In all insect and crustacean CE visual systems studied, feature extraction operations have been observed. Feature extraction is the result of spatiotemporal filtering operations on basic retinula cell outputs by the various ganglion masses in the OLs. Why are feature extraction operations performed in the OLs and not the CNS? The answer is simple. It is physiologically more efficient to perform "hard-wired," feature extraction operations on basic retinula cell analog outputs before they become attenuated by long-distance, passive propagation on retinula cell axons. Hence, feature extraction operations begin in the lamina ganglionaris, the first neural layer immediately under the receptors, and also occur in the medulla and lobula neuropile masses in the OLs.
Feature extraction provides other parts of the insect's nervous system with infornation needed for survival. A prime example is DS (vector) moving edge units that presumably project into motor centers that control flight attitude (roll, pitch, and yaw), or walking centers that control direction and speed on the ground. Jittery spot units (multimodal units in grasshoppers) may be involved with general arousal of the animal's nervous system, in effect preparing the animal for flight, or feeding.
Crustacean CE systems also perform feature extraction operations. As underwater swimming is a three-dimensional activity like flying, the animal probably uses DS visual movement information for attitude regulation. However, crustaceans also have statocysts to sense static body attitude in the Earth's gravity field (system redundancy is good for survival). When a crustacean is accelerating, the statolith is
"pinned" and the statocyst does not work as an up/down sensor, so visual moving edge information can be important. Crustacean CEs are unique because they are on movable eyestalks. Visual control of eyestalk position may make use of vector unit outputs from the OLs.
The existence of neural information exchange pathways between the eyes of crustaceans suggests that left-right comparisons of objects are occuring, and operations such as object range estimation by vergence (as well as by looming) could be taking place. It is hoped that someone will investigate vergence in CE systems in the future.
As you will be seen in the next chapter, feature extraction also takes place in the retinas of certain vertebrates.
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