Orientation tuning of shapefrom shading

Four experiments were performed to assess the effect of different orientations and direction of lighting on the visual processing of shaded or bipartite disks. In the first two experiments, observers were presented with nine different shading orientations from 0° to 180°. Targets were detected in a rapid and parallel fashion for shaded disks when the orientation of the shading gradient was not horizontal (90°) or oriented at 67.5°. Search asymmetries favoring the detection of “pock” targets over “ball” targets were found for all orientations. The search rates for bipartite disks were similar to the shaded disks at 0°, 22.5°, and 90° but not for intermediate orientations, and no search asymmetries were found. These differences suggest that shaded displays and bipartite displays are processed by different underlying mechanisms. The third experiment showed that the direction of the light source (left or right) had no influence on search asymmetries around the 90° point. Shading gradient orientation affected magnitude estimates of depth in the fourth experiment. These experiments show that the visual system’s “as-sumption” of overhead lighting is broadly tuned.     

Asymmetry in the detection of shapes from shading in infants1

Abstract: We investigated 3‐ and 4‐month‐old infants’ sensitivity to differences defined by shading using a paired‐comparison familiarity/novelty preference procedure. Infants were familiarized with a pair of displays consisting of homogeneous shaded disks, and then were tested with two displays: the familiar display and a novel one containing shaded disks with reversed polarity (defined as the target). Experiment 1 examined two assumptions on discerning shapes from shading in infants by manipulating the orientations in the shading gradient of stimuli. When the orientation of the shading gradient was vertical, 4‐month‐old infants looked at the novel display for a longer time during the test trial. However, they failed to detect differences when the orientation of shading gradients was horizontal. Three‐month‐old infants did not detect differences in either orientation of the shading gradient. Experiment 2 examined asymmetry in the detection of convex versus concave shapes. Four‐month‐old infants failed to detect the target when the orientation of the shading grating was vertical and the target was convex. Taken with the results of Experiment 1, concave shapes were much easier to detect than convex shapes for 4‐month‐olds. This asymmetry suggests that 4‐month‐old infants process shading information in the same manner as adults.     

Guided Search: The Effects of Learning

The previous report (Efron & Yund, 1996) offered an interpretation of the results of a number of search experiments within the theoretical context of the guided search model of Cave and Wolfe (1990) and Wolfe (1994). The present report extends this interpretation to the effects of extended practice when subjects search for a target defined by its orientation in the presence of a number of heterogeneous distractor items having differing orientations. Three experiments are described: The first revealed that over the course of 21 experimental sessions extending for a period of 6 weeks there were marked decreases in the magnitude of the reaction time gradient (RTG) and the right visual field superiority observed in the previous experiments. This learning persisted for more than 3 years. The second experiment revealed an interference in the capacity to learn to detect a target of one orientation when subjects had previously learned to detect a target of a different orientation at the same locations. The third experiment revealed that the learning was restricted to the area of the visual field where the target had been presented and that subjects could learn to detect two different targets concurrently. The results of these experiments indicate that the learning is orientation-specific and location-specific and is consistent with a localized increase in the selectivity of the top-down selection mechanism of the guided search model.     

Learning to search for visual targets defined by edges or by shading: Evidence for non-equivalence of line drawings and surface representations

A visual search task was used to test the idea that shaded images and their line-drawn analogues are treated identically from an early stage onwards in human vision. Reaction times and error rates were measured to locate the presence or absence of a target in an array of a variable number of distractors. The target was a cube in one orientation and the distractors cubes in a different orientation. The stimuli were defined by lines alone, shading alone, or lines plus shading. Both the slopes and the intercepts of the search functions (graphs of search time against number of displayed items) were higher for the line drawings than for the stimuli defined by shading. Over six experimental sessions, both the slopes and the intercepts fell for all stimuli, but the relative differences between them were maintained. The data suggest that, at an equivalent stage of practice, line-drawn stimuli are processed more slowly than shaded stimuli in early vision.     

Visual search for direction of shading is influenced by apparent depth.

Recent reports of rapid visual search for some feature conjunctions suggested that preattentive vision might be sensitive to scene-based as well as to image-based features (Enns & Rensink, 1990a, 1990b). This study examined visual search for targets defined by the direction of a luminance gradient, a conjunction of luminance and relative location that often corresponds to object curvature and direction of lighting in naturalistic scenes. Experiment 1 showed that such search is influenced by several factors, including the type of gradient, the shape of the contour enclosing the gradient, and the background luminance. These factors were varied systematically in Experiment 2 in a three-dimensionality rating task and in a visual-search task. The factors combined interactively in the rating task, supporting the presence of an emergent property of three-dimensionality. In contrast, each factor contributed only additively to the speed of the visual-search task. This is inconsistent with the view that search is guided by specialized detectors for surface curvature or direction of lighting. Rather, it is in keeping with the view that search is governed by a number of "quick and dirty" processes that are implemented rapidly and in parallel across the visual field.     

Shape from shading in pigeons

Light is the origin of vision. The pattern of shading reflected from object surfaces is one of several optical features that provide fundamental information about shape and surface orientation. To understand how surface and object shading is processed by birds, six pigeons were tested with differentially illuminated convex and concave curved surfaces in five experiments using a go/no-go procedure. We found that pigeons rapidly learned this type of visual discrimination independent of lighting direction, surface coloration and camera perspective. Subsequent experiments varying the pattern of the lighting on these surfaces through changes in camera perspective, surface height, contrast, material specularity, surface shape, light motion, and perspective movement were consistent with the hypothesis that the pigeons were perceiving these illuminated surfaces as three-dimensional surfaces containing curved shapes. The results suggest that the use of relative shading for objects in a visual scene creates highly salient features for shape processing in birds.     

Target-tilt and vertical-hemifield asymmetries in free-scan search for 3-D targets.

In this study, asymmetries in finding pictorial 3-D targets defined by their tilt and rotation in space were investigated by means of a free-scan search task. In Experiment 1, feature search for cube tilt and rotation, as assessed by a spatial forced-choice task, was slow but still exhibited a characteristic "flat" slope; it was also much faster to upward-tilted cubes and to targets located in the upper half of the search field. Faster search times for cubes and rectangular solids in the upper field, an advantage for upward-tilted cubes, and a strong interaction between target tilt and direction of lighting (upward or downward) for the rectangular solids were all demonstrated in Experiment 2. Finally, an advantage in searching for tilted cubes located in the upper half of the display was shown in Experiment 3, which used a present-absent search task. The results of this study confirm that the upper-field bias in visual search is due mainly to a biased search mechanism and not to the features of the target stimulus or to specific ecological factors.     

New reflections on visual search: Interitem symmetry matters!

A 90 degrees rotation of a display can turn a relatively easy visual search into a more difficult one. A series of experiments examined the possible causes of this effect, including differences in overall item shape and response mapping (Experiment 1), the interpretation of scene lighting (Experiment 2), the axis of internal symmetry of the search items (Experiment 3), and the axes of interitem symmetry between target and distractor items (Experiment 4). Only the elimination of differences in interitem mirror symmetry resulted in equal search efficiency in the upright and rotated displays. This finding is strong support for the view that visual search is guided by an analysis that considers interitem relations.     

Visual search for direction of shading is influenced by apparent depth

Recent reports ofrapid visual search for some feature conjunctions-suggested that preattentive vision might be sensitive to scene-based as well as to image-based features (Enns & Rensink, 1990a, 1990b). This study examined visual search for targets defined by the direction of a luminance gradient, a conjunction of luminance and relative location that often: corresponds to ob-= ject curvature and direction of lighting in naturalistic scenes. Experiment 1 showed that such search is influenced by several factors, including the type of gradient, the shape of the contour enclosing the gradient, and the background luminance. These factors were varied systematically in Experiment 2 in a three-dimensionality rating task and in a visual-search task. The factors combined interactively in the rating task, supporting the presence of an emergent property of three-dimensionality. In contrast, each factor contributed only additively to the speed of the visualsearch task. This is inconsistent with the view that search is guided by specialized detectors for surface curvature or direction of lighting. Rather, it is in keeping with the view that search is governed by a number of “quick and dirty” processes that are implemented rapidly and in parallel across the visual field.     

Orientation perception in rhesus monkeys (<Emphasis Type="Italic">Macaca mulatta</Emphasis>)

It was previously demonstrated that monkeys divide the orientation continuum into cardinal and oblique categories. However, it is still unclear how monkeys perceive within-category orientations. To better understand monkeys' perception of orientation, two experiments were conducted using five monkeys. In experiment 1, they were trained to identify either one cardinal or one oblique target orientation out of six orientations. The results showed that they readily identified the cardinal target whether it was oriented horizontally or vertically. However, a longer training period was needed to identify the oblique target orientation regardless of its degree and direction of tilt. In experiment 2, the same monkeys were trained to identify two-oblique target orientations out of six orientations. These orientations were paired, either sharing the degree of tilt, direction of tilt, or neither property. The results showed that the monkeys readily identified oblique orientations when they had either the same degree or direction of tilt. However, when the target orientations had neither the same degree nor direction of tilt, the animals had difficulty in identifying them. In summary, horizontal and vertical orientations are individually processed, indicating that monkeys do not have a category for cardinal orientation, but they may recognize cardinal orientations as non-obliques. In addition, monkeys efficiently abstract either the degree or the direction of tilt from oblique orientations, but they have difficulty combining these features to identify an oblique orientation. Thus, not all orientations within the oblique category are equally perceived.     

The role of categorization in visual search for orientation.

Visual search for 1 target orientation is fast and virtually independent of set size if all of the distractors are of a single, different orientation. However, in the presence of distractors of several orientations, search can become inefficient and strongly dependent on set size (Exp. 1). Search can be inefficient even if only 2 distractor orientations are used and even if those orientations are quite remote from the target orientation (e.g. 20 degrees or even 40 degrees away, Exp. 2). Search for 1 orientation among heterogeneous distractor orientations becomes more efficient if the target orientation is the only item possessing a categorical attribute such as steep, shallow (Exp. 3), tilted left or tilted right (Exp. 4), or simply tilted (Exps. 5 and 6). Orientation categories appear to be 1 of several strategies used in visual search for orientation. These serve as a compromise between the limits on parallel visual processing and the demands of a complex visual world.     

Tracking objects that move where they are headed

Previous work has demonstrated that the ability to keep track of moving objects is improved when the objects have unique visual features, such as color or shape. In the present study, we investigated how orientation information is used during the tracking of objects. Orientation is an interesting feature to explore in moving objects because it is directional and is often informative of the direction of motion. Most objects move forward, in the direction they are oriented. In the present experiments, participants tracked a subset of moving isosceles triangles whose orientations were constant, related, or unrelated to the direction of motion. In the standard multiple object tracking (MOT) task, tracking performance improved when orientations were unique and remained constant, but not when orientation and direction of motion were aligned. In the target recovery task, in which MOT was interrupted by a brief blanking of the display, performance did improve when orientation and direction were aligned. In the final experiment, results showed that orientation was not used before the blank to predict future target locations, but was instead used after the blank. We concluded that people use orientation to compare a stored representation to target position for recovery of lost targets.     

视觉搜索任务中直视探测优势的眼动研究

已有研究发现在视觉搜索任务中对直视的探测比斜视更快且更准确, 该现象被命名为“人群中的凝视效应”。大多数研究者将该效应的产生归因于直视会捕获更多的注意。然而, 直视条件下对搜索项的匹配加工更容易也有可能导致对直视的探测比斜视快。此外,已有研究还发现头的朝向会影响对注视方向的探测, 但对于其产生原因缺乏实验验证。本研究采用视觉搜索范式, 运用眼动技术, 把注视探测的视觉搜索过程分为准备阶段、搜索阶段和反应阶段, 对这两个问题进行了探讨。结果显示：对直视的探测优势主要表现在搜索阶段和反应阶段; 在搜索阶段直视的探测优势获益于搜索路径的变短和分心项数量的变少以及分心项平均注视时间的变短; 头的朝向仅在搜索阶段对注视探测产生影响。该结果表明, 在直视探测中对搜索项的匹配加工比在斜视探测中更容易也是导致“人群中的凝视效应”的原因之一; 头的朝向仅仅影响了对注视方向的搜索并没有影响对其的确认加工。     

Visual search for simple volumetric shapes.

Five experiments measured reaction time (RT) to detect the presence or absence of a simple volumetric shape (target) dependent on the number of display items (distractors) and their depicted three-dimensional (3-D) orientation. Experiments 1-4 examined every pairwise combination of two different simple volumetric shapes in two 3-D orientations. Conditions exhibiting "pop-out" could be predicted by differences in their two-dimensional (2-D) features. Conditions in which search was slower support previously found search asymmetries for particular 2-D features. When the distractors were a mixture of the other shapes in the same 3-D orientation, search was serial, except when the target had a curved principal axis (Experiment 5). The results suggest that these simple volumetric shapes are not processed preattentively.     

The Thatcher illusion: rotating the viewer instead of the picture

Faces are difficult to recognise when presented upside down. This effect of face inversion was effectively demonstrated with the 'Thatcher illusion' by Thompson (1980 Perception 9 483-484). It has been tacitly assumed that this effect is due to inversion relative to retinal coordinates. Here we tested whether it is due to egocentric (i.e. retinal) inversion or whether the orientation of the body with respect to gravity also influences the face-inversion effect. A 3-D human turntable was used to test subjects in 5 different body-tilt (roll) orientations: 0 degree, 45 degrees, 90 degrees, 135 degrees, and 180 degrees. The stimuli consisted of 4 'normal' and 4 'thatcherised' faces and were presented in 8 different orientations in the picture plane. The subjects had to decide in a yes-no task whether the faces were 'normal' or 'thatcherised'. Analysis of the d' values revealed a significant effect of stimulus orientation and body tilt. The significant effect of body tilt was due to a drop in d' values in the 135 degrees orientation. This result is compared to findings of studies on the subjective visual vertical, where larger errors occurred in body-tilt orientations between 90 degrees and 180 degrees. The present findings suggest that the face-inversion effect relies mainly on retinal coordinates, but that in head-down body-tilt orientations around 135 degrees the gravitational reference frame has a major influence on the perception of faces.     

Visual search in a multi-element asynchronous dynamic (MAD) world

In visual search tasks participants search for a target among distractors in strictly controlled displays. We show that visual search principles observed in these tasks do not necessarily apply in more ecologically valid search conditions, using dynamic and complex displays. A multi-element asynchronous dynamic (MAD) visual search was developed in which the stimuli could either be moving, stationary, and/or changing in luminance. The set sizes were high and participants did not know the specific target template. Experiments 1 through 4 showed that, contrary to previous studies, search for moving items was less efficient than search for static items and targets were missed a high percentage of the time. However, error rates were reduced when participants knew the exact target template (Experiment 5) and the difference in search efficiency for moving and stationary targets disappeared when lower set sizes were used (Experiment 6). In all experiments there was no benefit to finding targets defined by a luminance change. The data show that visual search principles previously shown in the literature do not apply to these more complex and "realistically" driven displays.     

On the perception of shape from shading.

The extraction of three-dimensional shape from shading is one of the most perceptually compelling, yet poorly understood, aspects of visual perception. In this paper, we report several new experiments on the manner in which the perception of shape from shading interacts with other visual processes such as perceptual grouping, preattentive search ("pop-out"), and motion perception. Our specific findings are as follows: (1) The extraction of shape from shading information incorporates at least two "assumptions" or constraints--first, that there is a single light source illuminating the whole scene, and second, that the light is shining from "above" in relation to retinal coordinates. (2) Tokens defined by shading can serve as a basis for perceptual grouping and segregation. (3) Reaction time for detecting a single convex shape does not increase with the number of items in the display. This "pop-out" effect must be based on shading rather than on differences in luminance polarity, since neither left-right differences nor step changes in luminance resulted in pop-out. (4) When the subjects were experienced, there were no search asymmetries for convex as opposed to concave tokens, but when the subjects were naive, cavities were much easier to detect than convex shapes. (5) The extraction of shape from shading can also provide an input to motion perception. And finally, (6) the assumption of "overhead illumination" that leads to perceptual grouping depends primarily on retinal rather than on "phenomenal" or gravitational coordinates. Taken collectively, these findings imply that the extraction of shape from shading is an "early" visual process that occurs prior to perceptual grouping, motion perception, and vestibular (as well as "cognitive") correction for head tilt. Hence, there may be neural elements very early in visual processing that are specialized for the extraction of shape from shading.     

Orientation-invariant object recognition: evidence from repetition blindness

The question of whether object recognition is orientation-invariant or orientation-dependent was investigated using a repetition blindness (RB) paradigm. In RB, the second occurrence of a repeated stimulus is less likely to be reported, compared to the occurrence of a different stimulus, if it occurs within a short time of the first presentation. This failure is usually interpreted as a difficulty in assigning two separate episodic tokens to the same visual type. Thus, RB can provide useful information about which representations are treated as the same by the visual system. Two experiments tested whether RB occurs for repeated objects that were either in identical orientations, or differed by 30, 60, 90, or 180 degrees . Significant RB was found for all orientation differences, consistent with the existence of orientation-invariant object representations. However, under some circumstances, RB was reduced or even eliminated when the repeated object was rotated by 180 degrees , suggesting easier individuation of the repeated objects in this case. A third experiment confirmed that the upside-down orientation is processed more easily than other rotated orientations. The results indicate that, although object identity can be determined independently of orientation, orientation plays an important role in establishing distinct episodic representations of a repeated object, thus enabling one to report them as separate events.     

Attentional set for axis of symmetry in symmetry-defined visual search

Olivers and van der Helm (1998) showed that symmetry-defined visual search (for both symmetry and asymmetry) requires selective spatial attention. We hypothesize that an attentional set for the orientation of a symmetry axis also is involved in symmetry-defined visual search. We conducted three symmetry-defined visual search experiments with manipulations of the axis of symmetry orientations, and performance was better when the axis orientations within the search array were uniform, rather than a mixture of two orientations, and the attentional set for the axis orientation could be kept. In addition, search performance when the target was defined by the presence of symmetry was equivalent to that when the target was defined by a difference of symmetry axis orientation. These results suggest that attentional set for axis orientation plays a fundamental role in symmetry-defined visual search.