The control parameters used by the CNS to guide the hand depend on the visuo-motor task: evidence from visually guided pointing.

To perform visually guided hand movements the visuo-motor system transforms visual information into movement parameters, invoking both central and peripheral processes. Central visuo-motor processes are active in the CNS, whereas peripheral processes are active at the neuromuscular junction. A major share of research attention regarding central visuo-motor processes concerns the question which parameters the CNS controls to guide the hand from one point to another. Findings in the literature are inconsistent. Whereas some researchers suggest that the CNS controls the hand displacement vector, others suggest that it controls final hand position. The current paper introduces a paradigm and analysis method designed to identify the parameters that the CNS controls to guide the hand. We use simulations to validate our analysis in the presence of peripheral visuo-motor noise and to estimate the level of peripheral noise in our data. Using our new tools, we show that hand movements are controlled either in terms of the hand displacement vector or in terms of final hand position, depending on the way visual information relevant for movement production is specified. Interestingly, our new analysis method reveals a difference in central visuo-motor processes, even though a traditional analysis of movement endpoint distributions does not. We estimate the level of peripheral noise in our data to be less than or equal to 40%. Based on our results we conclude that the CNS is flexible with regard to the parameters it controls to guide the hand; that spatial distributions of movement endpoints are not necessarily indicative of central visuo-motor processes; and that both peripheral and central noise has to be carefully considered in the interpretation of movement data.

Extracting 3D from motion: differences in human and monkey intraparietal cortex.

We compared three-dimensional structure-from-motion (3D-SFM) processing in awake monkeys and humans using functional magnetic resonance imaging. Occipital and midlevel extrastriate visual areas showed similar activation by 3D-SFM stimuli in both species. In contrast, intraparietal areas showed significant 3D-SFM activation in humans but not in monkeys. This suggests that human intraparietal cortex contains visuospatial processing areas that are not present in monkeys.

On the affine structure of perceptual space.

Affine geometry is a generalization of Euclidean geometry in which distance can be scaled along parallel directions, though relative distances in different directions may be incommensurable. This article presents a new procedure for testing the intrinsic affine structure of a psychological space by having subjects perform bisection judgments over multiple directions. If those judgments are internally consistent with one another, they must satisfy a theorem first proved by Pierre Varignon around 300 years ago. In the experiment reported here, this procedure was employed to measure the perceived structure of a visual ground surface. The results revealed that observers' judgments were systematically distorted relative to the physical environment, but that the judged bisections in different directions had an internally consistent affine structure. Implications of these findings for other possible response tasks are considered.

Ambiguity and the 'mental eye' in pictorial relief.

Photographs of scenes do not determine scenes in the sense that infinitely many different scenes could have given rise to any given photograph. In psychophysical experiments, observers have (at least partially) to resolve these ambiguities. The ambiguities also allow them to vary their response within the space of 'veridical' responses. Such variations may well be called 'the beholder's share' since they do not depend causally on the available depth cues. We determined the pictorial relief for four observers, four stimuli, and four different tasks. In all cases we addressed issues of reliability (scatter on repeated trials) and consistency (how well the data can be explained via a smooth surface, any surface). All data were converted to depth maps which allows us to compare the relief from the different operationalisations. As expected, pictorial relief can differ greatly either between observers (same stimulus, same task) or between operationalisations (same observer, same stimulus). However, when we factor out the essential ambiguity, these differences almost completely vanish and excellent agreement over tasks and observers pertains. Thus, observers often resolve the ambiguity in idiosyncratic ways, but mutually agree--even over tasks--in so far as their responses are causally dependent on the depth cues. A change of task often induces a change in 'mental perspective'. In such cases, the observers switch the 'beholder's share', which resolves the essential ambiguity through a change in viewpoint of their 'mental eye'.

The visual perception of surface orientation from optical motion.

Observers viewed monocular animations of rotating dihedral angles and were required to indicate their perceived structures by adjusting the magnitude and orientation of a stereoscopic dihedral angle. The motion displays were created by directly manipulating various aspects of the image velocity field, including the mean translation, the horizontal and vertical velocity gradients, and the manner in which these gradients changed over time. The adjusted orientation of each planar facet was decomposed into components of slant and tilt. Although the tilt component was estimated with a high degree of accuracy, the judgments of slant exhibited large systematic errors. The magnitude of perceived slant was determined primarily by the magnitude of the velocity gradient scaled by its direction. The results also indicate that higher order temporal derivatives of the moving elements had little effect on observers' judgments.

Human cortical regions involved in extracting depth from motion.

We used functional magnetic resonance imaging (fMRI) to investigate brain regions involved in extracting three-dimensional structure from motion. A factorial design included two-dimensional and three-dimensional structures undergoing rigid and nonrigid motions. As predicted from monkey data, the human homolog of MT/V5 was significantly more active when subjects viewed three-dimensional (as opposed to two-dimensional) displays, irrespective of their rigidity. Human MT/V5+ (hMT/V5+) is part of a network with right hemisphere dominance involved in extracting depth from motion, including a lateral occipital region, five sites along the intraparietal sulcus (IPS), and two ventral occipital regions. Control experiments confirmed that this pattern of activation is most strongly correlated with perceived three-dimensional structure, in as much as it arises from motion and cannot be attributed to numerous two-dimensional image properties or to saliency.

Stereoscopic discrimination of interval and ordinal depth relations on smooth surfaces and in empty space.

In a series of three experiments, observers judged the perceived relative depths of small probe dots, which could be presented in empty space or attached to a smoothly curved surface. Discriminations of ordinal depth were found to be more precise than discriminations of depth intervals. The amount of separation in the projected image between the locations in depth was also manipulated. Performance was higher when observers evaluated the depth relationships between nearby points in the projected images, and lower when the points were more widely separated. This effect was most pronounced when there was a continuous surface in between the points, suggesting that accurate knowledge of the three-dimensional structure of surfaces is primarily limited to relatively small local neighborhoods.

On the relative salience of Euclidean, affine, and topological structure for 3-D form discrimination.

A match-to-sample task was performed, in which observers compared configurations of line segments presented stereoscopically in different three-dimensional orientations. Several different structural properties of these configurations were manipulated, including the relative orientations of line segments (a Euclidean property), their coplanarity (an affine property), and their patterns of cointersection (a topological property). Although the differences in these properties to be detected were all metrically equivalent, they varied dramatically in their relative perceptual salience, such that the error rates and reaction times in the three conditions varied by as much as 400%. Performance was highest in the topological condition, intermediate in the affine condition, and lowest in the Euclidean condition. These findings suggest that the relative perceptual salience of object properties may be systematically related to their structural stability under change, in a manner that is similar to the Klein hierarchy of geometries.

Opponent motion interactions in the perception of transparent motion.

Interactions in the perception of motion transparency were investigated using a signal-detection paradigm. The stimuli were the linear sum of two independent, moving, random-check "signal" textures and a third texture consisting of dynamic random "noise." Performance was measured as the ratio of squared signal and noise contrasts was varied (S2/N2). Motion detectability was poorest when the two signal textures moved in opposite directions (180 degrees), intermediate when they moved in the same direction (0 degrees), and best when the textures moved in directions separated by 90 degrees in the stimulus plane. This pattern of results held across substantial variations in velocity, field size, duration, and texture-element size. Motion identification was also impaired, relative to 0 degrees, in the 180 degrees but not in the 90 degrees condition. These results are consistent with the idea that performance in the opponent-motion condition is limited by inhibitory (or suppressive) interactions. These interactions, however, appear to be direction specific: little, if any, inhibition was observed for perpendicular motion.

The perception of surface curvature from optical motion.

Observers viewed the optical flow field of a rotating quadric surface patch and were required to match its perceived structure by adjusting the shape of a stereoscopically presented surface. In Experiment 1, the flow fields included rigid object rotations and constant flow fields with patterns of image acceleration that had no possible rigid interpretation. In performing their matches, observers had independent control of two parameters that determined the surface shape. One of these, called the shape characteristic, is defined as the ratio of the two principle curvatures and is independent of object size. The other, called curvedness, is defined as the sum of the squared principle curvatures and depends on the size of the object. Adjustments of shape characteristic were almost perfectly accurate for both motion conditions. Adjustments of curvedness, on the other hand, were systematically over-estimated and were not highly correlated with the simulated curvedness of the depicted surface patch. In Experiment 2, the same flow fields were masked with a global pattern of curl, divergence, or shear, which disrupted the first-order spatial derivatives of the image velocity field, while leaving the second-order spatial derivatives invariant. The addition of these masks had only negligible effects on observers' performance. These findings suggest that observers' judgments of three-dimensional surface shape from motion are primarily determined by the second-order spatial derivatives of the instantaneous field of image displacements.

Perceptual localization of surface position.

In 2 experiments, observers were required to identify corresponding points on an object viewed from multiple orientations. On each trial, a surface was presented initially with a single target location marked by a small dot. Following a brief blank interval, the same surface was presented again at a different orientation. The observer was required to position an adjustable probe dot in this 2nd display to match the location of the target in the 1st view. Under optimal conditions, the variance in their settings over multiple trials was just a few minutes of arc, though these errors varied significantly with the structural complexity of the depicted surface.

The visual contour in depth.

We asked subjects to match points on the surface of a smooth three-dimensional (3-D) shape with points on the surface of another object that was geometrically identical to the first object but was placed in a different pose, was differently textured, and was differently shaded. In all cases, the fiducial point was on the rim of one of the objects (i.e., the boundary of the visible region of the surface), whereas the matching point was well within the silhouette of the other object. This allowed us to draw (preliminary) conclusions concerning the way monocular human observers are able to handle the neighborhood of the rim, where the local slant assumes arbitrarily high values. All experiments were done in real space with real objects (no computer-simulated scenes), the points being indicated with laser beam illumination. The subject was given control over the direction of the laser beams and was thus able to perform the task by adjustment from the vantage position. We studied both consistency (whether the subject's judgments were invariant against changes of relative pose) and veridicality (whether the depth of the visual contour as calculated from the settings agreed with the true distance as measured by mechanical means). Subjects caught much of the 3-D structure of the contour but did deviate appreciably and apparently idiosyncratically from the true geometry.

Effects of texture, illumination, and surface reflectance on stereoscopic shape perception.

Observers viewed computer-generated stereograms of randomly structured smooth surfaces and were required to judge the perceived local orientation at numerous probe points by adjusting a monocular gauge figure. The surfaces were depicted with specular or Lambertian reflectance functions, either with or without identifiable texture elements, and with varying directions of illumination. The results revealed a strong linear correlation between the judged patterns of relief and the actual depicted objects, though there were systematic differences in the magnitude of depth scaling in the different conditions. In general, the accuracy and reliability of observers' judgments for the smoothly shaded shiny surfaces was slightly lower than for the textured surfaces and slightly higher than for the smoothly shaded Lambertian surfaces. The direction of illumination had no detectable effect on the observers' judgments.

The generation and maintenance of schedule-induced polydipsia in normal male rats without weight reduction.

Experiment One demonstrated that two normal male Sprague-Dawley rats (approximately 60 days old) with free access to food and two control rats whose weights were held constant by dietary restriction acquired schedule-induced polydipsia (SIP) in daily 33-35 min sessions of fixed-time 60-s food delivery. Three of the rats showed rapid acquisition of SIP; the fourth acquired SIP more slowly and consumed less per session the other three rats. After a 36-40 day period without sessions, the constant-weight rats showed a 37% decrease in overall consumption due to reduced drinking bout length. The SIP of the free-feeding rats was not affected by the interruption. After 90-100 periodic food delivery sessions, all subjects consumed an average of 11.2-12.2 mL per session compared with 1.8-4.8 mL per session in baseline sessions with massed food presentations. Experiment Two replicated the acquisition phase of Experiment One using two non-weight-reduced rats of the age and size of those typically used in SIP studies (approximately 30 weeks old). Both acquired SIP, although one showed only a small average increase in consumption per session over baseline (2.8 mL/session under periodic food vs. 0.8 mL following massed-food presentations). Before weight reduction, the stronger drinker consumed approximately 8.8 mL per session compared with an average of 0.6 mL per session in baseline. After weight reduction, both exhibited strong SIP (18-19 mL per session in the final five sessions). This study demonstrates that weight reduction is not a necessary condition for the generation and maintenance of SIP in rats.

Surface range and attitude probing in stereoscopically presented dynamic scenes.

The authors report on different methods to probe the structure of visually perceived surfaces in 3 dimensions. The surfaces are specified by patterns of shading with Lambertian and specular components, which deform over time and over stereoscopic views. Five observers performed 2 probe tasks, 1 involving the adjustment of a punctate probe so as to be on the apparent surface and the other involving the adjustment of a small gauge figure that indicates surface attitude. The authors found that these rather different methods yielded essentially identical depth maps up to a linear transformations and that the observers all deviate slightly from veridicality in basically identical ways. The nature of this deviation appears to be correlated with the rough topography of the specularities.

Perception of local three-dimensional shape.

The authors present a series of 4 experiments designed to test the ability to perceive local shape information. Observers were presented with various smoothly varying 3-dimensional surfaces where they reported shape index and sign of Gaussian curvature at several probe locations. Results show that observers are poor at making judgments based on these local measures, especially when the region surrounding the local point is restricted or manipulated to make it noncoherent. Shape index judgments required at least 2 degrees of context surrounding the probe location, and performance on sign of Gaussian curvature judgments deteriorated as the contextual information was restricted as well.

The visual perception of rigid motion from constant flow fields.

Four experiments investigated observers' judgments of rigidity for different types of optical motion. The depicted structural deformations were of two types: (1) those with nonparallel image trajectories that are detectable from the first-order spatiotemporal relations between pairs of views; and (2) those with parallel image trajectories that can only be detected from higher order relations among three or more views. Patterns were composed of smooth flow fields in Experiments 1 and 3, and of wire frame figures in Experiments 2 and 4. For both types of display, the nonrigidity detectable from the first-order spatiotemporal structure of the motion sequence was much more salient than the deformation detectable only from the higher order spatiotemporal structure. These results indicate that observers' judgments of rigidity are based primarily on a two-view analysis, but that some useful information can be obtained under appropriate circumstances from higher order spatiotemporal relations among three or more views.

Effects of changing viewing conditions on the perceived structure of smoothly curved surfaces.

Observers' perceptions of a male and a female torso were investigated using monocular and stereoscopic images under varying conditions of illumination. Observers judged the shapes of these torsos by adjusting a gauge figure to estimate the local slant and tilt at numerous probe points arranged in a lattice over the torso's surface. The results revealed that the judged surfaces in the monocular and stereoscopic conditions were related by an affine stretching transformation in depth that accounted for approximately 95% of the between-condition variance. There was also a strong affine component between the judgments obtained for the different illumination directions, although a further analysis of the residuals indicated that changing the direction of illumination influenced perceived structure in a piecewise manner.

The visual perception of three-dimensional length.

A set of 4 experiments evaluated observers' sensitivity to three-dimensional (3-D) length, using both discrimination and adjustment paradigms with computer-generated optical patterns and real objects viewed directly in a natural environment. Although observers were highly sensitive to small differences in two-dimensional length for line segments presented in the frontoparallel plane, their discrimination thresholds increased by an order of magnitude when the line segments were presented at random orientations in 3-D space. There were also large failures of constancy, such that the perception of 3-D length varied systematically with viewing distance, even under full-cue conditions.

The discriminability of local surface structure.

The ability of observers to discriminate depth and orientation differences between separated local regions on object surfaces was examined. The objects were defined by many optical sources of information simultaneously, including shading, texture, motion, and binocular disparity. Despite the full-cue nature of the displays, the observers' performance was relatively poor, with Weber fractions ranging from 10% to 40%. The Weber fractions were considerably lower for discriminations of surface-orientation differences than for similar discriminations of depth differences. The ability of observers to discriminate surface-orientation differences was approximately invariant over the separation of the regions in the projected image. In contrast, the ability to discriminate depth differences was highly influenced by the amount of image separation. This qualitative difference between the perception of depth intervals and surface-orientation differences suggests that knowledge of depths and orientations may be represented separately within the human visual system.