Notes: Summary 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°) 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.

Notes: Abstract Experiments by Fabre (1915), Thorpe (1950), Chmurzynski (1964), and most recently Gould (1986) suggest that insects have “maps” of their terrain which enable them to find their way directly to a goal when they are displaced several hundred metres from it. This paper discusses what might constitute an insect's map in terms of a two-part computational model. The first part describes how an insect reaches a goal when the insect is sufficiently close that it can see some of the landmarks which are visible from the goal. The second part considers the problem of navigating when there is no similarity between the view from the release-site and the view from the goal. We start from a model designed to explain how a bee might return to a goal using a two-dimensional “snapshot” of the landscape seen from the goal (Collett and Cartwright 1983). To guide its return, the model bee continuously compares its snapshot with its current retinal image and moves so as to reduce the discrepancy between the two. Bees can only be guided in the right direction by the difference between current retinal image and snapshot when there is some resemblance between the two. In a realistically cluttered world, snapshot and retinal image become very dis-similar only a short distance from the goal. To increase the distance from which a model bee can return, the bee takes two snapshots at the goal. The first snapshot excludes landmarks near to the goal and the second snapshot includes them. With close landmarks filtered from both snapshot and retinal image, the match between the two deteriorates gradually as the bee moves away from the goal. A model bee using a filtered snapshot and image finds its way back to the neighbourhood of the goal from a relatively long distance (Fig. 2). The bee then switches to the second snapshot and is guided to the precise spot by its memory of the close landmarks. For longer range guidance, the model bee is equipped with an album of snapshots, each taken at a different location within the terrain. Linked to each snapshot is a vector encoding the distance and direction from the place where the snapshot was taken to the hive. When the bee is displaced to a new position, it selects the snapshot which best matches its current image and follows the associated home-vector back to the hive (Fig. 3). Such a hive-centred map can also be used to devise novel routes to places other than the hive. For instance, a bee can reach a foraging site from anywhere in its terrain by adding the home-vector recalled at the starting position to a vector specifying the distance and direction of the foraging site from the hive. The sum of these two vectors defines a direct trajectory to the foraging site.

Notes: Summary 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.

Notes: Summary 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 groundplane.

Notes: Summary 1. To discover whether bees learn the colours of landmarks, individually marked foragers were trained to collect sucrose from a small reservoir on the floor of a room. The reservoir was placed at one of two sites each defined by its position relative to one of two different arrays of cylindrical landmarks. On each foraging trip, a bee encountered one of the two arrays. Once a bee was trained to both arrays, its pattern of search was occasionally recorded on videotape during test trials in which one array of landmarks was present and the sucrose absent. 2. Both training arrays were composed of two dark blue and two light yellow landmarks placed at the corners of a square. The arrays differed only in the arrangement of coloured landmarks. When bees were tested separately with each array, they searched close to the reward-site defined by that array (Figs. 1 and 2). They behaved similarly on tests in which dark yellow and light blue landmarks replaced the dark blue and light yellow landmarks respectively (Fig. 3). To distinguish between the two arrays, the bees must have used the arrangement of colours.

Notes: Summary 1. In order to analyse the mechanism of accommodation in anurans, drugs (miotic or atropine) were applied to the cornea of anaesthetized animals to change the refractive state of their eyes. During such changes, the lens and cornea were photographed and the refractive state of the eye was measured using laser speckle refractometry. Measurements taken from the photographs confirmed suggestions by Beer (1898) that accommodation is achieved by moving the lens and not by changing the shape of the lens or cornea. The change in refractive state induced by pharmacological manipulation was about 10 diopters with an accompanying shift in lens position of about 150 μm. Calculations based on a schematic eye suggest a disparity between the amount of lens movement theoretically needed to produce a 10 D shift in refractive state and the amount actually observed. 2. The lens is probably moved by two protractor lentis muscles which are positioned so as to pull the lens towards the cornea (Tretjakoff 1906, 1913). Dissection and HRP preparations revealed that these muscles are innervated by fibres of the oculomotor nerve which relay in the ciliary ganglion. InR. esculenta andR. pipiens, the ciliary ganglion consists of only 8 to 12 nerve cells. 3. MS222 anaesthesia and lymphatic injection of curare cause the lens to move away from the cornea, presumably because they destroy the resting tonus of the protractor lentis muscles. We discuss this finding in relation to the frog's ‘resting’ accommodative state, and conclude that unparalysed frogs are likely to be myopic, and not emmetropic as previous work suggests. 4. Prey capture was analysed inR. pipiens after the disruption of accommodation by bilateral section of the oculomotor nerve. Estimates of prey distance remained accurate when vision was binocular. However, during monocular vision, when the oculomotor nerve was sectioned on one side and the other eye was either occluded or had its optic nerve cut, frogs consistently underestimated the distance of their prey. This result suggests, in agreement with earlier evidence, that accommodation is used for judging depth when vision is limited to one eye, but that binocular information predominates when it is available. 5. Atropine applied to the cornea of monocular frogs also causes distance to be underestimated. It is argued from this that frogs assess distance by monitoring the motor commands sent to their accommodative muscles, rather than by using sensory information from the muscles themselves.

Notes: Summary 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. 1C). 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.

Notes: Summary The strengths of the rotational and possible translational optomotor reflexes were measured over a wide range of closed-loop conditions in walking ladybirds (Coccinella septempunctata), and less thoroughly in three other species of insect. While ladybirds exhibit a strong rotational optomotor reflex, any visual control of speed there might be was found to be too feeble to be biologically significant. To see whether walking speed is instead controlled proprioceptively, changes in speed were measured when ladybirds pulled small weights. But there was no evidence of proprioceptive control either. Flying and swimming insects, on the other hand, do use visual feedback to control their translational velocity, and, unlike walking insects,must do so to cope with winds or watercurrents.

Notes: Summary 1. The aim of this study is to understand what a rodent (Meriones unguiculatus) learns about the geometrical relations between a goal and nearby visual landmarks and how it uses this information to reach a goal. Gerbils were trained to find sunflower seeds on the floor of a light-tight, black painted room illuminated by a single light bulb hung from the ceiling. The position of the seed on the floor was specified by an array of one or more landmarks. Once training was complete, we recorded where the gerbils searched when landmarks were present but the seed was absent. In such tests, gerbils were confronted either with the array of landmarks to which they were accustomed or with a transformation of this array. 2. Animals searched in the appropriate spot when trained to find seeds placed in a constant direction and at a constant distance from a single cylindrical landmark (Fig. 1). Since gerbils look in one spot and not in a circle centred on the landmark, the direction between landmark and goal must be supplied by cues external to the landmark array. Distance, on the other hand, must be measured with respect to the landmark. Tests in which the size of the landmark was altered from that used in training suggest that distance is not learned solely in terms of the apparent size of the landmark as seen from the goal (Fig. 3). 3. Gerbils can still reach a goal defined by an array of landmarks when the room light is extinguished during their approach (Figs. 4, 5). This ability implies that they have already planned a trajectory to the goal before the room is darkened. In order to compute such a trajectory, their internal representation of landmarks and goal needs to contain information about the distances and bearings between landmarks and goal. 4. For planning trajectories, each landmark of an array can be used separately from the others (Fig. 7). Gerbils trained to a goal specified by an array of several landmarks were tested with one or more of the landmarks removed or with the array expanded. They then searched as though they had computed an independent trajectory for each landmark. For instance, gerbils trained with an array of two landmarks were tested with the distance between two landmarks doubled. The animals then searched for seeds in two positions, which were at the correct distance and in the right direction from each landmark. 5. If an internal representation of an array of landmarks is to be used to plan a trajectory, landmarks seen on the ground must be matched to those held in memory. One way in which gerbils do this is by learning properties of individual landmarks, such as their shape, size or surface markings (Figs. 10, 11, 13). For example, gerbils were able to locate seeds defined by a single relevant landmark while ignoring an irrelevant landmark with different features which was placed randomly with respect to the goal. 6. Several experiments (Figs. 4, 12, 13, 14) suggested that, although landmarks may be used independently for computing trajectories, the process of matching landmarks to the gerbil's representation requires a knowledge of the distances and directionsbetween landmarks. 7. We conclude that a gerbil's representation of its environment is complete in that it stores explicitly or can compute from what it has stored the geometric arrangement of landmarks and goal. We discuss the possibility that its spatial memories consist of a set of vectors describing the distance and direction from the goal to each landmark (Fig. 18) and consider the advantages and disadvantages of such a goal-centred memory.

Notes: Summary 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. It is argued that one role of distant landmarks and other contextual cues is to ensure that bees retrieve the correct memory of a constellation of local landmarks while the bees are still some distance away from their goal. Even at a short distance, a bee's current image of local landmarks may differ considerably from its stored representation of those landmarks as seen from the goal. Accurate recall of the appropriate memory will be more certain if it is primed by relatively distant landmarks which present a more constant image as a bee moves in the vicinity of its goal.