Frontal Retina Research Articles

Snapshot Memories and Landmark Guidance in Wood Ants

Insects are thought to pinpoint a place by using memorized “snapshots,” i.e., two-dimensional retinotopic views of the surrounding landmarks recorded when at the place (reviewed in [1]). Insects then reach the place by moving until their current view matches their snapshot. To determine when snapshots are recalled, and how differences between view and snapshot are translated into appropriate movements, we analyzed the approaches of wood ants to a feeding site that was located in the center of an array of two or three cylinders. In ants, contrary to flying hymenopterans [2], body orientation and direction of travel are collinear, so that an ant approaching an object always looks at it with frontal visual field. On their way to a food site, ants fixated and approached a cylinder predominantly when its angular size was smaller than when viewed from the food site. This finding implies that ants store snapshots at this place while fixating landmarks with frontal retina, so simplifying the later alignment of snapshots with their current view. It also means that ants recall snapshots well in advance of reaching the place. Although snapshots are centered on a landmark, we show that they extend at least 120° into the periphery.

View-based navigation in insects: how wood ants (Formica rufa L.) look at and are guided by extended landmarks.

Bees, wasps and ants learn landmarks as views from particular vantage points, storing the retinal positions of landmark edges. By moving so as to minimise the difference between their stored and current view, they can return to the vantage point from which a view was taken. We have examined what wood ants learn about a laterally placed, extended landmark, a wall, while walking parallel to it to reach a feeder and how they use this stored information to guide their path. Manipulation of the height of the wall and the ant's starting distance from it reveals that ants maintain a desired distance from the wall by keeping the image of the top of the wall at a particular retinal elevation. Ants can thus employ image matching both for returning to a place and for following a fixed route. Unlike many flying insects, an ant's direction of motion while walking is always along its longitudinal body axis and, perhaps for this reason, it favours its frontal retina for viewing discrete landmarks. We find that ants also use their frontal retina for viewing a laterally placed wall. On a coarse scale, the ant's path along the wall is straight, but on a finer scale it is roughly sinusoidal, allowing the ant to scan the surrounding landscape with its frontal retina. The ant's side-to-side scanning means that the wall is viewed with its frontal retina for phases of the scanning cycle throughout its trajectory. Details of the scanning pattern depend on the scene. Ants scan further to the side that is empty of the wall than to the side containing the wall, and they scan further into the wall side when the wall is of a lower apparent height. We conclude that frontal retina is employed for image storage and for path control.

View-based navigation in Hymenoptera: multiple strategies of landmark guidance in the approach to a feeder

In order to analyse how landmarks guide the last stages of an insect's approach to a goal, we recorded many flights of individual wasps and honeybees as they flew to an inconspicuous feeder on the ground that was marked by one or by two nearby landmarks. An individual tends to approach the feeder from a constant direction, flying close to the ground. Its body is oriented in roughly the same horizontal direction during the approach so that the feeder and landmarks are viewed over a narrow range of directions. Consequently, when the insect arrives at the feeder, the landmarks take up a standard position on the retina. Three navigational strategies govern the final approach. The insect first aims at a landmark, treating it as a beacon. Secondly, bees learn the appearance of a landmark with frontal retina and they associate with this stored view a motor trajectory which brings them from the landmark sufficiently close to the goal that it can be reached by image matching. Insects then move so as to put the landmark in its standard retinal position. Image matching is shown to be accomplished by a control system which has as set points the standard retinal position of the landmark and some parameter related to its retinal size.

Insect navigation en route to the goal: multiple strategies for the use of landmarks

There are at least four distinct ways in which familiar landmarks aid an insect on its trips between nest and foraging site. Recognising scenes: when bees are displaced unexpectedly from their hive to one of several familiar locations, they are able to head in the direction of home as though they had previously linked an appropriate directional vector to a view of the scene at the release site. Biased detours: ants recognise familiar landmarks en route and will correct their path by steering consistently to the left or to the right around them. Aiming at beacons: bees and ants also guide their path by approaching familiar landmarks lying on or close to the direct line between start and finish. Simulations suggest that such mechanisms acting together may suffice to account for the routes taken by desert ants through a landmark-strewn environment: the stereotyped trajectories of individual ants can be modelled by a weighted combination of dead reckoning, biased detours and beacon-aiming. These mechanisms guide an insect sufficiently close to an inconspicuous goal for image matching to be successfully employed to locate it. Insects then move until their current retinal image matches a stored view of the surrounding panorama seen from a vantage point close to the goal. Bees and wasps perform learning flights on their first departure from a site to which they will return. These flights seem to be designed to pick up the information needed for several navigational strategies. Thus, a large portion of the learning flight of a bee leaving a feeder tends to be spent close to the feeder so aiding the acquisition of a view from that vantage point, as is needed for image matching. Bees and social wasps also tend to inspect their surroundings while facing along preferred directions and to adopt similar bearings before landing, thereby making it easy to employ retinotopically stored patterns in image matching. Aiming at beacons, in contrast, requires a landmark to be familiar to the frontal retina. Objects tend to be viewed frontally while the insect circles through arcs centred on the goal. This procedure may help insects to pick out those objects close to the goal that are best suited for guiding later returns.

Making learning easy: the acquisition of visual information during the orientation flights of social wasps

The flights of individual wasps (Vespula) were recorded as they approached a small feeder on the ground that was marked by a black cylinder ca 15 cm away. Two navigational strategies are used in these approaches. Initially, the wasp aims at the cylinder, treating it as a beacon and fixating it with frontal retina. In the last stage of the flight, the wasp assumes a preferred orientation so that the cylinder takes up a constant, more peripheral retinal position as the wasp nears the feeder. Path guidance by image-matching is likely to be limited to this final segment of the return. Wasps could gain the information needed for these distinct navigational strategies during the learning flights that they perform on their initial departures from the feeder. They fly away from the feeder in a series of arcs while turning at a mean angular velocity of 226°/s. The cylinder tends to be viewed with frontal retina during the arcs suggesting that the information required for aiming at the cylinder is acquired then. For image matching, the appearance of the cylinder needs to be learnt when the wasp is in the orientation that it adopts close to the feeder on its return flight. Wasps tend to assume this orientation during learning flights while they face the feeder. Such inspections of the feeder occur at the ends of arcs when a wasp's turning velocity is low.

How ladybirds approach nearby stalks: a study of visual selectivity and attention

1. An open- and closed-loop study of the way ladybirds,Coccinella septempunctata, approach vertical posts emphasizes two features of this reflex. First, ladybirds turn preferentially towards close objects, obtaining the necessary distance information from optic flow. Secondly, once an object has been fixated, movement of its image over frontal retina tends to suppress any response to other laterally viewed targets. 2. Recordings were made of the trajectories followed by ladybirds in an open arena which contained a single, vertical post on the floor. Ladybirds often approached and climbed up the post. Many approaches consisted of straight-line segments interrupted by abrupt turns. These turns are either appropriately sized so that the ladybird faces the post after one turn or, more often, they are too small in which case the ladybird's trajectory is a spiral. The gain of the turn (i.e. the ratio of the size of the turn to the size which is needed to fixate the post) increases slightly as the insect approaches the post. 3. The ladybirds' preference for close objects was examined further on a Y-maze. Insects placed on the trunk of the maze ran towards the fork and down one arm. Each arm of the maze led to a rectangle. Rectangles were placed at various distances from the fork. Their size was adjusted so that viewed from the fork they all subtended 18° horizontally and 63° vertically. Ladybirds chose predominantly the arm which led to the closer rectangle. 4. Open-loop tests indicate that this preference for close objects is caused by the pattern of optic flow resulting from the ladybirds' normal forward locomotion. Insects were fixed in front of a computer screen and carried a small ring which they rotated beneath them. Any attempted turn was manifest in a turn of the ring in the opposite direction. Turns were regularly elicited by small backward movements of a vertical stripe across the retina, as would occur during forward walking. Forward motion of the stripe over the retina rarely evoked turns. Turns increased in frequency and size as the speed of backward image motion was raised from 3°/s to 70°/s. The largest turns were evoked with the stripe placed at an eccentricity of about 90 degs from the midline. Amplitude dropped as the stripe was positioned further frontally or posteriorly. 5. Approaches to a target were modelled using a saccadic system in which gain varied with distance from the target. This simulation generated spiral trajectories. Thus, the spirals described by a ladybird when it walks towards a post may be a consequence of the way the insect uses motion parallax to restrict its attention to nearby objects. 6. Open-loop turns to a stripe moving over peripheral retina are prevented by the concurrent motion of a stripe viewed by frontal retina. This longrange inhibition means that, once a stripe has been fixated, an insect's attention is less likely to be distracted by objects seen peripherally.

Sexually dimorphic interneuron arrangements in the fly visual system

Silver-intensified cobalt preparations reveal sex-specific neurons in the optic lobes of flies. The most pronounced of these are found in the lobula neuropil and subserve projections from the dorsal and frontal retina specifically. Two main types of male neurons have been resolved. These are single unique elements, two in each lobula, and small groups of columnar neurons. The present account describes three main features of sex-specific neurons. These are, first their shapes and dispositions; secondly, their relations with visual nerve cells that are common to both sexes, and thirdly, their exact mapping within the retinotopic mosaic of the lobula. The possible functional significance of these arrangements are discussed with respect to sex-specific visual behaviour.

Some operating rules for the optomotor system of a hoverfly during voluntary flight

1. Two rules are proposed which can account for the way image motion in a horizontal plane helpsSyritta pipiens cope with disturbances during voluntary flight. The fly seems to assume that it moves within a stationary environment and that externally generated image motion is the result of some disturbance to its intended flight pattern and should be minimized. 2. The first rule is concerned with the control of angular velocity. WhenSyritta cruises with no clear goal in view it keeps the angular orientation of its body constant apart from saccade-like reorientations. Thus during undisturbed flight there will be image motion all over the retina, except for regions which either look along the direction of flight or directly behind.Syritta can thus assume that any image motion over these regions (the poles of the flow field) is caused by a disturbance. Duringforward flight externally generated image motion only influences the fly's angular velocity if it occurs overfrontal retina (Fig. 6). But when the fly movessideways with respect to its long axis, so that the pole of the flow field shifts to lateral retina, image motion confined to frontal retina no longer has any effect on angular velocity (Fig. 6). It is therefore proposed that during cruising angular orientation is governed principally by image motion over the poles of the flow field. 3. The second rule covers the control of translational velocity. This parameter seems to be governed by externally generated image motion over a much wider area of retina. It is proposed that each small region of retina influences a component of thrust at right angles to the direction of regard of that retinal region. The total translational response is assumed to be caused by the summed effects of image motion over many such retinal areas. Support for this rule comes from the behaviour ofSyritta when flying parallel to a vertically striped wall that moves from side to side (Fig. 7). The movement of the wall influences a component of thrust parallel to the wall, whether the fly views the wall with frontal or with lateral retina. 4. It is explained how these two rules can resolve the image motion resulting from a disturbance like a gust of wind into rotational and translational components.