1. Purpose

Figure 1

Our eye-test PC is a computer-controlled procedure for testing the eyes in viewing conditions that resemble visual displays at computer workstations.
Comfortable vision at the workplace requires good coordination of the two eyes:
the ocular muscles adjust the vergence angle between the two visual axes

2. Scientific background

The eye-test PC is the result of our research on binocular vision and its impact on visual fatigue. In Chapter 6 are the most relevant publications and their abstracts listed. Reprints of entire articles can by ordered by e-mailDr. Wolfgang Jaschinski.

3.

User Network

We cooperate with optometrists and ophthalmologists who use the eye-test PC as an approach to testing the eyes in viewing conditions of visual displays at computer workstations. The eye-test PC provides a detailed diagnosis of the state of binocular vision in workplace conditions and allows one to address several open research question, including effects of prism spectacles or visual training.

The cooperation within the network is based on scientific exchange: the Institut für Arbeitsphysiologie provides the eye-test PC Software to be run on the user's own hardware components. We aim at joint scientific publications of the results.
If you think you might be interested in using the eye-test PC and to join the network, please contact Dr. Wolfgang Jaschinski.

4. Optometric measures

Figure 3

The eye-test PC allows to measure some optometric functions that complement the repertoire of conventional test devices. These include:

4.1. Static fixation disparity
4.2. Temporal fluctuation of fixation disparity
4.3 Dynamic changes in vergence
4.4 Limits of stereovision
4.5 Contrast sensitivit

4.1. Static fixation disparity
4.1.1. Definition of fixation disparity

Figure 4

Binocular vision is optimal when the fixated target is imaged onto the centre of the fovea in each eye, so that the principle visual directions of both eyes intersect at the fixation point. However, even subjects who have normal binocular vision (with good stereoscopic acuity) may have slight deviations from this optimal state. These small errors in convergence typically amount to a few minutes of arc and are called exo or eso fixation disparity depending on whether the eyes converge slightly behind or in front of the fixation point, respectively.

4.1.2 The nonius method for measuring fixation disparity

Figure 5

Fixation disparity can be measured as shown in the figure: a central fusion stimulus (XOXOXOX) is visible for both eyes, while a pair of nonius lines were presented dichoptically, i.e. the upper line is only visible for the right eye and the lower line for the left eye (by means of liquid crystal shutter spectacles in front of the eyes). In the case of eso (or exo) fixation disparity, the upper nonius line has to be placed to the left (or right) of the lower nonius line, in order to be perceived collinear. Then, each nonius line is lying on the principle visual direction that determines the direction "straight ahead" of each eye. The fixation disparity is the visual angle corresponding to the resulting physical nonius offset.

4.1.3 Psychometric procedure

The eye-test PC software uses the adaptive psychometric procedure Best PEST (Lieberman and Pentland, 1982) in order to determine the physical nonius offset of perceived alignment. In a series of at least 30 presentations (trials) the physical offset of the nonius lines is varied and the subject responds by pressing the right or left button of the computer mouse depending on whether the subject perceived the upper line to the right (red data point) or to the left (green data point) of the lower line. During this procedure, the subject is requested to fixate the central fusion character O.

4.1.4 Data analysis

Figure 6

After 6 trials, the Best PEST procedure has roughly found the point of perceived alignment; in the following series of trials, any reversal of the response from either "left" to "right" or "right" to left", the mean between the two corresponding amounts of nonius offset is taken as an estimation of the actual fixation disparity (blue data point). In cases of two reversals within a series of three trials (i.e. in a series of e.g. "right", "left", "right") only the first reversal was taken, to have independent estimations.
These reversals may not be normally distributed; thus, instead of mean and standard deviation, we calculated the median and half the inter-quartile range (Q3-Q1)/2.(see Jaschinski, 1998 for details).

4.1.5 Effect of viewing distance on fixation disparity

Figure 7

Fixation disparity is strongly affected by the viewing distance. Small or zero fixation disparity is observed at a viewing distance of 1 m. The shorter the viewing distance, the more fixation disparity increases in the exo direction. These proximity-fixation-disparity curves have been confirmed to be a reliable individual parameter of the vergence system in young adults with normal binocular vision (Jaschinski 1997). The slope of fixation disparity curves as a function of vergence stimulus (i.e. viewing distance in the present case) reflects the gain factor of the fusional vergence mechanism, as shown in vergence control studies (Schor 1983; Jaschinski 2001): a subject with a low vergence gain has a steep fixation disparity curve, i.e. a more exo fixation disparity at near.
The y-intercept, i.e the viewing distance of zero fixation disparity, is interpreted as the resting position of vergence, also referred to as tonic vergence. The more the position of the fusion target deviates from the resting position of vergence (to shorter or longer viewing distances), the more the vergence error, i.e. fixation disparity tends to increase in the exo or eso direction, respectively.

4.1.6 Fixation disparity and visual fatigue

Figure 8

Measures of fixation disparity are used for a long time in optometric research and practice: subjects with larger exo fixation disparity (tested conventionally at a reading distance of 40 cm) tend to suffer from visual fatigue during near work or reading (Evans 1997; Scheiman and Wick 1994). The individual slopes of the proximity fixation disparity curve have been shown to be related to the viewing distance preferred as comfortable: those subjects with a steep proximity-fixation-disparity curve tended to avoid near vision in two laboratory studies: they moved more quickly away from a near screen (that was initially located at 40 cm) in Jaschinski (1998) and assumed longer preferred viewing distances in Jaschinski (2002). This latter result is illustrated in the figure the complete sample of 40 subjects had preferred viewing distances in the range of 43 - 99 cm, and subjects with preferred viewing distances longer or shorter than the median (63 cm) had steeper or more flat proximity-fixation-disparity curves, respectively.
This shows that measures of fixation disparity can be useful for the ergonomical design of computer workstations.

5. Technical components

The eye-test PC system comprises two personal computers, the Control Computer and the Test Computer (each with appropriate monitors) and Shutter glasses for dichoptic presentation of tests.

5.1
The experimenter operates the Control Computer and observes the currently running test on the Control Monitor
Technical requirement: the Control Computer can be any Pentium PC, with any kind of Control Monitor. A Notebook is convenient
5.2.
The Test Computer generates the visual stimuli on the Test Monitor and operates the LCD-Shutter glasses.
Technical requirement:
Test Computer: Pentium™ 500 (or faster) with
Graphic Board: 32 MB, e.g. geforce™ 2MX with nVidia™ Chips
Test Monitor: CRT-Monitor with 120 Hz vertical refresh rate (flat screens not applicable)
5.3.
LCD-Shutter glasses are attached to a swivel and connected to the printer port of the Test Computer via a special adapter (provided by the IfADo). The shutter glasses must be oriented so that the right eye observes the upper nonius line, and the left eye the lower nonius line (see Figure 5).
Technical Requirement:
Shutter glasses: 3D revelator (Elsa™) or CrystalEyes™ (StereoGraphics Corporation),

5.4.
A Photometer is used for Gamma-correction of the Test Monitor. This calibration of luminance is necessary in order to adjust the horizontal offset of the nonius lines (see below) with an accuracy smaller than the pixel size (by an anti-aliasing method, e.g. Morgan and Aiba, 1985).
5.5.
When illumination in the test room is incandescent or ordinary fluorescent light, the electric mains frequency produces flickering light at 100 Hz which is not visible. However, the interference with the 120 Hz operation of the shutter glasses produces irritating flicker in the room lighting for the subject when being tested. This can be avoided by testing under daylight or with high-frequency ballasts in fluorescent luminaires or in energy saver light bulbs (e.g. OSRAM DULUX™ EL Economy 21 W, or similar).

6. Literatur

6.1. Abstracts (all abstracts in one file)
Jainta S, Jaschinki W: Fixation Disparity: Binocular Vergence Accuracy for a Visual Display at Different Positions Relative to the Eyes. Hum. Factors, (2002, in press).
Jaschinski W: The Proximity-Fixation-Disparity Curve and the Preferred Viewing Distance at a Visual Display as an Indicator of Near Vision Fatigue, Optom. Vis. Sci. 79: 158-169 (2002). (3946)
Jaschinski W: Methods for measuring the proximity-fixation-disparity curve. Ophthal. Physiol. Opt. 21, 368-375 (2001). (3813)
Jaschinski W: Fixation disparity and accommodation for stimuli closer and more distant than oculomotor tonic positions. Vision Res 41, 923-933 (2001). (3764)
Fixation disparity, accommodation, dark vergence and dark focus during inclined gaze. Ophthal. Physiol. Opt. 18, 351-359 (1998). (3082)
Jaschinski W, Bröde P, Griefahn B: Fixation disparity and nonius bias. Vision Res. 39, 669-677 (1999). (3068a)
Jaschinski W: Fixation disparity at different viewing distances and the preferred viewing distance in a laboratory near-vision task. Ophthal. Physiol. Opt. 18, 30-39 (1998). (2915)
Jaschinski, W: Fixation disparity and accommodation as a function of viewing distance and prism load. Ophthal. Physiol. Opt. 17, 324-339 (1997). (2791)
Jaschinski-Kruza W: Dark vergence in relation to fixation disparity at different luminance and blur levels. Vision Res. 34, 1197-1204 (1994). (2118)
Jaschinski-Kruza W: Fixation disparity at different viewing distances of a visual display unit. Ophthal. Physiol. Opt. 13, 27-34 (1993). (1911)
Jaschinski-Kruza W, Schubert-Alshuth E: Variability of fixation disparity and accommodation when viewing a CRT visual display unit. Ophthal. Physiol. Opt. 12, 411-419 (1992). (1864)
Jaschinski-Kruza W.: Eyestrain in VDU users: Viewing distance and the resting position of ocular muscles. Hum. Factors 33, 69-83 (1991). (1649)
6.2. Other References
Evans BJW. Pickwell's Binocular Vision Anomalies: Investigation and Treatment. London: Butterworths, 1997.
Lieberman, H R. & Pentland, A P. (1982). Microcomputer-based estimation of psychophysical thresholds: The Best-PEST. Behavior Research Methods and Instruments, 14, 21-25.
Morgan, M.J & Aiba, T.S. (1985) Vernier acuity predicted from changes in the light distribution of the retinal image. Spatial Vision, 1, 151-161.
Scheiman M, Wick B. Clinical Management of Binocular Vision. Philadelphia: Lippincott, 1994.
Schor, C. M. Fixation disparity and vergence adaptation. In: C. M. Schor, & K. J. Ciuffreda. Vergence Eye Movements. Basic and Clinical Aspects (pp. 465-516). Boston, MA: Butterworths, 1983