Short term modification of vergence ramp eye movements in the convergent direction

Prior oculomotor studies have investigated the various effects of short-term modification on vergence, saccadic and smooth pursuit eye movements. Previous vergence studies have concentrated on step modification stimuli. Few have investigated the effects of short-term modification on vergence ramp movements. Thus, this study explores the trends observed within a short-term modification experiment studying smoothly tracking vergence eye movements responses elicited from convergent ramp stimuli. A short-term modification experiment is composed of three phases: baseline, modification and recovery. Baseline and recovery phases contain only test stimuli; whereas, during modification, the subject is presented test and conditioning stimuli in a ratio of 1:5 test to conditioning. The test stimulus is a 0.5 deg/sec vergence ramp presented from a 3 deg vergence angle to a 5 deg vergence angle. The conditioning stimulus is a 2 deg/sec ramp presented over the same visual range. The root mean square error (RMSE) is calculated on all slower (0.5 deg/ sec) ramp responses and compared over the three phases. A significant statistical change is observed between the three stages on day one, but not on day two. A trend that can be attributed to motor memory. This study additionally explores for potential differences between the left and right eye movements. No statistical significant difference of the RMSE is observed between the left and right eye movements. Data supports that the preprogrammed portion of vergence is significantly influenced by the short-term modification experiment described here

Dynamics of the disparity vergence fusion sustain component

The stereotypical vergence response to a step stimulus consists of two dynamic components: a high velocity fusion initiating component followed by a slower component that may mediate sustained fusion.  The initial component has been well-studied and is thought to be controlled by an open-loop mechanism. Less is known about the slow, or fusion sustaining component except that it must be feedback controlled to achieve the positional precision of sustained fusion.  Given the delays in disparity vergence control, a feedback control system is likely to exhibit oscillatory behavior.  Vergence responses to 4 deg step changes in target position were recorded in eight subjects. The slow component of each response was isolated manually using interactive graphics and the frequency spectrum determined.  The frequency spectra of all isolated slow vergence movements showed a large low frequency peak between 1.0 and 2.0 Hz and one or more higher frequency components.  The higher frequency components were found to be harmonics of the low frequency oscillation.  A feedback model of the slow component was developed consisting of a time delay, an integral/derivative controller and an oculomotor plant based on Robinson’s model.  Model simulations showed that a direction dependent asymmetry in the derivative element was primarily responsible for the higher frequency harmonic components. Simulations also showed that the base frequencies are primarily dependent on the time delay in the feedback control system. The fact that oscillatory behavior was found in all subjects provides strong support that the slow, fusion sustaining component is mediated by a feedback system

Vergence fusion sustaining oscillations

Introduction:  Previous studies have shown that the slow, or fusion sustaining, component of disparity vergence contains oscillatory behavior.  Given the delays in disparity vergence control, a feedback control system would be expected to exhibit oscillations following the initial transient period.  This study extends the examination of this behavior to a wider range of frequencies and a larger number of subjects.  Methods:  Disparity vergence responses to symmetrical 4.0 deg step changes in target position were recorded in 15 subjects. Approximately three seconds of the late component of each response were isolated using interactive graphics and the frequency spectrum calculated.  Peaks in these spectra associated with oscillatory behavior were identified and examined.  Results: All subjects exhibited oscillatory behavior with primary frequencies ranging between 0.45 and 0.6 Hz; much lower than those identified in the earlier study.  All responses showed significant higher frequency components.  These higher frequency components were related in both frequency and amplitude with the primary frequency indicating that they are harmonics probably generated by nonlinearities in the neural control processes. A correlation was found across subjects between the amplitude of the primary frequency and the maximum velocity of the fusion initialing component probably due the gain of shared neural pathways. Conclusion:  Low frequency oscillatory behavior was found in all subjects adding support that the slow, or fusion sustaining, component is mediated by a feedback control. Data have clinical implications in that dysfunction in feedback control may manifest as additional vergence error which may be reflected in the frequency spectrum

Target eccentricity and form influences disparity vergence eye movements responses: A temporal and dynamic analysis

This study sought to investigate whether stimulation to the fovea or the parafovea with different color combinations influenced the temporal and dynamic features of 4° disparity vergence step responses. Twelve unique types of stimuli were displayed within a haploscope presented along the participant’s midsagittal plane. Vergence eye movement responses from fifteen naive participants were recorded using video-based infrared eye tracking instrumentation. Latency and peak velocity from left and right eye movement responses were quantified. Results show that the type of stimulus projection (foveal versus parafoveal) significantly (p<0.001) influences the vergence response latency but did not impact peak velocity. Vergence responses to eccentric circles with 6° eccentricity targeting the parafovea resulted in a significantly faster response latency compared to vergence responses to a cross with 2° eccentricity stimuli targeting the fovea. Results have implications for the stimulus design of a variety of applications from virtual reality to vision therapy interventions

Changes in the disparity vergence main sequence after treatment of symptomatic convergence insufficiency in children

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

Epidemiology and Incidence of Pediatric Concussions in General Aspects of Life

Background: Concussions are one of the most common head injuries acquired within the pediatric population. While sport-related concussions are well documented, concussions within other aspects of a child’s life are not as well researched. The purpose of this study is to examine the incidence of a large pediatric concussion population in a broad range of daily activities. Methods: Patients’ gender and nature of injury were extracted from 1408 medical records of patients who were diagnosed with a concussion at Saint Peter’s Sports Medicine Institute. Statistical analyses were conducted for activities and environmental settings using chi-squared tests. Results: Concussions were most prevalent in organized sports (53.3%), followed by injuries within the following settings: school (16.5%), recreational (6.7%), motor vehicle collisions (6.6%), home (5.5%), and other (11.3%). Specifically, soccer (12.9%), school physical education (PE) class (10.6%), and football (9.8%) subcategories recorded the most incidences of concussion. For the PE class cohort (n = 149), significantly more females were diagnosed with a concussion compared to males (p < 0.001). Conclusions: PE-related concussions had the second highest incidence rate after organized sports. A significant gender difference was observed in PE class. Awareness about concussions and methods to reduce the risk of concussion is suggested for PE classes