The detection of different kinds of movements plays a very significant part in controlling the behaviour of most fast-moving animals, such as flies. These insects are able to make high-speed turns during flight, and are able to land precisely on small targets, like a blade of grass or the edge of a cup. They mate on the wing, and males pursue females under visual control (Land & Collett, 1974). Many of these behaviours are almost certainly under the control of a group of large, fan-shaped neurons that occupy a distinct region of neuropile towards the posterior of the lobula called the lobula plate. There are about 60 of these neurons (Hausen & Egelhaaf, 1989), and many of them are involved in optomotor responses. Unlike the large monopolar cells, they are not concerned with stimuli that affect single omma-tidia, but they respond to particular movements that occur anywhere within a large receptive field.
Optomotor responses result in movements that tend to keep images of the environment in constant positions on the retina. These simple behaviours play vital roles in enabling more complex behaviour patterns to be executed smoothly. They cause a stationary animal to maintain a constant position and orientation, or help a moving animal to maintain a particular course of locomotion.
An easy way to observe an optomotor response is to place an animal in the centre of an upright cylinder that has vertical dark and light stripes painted on its inner wall. If the cylinder is rotated, the animal will swivel around its vertical axis, following the movement of the stripes. Measurements of optomotor responses in a variety of types of animal have shown that the animal attempts to minimise slippage of the images of the stripes over the retina. Because most of the visual field of the animal is occupied by the stripes, the animal interprets movement of the drum as movement of itself relative to the environment, and optomotor responses naturally tend to stabilise the animal's eyes.
Flies reliably exhibit optomotor responses during flight. The strengths and directions of these responses can be measured in the laboratory by gluing a holder to the fly's back and suspending it in the air. When the fly's feet lose contact with the ground, it beats its wings up and down as if flying, and it will attempt to twist around its holder when it sees vertically oriented stripes drifting past its eyes. The strength of the twisting force that the fly applies to its holder can be measured, and the direction of the force changes whenever the direction of travel of the stripes is reversed (Fig. 5.5a, b). This kind of movement occurs in the yaw plane, and will tend to correct the animal's flight path when it twists around its vertical axis. Similar optomotor responses also operate in the roll and pitch planes (Fig. 5.5c). A twisting movement by the fly, such as yawing, is characterised by a difference in the direction in which images move over the two eyes, whereas a translating movement, such as flying straight forwards, is characterised by images that move over the two eyes in the same direction and at the same speed. When an animal moves relative to its environment, a particular pattern of direction of movements is detected by the eyes and that pattern depends on the type of movement. During forward movement, for example, images move from the front of the animal towards the rear. The slowest movements will be detected by the parts of the eyes directed towards the front of the animal, and the fastest movements will be detected by the parts of the eyes directed towards the sides of the animal. On the other hand, when a flying animal pitches downwards, a rotating pattern of movements is set up, with the front part of each eye receiving upward motion and the back part receiving downward motion. In Fig. 5.5d speed and direction of movement
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