Matthew Collett

Memory use in insect visual navigation

The navigational strategies that are used by foraging ants and bees to reach a goal are similar to those of birds and mammals. Species from all these groups use path integration and memories of visual landmarks to navigate through familiar terrain. Insects have far fewer neural resources than vertebrates, so data from insects might be useful in revealing the essential components of efficient navigation. Recent work on ants and bees has uncovered a major role for associative links between long-term memories. We emphasize the roles of these associations in the reliable recognition of visual landmarks and the reliable performance of learnt routes. It is unknown whether such associations also provide insects with a map-like representation of familiar terrain. We suggest, however, that landmarks act primarily as signposts that tell insects what particular action they need to perform, rather than telling them where they are.

Local and global vectors in desert ant navigation.

Desert ants returning from a foraging trip to their nest navigate both by path integration and by visual landmarks. In path integration, ants compute their net distance and direction from the nest throughout their outward and return journeys, and so can always return directly home from their current location. As the path-integration vector is updated over the entire journey, we call it a global vector. On a familiar route, when ants can steer by visual landmarks, they adopt a fixed and often circuitous path consisting of several separate segments that point in different directions,,. Here we show that, as in honeybees, such multisegment journeys are composed partly of stored local movement vectors, which are associated with landmarks and are recalled at the appropriate place. We also show that a local vector learnt at one value of the global vector can be recalled at many values, and that expression of the global vector is temporarily inhibited while the local vector is used. These results indicate that the global vector is ignored during navigation through familiar, cluttered territory, but that it re-emerges to take the ant home once the insect leaves the clutter and other guidance strategies cease to operate.

How do insects use path integration for their navigation

Abstract.u2002We 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 insects 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.

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 accumulators 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 ants trajectories to be misdirected. We propose a model of vector navigation to suggest how calibration could produce such trajectories.

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 bees 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.

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

SUMMARY 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.

Insect navigation : Measuring travel distance across ground and through air

Walking insects probably monitor leg movements to estimate how far they travel, whereas flying insects monitor optic flow.

The learning and maintenance of local vectors in desert ant navigation

SUMMARY The desert ant Cataglyphis fortis has at least three types of navigational strategy that can guide it between its nest and a familiar food site. The initial strategy after first finding a food site is based on a path integration memory of the position of the food site with respect to the nest. A second strategy is based on visual snapshot memories of features viewed from near or on the way to the food site. A third strategy uses local vector memories of the direction and length of habitual route segments. We show here that while such local vectors encode sufficient information to guide an individual along both the direction and distance of a route segment, its acquisition and long-term maintenance requires support from the other two strategies. We trained ants along an L-shaped route, designed to show that ants can learn local vectors on the way to a food site. The sharp turn appears to present particular difficulties for the ants. When low bushes 20–30 m from the route were removed, local vectors were briefly unaffected, but then deteriorated. The vectors improved again once the missing bushes were replaced by artificial landmarks. The fragility of local vector memories may permit an ant the flexibility to adapt its route to fluctuations in the distribution of its resources.

Local and global navigational coordinate systems in desert ants

SUMMARY While foraging, the desert ant Cataglyphis fortis keeps track of its position with respect to its nest through a process of path integration (PI). Once it finds food, it can then follow a direct home vector to its nest. Furthermore, it remembers the coordinates of a food site, and uses these coordinates to return to the site. Previous studies suggest, however, that it does not associate any coordinates remembered from previous trips with familiar views such that it can produce a home vector when displaced to a familiar site. We ask here whether a desert ant uses any association between PI coordinates and familiar views to ensure consistent PI coordinates as it travels along a habitual route. We describe an experiment in which we manipulated the PI coordinates an ant has when reaching a distinctive point along a habitual route on the way to a feeder. The subsequent home vectors of the manipulated ants, when displaced from the food-site to a test ground, show that also when a route memory is evoked at a significant point on the way to a food site, C. fortis does not reset its PI coordinates to those it normally has at that point. We use this result to argue that local vector memories, which encode the metric properties of a segment of a habitual route, must be encoded in a route-based coordinate system that is separate from the nest-based global coordinates. We propose a model for PI-based guidance that can account for several puzzling observations, and that naturally produces the route-based coordinate system required for learning and following local vectors.

Insect navigation : No map at the end of the trail?

Although the hunt for cognitive maps in insects may not have reached the end, the search itself has been fruitful in sharpening our understanding of the ways that insects navigate through familiar surroundings.