Feature detection (nervous system)

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 neurons—or groups of neurons—in 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 edges—a 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 noisy—emphasizing high spatial frequencies but lacking in extended edges. Responding selectively to an extended edge—either a bright line on a dark background, or the reverse—highlights 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.

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 cells—which respond to both the transition from light to dark and the transition from dark to light—and 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.

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