While an insect is flying around, it needs to detect and react to nearby objects as well as to monitor the way the background moves. A fly, for example, tends to chase other flies, and to select suitable targets such as twigs on which to land. When a fly approaches a twig, the image of the twig will move more rapidly over the fly's eyes than will the images of more distant objects. Tethered, suspended flies turn strongly towards a small object that is moving backwards relative to them, and this response is enhanced if the object is moving over a background pattern that is also moving, rather than remaining stationary (Kimmerle, Warzecha & Egelhaaf, 1997). Likely candidate neurons for recognising a small object and initiating a turn by the fly towards it are a group of four figure-ground neurons in each lobula plate (Egelhaaf, 1985). These neurons select a figure, or object, that is distinct from the general background. Like the HS neurons, they are excited by movements backwards over the eye. They also have large receptive fields but, unlike the HS neurons, they respond much more briskly to movements of small targets than to large parts of the visual field. This is because they receive a balance of excitatory and inhibitory synaptic inputs in response to moving stimuli (Warzecha, Egelhaaf & Borst, 1993). A figure-ground neuron responds strongly to a stimulus that extends over about 5° of the visual field, but progressively less strongly to stimuli that are seen by larger regions of the eye.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.