A binocular test of sensory eye dominance based on the Pulfrich effect

Many studies have shown inconsistent results among traditional measures of sensory ocular dominance (SED), seriously questioning the very concept of SED as a unitary aspect of the visual system (e.g., Laby & Kirschen, 2011; Mapp et al., 2003; Walls, 1951).The test outcomes may also change even if the same measure is repeated under different conditions, for example by varying the distance and eccentricity of the target (e.g., Rice et al., 2008; Kahn & Crawford, 2001).On the other hand, some authors suggest that such inconsistencies may result from uncontrolled variables. A candidate to produce confusing variables is the frequent format of these tests, often dichotomous and introspective.In this paper, we propose a possibility of measuring SED on a continuous scale and in a comparative way using a stimulus that induces a Pulfrich effect. Here a dichoptic motion stimulus, borrowed from a previous study (Reynaud & Hess, 2017), was used, which produced different degrees of 3D illusory perception strength through the variation of retinal disparity. We observed that the responses of the subjects varied according to their classical SED test outcomes and we estimated the differences in terms of the time delay of the information coming from the two eyes. In our sample, it appears that information from the dominant eye was processed 8.2±5.8 ms faster than that of the fellow eye. People with a left dominant eye showed more marked differences in processing time (6.8±2.0 ms) than people with a right dominant eye (1.8±0.9 ms). Eyes without a clear dominance did not show significant differences in processing time (1.2±1.7 ms). These results are consistent with the previous literature and could lead to the development of a new continuous-scaled SED test.

Contact lenses, the reverse Pulfrich effect, and anti-Pulfrich monovision corrections

Interocular differences in image blur can cause processing speed differences that lead to dramatic misperceptions of the distance and three-dimensional direction of moving objects. This recently discovered illusion—the reverse Pulfrich effect—is caused by optical conditions induced by monovision, a common correction for presbyopia. Fortunately, anti-Pulfrich monovision corrections, which darken the blurring lens, can eliminate the illusion for many viewing conditions. However, the reverse Pulfrich effect and the efficacy of anti-Pulfrich corrections have been demonstrated only with trial lenses. This situation should be addressed, for clinical and scientific reasons. First, it is important to replicate these effects with contact lenses, the most common method for delivering monovision. Second, trial lenses of different powers, unlike contacts, can cause large magnification differences between the eyes. To confidently attribute the reverse Pulfrich effect to interocular optical blur differences, and to ensure that previously reported effect sizes are reliable, one must control for magnification. Here, in a within-observer study with five separate experiments, we demonstrate that (1) contact lenses and trial lenses induce indistinguishable reverse Pulfrich effects, (2) anti-Pulfrich corrections are equally effective when induced by contact and trial lenses, and (3) magnification differences do not cause or impact the Pulfrich effect.

Contact lenses can cause the reverse Pulfrich effect and anti-Pulfrich monovision corrections can eliminate it

Interocular differences in image blur can cause dramatic misperceptions of the distance and three-dimensional direction of moving objects. This new illusion—the reverse Pulfrich effect—is caused by the optical conditions induced by monovision, a common correction for presbyopia. Fortunately, anti-Pulfrich monovision corrections, in which the blurring lens is slightly darkened, can eliminate the illusion for a wide range of viewing conditions. However, the reverse Pulfrich effect and the efficacy of anti-Pulfrich corrections have previously been demonstrated only with trial lenses. This situation should be addressed, for both clinical and scientific reasons. First, monovision is most commonly prescribed with contact lenses. It is important to replicate these effects in the most common monovision delivery system. Second, trial lenses of different powers, unlike contacts, cause large magnification differences between the eyes. To confidently attribute the reverse Pulfrich effect to differences in optical blur between the eyes, and to ensure that the reported effect sizes are reliable, one must control for magnification. Here, in a within observer study with five separate experiments, we demonstrate i) that contact lenses induce reverse Pulfrich effects that are indistinguishable from those induced by trial lenses, ii) that overall magnification differences do not cause or impact the Pulfrich effect, and iii) that anti-Pulfrich corrections (i.e. darkening the blurring lens) are equally effective when induced by contact lenses and by trial lenses.

Interocular Differences in Spatial Frequency Influence the Pulfrich Effect

The Pulfrich effect is a stereo-motion phenomenon. When the two eyes are presented with visual targets moving in fronto-parallel motion at different luminances or contrasts, the perception is of a target moving-in-depth. It is thought that this percept of motion-in-depth occurs because lower luminance or contrast delays the speed of visual processing. Spatial properties of an image such as spatial frequency and size have also been shown to influence the speed of visual processing. In this study, we use a paradigm to measure interocular delay based on the Pulfrich effect where a structure-from-motion defined cylinder, composed of Gabor elements displayed at different interocular phases, rotates in depth. This allows us to measure any relative interocular processing delay while independently manipulating the spatial frequency and size of the micro elements (i.e., Gabor patches). We show that interocular spatial frequency differences, but not interocular size differences of image features, produce interocular processing delays.

Monovision and the Misperception of Motion

Monovision corrections are a common treatment for presbyopia. Each eye is fit with a lens that sharply focuses light from a different distance, causing the image in one eye to be blurrier than the other. Millions of people in the United States and Europe have monovision corrections, but little is known about how differential blur affects motion perception. We investigated by measuring the Pulfrich effect, a stereo-motion phenomenon first reported nearly 100 years ago. When a moving target is viewed with unequal retinal illuminance or contrast in the two eyes, the target appears to be closer or further in depth than it actually is, depending on its frontoparallel direction. The effect occurs because the image with lower illuminance or contrast is processed more slowly. The mismatch in processing speed causes a neural disparity, which results in the illusory motion in depth. What happens with differential blur? Remarkably, differential blur causes a reverse Pulfrich effect, an apparent paradox. Blur reduces contrast and should therefore cause processing delays. But the reverse Pulfrich effect implies that the blurry image is processed more quickly. The paradox is resolved by recognizing that: i) blur reduces the contrast of high-frequency image components more than low-frequency image components, and ii) high spatial frequencies are processed more slowly than low spatial frequencies, all else equal. Thus, this new illusion—the reverse Pulfrich effect—can be explained by known properties of the early visual system. A quantitative analysis shows that the associated misperceptions are large enough to impact public safety.

Ken Jacobs and the Perverted Archival Image

This paper analyses two recent works by American filmmaker Ken Jacobs that deal with aspects of remediation. The first is A Tom Tom Chaser, in which Jacobs records the telecine process that transforms the classic silent film Tom, Tom, the Piper’s Son from chemical into electronic media. The film is riddled with poetic turns inviting the audience to rediscover the medial noise hidden by images. Moreover, Jacobs focuses on the moment of transition from a material medium (the film strip) to the immaterial (the image, the video), so that the noise brings the viewer closer to a perception or brief capture of the medium in itself. Images are both figured and disfigured along this process. The second work is The Guests, an unconventional 3D film in which Jacobs transforms a short take from a Lumière Brothers film by discovering unseen views of the original footage. In his remediation of the 3D technology, Jacobs employs the Pulfrich effect, which allows him to blur the images of the archival film and to create instances of uncertainty between the views coming from the two human eyes. As a result of this procedure, the characters in the film seem to look directly at the audience. The analysis of both films highlights the poetry of the typical manoeuvre by which Jacobs perverts the archival medium, whereupon the viewing mode between media denaturalizes the usual media gaze (framed and representational), focusing on the moment of viewing in itself. This, as a result, favours the medium for what it is and subverts the gaze that expects something representational, discursive, perhaps story-driven.