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T S Collett

Publications and source records attributed to T S Collett.

At least 19 recordsLinked to original sources
PubMed2003

Do familiar landmarks reset the global path integration system of desert ants?

It is often suggested that animals may link landmark memories to a global coordinate system provided by path integration, thereby obtaining a map-like representation of familiar terrain. In an attempt to discover if desert ants form such associations we have performed experiments that test whether desert ants recall a long-term memory of a global path integration vector on arriving at a familiar food site. Ants from three nests were trained along L-shaped routes to a feeder. Each route was entirely within open-topped channels that obscured all natural landmarks. Conspicuous artificial landmarks were attached to the channelling that formed the latter part of the route. The homeward vectors of ants accustomed to the route were tested with the foodward route, either as in training, or with the first leg of the L shortened or extended. These ants were taken from the feeder to a test area and released, whereupon they performed a home vector. If travelling the latter part of a familiar route and arriving at a familiar food site triggers the recall of an accustomed home vector, then the home vector should be the same under both test conditions. We find instead that the home vector tended to reflect the immediately preceding outward journey. In conjunction with earlier work, these experiments led us to conclude in the case of desert ants that landmark memories do not prime the recall of long-term global path integration memories. On the other hand, landmark memories are known to be linked to local path integration vectors that guide ants along a segment of a route. Landmarks thus seem to provide procedural information telling ants what action to perform next but not the positional information that gives an ant its location relative to its nest.

Animals↗
PubMed2001

Using artificial evolution and selection to model insect navigation.

BACKGROUND: An animal's behavioral strategies are often constrained by its evolutionary history and the resources available to it. Artificial evolution allows one to manipulate such constraints and explore how they influence evolved strategies. Here we compare the navigational strategies of flying insects with those of artificially evolved "animats" endowed with various motor architectures. Using evolutionary algorithms, we generated artificial neural networks that controlled a virtual animat's navigation within a 2D, simulated world. Like a flying insect, the animat possessed motors that generated thrust and torque, a compass, and visual sensors. Some animats were limited to forward motion, while others could also move sideways. Animats were selected for the precision with which they reached a target specified by a visual landmark. RESULTS: Animats given sideways motors could alter flight direction without changing body orientation and evolved strategies similar to those of flying bees or wasps performing the same task. Both animats and insects first aimed at the landmark. In the last phase, both adopted a fixed body orientation and adjusted their position to keep the landmark at a fixed retinal location. Animats unable to uncouple flight direction and body orientation evolved subtly different strategies and performed less robustly. CONCLUSIONS: This convergence between the navigational strategies of animals and animats suggests that the insect's strategies are primarily an adaptation to the demands of using visual information and compass direction to reach a position in space and that they are not significantly compromised by the insect's evolutionary history.

Animals↗
PubMed2001

The guidance of desert ants by extended landmarks.

Desert ants (Cataglyphis fortis) were trained to follow a fixed route around a barrier to a feeder. Their homeward trajectories were recorded on a test field containing a similar barrier, oriented either as in training or rotated through 22 or 45 . Under one set of experimental conditions, the homeward trajectories rotated with the orientation of the barrier, implying that the visual features of this extended landmark can determine the route independently of compass cues: the barrier provided a "visual scene" that controlled the trajectories of the ants. Under other conditions, the trajectories after rotation were a compromise between the habitual compass direction and the direction with respect to the rotated barrier. Trajectories were determined primarily by the visual scene when ants were allowed to return close to the nest before being caught and tested. The compromise trajectories were observed when ants were taken from the feeder. It seems that ants exhibit at least two separate learnt responses to the barrier: (i) a habitual compass direction triggered by the sight of the barrier and (ii) a visual scene direction that is compass-independent. We suggest that the weighting accorded to these different learnt responses changes with the state of the path integration system.

Animals↗
PubMed2000

Animal navigation: birds as geometers?

New experiments on a bird species able to remember the sites of thousands of cached seeds have revealed how a site can be specified by combining distance information from several landmarks.

Animals↗
PubMed2000

How do insects use path integration for their navigation?

We combine experimental findings on ants and bees, and build on earlier models, to give an account of how these insects navigate using path integration, and how path integration interacts with other modes of navigation. At the core of path integration is an accumulator. This is set to an initial state at the nest and is updated as the insect moves so that it always reports the insect's current position relative to the nest. Navigation that uses path integration requires, in addition, a way of storing states of the accumulator at significant places for subsequent recall as goals, and a means of computing the direction to such goals. We discuss three models of how path integration might be used for this process, which we call vector navigation. Vector navigation is the principal means of navigating over unfamiliar terrain, or when landmarks are unavailable. Under other conditions, insects often navigate by landmarks, and ignore the output of the vector navigation system. Landmark navigation does not interfere with the updating of the accumulator. There is an interesting symmetry in the use of landmarks and path integration. In the short term, vector navigation can be independent of landmarks, and landmark navigation needs no assistance from path integration. In the longer term, visual landmarks help keep path vector navigation calibrated, and the learning of visual landmarks is guided by path integration.

Animals↗
PubMed2000

Path integration in insects.

The most notable advance in our knowledge of path integration in insects is a new understanding of how the honeybee measures the distance that it travels during its foraging trips. Data from two groups show that the bee's odometer records distance in terms of the net amount of image motion over the retina that is accumulated during a flight. Progress has also been made in clarifying the relation between path integration and other navigational strategies. On unfamiliar ground, path integration is the only available means of navigation. In familiar surroundings, however, guidance by landmarks may override guidance by path integration. Path integration then becomes a back-up strategy that is used primarily when landmarks fail.

Animals↗
PubMed1999

Calibration of vector navigation in desert ants.

Desert ants (Cataglyphis sp.) monitor their position relative to the nest using a form of dead reckoning [1] [2] [3] known as path integration (PI) [4]. They do this with a sun compass and an odometer to update an accumulator that records their current position [1]. Ants can use PI to return to the nest [2] [3]. Here, we report that desert ants, like honeybees [5] and hamsters [6], can also use PI to approach a previously visited food source. To navigate to a goal using only PI information, a forager must recall a previous state of the accumulator specifying the goal, and compare it with the accumulator's current state [4]. The comparison - essentially vector subtraction - gives the direction to the goal. This whole process, which we call vector navigation, was found to be calibrated at recognised sites, such as the nest and a familiar feeder, throughout the life of a forager. If a forager was trained around a one-way circuit in which the result of PI on the return route did not match the result on the outward route, calibration caused the ant's trajectories to be misdirected. We propose a model of vector navigation to suggest how calibration could produce such trajectories.

Animals↗
PubMed1996

Vision: simple stereopsis.

By limiting stereopsis to measuring the distance of moving objects against a stationary background, insects can employ simple stratagems for solving the problems of stereoscopic range finding.

Animals↗
PubMed1993

Do frogs use retinal elevation to measure the distance of a barrier?

Grass frogs, Rana pipiens, will detour around a barrier to reach prey on the other side. However, if the distance between prey and barrier is short, frogs attempt to push through the barrier and reach the prey directly. The relationship between the probability of detouring and the distance between prey and barrier is the same whether the frog's starting position is 4 cm or 8 cm from the barrier. This suggests that frogs measure the absolute separation between the two objects. To discover whether the retinal elevation of the bottom of the barrier contributes to measuring this distance, the relationship between the frequency of detouring and barrier-prey distance was examined in several experiments in which the retinal position of the bottom of the barrier was manipulated. No evidence was obtained that the barrier's retinal elevation helps in gauging distance. On the other hand, retinal elevation influences strongly how far a frog lunges to reach its prey. It is suggested that different cues to distance are applied to the two classes of object because, under natural circumstances, it is difficult to judge where a barrier emerges from the ground. A barrier may be hard to detect below the horizon because of the low contrast between it and the ground, or because vegetation and ground litter mask where the barrier meets the ground. In contrast, the prey's movements make it easily detectable against a stationary background and the prey's short height means that partial occlusion will have little effect on its apparent vertical position in the visual field.

Animals↗
PubMed1991

Does vertical disparity scale the perception of stereoscopic depth?

It has been suggested that a measure of the gradients of vertical disparity over a surface may scale the mapping between horizontal disparity and perceived depth. We have investigated this possibility by obtaining estimates of the depth within stereograms that simulated two apposed fronto-parallel planes placed at different distances from an observer. The gradients of vertical disparity in a stereogram were set to simulate those appropriate to a viewing distance of 12.5 cm, 25 cm, 50 cm or 100 cm, whereas the distance specified by vergence and accommodative cues was always fixed at 50 cm. Judgements of the perceived depth between the two planes were uninfluenced by changes in the gradients of vertical disparity. It thus seems that the human visual system does not employ vertical disparity as a scaling parameter in stereoscopic depth judgements.

Depth Perception↗
PubMed1991

The interaction of oculomotor cues and stimulus size in stereoscopic death constancy.

In the natural world, observers perceive an object to have a relatively fixed size and depth over a wide range of distances. Retinal image size and binocular disparity are to some extent scaled with distance to give observers a measure of size constancy. The angle of convergence of the two eyes and their accommodative states are one source of scaling information, but even at close range this must be supplemented by other cues. We have investigated how angular size and oculomotor state interact in the perception of size and depth at different distances. Computer-generated images of planar and stereoscopically simulated 3-D surfaces covered with an irregular blobby texture were viewed on a computer monitor. The monitor rested on a movable sled running on rails within a darkened tunnel. An observer looking into the tunnel could see nothing but the simulated surface so that oculomotor signals provided the major potential cues to the distance of the image. Observers estimated the height of the surface, their distance from it, or the stereoscopically simulated depth within it over viewing distances which ranged from 45 cm to 130 cm. The angular width of the images lay between 2 deg and 10 deg. Estimates of the magnitude of a constant simulated depth dropped with increasing viewing distance when surfaces were of constant angular size. But with surfaces of constant physical size, estimates were more nearly independent of viewing distance. At any one distance, depths appeared to be greater, the smaller the angular size of the image. With most observers, the influence of angular size on perceived depth grew with increasing viewing distance. These findings suggest that there are two components to scaling. One is independent of angular size and related to viewing distance. The second component is related to angular size, and the weighting accorded to it grows with viewing distance. Control experiments indicate that in the tunnel, oculomotor state provides the principal cue to viewing distance. Thus, the contribution of oculomotor signals to depth scaling is gradually supplanted by other cues as viewing distance grows. Binocular estimates of the heights and distances of planar surfaces of different sizes revealed that angular size and viewing distance interact in a similar way to determine perceived size and perceived distance.

Adult↗
PubMed1989

The appearance of surfaces specified by motion parallax and binocular disparity.

The experiments reported in this paper were designed to investigate how depth information from binocular disparity and motion parallax cues is integrated in the human visual system. Observers viewed simulated 3-D corrugated surfaces that translated to and fro across their line of sight. The depth of the corrugations was specified by either motion parallax, or binocular disparities, or some combination of the two. The amount of perceived depth in the corrugations was measured using a matching technique. A monocularly viewed surface specified by parallax alone was seen as a rigid, corrugated surface translating along a fronto-parallel path. The perceived depth of the corrugations increased monotonically with the amount of parallax motion, just as if observers were viewing an equivalent real surface that produced the same parallax transformation. With binocular viewing and zero disparities between the images seen by the two eyes, the perceived depth was only about half of that predicted by the monocular cue. In addition, this binocularly viewed surface appeared to rotate about a vertical axis as it translated to and fro. With other combinations of motion parallax and binocular disparity, parallax only affected the perceived depth when the disparity gradients of the corrugations were shallow. The discrepancy between the parallax and disparity signals was typically resolved by an apparent rotation of the surface as it translated to and fro. The results are consistent with the idea that the visual system attempts to minimize the discrepancies between (1) the depth signalled by disparity and that required by a particular interpretation of the parallax transformation and (2) the amount of rotation required by that interpretation and the amount of rotation signalled by other cues in the display.

Attention↗
PubMed1988

Frogs use retinal elevation as a cue to distance.

We have investigated the role that retinal elevation plays in a frog's (Rana pipiens) estimate of prey distance. We dissociated retinal elevation from other depth cues by artificially increasing the height of the frogs' eyes above the ground. Frogs then snapped short of their prey in their ventral visual field as if their estimate of distance were determined primarily by the retinal elevation of the image of the prey. The data suggest that the frog assumes its eyes to be about 3 cm above the ground. Other cues modify depth judgements when targets are close to this assumed ground-plane.

Animals↗
PubMed1988

The retrieval of visuo-spatial memories by honeybees.

In order to explore how honeybees manage to retrieve the right landmark-memory in the right place, we trained bees along a short foraging route which consisted of two identical huts 33 m apart. Bees entered each hut to collect a drop of sucrose on the floor. The location of the drop was defined by the same arrangement of four blue and yellow cylindrical landmarks. However, in one hut the drop was between two yellow cylinders and in two other it was to the east of the blue cylinders. On tests with the sucrose missing, bees tended to search in the appropriate area in each hut (Fig. 1), thus showing that they used cues other than the sight of the local landmarks to select the appropriate memory. In a second experiment, the position of the sucrose was specified by yellow cylinders in one hut and by blue triangles in the other. When the arrays were swapped between huts, bees searched in the position specified by the array they encountered (Fig. 2). Thus, memories can be triggered by visual features of local landmarks. Bees were also trained outside to collect food from two platforms 40 m apart. The location of sucrose on one platform was defined by yellow cylinders, and on the other it was defined by blue triangles. When these arrays were exchanged between platforms, bees searched on each platform as though the landmarks had not been swapped. It seems that the more distant surroundings, which fill most of the visual field, may be more potent than the local landmarks in deciding which memory should be retrieved.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

A possible mechanism for binocular depth judgements in anurans.

Two experiments were performed to analyze how anurans (Bufo marinus) use binocular cues to gauge the distance of their prey. In the first, bilateral lesions of the nucleus isthmi eliminated the major source of input from the ipsilateral eye to the tectum. These lesions did not disrupt the animals' ability to use binocular cues to judge distance, suggesting that frogs and toads may not employ binocular disparity-selective cells to assess prey distance. They may instead use a scheme more overtly akin to triangulation, with each tectum providing an output signal encoding the angular position of the prey with respect to the contralateral eye and with distance extracted from the difference between these tectal outputs. In the second experiment, prisms imposed large (13.5 degrees) vertical disparities between the two eyes' images. The toads continued to use binocular cues. The added vertical disparities, like added horizontal ones, caused toads to undershoot their prey. Thus the binocular system must tolerate such vertical disparities and fail to distinguish them from horizontal ones.

Animals↗
PubMed1987

The use of visual landmarks by gerbils: reaching a goal when landmarks are displaced.

Gerbils (Meriones unguiculatus) can specify the location of a goal by means of visual landmarks and will return to such a goal from different starting positions in the vicinity of the landmarks. To discover whether landmark-cues are used continuously during an approach to the goal, gerbils were trained to forage for sunflower seeds close to a single illuminated light-bulb on the floor of an arena. As they approached the bulb, it was switched off and another bulb in a variable position with respect to the first turned on. On 52 out of 71 trials the gerbils changed their trajectory (latency ca. 240 ms) to aim for the newly lit bulb (Fig. 1 A, B). On the remaining trials, gerbils maintained their original course towards the first bulb as though it were still lit and then paused after a longer delay before eventually changing direction (Fig. 1 C). Thus, an approach to a beacon is usually under continuous visual control. This ensures that the gerbil will reach its goal correctly despite any inaccuracies in its initial computation of its approach. When switches were made between more complex arrays of landmarks, the gerbils' behaviour was less clear-cut. Possible reasons for this difference are suggested.

Animals↗