Tara L. Alvarez

Vision therapy in adults with convergence insufficiency: clinical and functional magnetic resonance imaging measures.

Purpose. This research quantified clinical measurements and functional neural changes associated with vision therapy in subjects with convergence insufficiency (CI). Methods. Convergence and divergence 4° step responses were compared between 13 control adult subjects with normal binocular vision and four CI adult subjects. All CI subjects participated in 18 h of vision therapy. Clinical parameters quantified throughout the therapy included: nearpoint of convergence, recovery point of convergence, positive fusional vergence at near, near dissociated phoria, and eye movements that were quantified using peak velocity. Neural correlates of the CI subjects were quantified with functional magnetic resonance imaging scans comparing random vs. predictable vergence movements using a block design before and after vision therapy. Images were quantified by measuring the spatial extent of activation and the average correlation within five regions of interests (ROI). The ROIs were the dorsolateral prefrontal cortex, a portion of the frontal lobe, part of the parietal lobe, the cerebellum, and the brain stem. All measurements were repeated 4 months to 1 year post-therapy in three of the CI subjects. Results. Convergence average peak velocities to step stimuli were significantly slower (p = 0.016) in CI subjects compared with controls; however, significant differences in average peak velocities were not observed for divergence step responses (p = 0.30). The investigation of CI subjects participating in vision therapy showed that the nearpoint of convergence, recovery point of convergence, and near dissociated phoria significantly decreased. Furthermore, the positive fusional vergence, average peak velocity from 4° convergence steps, and the amount of functional activity within the frontal areas, cerebellum, and brain stem significantly increased. Several clinical and cortical parameters were significantly correlated. Conclusions. Convergence peak velocity was significantly slower in CI subjects compared with controls, which may result in asthenopic complaints reported by the CI subjects. Vision therapy was associated with and may have evoked clinical and cortical activity changes.

Divergence eye movements are dependent on initial stimulus position

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 degrees 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.

Short-term predictive changes in the dynamics of disparity vergence eye movements.

Repetitive stimulation of the disparity vergence system to large convergent step stimuli has been shown to increase the dynamics of subsequent responses to smaller step stimuli. Here we show that decreases in the dynamics of both disparity convergence and divergence eye movements can be induced using a frequently occurring small amplitude conditioning stimulus to modify responses to a larger, occasionally presented test stimulus. In one experiment, a simple conditioning stimulus consisting of repetitive 1 degrees step stimuli was used to modify the dynamic vergence response to an occasional 4 degrees step test stimulus. An experimental trial consisted of three phases: baseline, conditioning, and recovery. The baseline and recovery phases used only the 4 degrees test stimuli. The dynamic characteristics of the responses to test stimuli were quantified by measuring the magnitude of the peak velocity. A statistically significant change was observed between the dynamics of conditioned responses compared to baseline and recovery responses indicting modification by the conditioning stimuli. During recovery, the response dynamics returned to levels near baseline levels showing that the decrease in response dynamics was caused by the conditioning stimulus, not fatigue. Another experiment showed that the response dynamics to large stimuli could be decreased whereas the dynamics of small stimuli could be increased by the same intermediate conditioning stimulus. Other experiments suggest that the modifications are due to a predictive mechanism. The results indicate that the dynamics of disparity vergence eye movements are malleable and depend to some extent on the amplitude of preceding stimuli.

Concurrent vision dysfunctions in convergence insufficiency with traumatic brain injury.

Purpose This study assessed the prevalence of convergence insufficiency (CI) with and without simultaneous vision dysfunctions within the traumatic brain injury (TBI) sample population because although CI is commonly reported with TBI, the prevalence of concurrent visual dysfunctions with CI in TBI is unknown. Methods A retrospective analysis of 557 medical records from TBI civilian patients was conducted. Patients were all evaluated by a single optometrist. Visual acuity, oculomotor function, binocular vision function, accommodation, visual fields, ocular health, and vestibular function were assessed. Statistical comparisons between the CI and non-CI, as well as inpatient and outpatient subgroups, were conducted using &khgr;2 and Z tests. Results Approximately 9% of the TBI sample had CI without the following simultaneous diagnoses: saccade or pursuit dysfunction; third, fourth, or sixth cranial nerve palsy; visual field deficit; visual spatial inattention/neglect; vestibular dysfunction; or nystagmus. Photophobia with CI was observed in 16.3% (21 of 130), and vestibular dysfunction with CI was observed in 18.5% (24 of 130) of the CI subgroup. Convergence insufficiency and cranial nerve palsies were common and yielded prevalence rates of 23.3% (130 of 557) and 26.9% (150 of 557), respectively, within the TBI sample. Accommodative dysfunction was common within the nonpresbyopic TBI sample, with a prevalence of 24.4% (76 of 314). Visual field deficits or unilateral visual spatial inattention/neglect was observed within 29.6% (80 of 270) of the TBI inpatient subgroup and was significantly more prevalent compared with that of the outpatient subgroup (p < 0.001). Most TBI patients had visual acuities of 20/60 or better in the TBI sample (85%; 473 of 557). Conclusions Convergence insufficiency without simultaneous visual or vestibular dysfunctions was observed in about 9% of the visually symptomatic TBI civilian population studied. A thorough visual and vestibular examination is recommended for all TBI patients.

Functional anatomy of predictive vergence and saccade eye movements in humans: A functional MRI investigation

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 3T 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 < 0.001) active regions of interest through a combination of correlation threshold and cluster extent. A paired t-test of the GLM beta weight coefficients was computed to determine significant spatial differences between the saccade and vergence data sets. RESULTS Predictive saccadic and vergent eye movements induced many common sites of significant functional cortical activity including: the dorsolateral prefrontal cortex (DLPFC), parietal eye field (PEF), cuneus, precuneus, anterior and posterior cingulate, and the cerebellum. However, differentiation in spatial location was observed within the frontal lobe for the functional activity of the saccadic and vergent network induced while studying prediction. A paired t-test of the beta weights from the individual subjects showed that peak activity induced by predictive versus random vergent eye movements was significantly (t > 2.7, p < 0.03) more anterior within the frontal eye field (FEF) and the supplementary eye field (SEF) when compared to the functional activity from predictive saccadic eye movements. CONCLUSION This research furthers our knowledge of which cortical sites facilitate a subjects ability to predict within the vergence and saccade networks. Using a predictive versus random visual task, saccadic and vergent eye movements induced activation in many shared cortical sites and also stimulated differentiation in the FEF and SEF.

Quantification of heterophoria and phoria adaptation using an automated objective system compared to clinical methods.

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.

Sustained convergence induced changes in phoria and divergence dynamics.

PURPOSE The purpose of this study was to investigate how sustained convergence induces phoria adaptation and changes divergence dynamics. METHODS 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. RESULTS 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 ). CONCLUSION 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.

Dynamics of the disparity vergence step response: a model-based analysis

A new method to analyze the dynamics of vergence eye movements was developed based on a reconstruction of the presumed motor command signal. A model was used to construct equivalent motor command signals and transform an associated vergence transient response into an equivalent set of motor commands. This model represented only the motor components of the vergence system and consisted of signal generators representing the neural burst and tonic cells and a plant representing the ocular musculature and dynamics of the orbit. Through highly accurate simulations, dynamic vergence responses could be reduced to a set of five model parameters, each relating to a specific feature of the internal motor command. This dynamic analysis tool was applied to the analysis of inter-movement variability in vergence step responses. Model parameters obtained from a large number of response simulations showed that the width of the command pulse was tightly controlled while its amplitude, rising slope, and falling slope were less tightly regulated. Variation in the latter three parameters accounted for most of the movement-to-movement variability seen in vergence step responses. Unlike version movements, pulse width did not increase with increased stimulus amplitude, although the other command signal parameters were substantially influenced by stimulus amplitude.

The relationship between phoria and the ratio of convergence peak velocity to divergence peak velocity.

PURPOSE The purpose of this study was to investigate the relationship between phoria and the dynamics of vergence eye movements as described by the ratio of convergence average peak velocity to divergence average peak velocity, termed the vergence peak velocity ratio. METHODS Phoria and vergence step responses were recorded. Three measures of phoria were assessed: baseline phoria, which is the initial phoria measurement; adapted phoria, which is the phoria measured after a visual task; and change in phoria, which is defined as adapted phoria minus baseline phoria. Phoria was modified in two experiments: vergence steps located at different initial positions and different sustained convergent fixations. Four linear regression analyses were conducted to study the interactions among baseline phoria and vergence peak velocity ratio, adapted phoria and vergence peak velocity ratio, baseline and adapted phoria, and baseline phoria and change in phoria. RESULTS Baseline and adapted phoria were significantly correlated to vergence peak velocity ratio. Adapted phoria and baseline phoria were also significantly correlated. The change in phoria induced by the vergence steps or a sustained fixation task was independent of the baseline phoria. CONCLUSIONS These data support that phoria is a factor in the asymmetry between peak velocity of convergence and divergence and that baseline phoria level is not a factor in the amount of change observed in phoria level (adapted phoria minus baseline phoria). Future oculomotor models of vergence should incorporate phoria within the design.

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

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