Vincent R. Vicci

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.

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.

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.

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.

Sustained Fixation Induced Changes in Phoria and Convergence Peak Velocity

Purpose This study sought to investigate the influence of phoria adaptation on convergence peak velocity from responses located at different initial vergence positions. Methods Symmetrical 4° convergence step responses and near dissociated phoria (measured at 40 cm from the subjects midline) were recorded from six subjects with normal binocular vision using an infrared limbus tracking system with a haploscope. Two different sustained fixations (1° and 16° convergent rotation along the subjects midline) were used to study whether phoria had an influence on the peak velocity of convergence responses located at two initial vergence positions (1° or ‘far’ steps and 12° or ‘near’ steps). Results Phoria was significantly adapted after a sustained fixation task at near (16°) and far (1°) (p<0.002). A repeated measures ANOVA showed that convergence far steps were significantly faster than the near steps (p<0.03). When comparing convergence steps with the same initial vergence position, steps measured after near phoria adaptation were faster than responses after far adaptation (p<0.02). A regression analysis demonstrated that the change in phoria and the change in convergence peak velocity were significantly correlated for the far convergence steps (r = 0.97, p = 0.001). A weaker correlation was observed for the near convergence steps (r = 0.59, p = 0.20). Conclusion As a result of sustained fixation, phoria was adapted and the peak velocity of the near and far convergence steps was modified. This study has clinical considerations since prisms, which evoke phoria adaptation, can be prescribed to help alleviate visual discomfort. Future investigations should include a systematic study of how prisms may influence convergence and divergence eye movements for those prescribed with prisms within their spectacles.

Neuroplasticity in vision dysfunction

Adaptation is critical to the survival of any species and is present in many systems within the brain. Rehabilitation can evoke neuroplasticity through adaptive mechanisms. Four subjects with the vision dysfunction of convergence insufficiency where two have mild traumatic brain injury and two were congenital participated in 18 hours of vision training. Clinical, behavioral, functional imaging and diffusion tensor imaging (DTI) are measured before and after therapy. Clinical parameters show improvement. Vergence eye-movements show an increase in peak velocity where independent component analysis revealed an increase in the magnitude of the preprogrammed transient component. Functional imaging using an oculomotor learning task shows increased area and intensity of activation suggesting neuronal recruitment and synchronization. DTI shows an increase in fractional anisotropy and an increase in the number of fibers suggesting changes in structural connectivity. Preliminary data suggest that neuroplasticity from vision training results in a change in behavioral oculomotor neural control through an increased magnitude of the preprogrammed disparity transient component potentially caused in part by neuronal recruitment, synchronization and improved structural connectivity.

The correlation between change in near-dissociated phoria and vergence dynamics

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.

Saccade and vergence interaction during fatigued versus non-fatigued sessions

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.

Diffusion tensor imaging of neural plasticity in visual dysfunction

Diffusion tensor imaging (DTI) analysis is performed on a subject that exhibits visual dysfunction as a result of mild traumatic brain injury (TBI). A comparison is made of the subjects DTI data before and after undergoing vision training in order to quantify any differences in the subjects fractional anisotropy (FA) and white matter fiber tracts. A marked increase is seen in the FA values of the designated regions of interest (ROI) as well as in the number of fibers that are present in the ROIs.

Cortical location of FEF revealed using fMRI

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.