Vergence tracking: a tool to assess oculomotor performance in stereoscopic displays

Oculomotor conflict induced between the accommodative and vergence components in stereoscopic displays represents an unnatural viewing condition. There is now some evidence that stereoscopic viewing may induce discomfort and changes in oculomotor parameters. The present study sought to measure oculomotor performance during stereoscopic viewing. Using a 3D stereo setup and an eye-tracker, vergence responses were measured during 20-min exposure to a virtual visual target oscillating in depth, which participants had to track. The results showed a significant decline in the amplitude of the in-depth oscillatory vergence response over time. We propose that eye-tracking provides a useful tool to objectively assess the timevarying alterations of the vergence system when using stereoscopic displays.

Discharge Characteristics of Pursuit Neurons in MST During Vergence Eye Movements

For small objects moving smoothly in space close to the observer, smooth pursuit and vergence eye movements maintain target images near the foveae to insure high-resolution processing of visual signals about moving objects. Signals for both systems must be synthesized for pursuit-in-three-dimensions (3D). Recent studies have shown that responses of the majority of pursuit neurons in the frontal eye fields (FEF) code pursuit-in-3D. This area is known to have reciprocal connections with the medial superior temporal area (MST) where frontal pursuit neurons are found. To examine whether pursuit-in-3D signals are already present in MST and how MST neurons discharge during vergence-tracking induced by a small spot, we examined discharge of MST pursuit neurons in 2 monkeys. Of a total of 219 pursuit neurons examined during both frontal pursuit and vergence-tracking, 61% discharged only for frontal pursuit, 18% only for vergence-tracking, and 21% for both. A majority of vergence-related MST neurons exhibited sensitivity to vergence eye velocity. Their discharge was maintained during brief blanking of a vergence target. About 1/3 of vergence-related MST neurons exhibited visual responses to spot motion in depth. The preferred directions for visual motion and vergence-tracking were similar in half of our population. Some of the remaining neurons showed opposite preferred directions. A significant proportion (29%) of vergence-related neurons discharged before onset of eye movements with lead times longer than 20 ms. The results in this and previous studies indicate differences in discharge characteristics of FEF and MST pursuit neurons, suggesting different roles for the two in pursuit-in-3D.

Pursuit-Related Neurons in the Supplementary Eye Fields: Discharge During Pursuit and Passive Whole Body Rotation

The primate frontal cortex contains two areas related to smooth-pursuit: the frontal eye fields (FEFs) and supplementary eye fields (SEFs). To distinguish the specific role of the SEFs in pursuit, we examined discharge of a total of 89 pursuit-related neurons that showed consistent modulation when head-stabilized Japanese monkeys pursued a spot moving sinusoidally in fronto-parallel planes and/or in depth and with or without passive whole body rotation. During smooth-pursuit at different frequencies, 43% of the neurons tested (17/40) exhibited discharge amplitude of modulation linearly correlated with eye velocity. During cancellation of the vestibulo-ocular reflex and/or chair rotation in complete darkness, the majority of neurons tested (91% = 30/33) responded. However, only 17% of the responding neurons (4/30) were modulated in proportion to gaze (eye-in-space) velocity during pursuit-vestibular interactions. When the monkeys fixated a stationary spot, 20% of neurons tested (7/34) responded to motion of a second spot. Among the neurons tested for both smooth-pursuit and vergence tracking ( n = 56), 27% (15/56) discharged during both, 62% (35/56) responded during smooth-pursuit only, and 11% (6/56) during vergence tracking only. Phase shifts (relative to stimulus velocity) of responding neurons during pursuit in frontal and depth planes and during chair rotation remained virtually constant (≤1 Hz). These results, together with the robust vestibular-related discharge of most SEF neurons, show that the discharge of the majority of SEF pursuit-related neurons is quite distinct from that of caudal FEF neurons in identical task conditions, suggesting that the two areas are involved in different aspects of pursuit-vestibular interactions including predictive pursuit.

Vergence Eye Movements Elicited by Stimuli without Corresponding Features

We have observed quantitative depth perception with a dichoptic stimulus which possessed no contrast-defined binocular corresponding features (phantom stereogram). The depth perception can be the result of appreciation of a partial-occlusion situation depicted by the stimulus, or the result of activities of low-level disparity detectors which are capable of combining dissimilar local features in the stimulus. Although both mechanisms predict similar depth perception, they predict different vergence eye-movement outputs, especially in the vertical dimension. To identify the underlying mechanisms of the phantom stereopsis, we recorded vergence tracking eye movements to four types of dichoptic stimuli: (a) conventional stereogram with horizontal disparity (HD); (b) horizontal phantom stereogram (HP); (c) conventional stereogram with vertical disparity (VD); and (d) vertical phantom stereogram (VP). We found that HD, HP, and VD stimuli could elicit robust vergence tracking eye movements but VP stimulus could not. While the success of HP stimulus in eliciting vergence tracking may be explained by proximal vergence, the failure of VP stimulus in eliciting vergence tracking clearly indicates that phantom stereogram could not elicit coherent responses among low-level disparity detectors. Partial occlusion, therefore, has to play an important role in the depth perception from the phantom stereogram.

Ocular vergence under natural conditions. I. Continuous changes of target distance along the median plane

Horizontal binocular eye movements of four subjects were recorded with the scleral sensor coil - revolving magnetic field technique while they fixated a natural target, whose distance was varied in a normally illuminated room. The distance of the target relative to the head of the subject was changed in three ways: ( a ) the target was moved manually by the experimenter; ( b ) the target was moved manually by the subject; ( c ) the target remained stationary while the subject moved his upper torso towards and away from the target. The rate of change of target distance was varied systematically in four levels, ranging from ‘slow’ to ‘very fast’, corresponding to changes in target vergence from about 10° s -1 to about 100° s -1 . The dynamics of ocular vergence with regard to delay and speed were, under all three conditions, considerably better than could be expected from the literature on ocular vergence induced by disparity and/or blur. When ‘very fast’ changes in the distance of the target were made, subjects achieved maximum vergence speeds of up to about 100° s -1 . Delays of these fast vergence responses were generally smaller than 125 ms. Negative delays, i. e. ocular vergence leading the change in target distance, were observed. The eyes led the target (i. e. predicted target motion) by about 90 ms on average, when the subject used his hand to move the target. Vergence tracking was almost perfect when changes in distance were produced by moving the upper torso. In this condition, the eye led the target by about 5 ms. In the ‘slow’ and ‘medium’ conditions (stimulus speeds about 10-40° s -1 ) tracking was accurate to within 1-2°, irrespective of the way in which the target was moved. In the ‘fast’ and ‘very fast’ conditions (stimulus speeds about 40-100° s -1 ), the accuracy of vergence tracking was better for self-induced than for experimenter-induced target displacements, and accuracy was best during voluntary movements of the upper torso. In the last case, ocular vergence speed was within about 10% of the rate of change of the vergence angle formed by the eyes and the stationary target. The dynamics of convergent and divergent vergence responses varied considerably. These variations were idiosyncratic. They were consistent within, but not between, subjects. Ocular vergence associated with attempted fixation of an imagined target, changing distance in darkness, could only be made by two of the four subjects. The changes they could make were unreliable and poorly correlated with changes in the distance of the imagined target. Vergence changes did not occur when the distance to the target, imagined in darkness, was varied by keeping the target stationary and moving the torso back and forth. In conclusion, when ocular vergence was studied under relatively natural conditions in which there were many cues to the distance of the target, oculomotor vergence was both much faster and much more accurate than could have been anticipated from previous studies done under more restricted stimulating conditions.