1,721,041 research outputs found
Binocular vision with null disparity disrupts the effects of extra-retinal signals
No full textWe studied a novel phenomenon indicating that the integration of extra-retinal information and the optic flow is a necessary but not sufficient condition for slant constancy during active head motion; congruency between monocular and binocular cues to depth is also required.
In two experiments, we measured observers' performance in a rotation-detection task during active vision. Two viewing conditions were compared: binocular vision with null disparity (same image projected to the two eyes) and monocular vision. Static or rotating slanted planar surfaces were simulated with 90/270-deg tilt (Experiment 1) and 0/180-deg tilt (Experiment 2). Observers produced oscillatory lateral head movements which were recorded by an Optotrack Certus system. The position of random dots on a CRT monitor were updated in real time, simulating the correct projection of a random-dot planar surface on the image screen by taking into account the head position of the observer and the rotation of the surface.
Full perceptual constancy was found for simulated stationary surfaces under monocular vision, but not under binocular vision with null disparity. A stationary surface appeared to be stationary when it was viewed monocularly; however, it appeared to be rotating when it was viewed binoculary with null disparity. Moreover, in both experiments, rotation sensitivity was larger for monocular than for binocular vision. Response bias was shifted in opposite directions, with an overall tendency to judge surfaces as being stationary in monocular vs. rotating in binocular vision.
Monocular vision supports full slant constancy. Binocular vision with null disparity (1) disrupts the effect of extra-retinal signals produced by head movement, and (2) induces the perception of an apparent rotation counter to the heading direction
La valutazione delle funzioni di base in soggetti disabili attraverso una scala osservativa
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Reaching movement accuracy is mainly determined by visual online control
No full textHere we demonstrate that the accuracy of reaching a target is affected by neither the representation of the target location nor the visual feedback provided up to 250ms before movement completion. Observers performed reach-to-point movements for a real cylinder positioned at variable locations. We manipulated the visual information (monocular/binocular) provided in two contiguous segments. Main segment: trajectory from the movement start up to 250 ms before movement completion. Final segment: trajectory during the final 250 ms to reach the target. The accuracy of the end position (z-depth) of the reaching movement was measured. We found reliable effects of visual information sampled during the final segment, but not during the main segment. Reaching was accurate if disparity information was provided throughout the whole movement or only in the last 250 ms segment. Overshooting of 1 cm was found when disparity information was absent during the whole movement or in the last 250 ms segment. The latter is surprising, as the target appeared binocular for most of the movement duration allowing for ample time to plan the movement-end. Thus, accuracy in reaching performance can be explained by nulling the relative disparity between the finger and the target. Our results indicate the importance of the online component of visual-motor control
Impaired perception of rigidity induced by the amodal completion of 3D structures in active and passive vision
No full textThe perceived slant difference between twisted patches is known to be reduced by partial occlusion, a phenomenon known as slant assimilation (Fantoni, Gerbino & Kellman, 2004; Liu & Shor, 2005). Fantoni, Gerbino and Kellman (2008) demonstrated that this phenomenon is diagnostic of visual approximation, a process mediating Amodal Completion (AC) of 3D structures that causes a distorted representation of image-specified parts. Approximation-based distortions were previously studied in impoverished viewing conditions (immobile observer) and with impoverished stimuli (stereoscopic patches under orthographic projections). Here, we show that similar distortions occur in more general viewing and stimulus conditions, as when the observer is naturally moving and the twisted surfaces are specified by self-generated optic flows under perspective projections. The sensitivity to discriminate between rigid and non-rigid 3D structures was impaired when the twisted patches appeared as the unoccluded parts of a smooth surface partially hidden by a foreground frontoparallel surface, relative to cases in which they were perceived as separated patches, either in the absence of the foreground occluding surface (Experiment 1) or with the occluding surface in the background (Experiment 2). The exact same biases were also found for passive observers, who experienced from a static vantage point the same optic flows generated during active viewing. Results are compatible with a Bayesian model that disregards egomotion information and selects a reliable prior for stationarity/rigidity in presence but not in absence of amodal completion. Results are incompatible with a model predicting that image encoding is biased towards slant assimilation in presence of AC. This calls for an update of the current notion of visual approximation in AC
Systematic distortions of perceived planar surface motion in active vision
No full textRecent studies suggest that the active observer combines optic flow information with extra-retinal signals resulting from head motion. Such a combination allows, in principle, a correct discrimination of the presence or absence of surface rotation. In Experiments 1 and 2, observers were asked to perform such discrimination task while performing a lateral head shift. In Experiment 3, observers were shown the optic flow generated by their own movement with respect to a stationary planar slanted surface and were asked to classify perceived surface rotation as being small or large. We found that the perception of surface motion was systematically biased. We found that, in active, as well as in passive vision, perceived surface rotation was affected by the deformation component of the first-order optic flow, regardless of the actual surface rotation. We also found that the addition of a null disparity field increased the likelihood of perceiving surface rotation in active, but not in passive vision. Both these results suggest that vestibular information, provided by active vision, is not sufficient for veridical 3D shape and motion recovery from the optic flow
Transcranial magnetic stimulation improves rotation sensitivity for actively viewed planar surfaces
No full textIn previous experiments we measured observers' performance in a rotation-detection task during active vision of structure from motion (SfM) displays. Observers performed a lateral head shift while viewing either monocularly or binocularly the same optic flows consistent with either static or rotating random-dot planar surfaces. An Optotrack Certus system was used to update in real-time the optic flows as a function of observer's head position and orientation. Results showed that the addition of a null disparity field increased the likelihood of perceiving surface rotation causing reduced rotation sensitivity for the binocular relative to the monocular viewing condition. A possible hypothesis for this phenomenon is that the introduction of a null disparity field creates an inconsistency among the depth cues forcing the visual system to interpret the optic flow in a way consistent with disparity (rotating surface far from the point of view) rather than vergence information (static surface located at the level of the screen). In order to test this hypothesis we used low-frequency rTMS over the early visual cortex. Neurophysiological inactivation studies (Ponce et al., 2008) have found that visual areas V2/V3 are selective for the recovery of depth from binocular-disparity information. Two groups of subjects performed the same rotation detection task before and after rTMS or Sham-TMS delivered offline (10min, 1Hz) over V2/V3 targeting binocular disparity-sensitive neurons. Consistent with our hypothesis rTMS induced an improvement in the rotation sensitivity that was selective for binocular condition, while monocular performance remained intact. We conclude that low-frequency rTMS over V2/V3 inhibits binocular disparity-sensitive neurons allowing the visual system to interpret a binocularly viewed optic flow as consistent with retinal motion information and vergence regardless of disparity information
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