Insects depend upon optic flow to supply much of their information about the three-dimensional structure of the world. Many insects use translational flow to measure the distance of objects from themselves. A recent study has provided new insights into the way Drosophila use optic flow to pick out a close target to approach. Virtual reality is a powerful tool for probing the mechanisms of visually guided behaviour in insects. Walking Drosophila explore their surroundings, approaching stationary objects as possible sources of food, shelter or mates. If one object proves unrewarding or inaccessible, the fly approaches another. When a fly is restrained within a small area together with a collection of useless objects, it may run repeatedly between the objects for several hours. On leaving one object, the fly avoids revisiting it and selects between the remainder on the basis of their relative proximity [1]. As they report in this issue of Current Biology, Schuster et al. [2] have used virtual reality techniques that they developed [3] to discover how a fly decides which target is nearest. As a fly's eyes are close together and their lenses have a short and fixed focal length, the primary depth cues available to the fly come from optic flow -the retinal image motion that is produced as a consequence of the insect's movements. Of the two possible motion parallax cues, flies were found to attend to translational velocity but to ignore image expansion (looming). The use of optic flow often involves the performance of special patterns of locomotion to generate the particular image motion that is needed for a given task. Locusts or mantids leaping onto a stationary object assess how far they need to jump by facing the target and moving their heads from side to side before jumping [4] [5] [6] [7] . This information-gathering 'peering' behaviour has interesting design features. The task of extracting the distance of the target is made easier by making the side-to-side movements of the head strictly translational, with no rotational component to degrade the distance signal. When targets are distant and the image motion correspondingly smaller, the distance signal is amplified by increasing the amplitude and speed of the movements. But when Drosophila are choosing a target, they are concerned with the more qualitative task of picking out the closest one and seem to do so adventitiously while walking. To see how Drosophila measure target distance [2], a wingless fly was confined to a platform that was surrounded by a tall cylinder of close-packed light emitting diodes. All diodes were lit except for vertical bars of unlit diodes to which the fly was attracted. The position and width of each dark bar were linked to the fly's recorded movements. By adjusting the gain of this self-generated motion parallax, the bar could be made to represent a fixed virtual object placed at any distance from the platform. Put crudely, the greater the retinal image motion seen by the fly, the closer is the bar. First tests compared a fly's choice between virtual and real objects (pieces of black card). Flies preferred to approach the closer object whether it was real or virtual. No preference was detected when real and virtual objects were equidistant. These findings imply that, as expected, the fly bases its decision entirely on motion parallax. To tease apart which component of motion parallax determines the fly's choice of target, use was made of the finding that the fly's ability to differentiate distances is impaired if a delay is introduced between the fly's movements and their translation into transformations of the bar. Performance deteriorates gradually with increasing delay, remaining above chance for delays of at least 2 seconds. Delays of any magnitude that were restricted to the looming component were harmless, whereas delays restricted to translational motion of the bar degraded the fly's correct choice, just as if both components of flow were delayed. Looming cues are thus ignored in this task, even though flies are highly sensitive to them during landing and the looming signals that Schuster et al. [2] provided were large. During normal forward locomotion, the images of objects move from the front to the back of the retina. When the direction of feedback was reversed, distance discrimination failed, suggesting that for this task the fly may attend only to backward motion and screen out any forward motion across its retina. One difficulty in obtaining an accurate estimate of object distance from optic flow during locomotion is that translational image speed and the amount of looming depend not only on the insect's speed and the distance of the object, but also on the angle between the object's line of sight and the direction of motion of the insect (Figure 1) . The looming component is maximum for locomotion along the line of sight, and falls with the cosine of the angle between the line of sight and the insect's direction of motion. Conversely, translational image speed is maximum when the line of sight is perpendicular to the direction of motion, and drops with the sine of this angle. Locusts, when peering from side to side, maximise the translational component of optic flow by facing their target. Flies, when choosing between objects, do nothing equivalent and it is unclear whether they tolerate a sloppy process, or have mechanisms that normalise image motion signals for different angles of approach, or employ behavioural strategies for resolving this ambiguity. Dispatch
REF shows that the task of evaluating distances between objects is made easier by making side-to-side movements of the head strictly translational and disregarding any rotational components that can influence the distance to the objects.
Insect Vision: Controlling Actions through Optic Flow
Current Biology
Current Biology
Biology Medicine