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Feature detection is a process by which the nervous system sorts or filters complex natural stimuli in order to extract behaviorally relevant cues that have a high probability of being associated with important objects or organisms in their environment, as opposed to irrelevant background or noise. ''Feature detectors'' are individual neuronsor groups of neuronsin the brain which code for perceptually significant stimuli. Early in the sensory pathway feature detectors tend to have simple properties; later they become more and more complex as the features to which they respond become more and more specific. For example, simple cells in the visual cortex of the domestic cat (''Felis catus''), respond to edgesa feature which is more likely to occur in objects and organisms in the environment. By contrast, the background of a natural visual environment tends to be noisyemphasizing high spatial frequencies but lacking in extended edges. Responding selectively to an extended edgeeither a bright line on a dark background, or the reversehighlights objects that are near or very large. Edge detectors are useful to a cat, because edges do not occur often in the background “noise” of the visual environment, which is of little consequence to the animal. ==History== Early in the history of sensory neurobiology, physiologists favored the idea that the nervous system detected specific features of stimuli, rather than faithful copying of the sensory world onto a sensory map in the brain. For example, in the visual system, they favored the idea of detecting specific visual features of the visual world as opposed as the eye as a camera where the retina acts like film and the brain acts like a faithful camera which preserves all elements without making assumptions about what is important in the environment. It wasn't until the late 1950s that the feature detector hypothesis fully developed, and over the last fifty years, it has been the driving force behind most work on sensory systems. Horace B. Barlow was one of the first investigators to use the concept of the feature detector to relate the receptive field of a neuron to a specific animal behavior. In 1953, H.B. Barlow’s electrophysiological recordings from excised retina of the frog provided the first evidence for the presence of an inhibitory surround in the receptive field of a frog’s retinal ganglion cell. In reference to “on-off” ganglion cellswhich respond to both the transition from light to dark and the transition from dark to lightand also had very restricted receptive fields of visual angle (about the size of a fly at the distance that the frog could strike), Barlow stated, “It is difficult to avoid the conclusion that the ‘on-off’ units are matched to the stimulus and act as fly detectors”. In the same year, Stephen Kuffler published ''in vivo'' evidence for an excitatory center, inhibitory surround architecture in the ganglion cells of the mammalian retina which further supported Barlow’s suggestion that on-off units can code for behaviorally relevant events. Barlow’s idea that certain cells in the retina could act as “feature detectors” was influenced by E.D. Adrian and Nikolaas Tinbergen.〔 E.D. Adrian, Barlow’s advisor, was the discoverer of the frequency codethe observation that sensory nerves convey signal intensity though the frequency of their firing. On the other hand, during Barlow’s career, Nikolaas Tinbergen was introducing the concept of the innate release mechanism (IRM) and sign stimulus. IRMs are hard wired mechanisms that give an animal the innate ability to recognize complex stimuli. The sign stimulus is a simple, reduced stimulus including only the necessary features of the stimulus capable of evoking a behavioral response. Tinbergen’s examination of the pecking behavior in herring gull chicks illustrated that the pecking response could be evoked by any bill-shaped long rod with a red spot near the end. In his own paper, Barlow later compared a sign stimulus to a password which was either accepted or rejected by a feature detector. Accepted passwords would contain the features necessary to trigger specific behavioral responses in an animal. In the late 1950s, Jerome Lettvin and his colleagues began to expand the feature detection hypothesis and clarify the relationship between single neurons and sensory perception.〔 In their paper “What the Frog’s Eye Tells the Frog’s Brain,” Lettvin et al. (1959) looked beyond the mechanisms for signal-noise discrimination in the frog’s retina and were able to identify four classes of ganglion cells in the frog retina: ''sustained contrast detectors'', ''net convexity detectors'' (or ''bug detectors''), ''moving edge detectors'', and ''net dimming detectors.'' In the same year, David Hubel and Torsten Wiesel began investigating properties of neurons in the visual cortex of cats, processing in the mammalian visual system. In their first paper in 1959, Hubel and Wiesel took recording from single cells in the striate cortex of lightly anesthetized cats. The retinas of the cats were stimulated either individually or simultaneously with spots of light of various sizes and shapes. From the analysis of these recordings, Hubel and Wiesel identified orientation-selective cells in the cat’s visual cortex and generated a map of the receptive field of cortical cells. At the time, circular spots of light were used as stimuli in studies of the visual cortex.〔 However, Hubel and Wiesel noticed that rectangular bars of light were more effective stimuli (i.e. more natural stimuli) than circular spots of light, as long as the orientation was adjusted to the correct angle appropriate for each ganglion cell. These so-called simple cells were later called bar detectors or edge detectors. While comparing the receptive fields of neurons in the cat striate cortex with the concentric “on” and “off” receptive fields identified in cat ganglion cells by Kuffler et al., Hubel and Wiesel noticed that, although “on” and “off” regions were present in the striate cortex, they were not arranged in concentric circles. From their discovery of these uniquely orienting receptive fields, Hubel and Wiesel concluded that orientation-selective cells exist within the cat’s visual cortex. In their second major paper, Hubel and Wiesel extended their technique to more complex regions in the visual cortex in an effort to understand the difference between cortical receptive fields and lateral geniculate fields. They observed that the cat striate cortex contained more cells than the lateral geniculate, and they reasoned that the cortex needs a large number of neurons to digest the large amount of information it receives. Through experimentation, they found that each neuron in the cortex is responsible for a small region of the visual field and also has its own orientation specificity. From the results of these single cell readings in the striate cortex and lateral geniculate, Hubel and Wiesel postulated that simple cortical receptive fields gain complexity and an intricate spatial arrangement through the patterned convergence of multiple “on” or “off” projections from lateral geniculate cells onto single cortical cells. Hubel and Wiesel’s investigation of the cat visual cortex sparked interest in the feature detection hypothesis and its relevance to other sensory systems. In fact, T.H. Bullock contended in 1961 that the vestibular system was being ignored by most of the contemporary sensory system research, and he suggested that the equivalent stimulation of vestibular organs may yield similarly intriguing results. Hubel and Wiesel’s work also raised the question: How far does the hierarchy of visual processing go? In one answer to this question, Lettvin coined the term grandmother cells in 1969 to describe hypothetical cells that are so specific that they only fire when your grandmother’s face is viewed. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Feature detection (nervous system)」の詳細全文を読む スポンサード リンク
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