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Vergence eye movements are the inward and outward rotation of the eyes responsible for binocular coordination. While studies have mapped and investigated the neural substrates of vergence, it is not well understood whether vergence eye movements evoke the blood oxygen level-dependent signal reliably in separate experimental visits. The test–retest reliability of stimulus-induced vergence eye movement tasks during a functional magnetic resonance imaging (fMRI) experiment is important for future randomized clinical trials (RCTs). In this study, we established region of interest (ROI) masks for the vergence neural circuit. Twenty-seven binocularly normal young adults participated in two functional imaging sessions measured on different days on the same 3T Siemens scanner. The fMRI experiments used a block design of sustained visual fixation and rest blocks interleaved between task blocks that stimulated eight or four vergence eye movements. The test–retest reliability of task-activation was assessed using the intraclass correlation coefficient (ICC), and that of spatial extent was assessed using the Dice coefficient. Functional activation during the vergence eye movement task of eight movements compared to rest was repeatable within the primary visual cortex (ICC = 0.8), parietal eye fields (ICC = 0.6), supplementary eye field (ICC = 0.5), frontal eye fields (ICC = 0.5), and oculomotor vermis (ICC = 0.6). The results demonstrate significant test–retest reliability in the ROIs of the vergence neural substrates for functional activation magnitude and spatial extent using the stimulus protocol of a task block stimulating eight vergence eye movements compared to sustained fixation. These ROIs can be used in future longitudinal RCTs to study patient populations with vergence dysfunctions. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s12264-019-00455-9) contains supplementary material, which is available to authorized users.

The Vergence Endurance Test (VET), a quantitative and objective eye movement assessment, was utilized to differentiate control from concussed subjects. Nine symptomatic concussed (2 male; 30.8 ± 11 years) and 9 asymptomatic control (6 male; 25.1 ± 1.4 years) subjects participated in the VET. Symmetrical disparity vergence step targets were presented with and without visual distractors. A masked data analyst measured vergence latency, peak velocity, response amplitude, settling time, and the percentage of trials which contained blinks. A Binocular Precision Index (BPI) and a Binocular Accuracy Index (BAI) were calculated to quantify the changes that occur in the vergence parameters over the duration of the VET. Convergence and divergence peak velocity, divergence response amplitude, the percentage of trials that contained blinks during the transient portion of the response, and the BAI were significantly (p

SIGNIFICANCE This study establishes normative data and a testing procedure for the oculomotor assessment tool. The oculomotor assessment tool standardizes visual targets for the Vestibular/OculoMotor Screening assessment and provides additional metrics that may aid in the differentiation between those with normal and those with abnormal oculomotor function potentially caused by a concussion. PURPOSE This study aimed to assess the oculomotor endurance of healthy participants with no self-reported history of concussions using the oculomotor assessment tool. METHODS Healthy participants (n = 376, average age of 20.4 years, range of 11 to 34 years, with no self-reported history of concussions) were recruited to perform the following three tasks for 60 seconds each: (1) horizontal saccades, (2) vertical saccades, and (3) vergence jumps. The participants were instructed to alternate visual fixation between two targets for each of the tasks as fast as they could without overshooting or undershooting the visual target. The differences in the number of eye movements between the initial and latter 30 seconds of the 1-minute test were analyzed. RESULTS A statistical difference (P < .001) was observed in the number of eye movements for all three tasks (horizontal saccades [70 ± 15 for initial 30 seconds, 63 ± 13 for latter 30 seconds], vertical saccades [68 ± 14, 63 ± 13], and vergence jumps [43 ± 11, 39 ± 10]) between the initial and latter 30 seconds. No significant differences were identified in the number of eye movements or the change in eye movements between the initial and latter 30 seconds based on sex. CONCLUSIONS These results establish a normative database for various eye movements. These data could potentially be used to compare different patient populations who have binocular endurance dysfunctions potentially due to traumatic brain injury, such as patients with concussion(s).

Deficits of disparity divergence found with objective eye movement recordings may not be apparent with standard clinical measures of negative fusional vergence (NFV) in children with symptomatic convergence insufficiency.This study aimed to determine whether NFV is normal in untreated children with symptomatic convergence insufficiency and whether NFV improves after vergence/accommodative therapy.This secondary analysis of NFV measures before and after office-based vergence/accommodative therapy reports changes in (1) objective eye movement recording responses to 4° disparity divergence step stimuli from 12 children with symptomatic convergence insufficiency compared with 10 children with normal binocular vision (NBV) and (2) clinical NFV measures in 580 children successfully treated in three Convergence Insufficiency Treatment Trial studies.At baseline, the Convergence Insufficiency Treatment Trial cohort's mean NFV break (14.6 ± 4.8Δ) and recovery (10.6 ± 4.2Δ) values were significantly greater (P.001) than normative values. The post-therapy mean improvements for blur, break, and recovery of 5.2, 7.2, and 1.3Δ, respectively, were statistically significant (P.0001). Mean pre-therapy responses to 4° disparity divergence step stimuli were worse in the convergence insufficiency group compared with the NBV group for peak velocity (P.001), time to peak velocity (P = .01), and response amplitude (P.001). After therapy, the convergence insufficiency group showed statistically significant improvements in mean peak velocity (11.63°/s; 95% confidence interval [CI], 6.6 to 16.62°/s), time to peak velocity (-0.12 seconds; 95% CI, -0.19 to -0.05 seconds), and response amplitude (1.47°; 95% CI, 0.83 to 2.11°), with measures no longer statistically different from the NBV cohort (P.05).Despite clinical NFV measurements that seem greater than normal, children with symptomatic convergence insufficiency may have deficient NFV when measured with objective eye movement recordings. Both objective and clinical measures of NFV can be improved with vergence/accommodative therapy.

Significance This study establishes normative data for objective outcome measures of vergence and saccade eye movements for the pediatric population. These data should facilitate future clinical trial design. Purpose This study was designed to establish normative data for objective measures of disparity vergence and saccades in children between the ages 9 and 17 years using an objective binocular eye movement tracking system. Methods Participants (aged 9 to 17 years) had a vision examination including refraction, accommodative, and binocular vision testing. Eligibility criteria included 20/25 visual acuity with best correction, normal accommodation, and binocular vision. The ISCAN RK-826PCI binocular tracking system (ISCAN, Woburn, MA) was used to objectively record horizontal, symmetrical disparity vergence, and saccadic eye movements. Parameters assessed included peak velocity, time to peak velocity, latency, and response amplitude for both disparity vergence and saccades. Results One hundred eighteen participants were recruited (54.94% female; mean age, 13.5 years), and 77.1% (91/118) of the participants completed the assessment with usable data. A sample of the normative data included peak velocity (°/s), which had a mean ± standard deviation of 25.4 ± 2.9, 22.0 ± 3.0, 225 ± 16.7, and 332.5 ± 20.5 for 4° convergence, 4° divergence, 5° saccades, and 10° saccades, respectively. The mean ± standard deviation for the latency (seconds) measures were 0.28 ± 0.1, 0.28 ± 0.16, 0.23 ± 0.05, and 0.23 ± 0.05 for 4° convergence, 4° divergence, 5° saccades, and 10° saccades, respectively. Conclusions Normative data enable researchers to have benchmark results for comparison with patient populations with binocular dysfunction. These objective disparity vergence measures can serve as outcome measures in future clinical trials to assess the effectiveness of therapeutic interventions by determining whether post-treatment results are similar to normal data.

This study investigates the underlying physiological mechanisms that may lead to improved outcomes for symptomatic convergence insufficiency (CI) patients after 12 weeks of office-based vergence/accommodation therapy (OBVAT) by evaluating the change in the main sequence of vergence and saccadic eye movements. In this prospective trial, 12 participants with symptomatic CI were recruited and treated with 12 weeks of OBVAT. Outcome measures included the objective assessment of the following: peak velocity, time to peak velocity, latency, response amplitude, and clinical changes in the near point of convergence (NPC), positive fusional vergence (PFV) and symptoms via the Convergence Insufficiency Symptom Survey (CISS). Ten of the twelve participants (83%) were categorized as “successful” and two were “improved” based on pre-determined published criteria (CISS, NPC, PFV). There were statistically significant changes in peak velocity, time to peak velocity, and response amplitude for both 4° and 6° symmetrical convergence and divergence eye movements. There was a significant change in the main sequence ratio for convergence post-OBVAT compared to baseline measurements (P=0.007) but not for divergence or saccadic responses. Phasic/step vergence movements adjust the underlying neural control of convergence and are critical within a vision therapy program for CI patients.

A paucity of research exists to investigate whether the normal aging process influences the ability to adapt disparity vergence and phoria. Vergence eye movements and dissociated phoria were recorded from 49 healthy subjects (ages 20–70 years) using an objective eye movement tracking system. Four-degree vergence responses were modified using a double-step protocol. Dynamics of vergence were quantified via peak velocity. The phoria adaptation experiment measured the magnitude (net change in phoria level) and rate (magnitude divided by the time constant) of phoria adaption during 5 min of sustained fixation on a binocular target (40 cm/8.44° from midline). The magnitude of phoria adaptation decreased as a function of age (r =−0.33; p = 0.04). The ability to adapt vergence peak velocity and the rate of phoria adaptation showed no significant age-related influence (p > 0.05). The data suggest that the ability to modify the disparity vergence system and the rate of phoria adaptation are not dependent on age; whereas, the magnitude of phoria adaptation decreases as part of the normal adult aging process. These results have clinical and basic science implications because one should consider age when assessing the changes in the magnitude of phoria adaptation which can be abnormal in those with oculomotor dysfunctions.

Purpose The purpose of this study is to investigate the functional neural anatomy that generates vergence eye movement responses from predictive versus random symmetrical vergence step stimuli in humans and compare it to a similar saccadic task via the blood oxygenation level dependent signal from functional MRI. Methods Eight healthy subjects participated in fMRI scans obtained from a 3 T Siemens Allegra scanner. Subjects tracked random and predictable vergent steps and then tracked random and predictable saccadic steps each within a block design. A general linear model (GLM) was used to determine significantly (p 2.7, p

Visual attention is an important aspect of everyday life, which can be incorporated in the assessment of many diagnoses. Another important characteristic of visual attention is that it can be improved via therapeutic interventions. Fifteen subjects with normal binocular vision were presented with visual distractor stimuli at various spatial locations while initiating disparity vergence eye movements (inward or outward rotation of eyes) within a haploscope system. First, a stationary distractor stimulus was presented in either the far, middle, or near visual spaces while the subjects were instructed to follow a target stimulus that was either stationary, converging (moving toward subject), or diverging (moving away from subject). For the second experiment, a dynamic distractor stimulus within the far, middle, or near visual space that was converging or diverging was presented while the target stimulus was also converging or diverging. The subjects were instructed to visually follow the target stimulus and ignore the distractor stimulus. The vergence responses had a final vergence angle between the target and distractor stimuli which has been termed a center of gravity (CoG) effect. Statistically significant differences were observed between the convergence peak velocities (p < 0.001) and response amplitudes (p < 0.001) comparing responses without distractors to responses with the presence of a vergence distractor. The results support that vergence eye movements are influenced by visual distractors, which is similar to how distractors influence saccadic eye movements. The influence of visual distractors within vergence eye movements may be useful to assess binocular dysfunction and visual distraction which are common post brain injury.

Purpose: To develop a system with which to quantify objectively the heterophoria to be measured throughout eye movement experiments. This study compared precision, resolution and accuracy characteristics of a limbus eye movement tracking system to the alternate cover test and the Maddox rod. Methods: Precision testing was performed using each technique with binocularly-normal subjects at different test sessions. Resolution was measured and compared between the three systems. The heterophoria or phoria was measured from 40 cm/16 inches objectively with the limbus eye movement system and the Maddox rod during a sustained convergence-induced phoria adaptation experiment using physical and haploscope visual targets to compare accuracy. Results: Precision testing quantified that the responses objectively recorded using the limbus tracking system exhibited similar standard deviations to the Maddox rod and the alternate cover test techniques. The limbus tracking method has the ability to quantify the response decay to phoria and has better resolution when compared to the clinical methods. When physical targets were used to induce phoria adaptation, the correlation was significant between the limbus eye movement tracking system and the Maddox rod after near and far adaptation, where the Maddox rod measurements were slightly more esophoric compared to those measured with the limbus tracking system. Conclusion: This objective limbus tracking system offers a better means by which to study the phoria and its adaptation throughout an eye movement experiment. The broader aim of this research is to establish a tool that will further the basic science of oculomotor control and binocular dysfunctions.

The purpose of this study was to investigate how sustained convergence induces phoria adaptation and changes divergence dynamics.Near dissociated phoria and divergence step responses were recorded using an infrared eye movement monitor on four binocularly normal subjects. We tested three different adapting vergence positions (16 degrees , 6 degrees , and 0.5 degrees ) and measured 4 degrees divergence step responses for two different initial vergence positions (16 degrees and 4.5 degrees ). Dynamics were quantified by measuring peak velocities of the divergence responses.Phoria was significantly adapted after subjects fixated on a sustained convergent target at near (16 degrees ) and at far (0.5 degrees ). As a result of sustained convergence, divergence peak velocity from a 4 degrees step stimulus significantly changed. A regression analysis showed that when the phoria became more esophoric (near adapted) the peak velocity for the divergence steps with an initial position of 16 degrees decreased (R=0.54, p=0.04). A trend was observed between the change vergence velocity and the change in phoria. Change was defined as the post-adapted data minus the pre-adapted data for vergence steps with an initial position of 16 degrees (R=0.65) and 4.5 degrees (R=0.66). Furthermore, the modification of divergence dynamics was dependent on the initial position of those divergence steps (initial position of 16 degrees versus 4.5 degrees ).As a result of sustained convergence, phoria and divergence dynamics changed in a correlated manner. Such correlated changes in phoria and divergence dynamics are not explained by current models of disparity vergence eye movements.

Previous work has shown that the disparity vergence eye movement system responds to inward (i.e., convergent) ramp stimuli with both smooth and step-like behavior depending on target velocity. The responses to diverging ramp stimuli have not been previously studied, but convergence and divergence responses to other stimuli often show different behaviors. Converging and diverging 6 degrees/s ramps were presented to four subjects over a stimulus range of 2 degrees-20 degrees. Step-like behavior was seen in both convergence and divergence responses, but the dynamics was different. For divergent ramps, the peak velocity of each step-like movement decreased as the stimulus moved away from the subject, but no such trend was observed for convergence. The step-like behavior seen in divergence supports the hypothesis that the transient component is active in disparity divergence similar to the transient component proposed for convergent movements. However, the transient component in divergence may be dependent on stimulus position which is not the case for convergence.

Dry dissection, a concept developed by Lawrence Stark, includes a variety of techniques designed to isolate internal neural control components by using cleverly designed stimulus or measurement protocols. As envisioned by Stark, the concept applies only to motor systems that have multiple stimulus inputs and/or response behaviors. A new application of independent component analysis (ICA) can be used to extend the dry dissection concept to identify motor components from a single, isolated response. It is only necessary that multiple responses can be obtained to the same stimulus. This ''ensemble ICA'' technique is well suited to analyze various eye movement behaviors as even isolated motor systems often include multiple control processes. Here we apply ensemble ICA to vergence eye movements: the inward (convergence) or outward (divergence) turning of the eyes that allows us to view images at various distances. Previous studies concerning the dynamics of convergence and divergence eye movements have produced varied, sometimes contradictory, results: most studies report that convergence is considerably faster than divergence, but opposite results have also been reported. Experimental results have shown that the dynamics of divergence movements depend on the initial vergence position while those of convergence do not: divergence eye movements in response to targets initially near to the subject can attain peak velocities twice that of those driven by more distant targets. To determine the underlying cause of this behavior, ensemble ICA was applied to divergence responses. Results show that both convergence and divergence contain a sustained (step-like) and a transient (pulse-like) control component, but the amplitude of the transient component in divergence is dependent on initial stimulus position.

Purpose: This study examined the relationship between the near dissociated phoria and disparity vergence eye movements. Convergence insufficiency (CI) patients before vergence therapy were compared to: (1) the same patients after vergence therapy; and (2) binocularly normal controls (BNC). Methods: Sixteen subjects were studied—twelve BNC and four with CI. Measurements from the CI subjects were obtained before and after 18 h of vergence eye movement therapy. The near dissociated phoria was measured using the flashed Maddox rod technique. Vergence responses were stimulated from 4° symmetrical disparity vergence step stimuli. The peak velocity of the vergence response and the magnitude of the fusion initiating component (FIC) from an independent component analysis (ICA) were calculated. A linear regression analysis was conducted studying the vergence peak velocity as a function of the near dissociated phoria where the Pearson correlation coefficient was computed. Results: Before vergence therapy, the average with one standard deviation FIC magnitude of convergence responses from CI subjects was 0.29° ± 0.82 and significantly less than the FIC magnitude of 1.85° ± 0.84 for BNC (p < 0.02). A paired t-test reported that the FIC and near dissociated phoria before vergence therapy for CI subjects significantly increased to 1.49° ± 0.57 (p < 0.04) and became less exophoric to 3.5Δ ± 1.9 exo (p < 0.02) after vergence therapy. A significant correlation (r = 0.87; p < 0.01) was observed between the near dissociated phoria and the vergence ratio of convergence peak velocity divided by divergence peak velocity. Conclusion: The results have clinical translational impact in understanding the mechanism by which vergence therapy may be changing the vergence system leading to a sustained reduction in visual symptoms.

Previous studies on the speed and latency of convergence and divergence eye movements have produced varied, sometimes contradictory, results. Four subjects were studied and tracked 4° disparity step changes for convergence and divergence at different initial target positions. Here we report that the dynamics of divergence movements not only differ from convergence movement, but depend on the initial vergence position. Velocities of divergence eye movements in response to targets that were initially near to the subject were approximately twice that of responses to initially distant targets and also exhibited shorter temporal properties. Hence, while convergence responses are fairly similar irrespective of the initial position, divergence dynamic and temporal properties are dependent on the initial stimulus position. It is speculated that the differences observed in divergence may be the result of nonlinear properties of the extraocular muscles or a difference in the underlying neural controller potentially a difference in the magnitude of the fusion initiating component of divergence.

While a body of literature relating cognition and oculomotor performance exists, a better understanding of these processes would help facilitate the development of effective treatments for patients suffering various neurological disorders, such as Multiple Sclerosis (MS), Huntington's Disease, or a traumatic brain injury. To examine the relationship between the two, we sought to measure cognition and oculomotor functioning simultaneously, through a modification of a commonly used neuropsychological test, the Symbol Digit Modalities Test (SDMT). Measurement includes monitoring of eye movements in two dimensions with the aid of infrared tracking. This paper presents preliminary data and an overview of analytical methods to be performed in the future on patients with neurological disorders. Correlations between eye movements and the mSDMT were verified using signal characterization. Furthermore, saccadic velocity was shown to remain relatively constant in healthy controls.

Vergence eye movements are critical in daily routines. These movements navigate three-dimensional space. Convergence and divergence eye movements are used to create and maintain stereoscopic vision. This study investigates differences in vergence eye movements between the left and right eye movement. Eye movements were collected using a costume haploscope design paired with our stimulus-presentation software. The experimental design consists of four stimulus conditions from which convergence and divergence measures are derived. A multi-factorial ANOVA and correlation analyses are used to test for differences in the vergence measures. This study has the potential to highlight the asymmetry between left and right eye vergence movements in individuals while studying multiple stimulus conditions.

Disparity vergence eye movements occasionally exhibit two high-velocity components to a single step stimulus (Alvarez, T. L., Semmlow, J. L. & Yuan, W. (1998). Journal of Neurophysiology, 79, 37–44). This research investigates the neural strategy used to trigger the second component of double high-velocity vergence eye movements. Vergence doubles evoked by an experimental protocol that induces post-movement visual error were compared to doubles that occur normally. The second component of a visually evoked response double occurred later, and with slower dynamics, than that of a naturally occurring double. These differences in timing and dynamics indicate that natural double responses are mediated, at least in part, by a mechanism other than visual feedback. The faster dynamics and timing of natural doubles suggest that an internal monitoring process triggers these movements.

Classically, the primary tool for quantifying the dynamics of vergence and other eye movements has been the main sequence. The main sequence is a plot of peak velocity versus response amplitude and is particularly useful for comparing the dynamics of a large number of eye movements over a range of response amplitudes. However, the main sequence represents only the equivalent first-order behavior of a response and does not describe its dynamics in detail. Since the main sequence is based on only two points on the dynamic trajectory, it is sensitive to measurement artifacts and noise. A new methodology is presented which quantifies the equivalent second-order dynamics of eye movements using a larger region of the transient response. These new indexes were applied to vergence eye movements and were found to differentiate between subtle, but important differences in movement dynamics.

Dynamics of disparity vergence eye movements can be modified by adaptive stimuli that generate large transient disparities. These modifications were observed for convergence as well as divergence eye movements. After modification, the peak velocities of the step responses for convergence and divergence were substantially higher than in normal baseline responses, a change observed in all four subjects studied. The change in peak velocity of a step response occurred very rapidly after presentation of the adaptive stimuli. Main sequence plots showed that first-order dynamic characteristics increased for post-adaptive responses with respect to normal step responses. Hence, response modification could be quantified as a change in gain accompanied with an increase in the effective response time constant. The adaptive responses to convergent and divergent ‘disappearing’ step stimuli revealed that the adaptation process modifies the high-velocity component of both disparity convergence and divergence eye movements. Moreover, a gain change in this component alone could account for both the gain and the time constant modifications seen in the overall response. A process of recovery or de-adaptation was also observed for both convergence and divergence eye movements. This observed short-term modification demonstrates a unique control mechanism for vergence eye movements that is effective in either direction.

When a visual stimulus changes direction and distance simultaneously, Hering's Law argues that the resulting eye movements are the result of combined version and vergence control processes. Recently, it has been suggested that slow asymmetrical eye movements might be guided by monocular control processes wherein each eye is driven by its own retinal image. Experimental results presented here show behavioral differences between slow version and slow vergence eye movements, indicating that different control processes drive the two “pure” responses. Specifically, version tracking of constant velocity stimuli (i.e., smooth pursuit) is more precise, showing less variation in tracking velocity than movements of equal velocity produced by vergence stimuli. When the two stimuli are combined, the variability in tracking is consistent with the addition of the two components in proportion to their respective stimuli. These results provide support for Hering's Law, at least for low velocity, smooth tracking movements (i.e., slow version and slow vergence)

Saliency of the stimuli presumably plays a critical role in vision rehabilitation. This study sought to determine whether a peripheral or near central target influenced the peak velocity of convergence which may have an impact on visual saliency. Blue line stimuli, one without any distracting peripheral background and two with distracting background were given to four subjects within a haploscope setting. Eye movements were recorded with an infrared video camera system. Data were quantified using latency, amplitude, and peak velocity. Although a consistent decrease in peak velocity was observed statistical differences were not (p=0.13). The results suggest distraction does not significantly influence convergence step peak velocity. Future experiments include the study of other visual distracters.

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.

This investigation sought to systematic study the parahippocampal area using three types of vergence eye movement tasks that utilized different demands on visual memory. The percent volume change of the functional activity in the posterior parahippocampal area (PPHA) was quantified for three different experimental paradigms that stimulated vergence eye movements: random tracking versus fixation; predictable tracking versus random tracking; and memory-guided task. A general linear model (GLM) was used to determine functional activity for each of the three vergence experiments. The average signal percent change for both the PPHA and frontal eye fields (FEF) was compared for each vergence task. Significant percent change in the average signal from the voxels in the PPHA was observed for all experiments (p 0.06). Lastly, percent volume change was also quantified for the PPHA among experiments. Percent volume change in the PPHA was greater for the memory-guided vergence task and the change was significant across all the experiments, (p

The study of saccades (side to side eye movements) with various types of visual fields or distracters has lead to a deeper understanding of the mechanisms of attention. Yet, virtually no research exists on how the influence of attention modulates the neural control of disparity vergence, the inward or outward turning of the eyes to view targets in depth. Within our activities of daily living, we use a combination of eye movements including vergence and saccades to acquire visual information. This investigation describes instrumentation to systematically study the influence of attention on disparity vergence. Disparity vergence can be modeled as a preprogrammed coupled with a feedback component describing the system's speed and accuracy respectively. Hence, two types of central stimuli were generated: a step stimulus which predominantly evokes the preprogrammed component during the transient portion of the movement and a ramp stimulus to study changes within the feedback component. To stimulate only disparity while keeping accommodation constant, the system was designed using a haploscope configuration. Four types of periphery stimuli were generated that vary the amount and type of distracters. The influence of attention, or the effect of the peripheral stimulation, will be assessed by quantifying peak velocities and error defined as the difference between the stimulus and vergence eye movement response. Preliminary results show that central and peripheral visual stimuli can be programmed independently and this system can be utilized to systematically study the influence of different types of distracters on disparity vergence responses. Future directions include further data collection and analysis in healthy control and those from neurological dysfunctional populations.

Throughout the day, the human visual system acquires information using saccade and vergence eye movements. Previously, functional MRI (fMRI) experiments have shown both shared neural resources and spatial differentiation between these two systems. FMRI experiments can reveal which regions are activated within an experimental task but do not yield insight into how regions of interest (ROIs) interact with each other. This study investigated the number and direction of influences among ROIs using a Granger Causality Analysis (GCA) — a statistical technique used to identify if an ROI is significantly influencing or ‘connected’ to another ROI. Two stimulus protocols were used: first, a simple block design of fixation versus random eye movements; and second, a more cognitively demanding task using random versus predictable movements. Each protocol used saccadic movements and was then repeated using vergence movements. Eight subjects participated in each of the four experiments. Results show that when prediction was evoked, more connections between ROIs were observed compared to the simple tracking experiment. More connections were also observed during the vergence prediction task compared to the saccade prediction task. Differences within the number of connections may be due to the type of oculomotor eye movements, as well as to the amount of higher-level executive cognitive demand.

A pure vergence stimulus requires the two eyes to turn equally inward or outward theoretically resulting in a pure symmetrical vergence response. However, saccades, a rapid conjugate eye movement, are frequently observed in vergence responses. This investigation sought to systematically quantify whether the occurrence of saccades within symmetrical vergence responses is correlated to vergence peak velocity. Eye movements are quantified using a limbus tracking system from three subjects. Symmetrical convergence and divergence 4° step responses with an initial position located at far or near which are known to evoke different peak velocities are analyzed. Data are quantified via peak velocity. A saccade detecting algorithm is utilized to quantify the frequency of saccades in the transient portion (first second) of vergence responses. Near convergence responses are slower than far convergence and far divergence responses are slower compared to near divergence movements. The occurrence of saccades is negatively correlated to vergence peak velocity. When the velocity is slower, the number of saccades increases. This study suggests that the brain may initiate a saccade to facilitate a slow vergence movement, potentially to allow object recognition before binocular fusion.

In a natural environment, saccade and vergence eye movements shift gaze in different directions and distances. In a laboratory setting, targets can be positioned precisely to elicit symmetrical vergence movements; however, saccades occur during the vergence movement even though the stimulus should not stimulate a saccadic response. These saccades may facilitate the response when the kinematics of the vergence component are modest as indicated by reduced velocities. Hence, the purpose of this study is to assess whether the frequency of saccades within vergence responses are correlated with vergence peak velocity. Ten subjects with normal binocular vision participated in this study. Eye movements were quantified using a limbus tracking system. Stimuli included 4° symmetrical convergence and divergence steps with an initial vergence angle at far (2° and 6°, respectively) and near (12° and 16°, respectively) which are known to evoke different vergence peak velocities. A saccade detecting algorithm was utilized to compute the percentage of saccades present within all vergence responses. A repeated measures ANOVA confirmed with a post hoc Bonferroni test demonstrated that convergence steps at near were slower than convergence steps at far, whereas divergence steps at far were slower than divergence steps at near in all subjects (p

Purpose: Cortical and subcortical functional activity stimulatedviasaccade and vergence eye movements were investigated to examine the similarities and differences between networks and regions of interest (ROIs). Methods: Blood oxygenation level-dependent (BOLD) signals from stimulus-induced functional Magnetic Resonance Imaging (MRI) experiments were analyzed studying 16 healthy subjects. Six types of oculomotor experiments were conducted using a block design to study both saccade and vergence circuits. The experiments included a simple eye movement task and a more cognitively demanding prediction task. A hierarchical independent component analysis (ICA) process began by analyzing individual subject data sets with spatial ICA to extract spatial independent components (sIC), which resulted in three ROIs. Using the time series from each of the three ROIs per subject, per oculomotor experiment, a temporal ICA was used to compute individual temporal independent components (tICs). For each of the three ROIs, the individual tICs from multiple subjects were entered into a second temporal ICA to compute group-level tICs for comparison. Results: Two independent spatial maps were observed for each subject (one sIC showing activity in the frontoparietal regions and another sIC in the cerebellum) during the six oculomotor tasks. Analysis of group-level tICs revealed an increased latency in the cerebellar region when compared to the frontoparietal region. Conclusion: Shared neuronal behavior has been reported in the frontal and parietal lobes, which may in part explain the segregation of frontoparietal functional activity into one sIC. The cerebellum uses multiple time scales for motor learning. This may result in an increased latency observed in the BOLD signal of the cerebellar group-level tIC when compared to the frontal and parietal group-level tICs. The increased latency offers a possible explanation to why ICA dissects the cerebellar activity into an sIC. The hierarchical ICA process used to calculate group-level tICs can yield insight into functional connectivity within complex neural networks.

The ability to adapt is critical to survival and varies between individuals. Adaptation of one motor system may be related to the ability to adapt another. This study sought to determine whether phoria adaptation was correlated with the ability to modify the dynamics of disparity vergence. Eye movements from ten subjects were recorded during dynamic disparity vergence modification and phoria adaptation experiments. Two different convergent stimuli were presented during the dynamic vergence modification experiment: a test stimulus (4° step) and a conditioning stimulus (4° double step). Dynamic disparity vergence responses were quantified by measuring the peak velocity (°/s). Phoria adaptation experiments measured the changes in phoria over a 5-min period of sustained fixation. The maximum velocity of phoria adaptation was determined from an exponential fit of the phoria data points. Phoria and dynamic disparity vergence peak velocity were both significantly modified (P 0.001). The maximum velocity of phoria adaptation was significantly correlated with the changes in convergence peak velocity (r 0.89; P 0.001). There was a strong correlation between the ability to adaptively adjust two different oculomotor parameters: a tonic and dynamic component. Future studies should investigate additional interactions between these parameters, and the ability to adaptively change other oculomotor systems such as the saccadic or smooth pursuit system. Understanding the ability to modify phoria, dynamic disparity vergence, and other oculomotor parameters can yield insights into the plasticity of short-term adaptation mechanisms.

Visual fatigue is defined as weariness or discomfort localized in either the head or the eye after a visually demanding task. The etiology of visual fatigue is insufficiently understood in vision research. This study investigates the correlation between changes of near-dissociated phoria versus vergence dynamics defined as the ratio of convergence average peak velocity to divergence average peak velocity. Four degree convergence and divergence eye movements as well as phoria levels were recorded using an infrared eye movement monitor from five subjects. Preliminary data show a high correlation between the changes in phoria versus the change in vergence dynamics. Results suggest that the amount of changes in the two parameters, phoria and vergence dynamics, are one of many factors which may infer why some people are more prone to visual fatigue than others.

Vergence eye movements are the inward and outward turning of the eyes and contemporary views believe these movements are mediated by a preprogramming and feedback controlled mechanism. The individual left and right eye responses have been reported to have large dynamic asymmetries between the individual eye movements. One hypothesis to explain the observed asymmetry is the preprogrammed transient component is monocularly controlled. A visual stimulus was created to stimulate a single step change to one eye and a double step change to the other eye using a haploscope. Results show that in some responses a double high-velocity component was observed in one response with a single high-velocity response in the other eye. This behavior suggests the preprogrammed component within a vergence response may be monocularly controlled.

This study's aim is to ascertain whether the amount of saccades during convergence has any relation to a person's preference of progressive additive lenses (PALs). Since patients who are better adapters to PALs have faster vergence dynamics, we presume those who do not adapt well must have an alternative to compensate for their slower vergence system. The goal of this experiment is to compare the eye movements of those who adapt and those who do not adapt when performing a double-step stimulus experiment. Of the presbyopes studied, eight subjects wear PALs daily, while the other seven could not adapt to PALs. Results concluded that presbyopes who could not adapt to PALs have an increased amount of saccades when performing the convergence experiment as compared to that of those who could adapt.

This study investigated the underlying components of vergence responses in binocularly healthy individuals and subjects with convergence insufficiency (CI). Convergent and divergent eye movements were recorded from seven subjects using step stimuli. In addition, four CI subjects participated in six weeks of vision rehabilitation. The goal of this study was to use an independent component analysis (ICA) algorithm to dissect the vergence responses into two underlying components (transient and sustained) for comparison. Preliminary data suggest the healthy controls have greater peak velocity and the magnitude of the transient component was larger compared to patients with CI. After training, the peak velocity and magnitude of the transient component increased for the CI patients and these behaviors were maintained after the training. Data suggest that training modifies the cortical signal responsible for generating the transient component signal and these changes are sustained post rehabilitation.

Fatigue is also known as weariness. However, fatigue is insufficiently understood in vision research. This study investigates saccades during the transient portion of symmetrical vergence responses when subjects report fatigue and non-fatigue. Four degree convergence and divergence eye movements were recorded from five subjects using an infrared eye movement monitor. Preliminary data show that there is an increase in the number of saccades during the transient portion of vergence responses when subjects are fatigued. Results suggest that saccades may be a compensatory mechanism used by the vergence system when humans are fatigued.

The objective of this study is to determine whether the magnitude of a subject's peak velocity is related to a subject's ability to adapt to progressive lenses. Our presumption is that patients fully adapted to progressive additive lenses (PALs) are better motor learners, allowing them to modify their vergence dynamics more readily than patients who could not adapt to PALs. The experiment aims to compare the eye movement dynamic behavior from a 4° double step stimulus which is a 4° step followed by a subsequent 4° step after a 200 msec delay. Eight presbyopes who wear PALs daily and seven presbyopes who could not adapt to PALs participated. The results show that presbyopes adapted to PALs have a significantly greater dynamic ratio of the first to second peak velocity from the two high velocity components stimulated from the double step. This behavior may in part be due to better motor learning abilities.

The visual system is one of the most complex mechanisms for processing information obtained from the external world. Ocular movements can be categorized as fast or slow. Saccadic eye movements are conjugate and fast compared to vergence which are disjunctive and slow. Saccadic eye movements quickly shift the fovea to the object of interest. Vergence movements are disconjugate which provide depth perception. This investigation will compare the saccadic and vergence functional activity to investigate differences between the neural circuitry. There are two primary findings from this study. First, the vergence-related FEF region is more anterior compared to the saccade-related FEF area. Furthermore, there is additional activation within the parietal lobe for vergence.

Processing visual signals from our external world and executing actions based upon that afferent information require contributions of not only cortical regions but also subcortical areas. The brainstem is an essential subcortical region in which neurons related to the transmission of ocular movements reside. Saccadic and vergence eye movements, used throughout our day to acquire visual information, were studied using eight control subjects who participated in four oculomotor experiments with fMRI. There are two main discoveries from this investigation. First, the activity with the brainstem for conveying signals to the cranial nerves can be observed via fMRI. Second, the pontine sites of the brainstem in humans contain not only saccade related signals but also vergence activity.

Purely symmetrical vergence stimuli aligned along the midline (cyclopean axis) require only a pure vergence response. Yet, in most responses saccades are observed and these saccades must either produce an error in the desired midline response or correct an error produced by asymmetry in the vergence response. A previous study (Semmlow, et al. 2008) has shown that the first saccade to appear in a response to a pure vergence stimulus usually increased the deviation from the midline, although all subjects (N = 12) had some responses where the initial saccade corrected a vergence induced midline error. This study focuses on those responses where the initial saccade produces an increased midline deviation and the resultant compensation that ultimately brings the eyes to the correct binocular position. This correction is accomplished by a higher level compensatory mechanism that uses offsetting asymmetrical vergence and/or corrective saccades. While responses consist of a mixture of the two compensatory mechanisms, the dominant mechanism is subject-dependent. Since fixation errors are quite small (minutes of arc), some feedback controlled physiological process involving smooth eye movements, and possibly saccades, must move the eyes to reduce binocular error to fixation disparity levels.

Purpose: This study sought to determine the effect of very brief, single and multiple pulses of light on spatial and temporal aspects of saccadic eye movements. Methods: Twelve visually normal, young adult subjects participated in the experiments. Horizontal eye position was monitored as subjects attempted to track target step displacements in the presence of either single or multiple brief flashes of light in the visual field. Results: Three primary findings were observed: (1) increased saccadic latency, (2) increased time for target acquisition and (3) increased initial saccadic error. Conclusion: The present findings suggest the influence of attentional processes and/or visual masking effects on saccadic eye movement control.

This study sought to determine whether significant changes would be observed between vergence eye movements before and after 12 hr of repetitive vergence therapy (1 hr per day on different days) in subjects with normal binocular vision compared to controls. Disparity vergence responses from 23 subjects were studied. An assessment protocol that minimized the influence of the near dissociated phoria on the disparity vergence system was designed. The following parameters were quantified for the responses: latency, time to peak velocity, settling time, peak velocity, and accuracy (difference between the response and stimulus amplitudes). The following outcomes were observed when comparing the results after vergence therapy to the baseline measurements: (a) near point of convergence and near dissociated phoria did not significantly change (p > 0.15); (b) latency, time to peak velocity, and settling time significantly decreased (p ≤ 0.01); and (c) accuracy significantly improved (p < 0.01). Results support that vergence peak velocity is dependent on the subject's near dissociated phoria. The accuracy and temporal properties of vergence eye movement responses from subjects with normal binocular vision can be improved after vergence therapy. These methods can be utilized within future studies to quantitatively assess vergence therapy techniques for patients with binocular dysfunction.

PURPOSE: This study sought to quantify divergence eye movements and differences between divergence and convergence to smoothly moving ramp, step, and disappearing step stimuli. METHODS: Eight visually normal, adult subjects participated in three experiments investigating the dynamics of responses using an infrared limbal eye tracker. RESULTS: There were four primary findings: (1) a smooth tracking behavior was observed for slow ramps while the fast ramps elicited smooth tracking combined with a high-velocity, step-like behavior; (2) the high-velocity components observed in the faster ramps had a similar main sequence as divergence steps; (3) divergence dynamics to disappearing steps starting at the subject's near dissociated phoria level were similar to corresponding step responses; and (4) the high-velocity components from divergence ramps were dependent on initial vergence position, whereas the high-velocity components from convergence ramps were not. CONCLUSION: The results suggest a preprogrammed component is present in divergence similar to convergence; however, unlike convergence, the high-velocity components from divergence ramp responses are dependent on initial vergence position.

If two targets are carefully aligned, the required eye movement will be symmetrical with the two eyes turning equally inward or outward: a pure vergence movement. Yet almost all responses contain saccades, a rapid tandem movement of the eyes. When saccades occur, they must either be producing an error in the desired symmetrical response or correcting an error produced by an asymmetrical vergence response. Results show that in most subjects, error-producing saccades predominate. The symmetry error produced by these saccades is then corrected by either a compensatory asymmetrical vergence, offsetting saccades (including saccadic asymmetry), or a mixture of the two. The compensatory mechanism is subject-dependent with individual subjects favoring one of the three strategies in the majority of their responses. The factors that lead to symmetry errors and subsequent compensatory mechanisms are described, quantified, and discussed.

If two targets are carefully aligned so that they fall along the cyclopean axis, the required eye movement will be symmetrical with the two eyes turning equally inward or outward. When such “pure vergence stimuli” are used only a “pure vergence movement” is required, yet almost all responses include saccadic eye movements, a rapid tandem movement of the eyes. When saccades occur, they must either produce an error in the desired symmetrical response or correct an error from an asymmetrical vergence response. A series of eye movement responses to pure convergence stimuli (4.0 deg step stimuli) were measured in 12 subjects and the occurrence, timing and amplitude of saccades was measured. Early saccades (within 400 msec of the stimulus onset) appeared in 80% to 100% of the responses. In most subjects, the first saccade increased the asymmetry of the response, taking the eyes away from the midline position. In three subjects, these asymmetry-inducing saccades brought one eye, the preferred or dominant eye, close to the target, but in the other subjects these asymmetry-inducing saccades were probably due to the distraction caused by the transient diplopic image generated by a pure vergence stimulus. While many of these asymmetry-inducing saccades showed saccade-like enhancements of vergence, they were, with the exception of two subjects, primarily divergent and did not facilitate the ongoing convergence movement. All subjects had some responses where the first saccade improved response symmetry, correcting an asymmetry brought about by unequal vergence movements in the two eyes. In five subjects, large symmetry-inducing saccades corrected an asymmetrical vergence response, bringing the eyes back to the midline (to within a few tenths of a degree).

Vergence eye movements are the inward (convergence) or outward (divergence) turning of the eyes which allows humans to view images in depth. This study compares converging responses where the stimuli are approaching the subject to diverging responses where the stimuli are moving away from the subject from 10, 6, and 2 degree/s ramp stimuli. Results show that similar to convergence, divergence may be governed by a two component control system consisting of a transient and sustained component. However, the dynamics of divergence shows a dependency on initial stimulus position which is not apparent for convergent responses. This study also shows that for fast drawing divergent responses, high-velocity components were present lending further evidence that a transient component similar in form and function to that found in convergence also exists for divergence. Responses to slow moving responses exhibit smoother tracking where the dynamics of the movement were more uniform throughout the range of the visual field. This implies that the transient component is more prominent in faster responses thus showing a strong dependency on initial stimulus position. With slower moving responses, the transient component did not dominant the movement leading to more homogeneous dynamic responses where a dependency on initial stimulus position was not as apparent.

Presbyopia, a degenerative condition, which decreases accommodation, sets in approximately at the age of forty. One approach to correct presbyopia is the use of bifocal or progressive lenses. Naturally, some people are more prone to adapt to wearing progressive lenses than others. The vergence system, which controls the inward and outward turning of the eyes, is a system which supports viewing objects in depth. Identifying the two components (transient and sustained components) of a pure vergence eye movement is possible with ICA (independent component analysis). Preliminary results suggest a correlation with the magnitude of the transient component and whether or not a person can adapt to wearing progressive lenses or not. Furthermore, the transient component of vergence is hypothesized to be an index of how flexible the vergence system is in adapting to new environments.

The brain has a dynamic ability to change or adapt which is imperative for survival of a species. Research has shown that the dynamics of disparity vergence eye movements, the inward (convergence) or outward (divergence) turning of the eyes, are malleable and depend to some extent on the amplitude of preceding stimuli. Disparity convergence is composed of two components. The transient component is open loop and accounts for the system's speed; whereas the sustained component is assumed to be feedback controlled allowing the system to be very accurate. The purpose of this study was to investigate if the modification of convergence eye movements was a function of the magnitude of the subject's transient component. An experimental session consisted of three phases: baseline, modification, and recovery. The baseline and recovery phases used only 4 degrees step test stimuli. The modification phase consisted of a 4 degrees test randomly intermixed with a larger conditioning double step or step ramp stimulus presented in a 1:5 ratio. Eight subjects participated. Independent component analysis was used to decompose the vergence responses into the transient and sustained components. Results show the magnitude of the transient component is an indicator for the amount of dynamic change observed during the modification phase, R = 0.88.

Presbyopia occurs from the aging process where accommodation decreases due to ciliary muscle weakness and loss of elasticity in the crystalline lens. People begin to have symptoms such as blurred vision, approximately at 40 years of age. To facilitate vision, presbyopes will wear multiple glasses, bifocals or progressive lenses. Progressive lenses can be difficult to adapt or learn how to wear. To date, patients are given little to no instruction on how to wear progressive lenses. The goal of this research is to predict who will have difficulties wearing the lenses and who will not. Other biomedical studies have shown that entropy which quantifies the energy content in physiological time-series data can be an indicator for how a person will adapt to changes or stress within the environment. Four degree convergence eye movement data (the inward turning of the eyes) from 10 people (4 Non-Presbyopes, 3 Presbyopes who have tried to wear the lenses but could not adjust, and 3 Presbyope who wear the lenses daily) were collected over 3 seconds and analyzed using sample entropy. Entropy values were obtained for each second of the response (0 tol, 1 to 2. and 2 to 3) and for the entire response (0 to 3 sec). Results were compared using a T-test and categorized using a clustering algorithm. The results showed that Sample Entropy classified the presbyopes who could adapt to the lenses compared to those who could not with 83% accuracy where the difference in entropy resulted in a P value of 0.1059.

Vergence eye movements are the inward (convergence) or outward (divergence) turning of the eyes which allows humans to view images in depth. Divergence is dependent on initial stimulus conditions whereas convergence is not. This study will investigate if the forces of the extraocular muscles show a causal relationship to divergence dynamics. The neural command to the brain is a four degree stimulus; however responses were measured when the extraocular muscles experienced different forces. The results show the extraocular muscles specifically the lateral and medical recti muscles have an influence on the dynamics of divergence eye movements.