Binocular Eye Movements Are Adapted to the Natural Environment.

Humans and many animals make frequent saccades requiring coordinated movements of the eyes. When landing on the new fixation point, the eyes must converge accurately or double images will be perceived. We asked whether the visual system uses statistical regularities in the natural environment to aid eye alignment at the end of saccades. We measured the distribution of naturally occurring disparities in different parts of the visual field. The central tendency of the distributions was crossed (nearer than fixation) in the lower field and uncrossed (farther) in the upper field in male and female participants. It was uncrossed in the left and right fields. We also measured horizontal vergence after completion of vertical, horizontal, and oblique saccades. When the eyes first landed near the eccentric target, vergence was quite consistent with the natural-disparity distribution. For example, when making an upward saccade, the eyes diverged to be aligned with the most probable uncrossed disparity in that part of the visual field. Likewise, when making a downward saccade, the eyes converged to enable alignment with crossed disparity in that part of the field. Our results show that rapid binocular eye movements are adapted to the statistics of the 3D environment, minimizing the need for large corrective vergence movements at the end of saccades. The results are relevant to the debate about whether eye movements are derived from separate saccadic and vergence neural commands that control both eyes or from separate monocular commands that control the eyes independently.SIGNIFICANCE STATEMENT We show that the human visual system incorporates statistical regularities in the visual environment to enable efficient binocular eye movements. We define the oculomotor horopter: the surface of 3D positions to which the eyes initially move when stimulated by eccentric targets. The observed movements maximize the probability of accurate fixation as the eyes move from one position to another. This is the first study to show quantitatively that binocular eye movements conform to 3D scene statistics, thereby enabling efficient processing. The results provide greater insight into the neural mechanisms underlying the planning and execution of saccadic eye movements

Error Correction in Vergence Eye Movements: Evidence Supporting Hering’s Law

In pure symmetrical vergence eye movements, a fusion initiating component quickly brings the eyes close to the desired position. A small error usually remains after this response which must be corrected to attain the small final vergence error (i.e., fixation disparity). Error correction will usually involve both version and version movements so possible mechanisms include: small saccades, smooth pursuit, symmetrical vergence, or some combination. Alternatively, an asymmetrical vergence or uniocular slow eye movement could be used to achieve the highly precise final position. Saccade-free late fusion sustaining components during the steady state to a symmetrical vergence step stimulus are analyzed using independent component analysis. Results suggest that fine correction is most likely the product of closely coordinated version and vergence components

Development of new methodologies for the clinical, objective and automated evaluation of visual function based on the analysis of ocular movements : application in visual health

Premi Extraordinari de Doctorat, promoció 2018-2019. Àmbit de CiènciesHealthy visual function not only relies on good visual acuity. Other systems such as accommodation or binocular vision need to be effective. Binocular vision is the ability of the visual system to coordinate and integrate the information received separately from the two eyes into a single binocular percept. Anomalies in the binocular vision system lead to dysfunctions which are often associated with symptoms like asthenopia, diplopia, or ocular strain. The most prevalent non-strabismic binocular dysfunction is convergence insufficiency. The tests typically used to evaluate binocular vision and diagnose binocular dysfunctions essentially consist in eliciting eye movements and asking the patients to report when they perceive diplopia or single vision. In a conventional optometric clinical setting, they are generally run subjectively, as their results depend on the answers of the patients or on the examiner’s criteria. However, there exist instruments to determine the gaze position and measure ocular movements objectively, i.e. eye-trackers. These instruments are used as research tools in a wide range of applications (neuroscience, psychology, marketing, computer science, ophthalmology, etc.). Eye tracking systems are seldom used in optometric clinical practice, although it seems reasonable to think that the evaluation of binocular vision could better rely on eye tracking systems rather than subjective observations. In this context, the main objective of this thesis is to develop new methodologies for the clinical, objective and automated evaluation of visual function based on the analysis of ocular movements. This thesis is divided in 4 studies. In the first study, new methods for an eye tracking system based on multiple corneal reflections are proposed. The other 3 studies aim to analyze ocular movements in clinically interesting situations for the objective and automated evaluation of binocular vision. The results of the first study showed that the light sources configurations that produced the reflections in the lower region of the cornea showed higher accuracy. Vertical accuracy was slightly better with a higher number of corneal reflections. However, the proposed normalization methods improved vertical accuracy and counteracted the tendency for increasing accuracy with the number of glints. As a result, if the light sources are optimally positioned to avoid the interference of the eyelids and the normalization methods are applied, there is no need for more than two light sources. In the second study, an automated and objective method to measure phoria was proposed. It was significantly more repeatable than two other conventional clinical methods. However, the phoria results of the three tests were not interchangeable. This study brings to light several advantages of using eye-trackers in optometric clinical settings. The third study analyzes the characteristics of saccadic movements that occur during the near point of convergence test. The results showed that saccadic amplitude increased and rate decrease at closer viewing distances. These changes might be explained by the more rapid change of vergence demand and the greater angular size of the fixation target at near than at far. In general, saccades contributed to correct vergence errors and fixation position errors of the dominant eye. Finally, the fourth study focuses on the effects of the stimulus’ predictability on the latency and response time of vergence step movements. The results confirmed that vergence movements to predictable stimulus had shorter latency and response time than when the stimulus was random. Latency of convergence and divergence movements was influenced by the direction of the phoria. Other factors such as attention or voluntary effort might also affect vergence responses. All these effects might influence the final result of the vergence facility test, although further research is needed to specify the impact on the clinical test.Una bona funció visual no és sinònim exclusivament de bona agudesa visual. Cal que altres sistemes com l’acomodatiu o la visió binocular siguin eficaços. La visió binocular és la capacitat del sistema visual per coordinar i integrar la informació que reben els dos ulls en una única percepció. Anomalies en el sistema de visió binocular poden donar lloc a disfuncions associades a símptomes com astenopia, o visió doble. La disfunció no estràbica de la visió binocular més prevalent és la insuficiència de convergència. Els tests que normalment es fan per avaluar la visió binocular es basen en estimular moviments oculars i demanar als pacients que indiquin quan veuen doble i quan fusionen. A la pràctica clínica convencional, aquests tests solen ser subjectius. No obstant, hi ha instruments que serveixen per determinar objectivament la posició de mirada i mesurar els moviments oculars: els eye-trackers o instruments de seguiment de mirada. Aquests instruments s’utilitzen en recerca en moltes disciplines (neurociència, psicologia, marketing, oftalmologia, etc.). Tot i que una aplicació directa dels eye-trackers podria ser en l’avaluació de la visió binocular, els sistemes de seguiment de mirada gairebé no s’utilitzen en la pràctica clínica optomètrica. En aquest context, l’objectiu principal d’aquesta tesi és desenvolupar noves metodologies per a l’avaluació clínica, objectiva i automatitzada de la funció visual basades en l’anàlisi dels moviments oculars. La tesi està estructurada en 4 estudis. En el primer, es proposen nous mètodes per a un eye-tracker basat en múltiples reflexos corneals. Els altres 3 estudis tenen com a objectiu analitzar els moviments oculars en situacions d’interès clínic per avaluar objectiva i automàticament la visió binocular. Els resultats del primer estudi demostren que les configuracions d’il·luminació dels eye-trackers que formen les reflexions corneals a la zona inferior de la còrnia són més precises. La precisió vertical és lleugerament millor amb més fonts de llum. Tot i això, els mètodes de normalització proposats milloren considerablement la precisió vertical i contraresten la tendència de més precisió amb més fonts de llum. D’aquesta manera, si les fonts de llum no interfereixen amb les parpelles i s’apliquen els mètodes de normalització, no cal que els eye-trackers tinguin més de dues fonts de llum. En el segon estudi s’ha proposat un mètode automàtic i objectiu per mesurar la fòria. Aquest mètode és significativament més repetitiu que dos altres mètodes clínics. Tot i això, els resultats amb els tres mètodes no són intercanviables. Aquest estudi posa de manifest avantatges que podrien tenir els eye-trackers a la pràctica clínica optomètrica. En el tercer estudi s’analitzen les característiques dels moviments sacàdics que es produeixen durant la prova del punt proper de convergència. Els resultats demostren que l’amplitud dels sacàdics augmenta i la freqüència disminueix a mesura que s’escurça la distància. Aquests canvis poden ser deguts a que la demanda de vergència canvia més ràpid a distàncies properes que llunyanes, i a l’increment de la mida angular de l’objecte de fixació. En general, els sacàdics contribueixen a corregir els errors de vergència i els errors de fixació de l’ull dominant. Finalment, el quart estudi se centra en els efectes de la predictibilitat de l’estímul en la latència i temps de resposta dels salts de vergència. Els resultats confirmen que els moviments de vergència tenen una latència i temps de resposta més curts quan l’estímul es predictible que quan és aleatori. La latència dels moviments de convergència i divergència està influenciada per la direcció de la fòria. Altres factors com el grau d’atenció o d’esforç voluntari pot ser que afectin els moviments de vergència. Tots aquests efectes probablement influencien en el resultat final de la prova de la instal·lació de vergència, encara que es necessiten més investigacions per especificar l'impacte en la prova clínica.Award-winningPostprint (published version

Development of new methodologies for the clinical, objective and automated evaluation of visual function based on the analysis of ocular movements : application in visual health

Healthy visual function not only relies on good visual acuity. Other systems such as accommodation or binocular vision need to be effective. Binocular vision is the ability of the visual system to coordinate and integrate the information received separately from the two eyes into a single binocular percept. Anomalies in the binocular vision system lead to dysfunctions which are often associated with symptoms like asthenopia, diplopia, or ocular strain. The most prevalent non-strabismic binocular dysfunction is convergence insufficiency. The tests typically used to evaluate binocular vision and diagnose binocular dysfunctions essentially consist in eliciting eye movements and asking the patients to report when they perceive diplopia or single vision. In a conventional optometric clinical setting, they are generally run subjectively, as their results depend on the answers of the patients or on the examiner’s criteria. However, there exist instruments to determine the gaze position and measure ocular movements objectively, i.e. eye-trackers. These instruments are used as research tools in a wide range of applications (neuroscience, psychology, marketing, computer science, ophthalmology, etc.). Eye tracking systems are seldom used in optometric clinical practice, although it seems reasonable to think that the evaluation of binocular vision could better rely on eye tracking systems rather than subjective observations. In this context, the main objective of this thesis is to develop new methodologies for the clinical, objective and automated evaluation of visual function based on the analysis of ocular movements. This thesis is divided in 4 studies. In the first study, new methods for an eye tracking system based on multiple corneal reflections are proposed. The other 3 studies aim to analyze ocular movements in clinically interesting situations for the objective and automated evaluation of binocular vision. The results of the first study showed that the light sources configurations that produced the reflections in the lower region of the cornea showed higher accuracy. Vertical accuracy was slightly better with a higher number of corneal reflections. However, the proposed normalization methods improved vertical accuracy and counteracted the tendency for increasing accuracy with the number of glints. As a result, if the light sources are optimally positioned to avoid the interference of the eyelids and the normalization methods are applied, there is no need for more than two light sources. In the second study, an automated and objective method to measure phoria was proposed. It was significantly more repeatable than two other conventional clinical methods. However, the phoria results of the three tests were not interchangeable. This study brings to light several advantages of using eye-trackers in optometric clinical settings. The third study analyzes the characteristics of saccadic movements that occur during the near point of convergence test. The results showed that saccadic amplitude increased and rate decrease at closer viewing distances. These changes might be explained by the more rapid change of vergence demand and the greater angular size of the fixation target at near than at far. In general, saccades contributed to correct vergence errors and fixation position errors of the dominant eye. Finally, the fourth study focuses on the effects of the stimulus’ predictability on the latency and response time of vergence step movements. The results confirmed that vergence movements to predictable stimulus had shorter latency and response time than when the stimulus was random. Latency of convergence and divergence movements was influenced by the direction of the phoria. Other factors such as attention or voluntary effort might also affect vergence responses. All these effects might influence the final result of the vergence facility test, although further research is needed to specify the impact on the clinical test.Una bona funció visual no és sinònim exclusivament de bona agudesa visual. Cal que altres sistemes com l’acomodatiu o la visió binocular siguin eficaços. La visió binocular és la capacitat del sistema visual per coordinar i integrar la informació que reben els dos ulls en una única percepció. Anomalies en el sistema de visió binocular poden donar lloc a disfuncions associades a símptomes com astenopia, o visió doble. La disfunció no estràbica de la visió binocular més prevalent és la insuficiència de convergència. Els tests que normalment es fan per avaluar la visió binocular es basen en estimular moviments oculars i demanar als pacients que indiquin quan veuen doble i quan fusionen. A la pràctica clínica convencional, aquests tests solen ser subjectius. No obstant, hi ha instruments que serveixen per determinar objectivament la posició de mirada i mesurar els moviments oculars: els eye-trackers o instruments de seguiment de mirada. Aquests instruments s’utilitzen en recerca en moltes disciplines (neurociència, psicologia, marketing, oftalmologia, etc.). Tot i que una aplicació directa dels eye-trackers podria ser en l’avaluació de la visió binocular, els sistemes de seguiment de mirada gairebé no s’utilitzen en la pràctica clínica optomètrica. En aquest context, l’objectiu principal d’aquesta tesi és desenvolupar noves metodologies per a l’avaluació clínica, objectiva i automatitzada de la funció visual basades en l’anàlisi dels moviments oculars. La tesi està estructurada en 4 estudis. En el primer, es proposen nous mètodes per a un eye-tracker basat en múltiples reflexos corneals. Els altres 3 estudis tenen com a objectiu analitzar els moviments oculars en situacions d’interès clínic per avaluar objectiva i automàticament la visió binocular. Els resultats del primer estudi demostren que les configuracions d’il·luminació dels eye-trackers que formen les reflexions corneals a la zona inferior de la còrnia són més precises. La precisió vertical és lleugerament millor amb més fonts de llum. Tot i això, els mètodes de normalització proposats milloren considerablement la precisió vertical i contraresten la tendència de més precisió amb més fonts de llum. D’aquesta manera, si les fonts de llum no interfereixen amb les parpelles i s’apliquen els mètodes de normalització, no cal que els eye-trackers tinguin més de dues fonts de llum. En el segon estudi s’ha proposat un mètode automàtic i objectiu per mesurar la fòria. Aquest mètode és significativament més repetitiu que dos altres mètodes clínics. Tot i això, els resultats amb els tres mètodes no són intercanviables. Aquest estudi posa de manifest avantatges que podrien tenir els eye-trackers a la pràctica clínica optomètrica. En el tercer estudi s’analitzen les característiques dels moviments sacàdics que es produeixen durant la prova del punt proper de convergència. Els resultats demostren que l’amplitud dels sacàdics augmenta i la freqüència disminueix a mesura que s’escurça la distància. Aquests canvis poden ser deguts a que la demanda de vergència canvia més ràpid a distàncies properes que llunyanes, i a l’increment de la mida angular de l’objecte de fixació. En general, els sacàdics contribueixen a corregir els errors de vergència i els errors de fixació de l’ull dominant. Finalment, el quart estudi se centra en els efectes de la predictibilitat de l’estímul en la latència i temps de resposta dels salts de vergència. Els resultats confirmen que els moviments de vergència tenen una latència i temps de resposta més curts quan l’estímul es predictible que quan és aleatori. La latència dels moviments de convergència i divergència està influenciada per la direcció de la fòria. Altres factors com el grau d’atenció o d’esforç voluntari pot ser que afectin els moviments de vergència. Tots aquests efectes probablement influencien en el resultat final de la prova de la instal·lació de vergència, encara que es necessiten més investigacions per especificar l'impacte en la prova clínica

Assessment of Dual-Mode and Switched-Channel Models with Experimental Vergence Responses

Controversy exists in the literature regarding the basic neural control structure that mediates convergence responses. This study constructed and simulated two models, the switched-channel feedback model and the dual-mode model consisting of preprogrammed with feedback control. Models were constructed and compared to experimental data. The stimuli consisted of 2 deg and 4 deg vergence steps. Both closed- and open-loop settings were utilized. After parameter adjustment, both models could accurately simulate step responses from subjects having a range of response dynamics. The model with a preprogrammed element required less parameter modification when stimulus amplitude changed. Both models could accurately simulate some attributes of vergence; however, neither model could represent the modifications commonly observed within the transient portion of the vergence response

Change blindness: eradication of gestalt strategies

Arrays of eight, texture-defined rectangles were used as stimuli in a one-shot change blindness (CB) task where there was a 50% chance that one rectangle would change orientation between two successive presentations separated by an interval. CB was eliminated by cueing the target rectangle in the first stimulus, reduced by cueing in the interval and unaffected by cueing in the second presentation. This supports the idea that a representation was formed that persisted through the interval before being 'overwritten' by the second presentation (Landman et al, 2003 Vision Research 43149–164]. Another possibility is that participants used some kind of grouping or Gestalt strategy. To test this we changed the spatial position of the rectangles in the second presentation by shifting them along imaginary spokes (by ±1 degree) emanating from the central fixation point. There was no significant difference seen in performance between this and the standard task [F(1,4)=2.565, p=0.185]. This may suggest two things: (i) Gestalt grouping is not used as a strategy in these tasks, and (ii) it gives further weight to the argument that objects may be stored and retrieved from a pre-attentional store during this task

Measuring gaze and pupil in the real world: object-based attention,3D eye tracking and applications

This dissertation contains studies on visual attention, as measured by gaze orientation, and the use of mobile eye-tracking and pupillometry in applications. It combines the development of methods for mobile eye-tracking (studies II and III) with experimental studies on gaze guidance and pupillary responses in patients (studies IV and VI) and healthy observers (studies I and V). Object based attention / Study I What is the main factor of fixation guidance in natural scenes? Low-level features or objects? We developed a fixation-predicting model, which regards preferred viewing locations (PVL) per object and combines these distributions over the entirety of existing objects in the scene. Object-based fixation predictions for natural scene viewing perform at par with the best early salience model, that are based on low-level features. However, when stimuli are manipulated so that low-level features and objects are dissociated, the greater prediction power of saliency models diminishes. Thus, we dare to claim, that highly developed saliency models implicitly obtain object-hood and that fixation selection is mainly influenced by objects and much less by low-level features. Consequently, attention guidance in natural scenes is object-based. 3D tracking / Study II The second study focussed on improving calibration procedures for eye-in-head positions with a mobile eye-tracker.We used a mobile eye-tracker prototype, the EyeSeeCam with a high video-oculography (VOG) sampling rate and the technical gadget to follow the users gaze direction instantaneously with a rotatable camera. For a better accuracy in eye-positioning, we explored a refinement in the implementation of the eye-in-head calibration that yields a measure for fixation distance, which led to a mobile eye-tracker 3D calibration. Additionally, by developing the analytical mechanics for parametrically reorienting the gaze-centred camera, the 3D calibration could be applied to reliably record gaze-centred videos. Such videos are suitable as stimuli for investigating gaze-behaviour during object manipulation or object recognition in real worlds point-of-view (PoV) perspective. In fact, the 3D calibration produces a higher accuracy in positioning the gaze-centred camera over the whole 3D visual range. Study III, eye-tracking methods With a further development on the EyeSeeCam we achieved to record gaze-in-world data, by superposing eye-in-head and head-in-world coordinates. This novel approach uses a combination of few absolute head-positions extracted manually from the PoV video and of relative head-shifts integrated over angular velocities and translational accelerations, both given by an inertia measurement unit (IMU) synchronized to the VOG data. Gaze-in-world data consist of room-referenced gaze directions and their origins within the environment. They easily allow to assign fixation targets by using a 3D model of the measuring environment – a strong rationalisation regarding fixation analysis. Applications Study III Daylight is an important perceptual factor for visual comfort, but can also create discomfort glare situations during office work, so we developed to measure its behavioural influences. We achieve to compare luminance distributions and fixations in a real-world setting, by also recording indoor luminance variations time-resolved using luminance maps of a scenery spanning over a 3pi sr. Luminance evaluations in the workplace environment yield a well controlled categorisation of different lighting conditions and a localisation as well as a brightness measure of glare sources.We used common tasks like reading, typing on a computer, a phone call and thinking about a subject. The 3D model gives the possibility to test for gaze distribution shifts in the presence of glare patches and for variations between lighting conditions. Here, a low contrast lighting condition with no sun inside and a high contrast lighting condition with direct sunlight inside were compared. When the participants are not engaged in any visually focused task and the presence of the task support is minimal, the dominant view directions are inclined towards the view outside the window under the low contrast lighting conditions, but this tendency is less apparent and sways more towards the inside of the room under the high contrast lighting condition. This result implicates an avoidance of glare sources in gaze behaviour. In a second more extensive series of experiments, the participants’ subjective assessments of the lighting conditions will be included. Thus, the influence of glare can be analysed in more detail and tested whether visual discomfort judgements are correlated in differences in gaze-behaviour. Study IV The advanced eye-tracker calibration found application in several following projects and included in this dissertation is an investigation with patients suffering either from idiopathic Parkinson’s disease or from progressive supranuclear palsy (PSP) syndrome. PSP’s key symptom is the decreased ability to carry out vertical saccades and thus the main diagnostic feature for differentiating between the two forms of Parkinson’s syndrome. By measuring ocular movements during a rapid (< 20s) procedure with a standardized fixation protocol, we could successfully differentiate pre-diagnosed patients between idiopathic Parkinson’s disease and PSP, thus between PSP patients and HCs too. In PSP patients, the EyeSeeCam detected prominent impairment of both saccade velocity and amplitude. Furthermore, we show the benefits of a mobile eye-tracking device for application in clinical practice. Study V Decision-making is one of the basic cognitive processes of human behaviours and thus, also evokes a pupil dilation. Since this dilation reflects a marker for the temporal occurrence of the decision, we wondered whether individuals can read decisions from another’s pupil and thus become a mentalist. For this purpose, a modified version of the rock-paper-scissors childhood game was played with 3 prototypical opponents, while their eyes were video taped. These videos served as stimuli for further persons, who competed in rock-paper-scissors. Our results show, that reading decisions from a competitor’s pupil can be achieved and players can raise their winning probability significantly above chance. This ability does not require training but the instruction, that the time of maximum pupil dilation was indicative of the opponent’s choice. Therefore we conclude, that people could use the pupil to detect cognitive decisions in another individual, if they get explicit knowledge of the pupil’s utility. Study VI For patients with severe motor disabilities, a robust mean of communication is a crucial factor for well-being. Locked-in-Syndrome (LiS) patients suffer from quadriplegia and lack the ability of articulating their voice, though their consciousness is fully intact. While classic and incomplete LiS allows at least voluntary vertical eye movements or blinks to be used for communication, total LiS patients are not able to perform such movements. What remains, are involuntarily evoked muscle reactions, like it is the case with the pupillary response. The pupil dilation reflects enhanced cognitive or emotional processing, which we successfully observed in LiS patients. Furthermore, we created a communication system based on yes-no questions combined with the task of solving arithmetic problems during matching answer intervals, that yet invokes the most solid pupil dilation usable on a trial-by-trial basis for decoding yes or no as answers. Applied to HCs and patients with various severe motor disabilities, we provide the proof of principle that pupil responses allow communication for all tested HCs and 4/7 typical LiS patients. Résumé Together, the methods established within this thesis are promising advances in measuring visual attention allocation with 3D eye-tracking in real world and in the use of pupillometry as on-line measurement of cognitive processes. The two most outstanding findings are the possibility to communicate with complete LiS patients and further a conclusive evidence that objects are the primary unit of fixation selection in natural scenes

Oculomotor responses and 3D displays

This thesis investigated some of the eye movement factors related to the development and use of eye pointing devices with three dimensional displays (stereoscopic and linear perspective). In order for eye pointing to be used as a successful device for input-control of a 3D display it is necessary to characterise the accuracy and speed with which the binocular point of foveation can locate a particular point in 3D space. Linear perspective was found to be insufficient to elicit a change in the depth of the binocular point of fixation except under optimal conditions (monocular viewing, accommodative loop open and constant display paradigm). Comparison of the oculomotor responses made between a stereoscopic 'virtual' and a 'real' display showed there were no differences with regards to target fixational accuracy. With one exception, subjects showed the same degree of fixational accuracy with respect to target direction and depth. However, close target proximity (in terms of direction) affected the accuracy of fixation with respect to depth (but not direction). No differences were found between fixational accuracy of large and small targets under either display conditions. The visual conditions eliciting fast changes in the location of the binocular point of foveation, i.e. saccade disconjugacy, were investigated. Target-directed saccade disconjugacy was confirmed, in some cases, between targets presented at different depths on a stereoscopic display. However, in general the direction of saccade disconjugacy was best predicted by the horizontal direction of the target. Leftward saccade disconjugacy was more divergent than rightward. This asymmetry was overlaid on a disconjugacy response, which when considered in relative terms, was appropriated for the level of vergence demand. Linear perspective depth cues did not elicit target-directed disconjugate saccades

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

A Portable Bio-Inspired Architecture for Efficient Robotic Vergence Control

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