<P> Feature detection is the ability to extract biologically relevant information from combinations of sensory signals . In the visual system, for example, sensory receptors in the retina of the eye are only individually capable of detecting "points of light" in the outside world . Second - level visual neurons receive input from groups of primary receptors, higher - level neurons receive input from groups of second - level neurons, and so on, forming a hierarchy of processing stages . At each stage, important information is extracted from the signal ensemble and unimportant information is discarded . By the end of the process, input signals representing "points of light" have been transformed into a neural representation of objects in the surrounding world and their properties . The most sophisticated sensory processing occurs inside the brain, but complex feature extraction also takes place in the spinal cord and in peripheral sensory organs such as the retina . </P> <P> Although stimulus - response mechanisms are the easiest to understand, the nervous system is also capable of controlling the body in ways that do not require an external stimulus, by means of internally generated rhythms of activity . Because of the variety of voltage - sensitive ion channels that can be embedded in the membrane of a neuron, many types of neurons are capable, even in isolation, of generating rhythmic sequences of action potentials, or rhythmic alternations between high - rate bursting and quiescence . When neurons that are intrinsically rhythmic are connected to each other by excitatory or inhibitory synapses, the resulting networks are capable of a wide variety of dynamical behaviors, including attractor dynamics, periodicity, and even chaos . A network of neurons that uses its internal structure to generate temporally structured output, without requiring a corresponding temporally structured stimulus, is called a central pattern generator . </P> <P> Internal pattern generation operates on a wide range of time scales, from milliseconds to hours or longer . One of the most important types of temporal pattern is circadian rhythmicity--that is, rhythmicity with a period of approximately 24 hours . All animals that have been studied show circadian fluctuations in neural activity, which control circadian alternations in behavior such as the sleep - wake cycle . Experimental studies dating from the 1990s have shown that circadian rhythms are generated by a "genetic clock" consisting of a special set of genes whose expression level rises and falls over the course of the day . Animals as diverse as insects and vertebrates share a similar genetic clock system . The circadian clock is influenced by light but continues to operate even when light levels are held constant and no other external time - of - day cues are available . The clock genes are expressed in many parts of the nervous system as well as many peripheral organs, but in mammals, all of these "tissue clocks" are kept in synchrony by signals that emanate from a master timekeeper in a tiny part of the brain called the suprachiasmatic nucleus . </P> <P> A mirror neuron is a neuron that fires both when an animal acts and when the animal observes the same action performed by another . Thus, the neuron "mirrors" the behavior of the other, as though the observer were itself acting . Such neurons have been directly observed in primate species . Birds have been shown to have imitative resonance behaviors and neurological evidence suggests the presence of some form of mirroring system . In humans, brain activity consistent with that of mirror neurons has been found in the premotor cortex, the supplementary motor area, the primary somatosensory cortex and the inferior parietal cortex . The function of the mirror system is a subject of much speculation . Many researchers in cognitive neuroscience and cognitive psychology consider that this system provides the physiological mechanism for the perception / action coupling (see the common coding theory). They argue that mirror neurons may be important for understanding the actions of other people, and for learning new skills by imitation . Some researchers also speculate that mirror systems may simulate observed actions, and thus contribute to theory of mind skills, while others relate mirror neurons to language abilities . However, to date, no widely accepted neural or computational models have been put forward to describe how mirror neuron activity supports cognitive functions such as imitation . There are neuroscientists who caution that the claims being made for the role of mirror neurons are not supported by adequate research . </P>

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