is to study the inferior temporal (IT) lobe in awake monkeys because of its importance in object perception and recognition, but the functional signal is degraded by strong susceptibility gradients. To allow fMRI of this region, we used an optimized SE-EPI, which recovers signal lost with GE-EPI and we corrected for susceptibility-induced image distortion. SE-EPI has the added advantage that, in contrast to GE-EPI, where the functional signal derives to a large extent from veins, the SE-EPI signal arises from the microvasculature, and hence it better represents the neural activation. We show fMRI at 7 T of the entire visual pathway in the awake primate with robust and widespread activation in all ventral areas of the brain, including areas adjacent to the ear canal. This allows fMRI of areas that normally suffer from artifact and thus more reliable whole-brain studies.

Functional MR imaging in the awake monkey: effects of motion on dynamic off-resonance and processing strategies

Functional MR imaging of the alert, behaving monkey is being used more and more often to detect activation patterns and guide electrophysiological research investigating the neural basis of behavior. Several labs have reported fMRI data from the awake monkey, but none of them has studied and systematically corrected the effects of monkeys' motion on fMRI time series. In this study, a significant refinement of acquisition and correction strategies is reported that can be used to minimize magnetic susceptibility artifacts induced by respiration and by jaw and body movement. Real-time acquisition of sensor signals (e.g., signals induced by jaw and body movement) and MR navigator data were combined to optimize fMRI signal-correction strategies. Within trials, the artifact-induced off-resonance changes were small and mainly reflected the effects of respiration; between trials, movements caused major changes of global frequency and shim (>20 Hz/cm). Several methods were used to assess the stability of the fMRI series: k-space analysis ('dynamic intensity and off-resonance changes in k-space', dubbed DICK and DORK) and image analysis using a Laplace operator and a center-of-mass metric. The variability between trials made it essential to correct for inter-trial variations. On the other hand, images were sufficiently stable with our approach to perform fMRI evaluations on single trials before averaging of trials. Different motion correction strategies were compared: DORK, McFLIRT (rigid body model with three translations and three rotations) and 2D image alignment based on a center-of-mass detection (in-plane translation). The latter yielded the best results and proved to be fast and robust for intra- and inter-trial alignment. Finally, fMRI in the behaving monkey was tested for spatial and temporal reproducibility on a trial-to-trial basis. Highly activated voxels also displayed good reproducibility between trials. On average, the BOLD amplitude response to a short 3-s visual stimulus was close to 2%.

Robust controlled functional MRI in alert monkeys at high magnetic field: effects of jaw and body movements

The use of functional magnetic resonance imaging (fMRI) in alert non-human primates is of great potential for research in systems neuroscience. It can be combined with invasive techniques and afford better understanding of non-invasively acquired brain imaging signals in humans. However, the difficulties in optimal application of alert monkey fMRI are multi-faceted, especially at high magnetic fields where the effects of motion and of changes in B0 are greatly amplified. To overcome these difficulties, strict behavioral controls and elaborate animal-training are needed. Here, we introduce a number of hardware developments, quantify the effect of movements on fMRI data, and present procedures for animal training and scanning for well-controlled and artifact-reduced alert monkey fMRI at high magnetic field. In particular, we describe systems for monitoring jaw and body movements, and for accurately tracking eye movements. A link between body and jaw movement and MRI image artifacts is established, showing that relying on the immobilization of an animal's head is not sufficient for high-quality imaging. Quantitative analysis showed that body and jaw movement events caused large instabilities in fMRI time series. On average, body movement events caused larger instabilities than jaw movement events. Residual baseline brain image position and signal amplitude shifts were observed after the jaw and body movement events ended. Based on these findings, we introduce a novel behavioral paradigm that relies on training the monkeys to stay still during long trials. A corresponding analysis method discards all data that were not obtained during the movement-free periods. The baseline position and amplitude shifts are overcome by motion correction and trial-by-trial signal normalization. The advantages of the presented method over conventional scanning and analysis are demonstrated with data obtained at 7 T. It is anticipated that the techniques presented here will prove useful for alert monkey fMRI at any magnetic field.