The advantage of noise
The idea that noise can improve a signal seems counterintuitive. In most systems, from audio engineering to telecommunications, noise is an unwanted element that obscures information. Yet, in a phenomenon called stochastic resonance (SR), a specific amount of random noise can actually amplify a weak, periodic signal that would otherwise be undetectable. This process requires three components: a weak, repeating signal below a system's detection threshold; a non-linear system, like a sensory neuron that only fires after a certain input level is reached; and the presence of random noise. The noise occasionally adds just enough energy to the weak signal, momentarily pushing it over the neuron's firing threshold. Too little noise has no effect, and too much noise drowns the signal out completely.
The concept was first proposed in 1981 by physicists Roberto Benzi, Giorgio Parisi, and colleagues to explain a long-standing climate puzzle: the 100,000-year cycle of Earth's ice ages. They suggested that small, regular variations in Earth's orbit (Milankovitch cycles) were the weak periodic signal. The Earth's climate system was the non-linear system, and random, short-term climate fluctuations acted as the noise. The combination, they argued, allowed the weak orbital signal to periodically push the global climate past a threshold, triggering the start or end of an ice age.
A world sensed through static
The first biological evidence for stochastic resonance was found not in climate, but in the tail fan of a crayfish (Procambarus clarkii). Experiments in 1993 showed that the crayfish's mechanoreceptors—hairs that detect water movement—were better at detecting a weak, 55.2 Hz mechanical vibration when a moderate level of random noise was added to the stimulus. Since then, SR has been an important mechanism in many sensory systems.
The American paddlefish (Polyodon spathula) is an example of SR in an important behavior: feeding. This fish navigates murky river waters using a long, paddle-shaped snout called a rostrum, which is covered with tens of thousands of passive electroreceptors. These organs, called ampullae of Lorenzini, are the same type found in sharks and rays, and they detect the faint electric fields produced by the muscle contractions of zooplankton like Daphnia. A single Daphnia generates a tiny, oscillating electrical signal between 0.5 and 20 Hz. Experiments showed that when a noisy electric field was added to the water, the paddlefish's success rate and strike distance for capturing plankton significantly increased. The swarms of plankton themselves generate a natural electrical noise, the paddlefish has evolved to exploit this phenomenon, using the background static of the swarm to pinpoint individual prey.
Evidence for SR is also present in human sensory systems. Studies have shown that adding low-level, random noise can improve the detection of faint visual images, improve the sense of touch, and even improve balance by providing noisy input to the vestibular system.