Academic journal article Human Factors

Fixation Disparity: Binocular Vergence Accuracy for a Visual Display at Different Positions Relative to the Eyes

Academic journal article Human Factors

Fixation Disparity: Binocular Vergence Accuracy for a Visual Display at Different Positions Relative to the Eyes

Article excerpt

INTRODUCTION

Binocular vision requires that the vergence angle between the two visual axes be adjusted for proper fusion of the two retinal images so that the point of regard is projected onto the fovea of each eye (i.e., the center region of the retina with the best spatial resolution). The vergence angle depends on the actual viewing distance: When a person looks at infinity, his or her visual axes are parallel; the closer the target, the stronger the force that the extraocular muscles have to exert in order to turn the visual axes toward each other. In an open-loop condition (i.e., without a fixation stimulus), the eyes assume a vergence resting position, also referred to as tonic vergence or dark vergence (when measured in a dark visual field). The resting position varies among individuals with an average of about 3.6[degrees], corresponding to a distance of 1 m (Jaschinski-Kruza, 1991; Rosenfield, 1997; Tyrrell & Leibowitz, 1990).

A muscular system can be evaluated by measuring how accurately it is able to operate. In optimal binocular vision, a fixated target is imaged onto the center of the fovea in each eye so that the principal visual directions of both eyes intersect at the fixation point (Figure 1a). Slight deviations from this optimal state may occur in people with normal binocular vision (indicated by good stereoscopic acuity), depending on individual disposition or the current viewing conditions. These small errors in vergence typically amount to a few minutes of arc; they are smaller than the Panum area, the region of sensory fusion where vergence errors do not lead to double vision (Howard & Rogers, 1995). These fixation disparities are called exo or eso when the eyes converge slightly behind or in front of the fixation point, respectively, as illustrated in Figures 1b and 1c. People with more exo fixation disparity at 40 cm tend to experience asthenopic complaints and visual fatigue when doing visual near work (Evans, 1997 ; Pickwell, Kaye, & Jenkins, 1991; Scheiman & Wick, 1994; Sheedy & Saladin, 1983).

However, the traditional condition of measuring fixation disparity at the typical reading distance of 40 cm does not apply to computer workstations, where the viewing distance is typically longer and the vertical gaze direction is also different compared with that for books or papers on the desk. The following two parameters of screen position can have an effect on fixation disparity. First, when the viewing distance is shortened from 100 to 30 cm, fixation disparity changes into the exo direction; this was tested at horizontal gaze (Jaschinski, 1997, 200 la, 200 ib). An exo fixation disparity at a close target means that the vergence near response (relative to the more distant resting position) lags behind the near stimulus. Second, depending on screen height relative to eye level, the eyes and/or the head will incline to adjust the required vertical gaze inclination, defined in the present study as the angle between horizontal and the line from eye to screen.

When gaze direction is lowered from +150 above horizontal to -45[degrees] below, fixation disparity changes into the eso direction (Jaschinski, Koitcheva, & Heuer, 1998). This effect was found at a fixed viewing distance of 40 cm while either the head was inclined (with eye position unchanged relative to the head) or the eyes were inclined (keeping the head upright); the latter procedure induced larger and more reliable effects on fixation disparity. These eso changes in fixation disparity were correlated with corresponding near shifts in the resting position of vergence when the gaze is lowered (Heuer & Owens, 1989), presumably because these measures of vergence are correlated (Jaschinski-Kruza, 1994). Mon-Williams, Plooy, Burgess-Limerick, and Wann (1998) and Mon-Williams, Burgess-Limerick, Plooy, and Wann (1999) reported similar effects of eye inclination in measurements of heterophoria. …

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