VisualEyes2020 (VE2020) is a custom scripting language that presents, records, and synchronizes visual eye movement stimuli. VE2020 provides stimuli for conjugate eye movements (saccades and smooth pursuit), disconjugate eye movements (vergence), accommodation, and combinations of each. Two analysis programs unify the data processing from the eye tracking and accommodation recording systems.
Through the purposeful stimulation and recording of eye movements, the fundamental characteristics of the underlying neural mechanisms of eye movements can be observed. VisualEyes2020 (VE2020) was developed based on the lack of customizable software-based visual stimulation available for researchers that does not rely on motors or actuators within a traditional haploscope. This new instrument and methodology have been developed for a novel haploscope configuration utilizing both eye tracking and autorefractor systems. Analysis software that enables the synchronized analysis of eye movement and accommodative responses provides vision researchers and clinicians with a reproducible environment and shareable tool. The Vision and Neural Engineering Laboratory's (VNEL) Eye Movement Analysis Program (VEMAP) was established to process recordings produced by VE2020's eye trackers, while the Accommodative Movement Analysis Program (AMAP) was created to process the recording outputs from the corresponding autorefractor system. The VNEL studies three primary stimuli: accommodation (blur-driven changes in the convexity of the intraocular lens), vergence (inward, convergent rotation and outward, divergent rotation of the eyes), and saccades (conjugate eye movements). The VEMAP and AMAP utilize similar data flow processes, manual operator interactions, and interventions where necessary; however, these analysis platforms advance the establishment of an objective software suite that minimizes operator reliance. The utility of a graphical interface and its corresponding algorithms allow for a broad range of visual experiments to be conducted with minimal required prior coding experience from its operator(s).
Concerted binocular coordination and appropriate accommodative and oculomotor responses to visual stimuli are crucial aspects of daily life. When an individual has reduced convergence eye movement response speed, quantified through eye movement recording, doubled vision (diplopia) may be perceived1,2. Furthermore, a Cochrane literature meta-analysis reported that patients with oculomotor dysfunctions, attempting to maintain normal binocular vision, experience commonly shared visual symptoms, including blurred/double vision, headaches, eye stress/strain, and difficulty reading comfortably3. Rapid conjugate eye movements (saccades), when deficient, can under-respond or over-respond to visual targets, thus meaning further sequential saccades are required to correct this error4. These oculomotor responses can also be confounded by the accommodative system, in which the improper focusing of light from the lens creates blur5.
Tasks such as reading or working on electronic devices demand coordination of the oculomotor and accommodative systems. For individuals with binocular eye movement or accommodative dysfunctions, the inability to maintain binocular fusion (single) and acute (clear) vision diminishes their quality of life and overall productivity. By establishing a procedural methodology for quantitatively recording these systems independently and concertedly through repeatable instrumentation configurations and objective analysis, distinguishing characteristics about the acclimation to specific deficiencies can be understood. Quantitative measurements of eye movements can lead to more comprehensive diagnoses6 compared to conventional methods, with the potential to predict the probability of remediation via therapeutic interventions. This instrumentation and data analysis suite provides insight toward understanding the mechanisms behind current standards of care, such as vision therapy, and the long-term effect therapeutic intervention(s) may have on patients. Establishing these quantitative differences between individuals with and without normal binocular vision may provide novel personalized therapeutic strategies and heighten remediation effectiveness based on objective outcome measurements.
To date, there is not a single commercially available platform that can simultaneously stimulate and quantitatively record eye movement data with corresponding accommodative positional and velocity responses that can be further processed as separate (eye movement and accommodative) data streams. The signal processing analyses for accommodative and oculomotor positional and velocity responses have respectively established minimum sampling requirements of approximately 10 Hz7 and a suggested sampling rate between 240 Hz and 250 Hz for saccadic eye movements8,9. However, the Nyquist rate for vergence eye movements has yet to be established, though vergence is about an order of magnitude lower in peak velocity than saccadic eye movements. Nonetheless, there is a gap in the current literature regarding eye movement recording and auto-refractive instrumentation platform integration. Furthermore, the ability to analyze objective eye movement responses with synchronous accommodation responses has not yet been open-sourced. Hence, the Vision and Neural Engineering Laboratory (VNEL) addressed the need for synchronized instrumentation and analysis through the creation of VE2020 and two offline signal processing program suites to analyze eye movements and accommodative responses. VE2020 is customizable via calibration procedures and stimulation protocols for adaptation to a variety of applications from basic science to clinical, including binocular vision research projects on convergence insufficiency/excess, divergence insufficiency/excess, accommodative insufficiency/excess, concussion-related binocular dysfunctions, strabismus, amblyopia, and nystagmus. VE2020 is complemented by the VEMAP and AMAP, which subsequently provide data analysis capabilities for these stimulated eyes and accommodative movements.
The study, for which this instrumentation and data analysis suite was created and successfully implemented was approved by the New Jersey Institute of Technology Institution Review Board HHS FWA 00003246 Approval F182-13 and approved as a randomized clinical trial posted on ClinicalTrials.gov Identifier: NCT03593031 funded via NIH EY023261. All the participants read and signed an informed consent form approved by the university's Institutional Review Board.
1. Instrumentation setup
Figure 1: Haploscope control and recording equipment configuration. Example of the VE2020's display indexing for clockwise monitor ordering and dimensioning. Here, 1 is the control monitor, 2 is the near-left display monitor, 3 is the far-left display monitor, 6 is the calibration board (CalBoard), 4 is the far-right display monitor, and 5 is the near-right display monitor. Please click here to view a larger version of this figure.
Table 1: BNC port map. The convention for BNC connections. Please click here to download this Table.
Figure 2: Breakout box switch references. Demonstration of the proper NI 2090A switch positions. Please click here to view a larger version of this figure.
2. Visual stimulation utilizing the VE2020 visual displays and VE2020 LED targets
Figure 3: Stimulated degrees to monitor pixels. Depiction of the operator view for calibrating the VE2020. From left to right, a table of values for the recorded pixels corresponding to a known degree value is provided for a given stimulus monitor selection (stretch mode ID) with a fixed aspect ratio, given file name, background stimulus (BG), and foreground stimulus (Line). Please click here to view a larger version of this figure.
Figure 4: Pixel to degree calibration slopes. Monocular calibration curve for known degree values and measured pixel values. Please click here to view a larger version of this figure.
3. LED calibration
Figure 5: Calculated degrees of rotation. Method of calculating the angular displacement for both saccadic eye movements and vergence movements with a known distance to the target (X) and inter-pupillary distance (IPD). Please click here to view a larger version of this figure.
4. Software programming
5. DC files
Table 2: DC file configuration. The table provides an overview of the DC text file format. Please click here to download this Table.
6. LED input file definition and stimulus library storing
Figure 6: Stimulus library. Utilizing text-editing software, the format shown for identifying the port communications, baud rate, data size, and parity, as well as the library of stimulus files (.vei), provides the VE2020 with the necessary configurations and stimulus file names to run successfully. Please click here to view a larger version of this figure.
7. Script creation for experimental protocols
Table 3: VE2020 function syntax. VE2020 has specific syntax, as demonstrated in the table for calling embedded functions and commenting. Please click here to download this Table.
8. Participant preparation and experiment initiation
9. VNEL eye movement analysis program (VEMAP)
Figure 7: Monocular calibration and correlation slopes. An example of the calibration of eye movement data from voltage values to degrees of rotation. Please click here to view a larger version of this figure.
Figure 8: Eye movement software classification. Classification of the stimulated eye movement responses. Please click here to view a larger version of this figure.
Figure 9: Eye movement response software analysis. An example of plotted convergence responses stimulated by a 4° symmetrical step change (right), with individual eye movement response metrics presented tabularly (left) and group-level statistics displayed tabularly below the response metrics. Please click here to view a larger version of this figure.
10. Accommodative Movement Analysis Program (AMAP)
Figure 10: AMAP software frontend. The figure displays the main user interface for the AMAP with highlighted sections for the graphical presentation (graphical options) of data and data analysis (metric modifications). Please click here to view a larger version of this figure.
Group-level ensemble plots of stimulated eye movements evoked by VE2020 are depicted in Figure 11 with the corresponding first-order velocity characteristics.
Figure 11: Eye movement response ensembles. The ensemble plots of vergence steps (left) and saccades (right) stimulated using the VE2020 are shown. Each eye movement position trace (degrees) is plotted as a uniquely colored line and overlayed with the group-level velocity response in red. Please click here to view a larger version of this figure.
The exported features from the AMAP enable the visualization of both the participant-level and group-level movement plots (ensembles) and corresponding metrics (export) in an accessible spreadsheet (Table 4). The exported data tables provide a quantitative overview of the participants' performances and can establish criteria for outlier removal.
Table 4: AMAP software analysis export. An example of the AMAP export function, in which individual eye movement responses are exported row-wise with the corresponding subject and movement type identification. Please click here to download this Table.
The visualization of the participant performances can also be accomplished within the AMAP, as shown in Figure 12, which shows an ensemble of 5° convergent responses and the corresponding 1.5 diopter accommodative responses that are the result of data processing.
Figure 12: Accommodative movement response ensembles. The figure demonstrates the AMAP ensemble function, which creates overlays of each individual movement response trace (gray) and the average response (green). Please click here to view a larger version of this figure.
Figure 11 and Figure 12 demonstrate the successful stimulation and recording of both vergence and saccadic eye movements as well as accommodative responses. Provided the calibration procedures from the VEMAP give the expected 4° vergence and 5° saccadic targets, Figure 11 shows that for a binocularly normal participant undergoing these visual tasks, the anticipated stimulation is met. For accommodative responses processed within the AMAP, Figure 12 demonstrates an approximate accommodative response of 1 diopter with an accommodative demand of 1.5 diopters, which is consistent with the variability of autorefractor systems for varying participant demographics17. These results can be further calibrated, utilizing a constant gain, following group-level statistics for various experimental participant groups with the export feature seen in Table 4. Hence, the establishment and successful implementation of VE2020, the VEMAP, and the AMAP can provide a quantitative understanding of the differences in stimulated eye movement and accommodative response metrics.
Applications of the method in research
Innovations from the initial VisualEyes2020 (VE2020) software include the expansibility of the VE2020 to project onto multiple monitors with one or several visual stimuli, which allows the investigation of scientific questions ranging from the quantification of the Maddox components of vergence18 to the influence of distracting targets on instructed targets19. The expansion of the haploscope system to VE2020 alongside the complementary development of the VEMAP and AMAP provide a self-contained stimulus and analysis platform that is compatible with currently accessible eye movement and accommodation recording equipment. Following the successful creation of the VE2020 stimulus routine and subsequent recording, the conversion of raw eye movement position and accommodation data into meaningful and analyzable subsets of data enables researchers with the necessary non-invasive tools to holistically investigate prevalent and underlying vision dysfunctions, such as typically occurring or mild traumatic brain injury-induced dysfunction and convergence insufficiency, which can be compared with function in binocularly normal control participants1,2,13,20. Providing eye movement analysis with corresponding accommodative responses enhances the scientific understanding of the unknown interplays between the vergence and accommodative systems in both healthy participants and those with oculomotor dysfunction21.
As demonstrated, with VE2020, the VEMAP, and the AMAP jointly configured, the underlying neural control mechanisms in dysfunction can be better understood22,23. Through VE2020's repeatable visual stimulation, latent neurological dysfunctions that may have early biomarkers expressed through abnormal accommodative, vergence, or version responses can now be quantitatively assessed by the AMAP and VEMAP. Unifying the accommodative response analysis with coupled eye movement recordings from previously isolated vision experiments helps studies to obtain more complete and quantifiable analysis outcomes. Objective methods of analysis and stimulation provide the ability to compare the efficacy of current standards of care and their therapeutic outcomes24,25. These quantifications, coupled with subjective participant symptom surveying, can aid in identifying personalized remediation strategies that improve outcomes. Furthermore, by evaluating these principal components that may evoke symptoms, early methods for detecting injury26 and severity assessments can be established with heightened efficacy.
Critical steps in the protocol
Eye movements are usually measured by the magnitude of rotation of the eye in degrees. As seen in Figure 5, the trigonometric conversion of a stimulus's translational movement to degrees requires a known inter-pupillary distance (IPD) and measured distance to the target. Utilizing known averages for the IPD can provide a generalized approximation for scripting the VE2020 stimulus sequences; however, these rely on proper calibrations. Sign conventions for the direction of movements can be altered; however, this will alter the application of gain values for the VEMAP. The VEMAP's current movement conventions are as follows for saccadic movements: rightward is positive, and leftward is negative. Additionally, for vergence movements, convergence (inward rotation) is positive, and divergence (outward rotation) is negative.
As seen in Figure 7, the stimulus targets were placed at 1°, 3°, and 5°, representing an inward monocular angular rotation from optical infinity. The bottom-left plot demonstrates a three-point linear regression for the left eye positional data, where for a 5° stimulus, the average recorded voltage was −1 V, for a 3° stimulus, the average recorded voltage was approximately 0.4 V, and for a 1° stimulus, the average recorded voltage was approximately 1.25 V. Similarly, for the right-eye position in the bottom-right plot, a 1° stimulus had a corresponding voltage of −1.25 V, a 3° stimulus had an average voltage of about 0 V, and a 5° stimulus had an average voltage of 1.1 V.
Limitations of the method
The current limitations of the method include the standardized output of the autorefractor and eye tracker data, as the AMAP and VEMAP are programmed to process these data formats. Another limitation includes the fact that if the experimentation is not engaging, participants may frequently blink (close) their eyes, leading to poor data recording quality. While other oculomotor dysfunctions such as strabismus, amblyopia, nystagmus, and suppression could leverage VE2020, the VEMAP, and the AMAP, modifications would need to be implemented for each of these specific oculomotor dysfunctions.
The authors have nothing to disclose.
This research was supported by National Institutes of Health grant R01EY023261 to T.L.A. and a Barry Goldwater Scholarship and NJIT Provost Doctoral Award to S.N.F.
Analog Terminal Breakout Box | National Instruments | 2090A | |
Convex-Sphere Trial Lens Set | Reichert | Portable Precision Lenses | Utilized for autorefractor calibration |
Graphics Cards | – | – | Minimum performance requirement of GTX980 in SLI configuration |
ISCAN Eye Tracker | ISCAN | ETL200 | |
MATLAB | MathWorks | v2022a | AMAP software rquirement |
MATLAB | MathWorks | v2015a | VEMAP software requirement |
Microsoft Windows 10 | Microsoft | Windows 10 | Required OS for VE2020 |
Plusoptix PowerRef3 Autorefractor | Plusoptix | PowerRef3 | |
Stimuli Monitors (Quantity: 4+) | Dell | Resolution 1920×1080 | Note all monitors should be the same model and brand to avoid resolution differences as well as physical configurations |