Quantification of neural substrates of vergence system via fMRI

Vergence eye movement is one of the oculomotor systems which allow depth perception via disconjugate movement of the eyes. Neuroimaging methods such as functional magnetic resonance imaging (fMRI) measure neural activity changes activity in the brain while subjects perform experimental tasks. A rich body of primate investigations on vergence is already established in the neurophysiology literature; on the other hand, there are a limited number of fMRI studies on neural mechanisms behind the vergence system. The results demonstrated that vergence system shares neural sources and also shows differentiation within the boundaries of frontal eye fields (FEF) and midbrain of the brainstem in comparison to saccadic, rapid conjugate eye movements, system with application of simple tracking experiment. Functional activity within the FEF was located anterior to the saccadic functional activity (z \u3e 2.3; p \u3c 0.03). Functional activity within the midbrain was observed as a result of application of vergence task, but not for the saccade data set. The novel memory-guided vergence experiment also showed a relationship between posterior parahippocampal area and memory where two other experiments were implemented for comparison of memory load in this region. Significant percent change in the functional activity was observed for the posterior parahippocampal area. Furthermore, an increase in the interconnectivity was observed for vergence tasks via utilization of Granger-Causality Analysis. When prediction was involved the increase in the number of causal interactions was statistically significant (p\u3c 0.05). The comparison of the number of influences between prediction-evoked vergence task and simple tracking vergence task was also statistically significant for these two experimental paradigms, p \u3c 0.0001. Another result observed in this dissertation was the application of hierarchical independent component analysis from to the fronto-parietal and cerebellar components within saccade and vergence tasks. Interestingly, cerebellar component showed delayed latency in the group level signal in comparison to fronto-parietal group level signals, which was evaluated to determine why segregation existed between the components acquired from the implementation of independent component analysis. Lastly, region of interet (ROI) based analysis in comparison to global (whole) brain analysis indicated more sensitive results on frontal, parietal, brainstem and occipital areas at both individual and group levels. Overall, the purpose of this dissertation was to investigate neural control of vergence movements by 1-spatial mapping of vergence induced functional activity, 2- applying different signal processing methods to quantify neural correlates of the vergence system at causal functional connectivity, underlying sources and region of interests (ROI) based levels. It was concluded that quantification of vergence movements via fMRI can build a synergy with behavioral investigations and may also shed light on neural differentiation between healthy individuals and patients with neural dysfunctions and injuries by serving as a biomarker

Differentiation between Vergence and Saccadic Functional Activity within the Human Frontal Eye Fields and Midbrain Revealed through fMRI

Eye movement research has traditionally studied solely saccade and/or vergence eye movements by isolating these systems within a laboratory setting. While the neural correlates of saccadic eye movements are established, few studies have quantified the functional activity of vergence eye movements using fMRI. This study mapped the neural substrates of vergence eye movements and compared them to saccades to elucidate the spatial commonality and differentiation between these systems.The stimulus was presented in a block design where the 'off' stimulus was a sustained fixation and the 'on' stimulus was random vergence or saccadic eye movements. Data were collected with a 3T scanner. A general linear model (GLM) was used in conjunction with cluster size to determine significantly active regions. A paired t-test of the GLM beta weight coefficients was computed between the saccade and vergence functional activities to test the hypothesis that vergence and saccadic stimulation would have spatial differentiation in addition to shared neural substrates.Segregated functional activation was observed within the frontal eye fields where a portion of the functional activity from the vergence task was located anterior to the saccadic functional activity (z>2.3; p<0.03). An area within the midbrain was significantly correlated with the experimental design for the vergence but not the saccade data set. Similar functional activation was observed within the following regions of interest: the supplementary eye field, dorsolateral prefrontal cortex, ventral lateral prefrontal cortex, lateral intraparietal area, cuneus, precuneus, anterior and posterior cingulates, and cerebellar vermis. The functional activity from these regions was not different between the vergence and saccade data sets assessed by analyzing the beta weights of the paired t-test (p>0.2).Functional MRI can elucidate the differences between the vergence and saccade neural substrates within the frontal eye fields and midbrain

Brain Perfusion MRI Findings in Patients with Behcet's Disease

Objective. To search brain perfusion MRI (pMRI) changes in Behcet's disease (BD) with or without neurological involvement. Materials and Method. The pMRI were performed in 34 patients with BD and 16 healthy controls. Based on neurologic examination and post-contrast MRI, 12 patients were classified as Neuro-Behcet (group 1, NBD) and 22 patients as BD without neurological involvement (group 2). Mean transit time (MTT), time to peak (TTP), relative cerebral blood volume (rCBV), and relative cerebral blood flow (rCBF) were obtained and compared to those of healthy control group (group 3). Results. There was a significant difference in the MTT and rCBF within the pons and parietal cortex in groups 1 and 2. rCBV increased in cerebral pedicle in group 1 compared with groups 2 and 3. In the temporal lobe white matter, prolonged MTT and decreased rCBF were found in groups 1 and 2. In the corpus striatum, internal capsule, and periventricular white matter, rCBF increased in group 1 compared with group 3 and decreased in groups 1 and 2. Conclusion. Brain pMRI is a very sensitive method to detect brain involvement in patients with BD and aids the clinical diagnosis of NBD, especially in patients with negative MRI findings

Cortical location of saccadic and vergence oculomotor learning using fMRI

Motor learning is critical to the survival of a species and changes throughout life via neuroplasticity. The brain receives most of its information about the external world via the visual system. Eye movements are used to direct the visual information of interest to the fovea, the area of the retina which has the highest density of photoreceptors, and the largest amount of cortical area. This research will study how two of the five eye movement systems utilize oculomotor learning. Saccadic eye movements are used to quickly shift the fovea to objects using conjugate movements typically used during reading. The vergence system encompasses disconjugate movements of the eyes and provides perception of the depth of the objects. When a visual task is learned by a person, the latency and the peak velocity, inversely modulate according to each other under predictable and non-predictable conditions. This research will compare neural activity results during predictable and non-predictable visual conditions using Functional Magnetic Resonance Imaging (fMRI) in humans. FMRI indirectly measures neural activity by directly measuring the hemodynamics of neural responses. There were three primary results from this research; 1) activation was observed in occipital, frontal, temporal and cerebellar regions, 2) short-term neuroplasticity via recruitment and synchronization was observed in the cerebellar vermis 4/5, and 3) the frontal eye fields within the frontal lobe had distinct areas of activity allocated for saccadic versus vergence eye movements. Activity was observed in the integration of oculo-motor functions and cognitive functions such as memory corresponding to the occipital lobe, the prefrontal cortex, the frontal lobe, and the parietal lobe of the brain was observed in subjects. Furthermore, software was written to quantify the amount of cortical area involved in different areas of activation. The saccadic and vergence systems show similarities in the use of predictive learning as well as distinct cortical locations allocated to each system. Neuroplasticity was observed which was person dependent

Percent signal change from baseline within the posterior (left) and anterior (right) portions of the frontal eye fields (FEF).

<p>Significantly more signal change is observed within the posterior portion of FEF in the saccade compared to the vergence data set. Significantly more signal changes is observed within the anterior portion of FEF in the vergence compared to the saccade data set.</p

Saccade minus vergence data sets / positive paired <i>t</i>-test and vergence minus saccade data sets / negative paired <i>t</i>-test statistics showing differentiation between FEF and midbrain in comparing fixation versus random saccade and vergence tasks.

<p>Peak activation per subject of the fixation versus random vergence oculomotor task in Talairach-Tournoux coordinates with the level of significance denoted as a z-score. For the x axis: positive is right (R) and negative is left (L); for the y axis: negative is posterior (P) and positive is anterior (A); and for the z axis: positive is superior (S) and negative is inferior (I).</p

Experimental block design of 3.5 cycles alternating between fixation and eye movements (Plot A).

<p>Fixation and saccadic eye movements to targets 10 degrees into the left or right visual field or along midline plotted as position (deg) as a function of time (sec) (Plot B). Functional activity within FEF during saccadic stimulation plotted as percent signal change from baseline as a function of time (sec) (Plot C). Fixation and vergence eye movements to targets 2, 3, or 4 degrees along midline plotted as position (deg) as a function of time (sec) (Plot D). Functional activity within FEF during vergence stimulation plotted as percent signal change from baseline as a function of time (sec) (Plot E).</p

Average peak activation of the fixation versus random vergence oculomotor task in Talairach-Tournoux coordinates with the level of significance denoted as a z-score.

<p>For the x axis: positive is right (R) and negative is left (L); for the y axis: negative is posterior (P) and positive is anterior (A); and for the z axis: positive is superior (S) and negative is inferior (I).</p

Functional activation for the group analysis of fixation versus random eye movements for the saccade (left side) and the vergence data set (right side) showing typical commonality.

<p>DLPFC = dorsolateral prefrontal cortex and BA = Brodmann Area. The number of mm above the bicommissural plane is indicated. The functional activation is denoted by the scale bar as a z-score from a minimum of 2.0 to a maximum value of 6.6. Data are overlaid onto a standardized Talairach-Tournoux normalized image. Semi-inflated images of the functional activity within the lateral hemispheric surface and cerebellum are displayed using Caret software.</p