Stimulus filtering occurs when an animal's nervous system fails to respond to stimuli that would otherwise cause a reaction to occur. The nervous system has developed the capability to perceive and distinguish between minute differences in stimuli, which allows the animal to only react to significant impetus. This enables the animal to conserve energy as it is not responding to unimportant signals.
Adaptive value
The proximate causes of stimulus filtering can be many things in and around an animal's environment, but the ultimate cause of this response may be the evolutionary advantage offered by stimulus filtering. An animal that saves energy by not responding to unnecessary stimuli may have increased fitness, which means that it would be able to produce more offspring, whereas an animal that does not filter stimuli may have reduced fitness due to depleted energy stores. An animal that practices stimulus filtering may also be more likely to respond appropriately to serious threats than an animal that is distracted by unimportant stimuli.
Physiological mechanism
When particular signals are received by the animal, the superior-ranking neurons determine which signals are important enough to preserve and which signals are insignificant and can be ignored. This process essentially works as a filter as the synapses of the neural network enhance certain signals and repress others, with simple stimuli receiving attention from lower-level neurons, and more complicated stimuli receiving attention from higher level neurons.
Relation to humans
Stimulus filtering is also seen humans from a day-to-day basis. The phenomenon is called the cocktail party effect. When in a crowded room people tend to ignore other conversations and just focus on the one they are participating in. This effect also works in that when an individual hears their name in another's conversation they immediately focus on that conversation.
Examples
Moths
The evolution of a moth’s auditory system has helped them escape a bat’s echolocation. Physically a moth has two ears on each side of the thorax where they receive ultrasonic indicators to hear the distinct vocalizations that then vibrate the membranes of the moths ears at one of two auditory receptors: A1 or A2. These are attached to the tympanum in the ear. Intense sound pressure waves sweep over the moth's body causing the tympanum to vibrate and deforming these receptor cells. This opens stretch-sensitive channels in the cell membrane and provides the effective stimuli for a moth auditory receptor. These receptors work in the same ways that most neurons do, by responding to the energy contained in selected stimuli and changing the permeability of their cell membranes to positively charged ions. Even though the A1 and A2 receptors work in a similar fashion, there are significant differences between them. The A1 receptor is the main bat detector, and as the rate of firing increases the moth turns away from the bat to reduce sonar echo. In other words, the A1 receptor is sensitive to low frequencies. To determine the relative position of the bat the differential firing rates of the A1 cells will fire on either side of the moth's head and if the bat is farther away cells receive a weaker signal and will fire at a slower rate. The A2 receptor is the emergency back-up system by initiating erratic flight movements as a last-ditch effort to evade capture. This differential sensitivity of the A1 and A2 sensory neurons leads to stimulus filtering of the bat vocalizations. Long-distance evasion tactics are engaged when the bat is far away and therefore the A1 sensory neurons fire. When the bat is in extremely close range, short-distance evasion tactics are engaged with the use of A2 sensory neurons. The adaptive value of the physiological mechanisms of two distinct receptors aids in the evasion of capture from bats.
Parasitoid flies
Female flies of the genus Ormia ochracea possess organs in their bodies that can detect frequencies of cricket sounds from meters away. This process is important for the survival of their species because females will lay their first instar larvae into the body of the cricket, where they will feed and molt for approximately seven days. After this period, the larvae grow into flies and the cricket usually perishes.