Vergence disparity visual stimuli were presented to the subjects via a haploscope system (
Figure 1). Two computer monitors, one for each eye, were used to present symmetrical vergence disparity stimuli. The target and distractor stimuli were projected onto the subject's field of view by two partially reflective (50% light transmission) mirrors. The subject was carefully situated in the apparatus so that all visual stimuli were along the subject's midline, the midsagittal plane. All stimuli that were presented within the haploscope were calibrated with physical targets placed at measured distance to evoke the following vergence angles: 1°, 3°, 5°, 7°, and 9°. The voltage values at these vergence angles were recorded to form linear relationships between vergence angles and voltage output. The subjects were placed in a customized space and covered by commercial blackout curtains (Blackout Curtains, St. Louis Park, MN) to minimize the amount of light emitted into the experimentation space. Prior to the start of each experimental session, each subject verbally confirmed that he or she did not perceive any light source other than the presented stimuli.
Eye movements were recorded with an ISCAN Eye Tracking Camera System (model ETL 400; ISCAN Inc., Burlington, MA). This system utilizes an infrared (λ = 950 nm) video-based system. The manufacturer specifies that the accuracy for this system is 0.3° over a ±20° horizontal range. The two cameras were placed in front of subject, one in front of the left eye and the other in front of the right eye at a distance of 38 cm, which is the distance recommended by the manufacturer. The cameras have a clear line of sight to the subject's eyes and were not blocked by any materials including the partially reflective mirrors. Individual eye movements were quantified using the centroid of the pupil movements at a sampling rate of 240 frames per second (fps). Each subject's eyes were illuminated using a board beam infrared source. The maximum infrared light power level was 1.2 mW/cm2, which is well below the ANSI Z136 specification safety limits of 10 mW/cm2.
The entire system was controlled by a custom LabVIEW
TM 2013 SP 1 Virtual Instrument (National Instrument, Austin, TX) called VisualEyes2020 which generated the visual stimuli that was digitally synchronized with the eye movement acquisition to ensure accurate temporal analyses. This system was a modified version of the 2011 system described by Guo, Kim, and Alvarez (
2011). Prior research calculated via a spectrum analysis showed that the power of saccadic eye movements are predominantly within the first 100 Hz (Zuber, Semmlow, & Stark,
1968). Since vergence eye movements are an order of magnitude slower than saccadic eye movements, our sampling rate of 240 fps satisfies the Nyquist criterion for digitizing both saccadic and vergence eye movements. While vergence eye movements were the primary purpose of this investigation, it is well known that even with symmetrical vergence stimuli, saccadic responses are commonly initiated (Jaswal, Gohel, Biswal, & Alvarez,
2014; Semmlow, Chen, Granger, Donnetti, & Alvarez,
2008). The signals were digitized using a 16-bit digital acquisition (DAQ) hardware card using the range of ±5 Volts (National Instruments PCIe-6351 X Series Data Acquisition, Austin, TX). The left- and right-eye movements were saved individually for offline data analysis using a custom MATLAB version R2015A code (MathWorks, Natick, MA).