Active sensory systems

Active sensory systems are sensory receptors that are activated by probing the environment with self-generated energy. Examples include echolocation of bats and dolphins and insect antennae. Using self-generated energy allows more control over signal intensity, direction, timing and spectral characteristics. By contrast, passive sensory systems involve activation by ambient energy (that is, energy that is preexisting in the environment, rather than generated by the user). For example, human vision relies on using light from the environment.

Active sensory systems receive information with or without direct contact. Teleceptive active sensory systems collect information by directing propagating energy and detecting objects using cues such as time delay and intensity of return signal. Examples include echolocation of bats and electrosensory detection of electric fish. Contact active sensory systems use physical contact between stimuli and organism. Insect antennae and whiskers are examples of contact active sensory systems.

Bioluminescence: Adult firefly uses self-generated light to locate mates. In deep oceans, barbeled dragonfish produces near infrared light.

Electrostatic field: Electric fishes probe the environment and create active electrodynamic imaging.

Active touching: Nocturnal animals depend on whiskers to navigate by gathering information about position, size, shape, orientation and texture of objects. Insects use antennae to probe the environment during locomotion. Human's reaching out to objects with hands is an analogy.

Echolocation: Active acoustic sensing of self-produced sounds. Bats emit echolocation calls for detecting prey in flight. Toothed whales use echolocation in water.

Because propagation of chemicals take longer than other sources, only organisms with slow locomotion can utilize chemical signals to probe the environment. The slime mold Dictyostelium discoideum uses ammonia to probe the environment to avoid obstacles during formation of fruiting body. Deploying chemical signal is also limited by lack of return signals.

An important constraint in teleceptive active sensory systems is generating energy with return signal above threshold of detection. Self-generated energy needs to be strong enough to detect objects at a distance. Due to geometric spreading, energy emitted uniformly will spread over a sphere of increasing surface area. Signal strength depends on the square of distance between organism and target. In teleceptive active sensing, geometric spread cost is doubled, because signal is emitted and returned. As a result, fraction of energy returned decreases as the fourth power of the distance between organism and target.

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