Stimulus filtering

Last updated November 06, 2019

Stimulus filtering occurs when an animal's nervous system fails to respond to stimuli that would otherwise cause a reaction to occur.^[1] 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.^[2] This enables the animal to conserve energy as it is not responding to unimportant signals.

The nervous system is a highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events. Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. In vertebrates it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to every other part of the body. Nerves that transmit signals from the brain are called motor or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory or afferent. Spinal nerves serve both functions and are called mixed nerves. The PNS is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems. Somatic nerves mediate voluntary movement. The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves.

In physiology, a stimulus is a detectable change in the physical or chemical structure of an organism's internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity. When a stimulus is applied to a sensory receptor, it normally elicits or influences a reflex via stimulus transduction. These sensory receptors can receive information from outside the body, as in touch receptors found in the skin or light receptors in the eye, as well as from inside the body, as in chemoreceptors and mechanoreceptors. An internal stimulus is often the first component of a homeostatic control system. External stimuli are capable of producing systemic responses throughout the body, as in the fight-or-flight response. In order for a stimulus to be detected with high probability, its level must exceed the absolute threshold; if a signal does reach threshold, the information is transmitted to the central nervous system (CNS), where it is integrated and a decision on how to react is made. Although stimuli commonly cause the body to respond, it is the CNS that finally determines whether a signal causes a reaction or not.

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.^[3] 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.^[2] 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.^[2]

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.

The cocktail party effect is the phenomenon of the brain's ability to focus one's auditory attention on a particular stimulus while filtering out a range of other stimuli, as when a partygoer can focus on a single conversation in a noisy room. Listeners have the ability to both segregate different stimuli into different streams, and subsequently decide which streams are most pertinent to them. Thus, it has been proposed that one's sensory memory subconsciously parses all stimuli, identifying discrete pieces of information and classifying them by salience. This effect is what allows most people to "tune into" a single voice and "tune out" all others. It may also describe a similar phenomenon that occurs when one may immediately detect words of importance originating from unattended stimuli, for instance hearing one's name among a wide range of auditory input.

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.^[4] 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.^[3] 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.^[4] The adaptive value of the physiological mechanisms of two distinct receptors aids in the evasion of capture from bats.

Moths are a polyphyletic group of insects that includes all members of the order Lepidoptera that are not butterflies, with moths making up the vast majority of the order. There are thought to be approximately 160,000 species of moth, many of which have yet to be described. Most species of moth are nocturnal, but there are also crepuscular and diurnal species.

Bats are mammals of the order Chiroptera; with their forelimbs adapted as wings, they are the only mammals naturally capable of true and sustained flight. Bats are more manoeuvrable than birds, flying with their very long spread-out digits covered with a thin membrane or patagium. The smallest bat, and arguably the smallest extant mammal, is Kitti's hog-nosed bat, which is 29–34 mm (1.14–1.34 in) in length, 15 cm (5.91 in) across the wings and 2–2.6 g (0.07–0.09 oz) in mass. The largest bats are the flying foxes and the giant golden-crowned flying fox, Acerodon jubatus, which can weigh 1.6 kg (4 lb) and have a wingspan of 1.7 m.

Echolocation, also called bio sonar, is the biological sonar used by several species of animal. Echolocating animals emit calls out to the environment and listen to the echoes of those calls that return from various objects near them. They use these echoes to locate and identify the objects. Echolocation is used for navigation and for foraging in various environments.

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.

Ormia ochracea is a small yellow nocturnal fly in the Tachinidae family. It is notable for its parasitism of crickets and its exceptionally acute directional hearing. The female is attracted by the song of the male cricket and deposits larvae on or around him, as was discovered in 1975 by the zoologist William H. Cade. O. ochracea is found throughout North America, South America, and the Caribbean, though its exact range is not known.

Cricket is a bat-and-ball game played between two teams of eleven players on a field at the centre of which is a 20-metre (22-yard) pitch with a wicket at each end, each comprising two bails balanced on three stumps. The batting side scores runs by striking the ball bowled at the wicket with the bat, while the bowling and fielding side tries to prevent this and dismiss each player. Means of dismissal include being bowled, when the ball hits the stumps and dislodges the bails, and by the fielding side catching the ball after it is hit by the bat, but before it hits the ground. When ten players have been dismissed, the innings ends and the teams swap roles. The game is adjudicated by two umpires, aided by a third umpire and match referee in international matches. They communicate with two off-field scorers who record the match's statistical information.

An instar is a developmental stage of arthropods, such as insects, between each moult (ecdysis), until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions, colors, patterns, changes in the number of body segments or head width. After moulting, i.e. shedding their exoskeleton, the juvenile arthropods continue in their life cycle until they either pupate or moult again. The instar period of growth is fixed; however, in some insects, like the salvinia stem-borer moth, the number of instars depends on early larval nutrition. Some arthropods can continue to moult after sexual maturity, but the stages between these subsequent moults are generally not called instars.

Researchers were puzzled about how precise hearing ability could arise from a small ear structure. Normal animals detect and locate sounds using the interaural time difference (ITD) and the interaural level difference (ILD).^[5] The ITD is the difference in the time it takes sound to reach the ear. ILD is the difference in sound intensity measure between both ears. At maximum, the ITD would only reach about 1.5 microseconds and the ILD would be less than one decibel.^[5] These small values make it hard to sense the differences. To solve these issues, researchers studied the mechanical aspects of flies’ ears. They found that they have a presternum structure linking both tympanal membranes that is critical in detecting sound and localization. The structure acts as a lever by transferring and amplifying vibrational energy between the membranes.^[5] After sound hits the membranes at different amplitudes, the presternum sets up symmetrical vibration modes through bending and rocking.^[5] This effect helps the nervous system distinguish which side the sound is coming from. Because the presternum acts as an intertympanal bridge, the ITD is increased from 1.5 us to 55 us and the ILD is increased from less than one decibel to over 10 decibels.^[5]

When looking at the nervous systems of flies, researchers found three auditory afferents. Type one fires only one spike to the stimulus onset, has low jitter (variability in timing over stimulus presentations), no spontaneous activity, and is the most common type.^[6] Type two fires two to four spikes to the stimulus onset, has increased jitter with subsequent spikes, and has low spontaneous activity.^[6] Finally, type three has tonic spiking to the presented stimulus, has low jitter only with the first spikes, has low spontaneous activity, and is the least common type. Researchers discovered that neurons responded the strongest to sound frequencies between 4 and 9 kHz, which includes the frequencies present in cricket songs.^[5] Also, neurons were found to have responded strongest at 4.5 kHz, which is the frequency of the Gryllus song.^[5] Despite the type of auditory afferent, all observed neurons revealed an inverse/latency relationship. The stronger the stimulus, the shorter the time until the neuron begins to respond. The difference in the number of afferents above the threshold on a side of the animal is called population code and can be used to account for sound localization.^[6]

Midshipman fish

Female midshipman fish undergo stimulus filtering when it comes time to mate with a male. Midshipman fish use stimulus filtering when listening to sounds produced by underwater species.^[7] Dominant signals underwater range between 60–120 Hz, which is the most normally the most sensitive to the fish's auditory receptor.^[3] However, the female auditory system changes seasonally to acoustical stimuli in the songs of male midshipman fish. In the summer when female midshipman fish are reproducing they listen to a male humming song that can be produce a frequency level of 400 Hz.^[3] The summer is reproducing season for the females so their hearing is more sensitive to the high frequency of the male humming.

Related Research Articles

Sensory neurons, also known as afferent neurons, are neurons in the central nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell bodies of the sensory neurons are located in the dorsal ganglia of the spinal cord.

Sound localization is a listener's ability to identify the location or origin of a detected sound in direction and distance.

Volley theory states that groups of neurons of the auditory system respond to a sound by firing action potentials slightly out of phase with one another so that when combined, a greater frequency of sound can be encoded and sent to the brain to be analyzed. The theory was proposed by Ernest Wever and Charles Bray in 1930 as a supplement to the frequency theory of hearing. It was later discovered that this only occurs in response to sounds that are about 500 Hz to 5000 Hz.

Virtual acoustic space (VAS), also known as virtual auditory space, is a technique in which sounds presented over headphones appear to originate from any desired direction in space. The illusion of a virtual sound source outside the listener's head is created.

The interaural time difference when concerning humans or animals, is the difference in arrival time of a sound between two ears. It is important in the localization of sounds, as it provides a cue to the direction or angle of the sound source from the head. If a signal arrives at the head from one side, the signal has further to travel to reach the far ear than the near ear. This pathlength difference results in a time difference between the sound's arrivals at the ears, which is detected and aids the process of identifying the direction of sound source.

Binaural fusion or binaural integration is a cognitive process that involves the "fusion" of different auditory information presented binaurally, or to each ear. In humans, this process is essential in understanding speech as one ear may pick up more information about the speech stimuli than the other.

Computational auditory scene analysis (CASA) is the study of auditory scene analysis by computational means. In essence, CASA systems are "machine listening" systems that aim to separate mixtures of sound sources in the same way that human listeners do. CASA differs from the field of blind signal separation in that it is based on the mechanisms of the human auditory system, and thus uses no more than two microphone recordings of an acoustic environment. It is related to the cocktail party problem.

Auditory masking occurs when the perception of one sound is affected by the presence of another sound.

Coincidence detection in the context of neurobiology is a process by which a neuron or a neural circuit can encode information by detecting the occurrence of temporally close but spatially distributed input signals. Coincidence detectors influence neuronal information processing by reducing temporal jitter, reducing spontaneous activity, and forming associations between separate neural events. This concept has led to a greater understanding of neural processes and the formation of computational maps in the brain.

Ultrasound avoidance is an escape or avoidance reflex displayed by certain animal species that are preyed upon by echolocating predators. Ultrasound avoidance is known for several groups of insects that have independently evolved mechanisms for ultrasonic hearing. Insects have evolved a variety of ultrasound-sensitive ears based upon a vibrating tympanic membrane tuned to sense the bat's echolocating calls. The ultrasonic hearing is coupled to a motor response that causes evasion of the bat during flight.

Sensory maps are areas of the brain which respond to sensory stimulation, and are spatially organized according to some feature of the sensory stimulation. In some cases the sensory map is simply a topographic representation of a sensory surface such as the skin, cochlea, or retina. In other cases it represents other stimulus properties resulting from neuronal computation and is generally ordered in a manner that reflects the periphery. An example is the somatosensory map which is a projection of the skin's surface in the brain that arranges the processing of tactile sensation. This type of somatotopic map is the most common, possibly because it allows for physically neighboring areas of the brain to react to physically similar stimuli in the periphery or because it allows for greater motor control.

Jamming avoidance response (JAR) is a behavior performed by some species of weakly electric fish. The JAR occurs when two electric fish with wave discharges meet – if their discharge frequencies are very similar, each fish will shift its discharge frequency to increase the difference between the two fish's discharge frequencies. By doing this, both fish prevent jamming of their sense of electroreception.

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.

Electrocochleography is a technique of recording electrical potentials generated in the inner ear and auditory nerve in response to sound stimulation, using an electrode placed in the ear canal or tympanic membrane. The test is performed by an otologist or audiologist with specialized training, and is used for detection of elevated inner ear pressure or for the testing and monitoring of inner ear and auditory nerve function during surgery.

Surface wave detection by animals is the process by which animals, such as surface-feeding fish are able to sense and localize prey and other objects on the surface of a body of water by analyzing features of the ripples generated by objects' movement at the surface. Features analyzed include waveform properties such as frequency, change in frequency, and amplitude, and the curvature of the wavefront. A number of different species are proficient in surface wave detection, including some aquatic insects and toads, though most research is done on the topminnow/surface killifish Aplocheilus lineatus. The fish and other animals with this ability spend large amounts of time near the water surface, some just to feed and others their entire lives.

Infrasound is sound at frequencies lower than the low frequency end of human hearing threshold at 20 Hz. It is known, however, that humans can perceive sounds below this frequency at very high pressure levels. Infrasound can come from many natural as well as man-made sources, including weather patterns, topographic features, ocean wave activity, thunderstorms, geomagnetic storms, earthquakes, jet streams, mountain ranges, and rocket launchings. Infrasounds are also present in the vocalizations of some animals. Low frequency sounds can travel for long distances with very little attenuation and can be detected hundreds of miles away from their sources.

Perceptual-based 3D sound localization is the application of knowledge of the human auditory system to develop 3D sound localization technology.

Most owls are nocturnal or crepuscular birds of prey. Because they hunt at night, they must rely on non-visual senses. Experiments by Roger Payne have shown that owls are sensitive to the sounds made by their prey, not the heat or the smell. In fact, the sound cues are both necessary and sufficient for localization of mice from a distant location where they are perched. For this to work, the owls must be able to accurately localize both the azimuth and the elevation of the sound source.

Binaural unmasking is phenomenon of auditory perception discovered by Ira Hirsh. In binaural unmasking, the brain combines information from the two ears in order to improve signal detection and identification in noise. The phenomenon is most commonly observed when there is a difference between the interaural phase of the signal and the interaural phase of the noise. When such a difference is present there is an improvement in masking threshold compared to a reference situation in which the interaural phases are the same, or when the stimulus has been presented monaurally. Those two cases usually give very similar thresholds. The size of the improvement is known as the "binaural masking level difference" (BMLD), or simply as the "masking level difference".

References

↑ "Stimulus filtering". Oxford Reference. 2012-02-17. Retrieved 2015-02-25.
1 2 3 "5 - Stimulus filtering: vision and motion detection - University Publishing Online". Ebooks.cambridge.org. Retrieved 2015-02-25.
1 2 3 4 Alcock, J. (2009). Animal Behavior (Ninth ed., Vol. 1). Sunderland, MA: Sinauer Associates, Inc.
1 2 3 4 5 6 7 "Neuroethology: Fly Hearing". Nelson.beckman.illinois.edu. 2003-04-29. Retrieved 2015-02-25.
1 2 3 Michael L. Oshinsky1 and Ronald R. Hoy2 (2002-08-15). "Physiology of the Auditory Afferents in an Acoustic Parasitoid Fly". Jneurosci.org. Retrieved 2015-02-25.
↑ Alderks, P. W., & Sisneros, J. A. (2011). "Ontogeny of auditory saccular sensitivity in the plainfin midshipman fish, Porichthys notatus." J Comp Physiol A, 127, 387-398.