Single Landmark Research Articles

Optic-Flow-Based Perception of Two-Dimensional Trajectories and the Effects of a Single Landmark

Human observers can detect their heading direction on a short time scale on the basis of optic flow. We investigated the visual perception and reconstruction of visually travelled two-dimensional (2-D) trajectories from optic flow, with and without a landmark. As in our previous study, seated, stationary subjects wore a head-mounted display in which optic-flow stimuli were shown that simulated various manoeuvres: linear or curvilinear 2-D trajectories over a horizontal plane, with observer orientation either fixed in space, fixed relative to the path, or changing relative to both. Afterwards, they reproduced the perceived manoeuvre with a model vehicle, whose position and orientation were recorded. Previous results had suggested that our stimuli can induce illusory percepts when translation and yaw are unyoked. We tested that hypothesis and investigated how perception of the travelled trajectory depends on the amount of yaw and the average path-relative orientation. Using a structured visual environment instead of only dots, or making available additional extra-retinal information, can improve perception of ambiguous optic-flow stimuli. We investigated the amount of necessary structuring, specifically the effect of additional visual and/or extra-retinal information provided by a single landmark in conditions where illusory percepts occur. While yaw was perceived correctly, the travelled path was less accurately perceived, but still adequately when the simulated orientation was fixed in space or relative to the trajectory. When the amount of yaw was not equal to the rotation of the path, or in the opposite direction, subjects still perceived orientation as fixed relative to the trajectory. This caused trajectory misperception because yaw was wrongly attributed to a rotation of the path: path perception is governed by the amount of yaw in the manoeuvre. Trajectory misperception also occurs when orientation is fixed relative to a curvilinear path, but not tangential to it. A single landmark could improve perception. Our results confirm and extend previous findings that, for unambiguous perception of ego-motion from optic flow, additional information is required in many cases, which can take the form of fairly minimal, visual information.

Visual landmark orientation by flying bats at a large-scale touch and walk screen for bats, birds and rodents

Orientation depends on multi-modal information about the locally perceptible environment (local view) in many situations. We developed a behavioural paradigm for investigating visual orientation of flying bats based on a large-scale touch screen (1.2 m × 1.8 m). It functions by a grid of rows and columns of infra-red beams just in front of a screen with back-projected visual stimuli. Approaching animals interrupt the beams and thus permit automatic recording of the time and place of an animal’s locational choice. We used it as a vertical touch surface. Installed as a horizontal walk surface, it may also serve as a more natural ‘firm ground’, circular arena analogue to the ‘Morris water maze’ for investigating orientation behaviour and spatial cognition from rodents to birds while offering automatic real-time recording of paths, times and latencies with enhanced possibilities to score details of motor behaviour and to control stimuli interactively. Bats offer a unique possibility to investigate the use of both echo-acoustic and visual information processing pathways for the process of self-localization and orientation. In our first experiment, a bat was presented with five identical targets, one central and four peripheral and had to choose the central target. After task acquisition, the array was shifted by the distance between targets, so that a formerly peripheral landmark was now in the absolute location of the formerly central target. At small inter-target distances, the bat ‘went with’ the array, and chose the new central target (at a new absolute location). With 30 cm or more of inter-target distance (60 cm across the landmark configuration), however, the bat went with absolute location, and chose a peripheral target. In experiment 2, the bat was presented with two landmarks 30 cm apart and an unmarked target located at midline beneath them. On tests, the landmarks either maintained training distance or were expanded to 50 cm apart. On such expansion tests, the bat chose most the location at the correct vector from the right landmark. This showed that the bat first identified a single landmark by the configuration and then applied a previously learnt vector (angle and distance) to locate the target. Glossophaga did not orient by pure angular geometry between landmarks and target.

Effect of absolute spatial proximity between a landmark and a goal

In two experiments rats were trained in a Morris pool to find a hidden platform in the presence of a single landmark. Circular black curtains surrounded the pool, with the single landmark inside this enclosure, so that no other room cues could provide additional information about the location of the platform. This landmark was hung from a false ceiling and rotated from trial to trial, and the position of the platform also changed on each trial, thus preserving a constant relation between the platform and the landmark. In Experiment 1, the position of the landmark was exactly above the hidden platform in Group Above and was relatively close to the hidden platform in Group Near. At the end of acquisition, test trials without the platform revealed a difference between the groups. Although a preference for searching in the correct quadrant of the pool, where the platform should have been, was found in both groups, this preference was significantly higher for Group Above. In Experiment 2, new rats in Group Near were compared to rats for which the position of the landmark was relatively far from the hidden platform in Group Far. Test trials revealed a preference for searching in the correct quadrant of the pool in both groups, but this preference was significantly higher for Group Near. The implication of these results is that the control acquired by a single landmark is different depending on its relative distance from a hidden platform: Closer landmarks acquire better control than distant ones. These results show a clear parallelism in comparison with the effect of absolute temporal proximity of the CS to the US in classical conditioning.

Clark's nutcrackers (Nucifraga columbiana) and the effects of goal--landmark distance on overshadowing.

Three groups of Clark's nutcrackers (Nucifraga columbiana) were trained to find a goal location defined by an array of 4 landmarks that varied in goal--landmark distance. The arrays for each group differed in the distance of the closest landmark and contained goal--landmark distances that were common across groups, allowing for the examination of the effects of both relative and absolute goal--landmark distance on encoding of a landmark array. All 3 groups readily learned the task and were subsequently tested in probe tests with only single landmarks from the array available. Search error in tests with single landmarks was compared both within and across groups. Results demonstrated that both relative and absolute goal-landmark distances are important in spatial search.

Indirect landmark use at 6 months of age in a spatial orientation task

It was hypothesized that 6-month-old infants may be able to use indirect landmarks to locate a goal if (a) the landmarks are sufficiently distinctive and (b) the goal location is between landmarks, rather than on the opposite side of the space as was used in earlier research. Six-month-old infants were tested in a peekaboo paradigm in which they had to turn to a target location after displacement to a novel position. Infants looked to the goal location significantly more in a beacon and an indirect landmarks condition relative to a control and a single landmark condition. These results are discussed in terms of current theories of spatial development.

Assessing landmark influence on shape variation

Given two sets of landmark data which differ in shape, it is useful to determine the extent to which shape variation can be explained by the perturbations of individual landmarks. We propose a method for assessing this, based on analysing the relative reduction in distance between the shapes that can be achieved by varying the location of a single landmark. This method is applied to a set of landmark data from the cervical vertebrae of two subspecies of gorillas.

A quantitative three-dimensional assessment of soft tissue facial asymmetry of cleft lip and palate adult patients.

The three-dimensional coordinates of 23 selected soft-tissue facial landmarks were digitized on 18 cleft lip and palate (CLP) white patients (11 male and 7 female patients aged 19-27 years) and 161 healthy controls (73 female and 89 male subjects aged 18-30 years) by an electromagnetic instrument. Facial asymmetry was quantified by detecting a plane of symmetry and the centers of gravity (CG) of the right and left hemifaces and by calculating the distance between the two CG (distance from symmetry [DFS]). Both absolute (millimeters) and percentage (of the nasion center of gravity distance) DFS was obtained. The asymmetry of single landmarks was also quantified. Overall, asymmetry in operated CLP patients appeared only moderately larger than that measured in the healthy reference population, with the largest value being only 5% larger than the maximum normal asymmetry. Female patients had a somewhat larger lateral asymmetry than male patients, and unilateral CLP patients (particularly the men) were more asymmetrical than bilateral CLP patients. Pronasale and subnasale landmarks were asymmetrical in 8 patients, whereas endocanthion, zygion, cheilion, and gonion landmarks were symmetrical in all patients. In conclusion, the facial soft-tissue structures of CLP patients operated on as adults were only moderately more asymmetrical than those measured in a reference group of the same age, sex, and ethnicity.

Spatial distortions induced by multiple visual landmarks: how local distortions combine to produce complex distortion patterns.

Visual landmarks introduce systematic distortions into spatial short-term memory for single target positions, the exact form of the distortion depending on the spatial layout of the landmarks. In two experiments, we investigated how the combined effect of two landmarks can be predicted from the effects of individual landmarks. Participants used a mouse cursor to reproduce the positions of briefly presented targets in the context of one, the other, or both landmarks. We found that distortions near a landmark are independent of whether another landmark is present, so that remembered space is partitioned into regions dominated by single landmarks. Interestingly, the display midpoint behaves like a "virtual landmark," with its own pattern of distortion. Results are inconsistent with current models of spatial memory distortions but suggest that attentional processes can lead to enhanced fidelity of salient regions in topographical neural networks while also introducing some spatial biases.

Fast Landmark Tracking and Localization Algorithm for the Mobile Robot Self-Localization

We present a simple artificial landmark model and a robust landmark tracking alg orithm for mobile robot localization. The landmark model, consisting of symmetric and repetitive color patches, produces color histograms that are invariant under the geometric and photometric distortions. A stochastic approach based on the CONDENSATION tracks the landmark model robustly even under the varying illumination conditions. After the landmark detection, relative position of the mobile robot to the landmark is calculated. Experimental results show that the proposed landmark model is effective and can be detected and tracked in a cluttered scenerobustly. The tracked single landmark is enough to detennine the position of the robot.

The effect of sex and age on facial asymmetry in healthy subjects: A cross-sectional study from adolescence to mid-adulthood

Purpose: The study assessed the effects of sex and age on 3-dimensional (3D) soft-tissue facial asymmetry. Patients and Methods: The 3D coordinates of selected soft-tissue facial landmarks were digitized on 314 healthy white subjects (40 male and 33 female adolescents, aged 12 to 15 years; 73 female and 89 male young adults, aged 18 to 30 years; and 41 male and 38 female adults, aged 31 to 56 years) by an electromagnetic instrument. Facial asymmetry was quantified by detecting a plane of symmetry and the centers of gravity of the right and left hemifaces and by calculating the distance between the 2 centers of gravity (distance from the symmetry [DFS]). Both absolute (millimeters) and percentage (of the nasion-center of gravity distance) DFS were obtained, as well as the maximum normal asymmetry. The asymmetry of single landmarks was also quantified. Results: No gender- or age-related differences were found for both absolute and percentage DFS (P >.05). The maximum normal asymmetry was slightly greater in females than in males of corresponding age; within each sex, the largest values were found in the adolescent group. Tragion, gonion, and zygion were the most asymmetric landmarks in all groups (about 10% to 12% of the nasion-facial center of gravity distance), whereas the least asymmetric was endocanthion (4% to 6%). Conclusions: A slight soft-tissue facial asymmetry was found in normal subjects. The maximum normal asymmetry could be useful in identifying borderline asymmetric patients. © 2001 American Association of Oral and Maxillofacial Surgeons

Blocking in landmark-based search in honeybees

Two experiments tested blocking in landmark-based search in honeybees. Honeybees in the experimental group were trained in Phase 1 with a single landmark in a constant spatial relation to the target (sugar water). In the compound training second phase, the landmark used in Phase 1 (blocking landmark) and a new landmark (blocked landmark) were presented at constant spatial relations to the target. The blocking and blocked landmarks differed from each other in color and position, and the blocking landmark retained the same spatial relationship to the target as in Phase 1. In Experiment 1, the control group experienced only Phase 2 training with two landmarks. In Experiment 2, the control group was trained with a different landmark in a different position in Phase 1. Blocking was found in both cases.

Influence of the shape of the experimental room on spatial learning in rats

Rats and other mammals can orient themselves in their habitat and locate various goals using different learning strategies. When a cartographic or local strategy is used, the animals construct and store in their long-term memories a complex representation of the extramaze space. However, exactly what constitutes the behaviorally key components of the extramaze space is still little known. The purpose of this study was to investigate whether the shape of the environment that surrounds the animal is an informative factor when a cartographic strategy is used. Two experiments were performed in which the rats were trained to find food in the goal arm of a four-arm, plus-shaped maze. Experiment 1 shows that lesions to the hippocampus impair the acquisition of this task, which suggests that in fact, a cartographic strategy is necessary to solve it. In Experiment 2, in which only one group of neurologically intact rats was used, we directly manipulated the shape of the space around the maze. The results showed that once the animals have learned the task, the hiding of all the extramaze single landmarks while keeping intact the general shape of the experimental room does not affect performance. Only when the shape of the surrounding space was modified did the performance deteriorate significantly. These results are discussed in relation to the different components of the extramaze environment and the contribution of each of them to the spatial learning.

Error Recovery of Autonomous Mobile Robot With Global Matching

This paper proposes a new navigation system for an autonomous mobile robot. An earlier version of the robot navigated by recognizing a single landmark at a time, but the landmark was often lost sight of, and as a result, the robot's self-position was lost. We propose a method that enables the robot to recover from such an error. The method determines the self-position by using information on the corner, windows and doors of a room simultaneously if the single landmark is lost.

Spatial localization of a goal: Beacon homing and landmark piloting by rats on a radial maze

We performed six experiments in order to examine the ability of rats to use moving beacons and landmarks as cues to the location of reward on an eight-arm radial maze. In Experiments 1–4, the cues and goals were moved before each trial, and groups in which a single beacon was placed on the rewarded arm, a single landmark indicated that reward was on the arm immediately to the left of a landmark, or two landmarks were placed on each side of the reward arm were compared. The rats rapidly learned to track the reward in the beacon condition, failed to find the reward sooner than chance expectation with a single landmark, and did only slightly better than chance with two landmarks. In Experiments 5 and 6, the rats were trained in five trials per day, with the landmark and goal locations constant over daily rewarded trials, and in two extinction trials that were inserted among the rewarded trials. The rats found the goal arm at substantially better than chance expectancy with both one and two landmarks. Our results, in agreement with data from recent swimming pool experiments (A. D. L. Roberts & Pearce, 1998), show that rats will use the relationship between moving landmarks and a goal in order to find reward.

Geometric rule learning by Clark's nutcrackers (Nucifraga columbiana).

Clark's nutcrackers (Nucifraga columbiana) were trained to search in a location defined by its geometric relationship to 2 landmarks. Two groups were trained to search at different points along the line connecting the landmarks, and 2 groups were trained to find the 3rd point of a triangle, on the basis of either direction or distance from the landmarks. All groups learned and transferred to new interlandmark distances. However, the constant-distance group learned more slowly, searched less accurately, and showed less transfer than the other 3 groups. When tested with new orientations of the landmarks, the birds tended to follow small but not large rotations. When tested with a single landmark, birds in the half, quarter, and constant-bearing groups searched in the appropriate direction from the landmark, but birds in the distance group did not. These results demonstrate that nutcrackers can learn a variety of geometric principles, that directional information may be weighted more heavily than distance information, and that the birds can use both absolute and relative information about spatial relationships.

Distances and directions are computed separately by honeybees in landmark-based search

Honeybees were trained with two landmarks at some angle (e.g., 120°) apart from the target. On crucial unrewarded tests, only a single landmark was present. If distances and directions to landmarks are computed separately (independent averaging), the search distance to the landmark should equal the landmark-target distance found in training. If entire vectors are averaged, the search distance should be much shorter. Three experiments with short target-landmark distances showed results in between the predictions of the two hypotheses. A fourth experiment used longer target-landmark distances and isolated double peaks on single-landmark tests: one predicted by the independent averaging hypothesis, and one very close to the landmark. The near peak is interpreted as arising from approach and exploration of a landmark in a new location, and not from searching.

Hippocampal lesions disrupt navigation based on cognitive maps but not heading vectors.

Animals can find a hidden goal in several ways. They might use a cognitive map that encodes information about the geometric relationship between the goal and two or more landmarks. Alternatively, they might use a heading vector that specifies the direction and distance of the goal from a single landmark. Rats with damage to the hippocampus have difficulty in finding a hidden goal. Here we determine which of the above strategies is affected by such damage. Rats were required to swim in a water maze to a submerged platform, which was always at the same distance and direction from a landmark. The platform and landmark remained in the same place for the four trials of each session, but they were moved to a new position at the start of a session. Rats with damage to the hippocampus found the platform more efficiently than did normal rats in the first trial of a session but, in contrast to normal rats, their performance did not improve during a session. Our results indicate that hippocampally damaged rats are able to navigate by means of heading vectors but not cognitive maps.

Landmark stability: further studies pointing to a role in spatial learning.

Two experiments were conducted to investigate the possible role of landmark stability in spatial learning. Rats were trained to search in a large arena for food hidden at a consistent distance and direction from either a single radially symmetric landmark or an array of two landmarks. We varied the relative degree to which the landmark array and/or the cues of the training context predicted the location of food, without varying the conditional probability of food being available given either cue. Experiment 1 used vestibular disorientation to ensure control of search location by experimenter-controlled cues. The results showed that making either a single landmark or a cluster of two adjacent landmarks the sole spatial predictor of reward location reduced the accuracy of search compared to a condition where both the landmark array and context cues were reliable spatial predictors. Varying global landmark stability had no effect when training was conducted using an array of two landmarks located some distance from each other. Context cues, when tested alone, triggered very little searching in appropriate locations, and the absolute magnitude of control over search was insufficient to account for the superiority of stable landmarks. The better learning with a stable landmark, and the dependence of this effect on the geometrical arrangement of landmarks, points to the conditions of spatial learning involving additional principles to those of simple associative conditioning. Experiment 2 examined landmark stability using a single landmark and fixed directional cues in the absence of vestibular disorientation. This also revealed a relative advantage of landmark stability, but animals with a landmark that moved from trial to trial did show some evidence of learning. Context cues when tested alone had minimal influence. Parametric manipulation of landmark stability offers a novel way of influencing spatial learning and thus understanding better the process through which egocentric representations of perceived space are transformed into allocentric representations of the real world.

Wind drift compensation, flyways, and conservation of diurnal, migrant Neotropical Lepidoptera

The identification and conservation of migration corridors requires that migrating organisms are goal-oriented and capable of adjusting for drifting off-course. Migrating birds are capable of wind drift compensation over water, but no insects have been demonstrated to possess such capabilities. Using vector analysis of individual airspeeds, track directions, ambient windspeeds and wind directions, we quantified within-individual variation in compensation for wind drift in two migrating butterfly and one moth species in natural free flight over a lake. The pierid Aphrissa statira and nymphalid Marpesia chiron butterflies were capable of wind drift compensation, whereas Urania fulgens (Uraniidae) moths were incapable of course correction. Changes in heading across the lake were not indicative of the use of a single landmark for orientation, and thus the use of two landmarks or the vector orientation of the surface beneath the insect were potential orientation cues. Among migrating Aphrissa statira, Marpesia chiron, Phoebis argante, and Urania fulgens, individual headings corrected at least partly for wind drift. For Aphrissa, short-distance compensation over water extended to a long-distance flyway across the isthmus of Panama that is suitable for conservation.

Learning the configuration of a landmark array: I. Touch-screen studies with pigeons and humans.

Pigeons and humans searched on a touch-screen monitor for an unmarked goal located relative to an array of landmarks presented in varied screen locations. After training with the goal centered in various square arrays of 4 landmarks, humans, but not pigeons, transferred accurately to arrays with novel elements. Humans searched in the middle of expanded arrays, whereas pigeons preserved the distance and direction to a single landmark. When trained with the goal centered below 2 identical horizontally aligned landmarks, humans responded to horizontal expansions or contractions of the array by shifting their search vertically, preserving angles from landmarks to goal. Pigeons did not adjust their search vertically. Humans trained with a single landmark adjusted search distance when landmark size was changed. Both pigeons and humans use the configuration of a landmark array, but the underlying processes seem to differ.