DLP-based dichoptic vision test system

Abstract

It can be useful to present a different image to each of the two eyes while they cooperatively view the world. Such dichoptic presentation can occur in investigations of stereoscopic and binocular vision (e.g., strabismus, amblyopia) and vision rehabilitation in clinical and research settings. Various techniques have been used to construct dichoptic displays. The most common and most flexible modern technique uses liquid-crystal (LC) shutters. When used in combination with cathode ray tube (CRT) displays, there is often leakage of light from the image intended for one eye into the view of the other eye. Such interocular crosstalk is 14% even in our state of the art CRT-based dichoptic system. While such crosstalk may have minimal impact on stereo movie or video game experiences, it can defeat clinical and research investigations. We use micromirror digital light processing (DLP) technology to create a novel dichoptic visual display system with substantially lower interocular crosstalk (0.3%; remaining crosstalk comes from the LC shutters). The DLP system normally uses a color wheel to display color images. Our approach is to disable the color wheel, synchronize the display directly to the computer's sync signal, allocate each of the three (former) color presentations to one or both eyes, and open and close the LC shutters in synchrony with those color events.

Figure 1

Ring scotoma with a bioptic telescope. (a) The enlarged image on the retina through a spectacle-mounted telescope necessarily blocks a portion of the see-through view around the telescope’s field. (b) A visual field plot shows that only the portions of the scene not blocked by the magnified image or actually seen through the telescope are detectable, creating a blind area known as a ring scotoma (gray area in the figure).

Figure 2

Schematic diagram of a raster scan. The electron beam scans later pixels in the lower lines not long before the next frame is initiated. Hence a “late” pixel, even with a fast phosphor, may not have decayed completely before the next frame sync (vsync signal).

Figure 3

Luminance measured (at display center) through the closed FE-1 (ferroelectric LC) shutter increased as luminance was increased. Interocular crosstalk, the slope of these functions, was substantially less for our DLP-based dichoptic visual test system (<0.3%, black squares) than the EIZO CRT-based dichoptic system (14%, open diamonds). The luminance data were normalized to a maximum value of one for a digital video input value of (255, 255, 255) for each system.

Figure 4

Temporal-luminance sequence during one frame of a single-chip DLP display. (a) to (d) are schematic illustrations of the luminance for (a) red only, (b) blue only, (c) green only, and (d) white. (e) Color wheel from a Davis Powerbeam VI. The temporal windows of each color are used in combination (temporal multiplexed) to provide the gamut of colors. As shown in Fig. 5, the luminance profile of a Davis Powerbeam VI is more complicated, with variations in luminance within each color window, and it includes two short white windows.

Figure 5

Temporal sequence of (a) the luminance output of the Davis Powerbeam VI DLP display showing bright white; (b) the frame sync; and the shutter controller (c) “red” circuit; (d) “blue” circuit; and (e) “green” circuit. The three “color” circuits control the shutters. Negative voltage opens the shutter (high light transmission) for the eye specified for that circuit. For example, the right eye might be driven by the “red” and “green” circuits, therefore opening twice per frame. Meanwhile the “blue” and “green” circuits might drive the left eye. “Green” elements of the image could be seen binocularly (e.g., fusion lock). The temporal-luminance sequence of the Powerbeam VI display is more complex than the schematic in Fig. 4, and actually even more complex at other luminances, as shown in (f) for various values of just the red channel. This is a function of the binary pulse-width modulation technique used to create gray levels and the addition of “white” to enhance perceived brightness and contrast. Measurements were made with the color wheel removed, so the appearance was always grayscale. The time of each segment of the frame sequence was measured empirically and used to establish the shutter control switch settings.

Figure 6

Our DLP-based dichoptic visual test system. (a) The frame sync relay allows us to switch between the color wheel, the computer, or a 60-Hz signal generator. (b) Conceptual diagram of the shutter controller. Delay after the onset of the frame sync pulse is used to time the shutter events (i.e., open and close) in each frame (Fig. 5). The rows of circles on the right represent two cycles (frames) of shutter events. Each cycle has three periods that are established by the on periods of three flip-flops, which, in turn, are set and reset at the times the counter reaches the values set in the switches of their associated comparators. First the “red” flip-flop opens the right goggle lens (open circle) while the left goggle lens remains closed (filled), then the “blue” flip-flop opens the left goggle lens while the right goggle lens is closed. The gray circles in the shutter event cycle (third and sixth events as shown) represent the state of the “green” counter, if connected. The four-way switch allows monocular or binocular or no shutter opening. At the switch location shown, the gray circles would represent both goggle lenses being open (a binocular view), although we have not used that setting in our applications.

Figure 7

Temporal modulation transfer function of the single-chip DLP used in our dichoptic system (Davis PowerBeam VI: solid squares) showed no loss up to the maximum possible 30 Hz, unlike the three-chip DLP (open triangles) reported by Packer et al.

Figure 8

Dichoptic visual fields. Monocular, right (right-tilted stripes) and left (left-tilted stripes) scotomas found in the visual fields when measured with our dichoptic visual test system while viewing binocularly for (a) a normally sighted subject, and (b) a patient with bilateral central scotomas. For the normally sighted subject, both physiological blind spots (optic nerve heads) are measured separately while the observer was fixating on a binocularly visible target and was seeing the physical screen binocularly. The patient used the same preferred retinal locus (PRL) for fixation here as when viewing monocularly with the right eye (i.e., not the left PRL). Note how the left eye central scotoma includes the fixation target. These visual fields illustrate the ability to measure each eye separately under binocular conditions.

Figure 9

Ring scotoma probe. Dichoptic visual field plot of a normally sighted subject fixating through a 3× bioptic telescope mounted on the right spectacle lens while the left eye was open. The clear central area represents the visual field visible to the right eye through the telescope, and the large hatched area is the ring scotoma caused by the 3× magnification of the telescope. The small cross-hatched area is not seen by either eye, as it is in the physiological blind spot of the left eye. The boundaries of those scotomas were found using kinetic stimuli. The left-pointing triangles represent locations at which static stimuli were presented only to the left eye. All of these static stimuli, presented within the ring scotoma of the right eye, were detected by the left eye. Under these conditions, vision in the left (nontelescope) eye was not suppressed, and therefore, objects appearing within the ring scotoma would be detected when viewing binocularly. The exception is the area of overlap of the ring scotoma with the blind spot, which is a (small) binocular scotoma.

Figure 10

Prism scotoma fitting. A peripheral prism for hemianopia creates visual field expansion (patient without the peripheral prism would only see to the right of the vertical midline). (a) Binocular visual field with an upper peripheral prism worn over the left eye. When probed binocularly, the fellow eye compensates (on the seeing side, here the right side) for the prism’s apical scotoma, so there is no reduction in the right portion of the visual field. (b) Monocular visual field of the left eye when stimuli are presented dichoptically so they were seen only by the prism eye: the effect of the apical scotoma is evident. A plot of this nature helps identify proper fitting of the peripheral prism. In this case, the peripheral prism could be moved to the right to reduce or eliminate the gap between the normal and expanded visual fields in binocular viewing. The fixation mark and background were presented binocularly in both plots. The difference in the extent of the visual field on the right side [ending at 30 deg in (a) and 20 deg in (b)] is due to the greater limitation from the frame of the goggles on the left eye than the right eye (available in the binocular test).

Figure 11

Threshold facilitation. The effect of interocular crosstalk when using our EIZO CRT-based dichoptic display and FE-1 (ferroelectric LC) shutter goggles. Contrast thresholds were determined with the left eye covered. When a mask was presented to the right (open) eye, there was facilitation of contrast detection for relative mask contrasts less than about three times the contrast threshold with no mask, and inhibition for higher contrast monocular mask conditions (open symbols). When a mask was presented to the closed (left) eye, it should not have been visible to the open (right) eye, and there should have been no effect on contrast thresholds (i.e., filled symbols should all be along the horizontal dashed line). crosstalk caused facilitation of contrast thresholds at all measured closed-eye mask contrasts, including contrasts that could not be detected (i.e., to the left of the vertical dashed line). Stimulus and mask were 1- cycle∕deg, 1 octave Gabor patches. Error bars are 95% confidence limits.