Orientation in Space

Probably the most fundamental requirement for a defined orientation relative to the world is imposed by vision and the need to appropriately direct motor commands. There are only two 'absolute' and reliable reference cues animals can use for their alignment with the world: one is the direction of gravity and the other is the division of the world into a celestial and a terrestrial hemisphere, separated by a visual horizon line, with light coming from above. Animals have evolved a number of sensor systems to determine 'what is up': they use collections of heavy materials bedded on shearing sensors (statocysts or otholiths) which signal the direction of gravitational pull. Some water insects trap air bubbles in sensory hair fields which serve the same role. In terrestrial animals, the distribution and orientation of body mass relative to gravity can be measured by pro-prioceptors, in vertebrates by muscle spindles and stretch receptors in the joints and around internal organs, and in insects by mechanoreceptors in the head-thorax and thorax-abdomen joints. The second absolute reference is the fact that light comes from above. Insects and fish are known to use this property of the world to align their vertical body axis and/or their eyes. The compound eyes of especially flying insects are aided in this task by an additional visual system, the ocelli, which are an assembly of three lens eyes with huge visual fields on the top of the head. In dragonflies and flies, ocelli have been shown to be exquisite horizon sensors, in addition to 'dorsal light sensors', which elicit compensatory roll and pitch movements of the head, whenever it is not aligned horizontally.

There are a number of other animal sensors that cannot by themselves assure proper alignment with the vertical, but are essential in controlling and maintaining orientation. Vertebrates, crustaceans, and flies possess vestibular systems that are designed to sense the rotational acceleration (in the case of semicircular canals in vertebrates and crustaceans) and the velocity (in the case of fly gyroscopes) of the body and the head. The principle of operation in semicircular canals is the fact that liquid in tubes embedded in the head does not immediately follow the acceleration of the head due to inertia. There is a brief moment then, in which the liquid moves relative to the canal walls and this movement is sensed by hair cell mechanoreceptors, which are sensitive to shearing forces. The relative motion is caused by acceleration only: during prolonged rotation at a constant velocity, the liquid comes to rest due to friction. The main function of these vestibular systems is to stabilize gaze around the three rotational axes, the yaw, pitch, and roll axes, by sending the appropriate commands to the eye muscles in vertebrates and crustaceans, and to the neck muscles in flies, which then move the eyes or the head into the opposite direction. The modified hindwings in flies, called halteres, are a special case of such inertial sensors: they consist of clubs on rigid stalks, which during flight rapidly oscillate up and down in antiphase to the wings. Any rotation of the flying insect creates an inertial force which resists the change of orientation of the plane in which the halteres oscillate. This Coriolis force (which also operates on the large gyral ocean and air current systems on Earth) is then sensed by fields of strain sensors at the base of the halteres and their output drives compensatory head movements of the fly. Because these inertial systems provide feedforward information (their effect does not feed back directly onto the sensor itself) with little delay, the reflex movements they elicit belong to the fastest biological sensor-action loops. However, the information they provide is dynamical, lacks an absolute reference, and is unreliable at low accelerations and low angular velocities. Inertial sensors by themselves therefore cannot prevent slow drift from the absolute reference orientation. They thus need to operate in concert with other sensory systems, most notably the visual system and its high sensitivity to image motion (Figure 1).

Control of orientation in files

Control of orientation in files

Figure 1 Orientation in space. (a) Head stabilization in flight of a sand wasp (Bembix sp.). The sequence of high-speed images shows the wasp as it executes a sideslip movement to the top left by rolling its body first to the left (top row) and then to the right as a breaking maneuver (bottom row). As the body rolls through nearly 180°, the head remains perfectly aligned with the horizontal. High-speed video images were taken at 250fps. (b) The different sensory inputs that contribute to head stabilization and orientation in flies. (a) Courtesy of Norbert Boeddeker, Jan M. Hemmi, and Jochen Zeil, unpublished. (b) Modified from Hengstenberg R (1993) Multisensory control in insect oculomotor systems. In: Miles FA and Wallman J (eds.) Visual Motion and Its Role in the Stabilization of Gaze, pp. 285-298. Amsterdam: Elsevier Science.

Figure 1 Orientation in space. (a) Head stabilization in flight of a sand wasp (Bembix sp.). The sequence of high-speed images shows the wasp as it executes a sideslip movement to the top left by rolling its body first to the left (top row) and then to the right as a breaking maneuver (bottom row). As the body rolls through nearly 180°, the head remains perfectly aligned with the horizontal. High-speed video images were taken at 250fps. (b) The different sensory inputs that contribute to head stabilization and orientation in flies. (a) Courtesy of Norbert Boeddeker, Jan M. Hemmi, and Jochen Zeil, unpublished. (b) Modified from Hengstenberg R (1993) Multisensory control in insect oculomotor systems. In: Miles FA and Wallman J (eds.) Visual Motion and Its Role in the Stabilization of Gaze, pp. 285-298. Amsterdam: Elsevier Science.

All visual animals exhibit a strong reflex which counteracts large-field image motion on the retina. Such large-field motion is produced by rotational movements of the eyes, the head, or the body, which cause the retinal image to shift in the opposite direction. Animals respond to such unintended rotations by eye, head, or body movements in the opposite direction. This so-called optomotor response, or optokinetic reflex, is part of a negative feedback loop, with the speed of image motion as input and a motor command as output which - within a certain range of speeds - reduces image motion to zero. As a result, the retinal image is said to be stabilized, which is not quite true, because there will always be some residual image motion experienced by an animal when it moves through the world, that is caused by the animal's translational movement. However, the retinal image is cleaned from the effects of rotations by the optomotor response (around all three rotational axes) and thus helps to stabilize gaze and keep the visual system aligned with the vertical. Although this sounds simple enough, there are some major computational problems animals have to solve, in order to achieve this optomotor stabilization: in flies, at least 60 large-field, motion-sensitive neurons, many of them tuned to a particular rotational image motion component, are involved in the task of controlling the rotational movements of the fly's head!

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