Binocular Eye Movements Are Adapted to the Natural Environment.

Humans and many animals make frequent saccades requiring coordinated movements of the eyes. When landing on the new fixation point, the eyes must converge accurately or double images will be perceived. We asked whether the visual system uses statistical regularities in the natural environment to aid eye alignment at the end of saccades. We measured the distribution of naturally occurring disparities in different parts of the visual field. The central tendency of the distributions was crossed (nearer than fixation) in the lower field and uncrossed (farther) in the upper field in male and female participants. It was uncrossed in the left and right fields. We also measured horizontal vergence after completion of vertical, horizontal, and oblique saccades. When the eyes first landed near the eccentric target, vergence was quite consistent with the natural-disparity distribution. For example, when making an upward saccade, the eyes diverged to be aligned with the most probable uncrossed disparity in that part of the visual field. Likewise, when making a downward saccade, the eyes converged to enable alignment with crossed disparity in that part of the field. Our results show that rapid binocular eye movements are adapted to the statistics of the 3D environment, minimizing the need for large corrective vergence movements at the end of saccades. The results are relevant to the debate about whether eye movements are derived from separate saccadic and vergence neural commands that control both eyes or from separate monocular commands that control the eyes independently.SIGNIFICANCE STATEMENT We show that the human visual system incorporates statistical regularities in the visual environment to enable efficient binocular eye movements. We define the oculomotor horopter: the surface of 3D positions to which the eyes initially move when stimulated by eccentric targets. The observed movements maximize the probability of accurate fixation as the eyes move from one position to another. This is the first study to show quantitatively that binocular eye movements conform to 3D scene statistics, thereby enabling efficient processing. The results provide greater insight into the neural mechanisms underlying the planning and execution of saccadic eye movements

Creating effective focus cues in multi-plane 3D displays.

Focus cues are incorrect in conventional stereoscopic displays. This causes a dissociation of vergence and accommodation, which leads to visual fatigue and perceptual distortions. Multi-plane displays can minimize these problems by creating nearly correct focus cues. But to create the appearance of continuous depth in a multi-plane display, one needs to use depth-weighted blending: i.e., distribute light intensity between adjacent planes. Akeley et al. [ACM Trans. Graph. 23, 804 (2004)] and Liu and Hua [Opt. Express 18, 11562 (2009)] described rather different rules for depth-weighted blending. We examined the effectiveness of those and other rules using a model of a typical human eye and biologically plausible metrics for image quality. We find that the linear blending rule proposed by Akeley and colleagues [ACM Trans. Graph. 23, 804 (2004)] is the best solution for natural stimuli

Is vertical disparity used to determine azimuth?

AbstractThe azimuth of a stimulus relative to the head can be determined from an extra-retinal, eye-position signal plus an estimate of the retinal eccentricity of the image. Alternatively, azimuth could be determined from retinal-image information alone. Specifically, stimulus azimuth could be estimated from two derivatives of vertical disparity: vertical size ratio (which varies with azimuth), and the horizontal gradient of vertical size ratio (a measure of distance). Here we examine the determinants of perceived azimuth in viewing conditions that, theoretically, should favor the use of vertical disparity. We find no evidence that vertical disparity is used. Perceived azimuth was determined completely by felt eye position and the retinal eccentricity of the image

Vestibular heading discrimination and sensitivity to linear acceleration in head and world coordinates

Effective navigation and locomotion depend critically on an observer\u27s ability to judge direction of linear self-motion, i.e., heading. The vestibular cue to heading is the direction of inertial acceleration that accompanies transient linear movements. This cue is transduced by the otolith organs. The otoliths also respond to gravitational acceleration, so vestibular heading discrimination could depend on (1) the direction of movement in head coordinates (i.e., relative to the otoliths), (2) the direction of movement in world coordinates (i.e., relative to gravity), or (3) body orientation (i.e., the direction of gravity relative to the otoliths). To quantify these effects, we measured vestibular and visual discrimination of heading along azimuth and elevation dimensions with observers oriented both upright and side-down relative to gravity. We compared vestibular heading thresholds with corresponding measurements of sensitivity to linear motion along lateral and vertical axes of the head (coarse direction discrimination and amplitude discrimination). Neither heading nor coarse direction thresholds depended on movement direction in world coordinates, demonstrating that the nervous system compensates for gravity. Instead, they depended similarly on movement direction in head coordinates (better performance in the horizontal plane) and on body orientation (better performance in the upright orientation). Heading thresholds were correlated with, but significantly larger than, predictions based on sensitivity in the coarse discrimination task. Simulations of a neuron/anti-neuron pair with idealized cosine-tuning properties show that heading thresholds larger than those predicted from coarse direction discrimination could be accounted for by an amplitude-response nonlinearity in the neural representation of inertial motion

Cultivation and anaerobic digestion of Scenedesmus spp. grown in a pilot-scale open raceway

Digestibility of a micro-algal mixture was evaluated by mesophilic anaerobic digestion in continuously-stirred tank reactors. The culture consisted primarily of Scenedesmus spp. continuously cultivated over a 6-month period in a 100 m2 raceway reactor instrumented to record pH, dissolved oxygen and temperature. The raceway received supplementary carbon in the form of flue gas from a diesel boiler (10% CO2) injected into a 1-m deep sump to control pH in the range 7.8–8.0. Dilution was optimised to biomass productivity and gave values of 10–15 and 20–25 g total suspended solids (TSS) m? 2 day? 1 in winter (December–February) and spring (April–May), respectively. The culture for the anaerobic digestion trial was harvested in February by centrifugation to give an algal paste containing 4.3% volatile solids (VS). Semi-continuous digestion at organic loading rates of 2.00, 2.75 and 3.50 g VS l? 1 day? 1 gave volumetric biogas productions of ~ 0.66, ~ 0.83 and ~ 0.99 l l? 1 day? 1, respectively. Specific methane yield ranged from 0.13 to 0.14 l CH4 g? 1 VSadded with biogas methane content ~ 62%. Overall the digestion process was stable, but only ~ 30% VS destruction was achieved indicating low biodegradability, due to the short retention times and the recalcitrant nature of this type of biomas

Perceived Visual Direction near an Occluder

When an opaque object occludes a more distant object, the two eyes often see different parts of the distant object. Hering s laws of visual direction make an interesting prediction for this situation: the part seen by both eyes should be seen in a different direction than the part seen by one eye. We examined whether this prediction holds by asking observers to align a vertical monocular line segment with a nearby vertical binocular segment. We found it necessary to correct the alignment data for vergence errors, which were measured in a control experiment, and for monocular spatial distortions, which were also measured in a control experiment. Settings were reasonably consistent with Hering's laws when the monocular and binocular targets were separated by 30 arcmin or more. Observers aligned the targets as if they were viewing them from one eye only when they were separated by 2 arcmin; this behavior is consistent with an observation reported by Erkelens and colleagues. The same behavior was observed when the segments were horizontal and when no visible occluder was present. Perceived visual direction when the two eyes see different parts of a distant target is assigned in a fashion that minimizes, but does not eliminate, distortions of the shape of the occluded object

An analysis of binocular slant contrast

When a small frontoparallel surface (a test strip) is surrounded by a larger slanted surface (an inducer), the test strip is perceived as slanted in the direction opposite to the inducer. This has been called the depth-contrast effect, but we call it the slant-contrast effect. In nearly all demonstrations of this effect, the inducers slant is specified by stereoscopic signals, and other signals, such as the texture gradient, specify that it is frontoparallel. We present a theory of slant estimation that determines surface slant via linear combination of various slant estimators; the weight of each estimator is proportional to its reliability. The theory explains slant contrast because the absolute slant of the inducer and the relative slant between test strip and inducer are both estimated with greater reliability than the absolute slant of the test strip. The theory predicts that slant contrast will be eliminated if the signals specifying the inducers slant are consistent with one another. It also predicts reversed slant contrast if the inducers slant is specified by nonstereoscopic signals rather than by stereo signals. These predictions were tested and confirmed in three experiments. The first showed that slant contrast is greatly reduced when the stereo- and nonstereo-specified slants of the inducer are made consistent with one another. The second showed that slant contrast is eliminated altogether when the stimulus consists of real planes rather than images on a display screen. The third showed that slant contrast is reversed when the nonstereo-specified slant of the inducer varies and the stereo-specified slant is zero. We conclude that slant contrast is a byproduct of the visual systems reconciliation of conflicting information while it attempts to determine surface slant

On the "Causality Argument" in Bouncing Cosmologies

We exhibit a situation in which cosmological perturbations of astrophysical relevance propagating through a bounce are affected in a scale-dependent way. Involving only the evolution of a scalar field in a closed universe described by general relativity, the model is consistent with causality. Such a specific counter-example leads to the conclusion that imposing causality is not sufficient to determine the spectrum of perturbations after a bounce provided it is known before. We discuss consequences of this result for string motivated scenarios.Comment: 4 pages, 1 figure, ReVTeX, to appear in Phys. Rev. Let