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^ Pitching

Figure 5.5 Optomotor responses and flow fields. (a) A fly, such as Calliphora or Musca, is suspended in the centre of a striped drum that can be rotated. The wire tethering the fly is attached to a meter that measures the torque, or turning force, produced by the fly. (b) Recordings of drum movements and torque produced by a fly. (c) The three axes for rotatory and translatory movements that an airborne fly can make. (d) Flow fields that stimulate the right eye when the insect flies forwards or pitches downwards. (b recordings from Egelhaaf, 1985; copyright © 1985 Springer-Verlag.).

^ Pitching of images over local regions of the eye are indicated by the lengths and directions of the arrows. The pattern of coherent motion associated with a particular kind of movement is called a flow field, and an animal can gather information about its movement through the environment by detecting the kind of flow field that its eyes are receiving.

Many of the large, fan-shaped neurons of the fly lobula plate are unique individuals, and generate their largest responses to particular types of stimulus movement. They filter out particular stimulus features and ignore others. The exact location of a moving stimulus is not significant to these neurons, but direction of movement is, and all these neurons show particular direction selectivity. For example, the three HS neurons respond most strongly to images that rotate around the animal in the horizontal plane, and the 11 VS neurons respond to movements upwards or downwards (Fig. 5.6a). Each HS neuron in the right lobula plate is most strongly excited by movements occurring in the clockwise direction around the animal (backwards over its own eye or forwards over the other eye), and those in the left lobula plate are most excited by anticlockwise movements. The most effective direction for exciting a neuron is called the preferred direction. Often, neurons are inhibited by movement in the opposite direction, which is called the null direction. Each VS neuron is tuned to respond most strongly to a particular type of flow field (Krapp & Hengstenberg, 1996). One VS neuron, for example, responds most strongly to one direction of rolling about the longitudinal axis, but other VS neurons are tuned to detect twisting movements that pitch the head upwards or downwards. The VS neurons, therefore, provide an array of filters, in which a particular combination of pitch and roll during flight corresponds to the strongest excitation of one of the VS neurons. Abstraction of particular stimulus features is often achieved by an array of neurons like this, each of which is tuned slightly differently from its neighbours.

Although the HS neurons have no branches that extend across the brain into the opposite optic lobe, they respond to movements over both eyes. They are most sensitive to stimuli that rotate about the animal in one direction, being excited by stimuli that move backwards over their own eye or forwards over the opposite eye. Neurons such as H1 (Fig. 5.6b) are responsible for carrying information from one lobula plate to the other, and provide pathways that enable the fly to compare stimuli that affect the two eyes. These pathways enable the fly to distinguish whether it is rotating or

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