In a sensory system, the receptors transmit signals to a series of interneu-rons and, together, these constitute a neuronal pathway in which signals are progressively modified as they are transmitted from stage to stage. Through their synaptic interactions, the interneurons act as selective filters, enhancing some aspects of the received signal and discarding others. A complex sense organ with many receptors is associated with a series of neuropiles made up of many parallel pathways, each processing information from a small group of receptors. Separate classes of interneuron within each pathway extract different types of information from the same receptor input. The neuronal pathways are arranged in an orderly manner, with each one maintaining a particular position relative to its neighbours, so that the central nervous system contains a map of the receptor array and hence, in a visual system, of the external world.
Early in the pathway, interneurons act as signal conditioners, providing a clear and unambiguous signal for the filtering networks that lie downstream. This is illustrated by the large monopolar cells of the insect lamina, in which the signal provided by photoreceptors is processed so that it is amplified and information about background light intensity is discarded. The signals received by interneurons in the medulla are, as a result, largely about contrasts in the light signal - changes both in time and in space. The kinds of operation that medulla interneurons perform are illustrated by the elementary motion detector in flies, or the array of laterally connected neurons thought to drive the LGMD in locusts. We have some good indications of the types of computation that these circuits perform, although direct experimentation, using microelectrodes to record from and stimulate different elements in these circuits, is extremely challenging.
Large neurons in the insect lobula are good examples of feature detectors. They abstract information about a particular aspect of the stimulus, usually the direction of movement, but discard others, such as the location of a target. Neurons of the lobula plate in flies, and the LGMD in the locust, are tuned quite tightly to respond most briskly to particular types of movement. Information about other aspects of a stimulus is processed by other neurons in the visual system, so that a particular scene in the environment is analysed by parcelling different aspects into an array of different inter-neurons, each concentrating on a different aspect. Understanding the manner in which a nervous system resynthesises a visual scene, combining together information from different feature-detecting pathways, provides a major challenge to neuroscientists.
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