Gezichtsbedrog onthult hersenwerking in drie-dimensionale waarneming

Gezichtsbedrog biedt een venster op de werking van de hersenen omdat het onderliggende waarnemings-mechanismen kan onthullen op een manier die niet mogelijk is tijdens waarheidsgetrouwe waarneming. Hier wordt getoond hoe een klassiek voorbeeld van gezichtsbedrog inspiratie geeft voor de ontwikkeling van een hersenmodel voor drie- dimensionale waarneming. Zulke modellen kunnen tal van praktische toepassingen vinden in de geneeskunde, luchtvaart en robotiek

Perceptual learning without feedback and the stability of stereoscopic slant estimation

Subjects were examined for practice effects in a stereoscopic slant estimation task involving surfaces that comprised a large portion of the visual field. In most subjects slant estimation was significantly affected by practice, but only when an isolated surface (an absolute disparity gradient) was present in the visual field. When a second, unslanted, surface was visible (providing a second disparity gradient and thereby also a relative disparity gradient) none of the subjects exhibited practice effects. Apparently, stereoscopic slant estimation is more robust or stable over time in the presence of a second surface than in its absence. In order to relate the practice effects, which occurred without feedback, to perceptual learning, results are interpreted within a cue interaction framework. In this paradigm the contribution of a cue depends on its reliability. It is suggested that normally absolute disparity gradients contribute relatively little to perceived slant and that subjects learn to increase this contribution by utilizing proprioceptive information. It is argued that---given the limited computational power of the brain---a relatively small contribution of absolute disparity gradients in perceived slant enhances the stability of stereoscopic slant perception

Retinal image shifts, but not eye movements per se, cause alternations in awareness during binocular rivalry.

Particularly promising studies on visual awareness exploit a generally used perceptual bistability phenomenon, "binocular rivalry"--in which the two eyes' images alternately dominate--because it can dissociate the visual input from the perceptual output. To successfully study awareness, it is crucial to know the extent to which eye movements alter the input. Although there is convincing evidence that perceptual alternations can occur without eye movements, the literature on their exact role is mixed. Moreover, recent work has demonstrated that eye movements, first, correlate positively with perceptual alternations in binocular rivalry, and second, often accompany covert attention shifts (that were previously thought to be purely mental). Here, we asked whether eye movements cause perceptual alternations, and if so, whether it is either the execution of the eye movement or the resulting retinal image change that causes the alternation. Subjects viewed repetitive line patterns, enabling a distinction of saccades that did produce foveal image changes from those that did not. Subjects reported binocular rivalry alternations. We found that, although a saccade is not essential to initiate percept changes, the foveal image change resulting from a (micro)saccade is a deciding factor for percept dominance. We conclude that the foveal image must change to have a saccade cause a change in awareness. This sheds new light on the interaction between spatial attention shifts and perceptual alternations

INVESTMENT ANALYSIS OF ALTERNATIVE FRUIT TREE SPRAYERS IN MICHIGAN ORCHARDS

Changing orchard sprayer technology and rising pesticide costs to fruit growers raise the need to analyze the profitability of alternative sprayer investments. This study analyzes investments in four orchard sprayers for use in Michigan apple production: an air blast sprayer, a tower boom sprayer, a tower boom sprayer equipped with electronic sensors that activate spray nozzles when foliage is detected, and an air curtain sprayer that targets spray with a layer of forced air. Assuming equal pest control efficacy, the study calculates the annualized net present cost per acre of owning and operating each sprayer for ten years using a baseline discount rate of 10 percent over 200 acres of semi-dwarf apple trees. The analysis found the annualized net present cost per acre, from least to greatest, to be $287 for the air curtain sprayer,$312 for the tower sprayer with electronic sensors, $345 for the plain tower sprayer, and$391 for the conventional air blast sprayer. Sensitivity analysis revealed that the ranking of these cost results was sensitive to farm size, but not to percentage of funds borrowed, discount rate, loan interest rate, or pesticide costs within the ranges investigated. The air curtain sprayer was lowest cost for orchards of 25 acres or more; the conventional air blast sprayer was lowest cost for 10-acre orchards.Crop Production/Industries,

Age effects on visual perceptual decisions of ambiguous stimuli

The brain is constantly making choices while interpreting the environment. To understand how age affects visual decision-making, we investigated age-related changes in spontaneous percept switches and percept choices during intermittent presentations of ambiguous stimuli. Spontaneous switches can be triggered by different visual stimuli, such as monocular ambiguous visual stimuli or binocular rivalry images. An ambiguous visual stimulus has multiple and equally plausible interpretations, such as the bi-stable rotating sphere. In such a sphere two transparently moving dots are moving in opposite directions and due to structure-from-motion the stimulus is perceived as a 3-dimensional rotating sphere moving in one or the opposite direction. During binocular rivalry experiments, the left and the right eye receive different input simultaneously. During stimulus-presentation only one of the two presented images is perceived, and the other image is suppressed. Dominance durations (the time a percept remains dominant) are typically in the order of several seconds. In this study, 52 observers ranging from 17 to 72 years old, viewed bi-stable rotating spheres and binocular rivalry stimuli and were forced to make a choice between two percepts. Stimuli were presented continuously for 2 minutes or intermittently for 1 second, with a range of inter-stimulus intervals (0.125 - 2 seconds). The results show that dominance durations during continuous viewing are longer for older subjects for the binocular rivalry stimulus but not for the bi-stable rotating spheres. For the intermittent stimulus presentation, perceptual alternations decrease at an older age in binocular rivalry, while for the bi-stable rotating sphere there are only differences in perceptual alternations among different age groups at a short off-duration. Based on these results, we conclude that the effect of age is not a general phenomenon for ambiguous stimuli. Visual decisions are more stimulus dependent, rather than experience dependent

Is there an interaction between perceived direction and perceived aspect ratio in stereoscopic vision?

In monocular vision, the horizontal/vertical aspect ratio (shape) of a fronto-parallel rectangle can be based upon the comparison of the perceived directions of the rectangle's edges. In binocular vision of a typical three-dimensional scene (when occlusions are present) this is not the case: fronto-parallel rectangles would be perceived in a distorted fashion if an observer were to base perceived aspect ratio on the perceived directions of the rectangle's edges. We psychophysically investigated stereoscopically perceived aspect ratios of fronto-parallel occluding and occluded rectangles for various distances and fixation depths. We found that observers did not perceive the distortions as predicted on the basis of the above-mentioned comparison of the perceived visual direction of the edges of the rectangle. Our results strongly suggest that the mechanism that determines perceived aspect ratio is dissociated from the mechanism that determines perceived direction. The consequences of the findings for the Kanizsa, Poggendorff, and horizontal/vertical illusions are discussed

Bi-stability in perceived slant when binocular disparity and monocular perspective specify different slants.

We examined how much depth we perceive when viewing a depiction of a slanted plane in which binocular disparity and monocular perspective provide different slant information. We exposed observers to a grid stimulus in which the monocular--and binocular-specified grid orientations were varied independently across stimulus presentations. The grids were slanted about the vertical axis and observers estimated the slant relative to the frontal plane. We were particularly interested in the metrical aspects of perceived slant for a broad spectrum of possible combinations of disparity--and perspective-specified slants. We found that observers perceived only one grid orientation when the two specified orientations were similar. More interestingly, when the monocular--and binocular-specified orientations were rather different, observers experienced perceptual bi-stability (they were able to select either a perspective--or a disparity-dominated percept)

Stability of binocular depth perception with moving head and eyes

We systematically analyse the binocular disparity field under various eye, head and stimulus positions and orientations. From the literature we know that certain classes of disparity which involve the entire disparity field (such as those caused by horizontal lateral shift, differential rotation, horizontal scale and horizontal shear between the entire half-images of a stereogram) lead to relatively poor depth perception in the case of limited observation periods. These classes of disparity are found to be similar to the classes of disparities which are brought about by eye and head movements. Our analysis supports the suggestion that binocular depth perception is based primarily (for the first few hundred milliseconds) on classes of disparity that do not change as a result of ego-movement

Temporal aspects of stereoscopic slant estimation: An evaluation and extension of Howard and Kaneko's theory

We investigated temporal aspects of stereoscopically perceived slant produced by the following transformations: horizontal scale, horizontal shear, vertical scale, vertical shear, divergence and rotation, between the half-images of a stereogram. Six subjects viewed large field stimuli (70 deg diameter) both in the presence and in the absence of a visual reference. The presentation duration was: 0.1, 0.4, 1.6, 6.4 or 25.6 s. Without reference we found the following: Rotation and divergence evoked considerable perceived slant in a number of subjects. This finding violates the recently published results of Howard and Kaneko. Slant evoked by vertical scale and shear was similar to slant evoked by horizontal scale and shear but was generally less. With reference we found the following: Vertical scale and vertical shear did not evoke slant. Slant due to rotation and divergence was similar to slant due to horizontal scale and shear but was generally less. According to the theory of Howard and Kaneko, perceived slant depends on the difference between horizontal and vertical scale and shear disparities. We made their theory more explicit by translating their proposals into linear mathematical expressions that contain weighting factors that allow for both slant evoked by rotation or divergence, subject-dependent underestimation of slant and other related phenomena reported in the literature. Our data for all stimulus durations and for all subjects is explained by this unequal-weighting extension of Howard and Kaneko's theory

Temporal aspects of binocular slant perception

We investigate temporal aspects of binocular slant perception in the presence and absence of a visual reference. Subjects judge slant induced by large-field stereograms of which one half-image is either horizontally scaled or sheared relative to the other half-image. Each stimulus is presented for different ob- servation periods ranging from 0.1 to 19.2 sec. We quantitatively corroborate earlier findings that perceived slant develops significantly faster and to higher levels with visual reference than without it. In daily life, when we are active, there will not be much time for slant to develop. We find that if observation periods are brief (a few seconds or less) slant is poorly perceived if there is no visual reference. We conclude that the visual system is relatively insensitive to large-field horizontal scale and shear