Coincidence detection in neurobiology
Encyclopedia
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 timely simultaneous yet spatially separate input signals. Coincidence detectors are important in 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.
Coincidence detection relies on separate inputs converging on a common target. Consider a basic neural circuit with two input neurons, A and B, that have excitatory synaptic terminals converging on a single output neuron, C (Fig. 1). If each input neuron's EPSP is subthreshold for an action potential
at C, then C will not fire unless the two inputs from A and B are temporally close together. Synchronous arrival of these two inputs may push the membrane potential
of a target neuron over the threshold required to create an action potential. If the two inputs arrive too far apart, the depolarization of the first input may have time to drop significantly, preventing the membrane potential of the target neuron from reaching the action potential threshold. This example incorporates the principles of spatial and temporal summation. Furthermore, coincidence detection can reduce the jitter formed by spontaneous activity. While random sub-threshold stimulations by neuronal cells may not often fire coincidentally, coincident synaptic inputs derived from a unitary external stimulus will ensure that a target neuron fires as a result of the stimulus.
along the azimuth plane in several organisms. In 1948, Lloyd Jeffress proposed that some organisms may have a collection of neurons that receive auditory input from each ear. The neural pathways to these neurons are called delay lines. Jeffress claimed that the neurons that the delay lines link act as coincidence detectors by firing maximally when receiving simultaneous inputs from both ears. When a sound is heard, sound waves may reach the ears at different times. This is referred to as the interaural time difference
(ITD). Due to differing lengths and a finite conduction speed within the axons of the delay lines, different coincidence detector neurons will fire when sound comes from different positions along the azimuth. Jeffress' model proposes that two signals even from an asynchronous arrival of sound in the cochlea of each ear will converge synchronously on a coincidence detector in the auditory cortex based on the magnitude of the ITD (Fig. 2). Therefore, the ITD should correspond to an anatomical map that can be found within the brain. Masakazu Konishi's
study on barn owls shows that this is true (Carr 1988). Sensory information from the hair cells of the ears travels to the ipsilateral nucleus magnocellularis. From here, the signals project ipsilaterally and contralaterally to two nucleus laminari. Each nucleus laminaris contains coincidence detectors that receive auditory input from the left and the right ear. Since the ipsilateral axons enter the nucleus laminaris dorsally while the contralateral axons enter ventrally, sounds from various positions along the azimuth correspond directly to stimulation of different depths of the nucleus laminaris. From this information, a neural map of auditory space was formed. The function of the nucleus laminaris parallels that of the medial superior olive in mammals (Zupanc 2004).
. Studies of LTP on multiple presynaptic cells stimulating a postsynaptic cell uncovered the property of associativity. A weak neuronal stimulation onto a pyramidal neuron may not induce long-term potentiation. However, this same stimulation paired with a simultaneous strong stimulation from another neuron will strengthen both synapses. This process suggests that two neuronal pathways converging on the same cell may both strengthen if stimulated coincidentally.
requires a prolonged depolarization that can expel the Mg2+ block of postsynaptic NMDA receptor
s. The removal of the Mg2+ block allows the flow of Ca2+ into the cell. A large elevation of calcium levels activate protein kinases that ultimately increase the number of postsynaptic AMPA receptor
s. This increases the sensitivity of the postsynaptic cell to glutamate. As a result, both synapses strengthen. The prolonged depolarization needed for the expulsion of Mg2+ from NMDA receptors requires a high frequency stimulation (Purves 2004). Associativity becomes a factor because this can be achieved through two simultaneous inputs that may not be strong enough to activate LTP by themselves.
Besides the NMDA-receptor based processes, further cellular mechanisms allow of the association between two different input signals converging on the same neuron, in a defined timeframe. Upon a simultaneous increase in the intracellular concentrations of cAMP and Ca2+, a transcriptional coactivator called TORC1 (CRTC1
) becomes activated, that converts the temporal coincidence of the two second messengers into long term changes such as LTP . This cellular mechanism, through calcium-dependent adenylate cyclase
activation, might also account for the detection of the repetitive stimulation of a given synapse
.
requires a coincident stimulation of parallel fibers and climbing fibers. Glutamate released from the parallel fibers activates AMPA receptors which depolarize the postsynaptic cell. The parallel fibers also activate metabotropic glutamate receptors that release the second messengers IP3 and DAG. The climbing fibers stimulate a large increase in postsynaptic Ca2+ levels when activated. The Ca2+, IP3
, and DAG
work together in a signal transduction pathway to internalize AMPA receptors and decrease the sensitivity of the postsynaptic cell to glutamate (Purves 2004).
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
or a neural circuit can encode information by detecting the occurrence of timely simultaneous yet spatially separate input signals. Coincidence detectors are important in information processing by reducing temporal jitter
Jitter
Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...
, 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.
Principles of coincidence detection
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
at C, then C will not fire unless the two inputs from A and B are temporally close together. Synchronous arrival of these two inputs may push the membrane potential
Membrane potential
Membrane potential is the difference in electrical potential between the interior and exterior of a biological cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with a variety of types of proteins embedded in it...
of a target neuron over the threshold required to create an action potential. If the two inputs arrive too far apart, the depolarization of the first input may have time to drop significantly, preventing the membrane potential of the target neuron from reaching the action potential threshold. This example incorporates the principles of spatial and temporal summation. Furthermore, coincidence detection can reduce the jitter formed by spontaneous activity. While random sub-threshold stimulations by neuronal cells may not often fire coincidentally, coincident synaptic inputs derived from a unitary external stimulus will ensure that a target neuron fires as a result of the stimulus.
Sound localization
Coincidence detection has been shown to be a major factor in sound localizationSound localization
Sound localization refers to a listener's ability to identify the location or origin of a detected sound in direction and distance. It may also refer to the methods in acoustical engineering to simulate the placement of an auditory cue in a virtual 3D space .The sound localization mechanisms of the...
along the azimuth plane in several organisms. In 1948, Lloyd Jeffress proposed that some organisms may have a collection of neurons that receive auditory input from each ear. The neural pathways to these neurons are called delay lines. Jeffress claimed that the neurons that the delay lines link act as coincidence detectors by firing maximally when receiving simultaneous inputs from both ears. When a sound is heard, sound waves may reach the ears at different times. This is referred to as the interaural time difference
Interaural time difference
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 localisation 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...
(ITD). Due to differing lengths and a finite conduction speed within the axons of the delay lines, different coincidence detector neurons will fire when sound comes from different positions along the azimuth. Jeffress' model proposes that two signals even from an asynchronous arrival of sound in the cochlea of each ear will converge synchronously on a coincidence detector in the auditory cortex based on the magnitude of the ITD (Fig. 2). Therefore, the ITD should correspond to an anatomical map that can be found within the brain. Masakazu Konishi's
Masakazu Konishi
is a Japanese neurobiologist, known for his research on prey capture auditory systems of barn owls and singing in songbirds.-Life:After growing up in wartime Kyoto, Konishi moved to study at Sapporo Agricultural College, Hokkaido University. Konishi studied for his doctoral thesis on properties of...
study on barn owls shows that this is true (Carr 1988). Sensory information from the hair cells of the ears travels to the ipsilateral nucleus magnocellularis. From here, the signals project ipsilaterally and contralaterally to two nucleus laminari. Each nucleus laminaris contains coincidence detectors that receive auditory input from the left and the right ear. Since the ipsilateral axons enter the nucleus laminaris dorsally while the contralateral axons enter ventrally, sounds from various positions along the azimuth correspond directly to stimulation of different depths of the nucleus laminaris. From this information, a neural map of auditory space was formed. The function of the nucleus laminaris parallels that of the medial superior olive in mammals (Zupanc 2004).
Synaptic plasticity and associativity
In 1949, Donald Hebb postulated that synaptic efficiency will increase through repeated and persistent stimulation of a postsynaptic cell by a presynaptic cell. This is often informally summarized as "cells that fire together, wire together". The theory was validated in part by the discovery of long-term potentiationLong-term potentiation
In neuroscience, long-term potentiation is a long-lasting enhancement in signal transmission between two neurons that results from stimulating them synchronously. It is one of several phenomena underlying synaptic plasticity, the ability of chemical synapses to change their strength...
. Studies of LTP on multiple presynaptic cells stimulating a postsynaptic cell uncovered the property of associativity. A weak neuronal stimulation onto a pyramidal neuron may not induce long-term potentiation. However, this same stimulation paired with a simultaneous strong stimulation from another neuron will strengthen both synapses. This process suggests that two neuronal pathways converging on the same cell may both strengthen if stimulated coincidentally.
Molecular mechanism of long-term potentiation
LTP in the hippocampusHippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
requires a prolonged depolarization that can expel the Mg2+ block of postsynaptic NMDA receptor
NMDA receptor
The NMDA receptor , a glutamate receptor, is the predominant molecular device for controlling synaptic plasticity and memory function....
s. The removal of the Mg2+ block allows the flow of Ca2+ into the cell. A large elevation of calcium levels activate protein kinases that ultimately increase the number of postsynaptic AMPA receptor
AMPA receptor
The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor is a non-NMDA-type ionotropic transmembrane receptor for glutamate that mediates fast synaptic transmission in the central nervous system . Its name is derived from its ability to be activated by the artificial glutamate analog AMPA...
s. This increases the sensitivity of the postsynaptic cell to glutamate. As a result, both synapses strengthen. The prolonged depolarization needed for the expulsion of Mg2+ from NMDA receptors requires a high frequency stimulation (Purves 2004). Associativity becomes a factor because this can be achieved through two simultaneous inputs that may not be strong enough to activate LTP by themselves.
Besides the NMDA-receptor based processes, further cellular mechanisms allow of the association between two different input signals converging on the same neuron, in a defined timeframe. Upon a simultaneous increase in the intracellular concentrations of cAMP and Ca2+, a transcriptional coactivator called TORC1 (CRTC1
CRTC1
CREB-regulated transcription coactivator 1 , previously referred to as TORC1 is a protein that in humans is encoded by the CRTC1 gene...
) becomes activated, that converts the temporal coincidence of the two second messengers into long term changes such as LTP . This cellular mechanism, through calcium-dependent adenylate cyclase
Adenylate cyclase
Adenylate cyclase is part of the G protein signalling cascade, which transmits chemical signals from outside the cell across the membrane to the inside of the cell ....
activation, might also account for the detection of the repetitive stimulation of a given synapse
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...
.
Molecular mechanism of long-term depression
Long-term depression also works through associative properties although it is not always the reverse process of LTP. LTD in the cerebellumCerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...
requires a coincident stimulation of parallel fibers and climbing fibers. Glutamate released from the parallel fibers activates AMPA receptors which depolarize the postsynaptic cell. The parallel fibers also activate metabotropic glutamate receptors that release the second messengers IP3 and DAG. The climbing fibers stimulate a large increase in postsynaptic Ca2+ levels when activated. The Ca2+, IP3
Inositol triphosphate
Inositol trisphosphate or inositol 1,4,5-trisphosphate , together with diacylglycerol , is a secondary messenger molecule used in signal transduction and lipid signaling in biological cells. While DAG stays inside the membrane, IP3 is soluble and diffuses through the cell...
, and DAG
Diglyceride
A diglyceride, or a diacylglycerol , is a glyceride consisting of two fatty acid chains covalently bonded to a glycerol molecule through ester linkages....
work together in a signal transduction pathway to internalize AMPA receptors and decrease the sensitivity of the postsynaptic cell to glutamate (Purves 2004).
See also
- Neurobiology
- Sound localizationSound localizationSound localization refers to a listener's ability to identify the location or origin of a detected sound in direction and distance. It may also refer to the methods in acoustical engineering to simulate the placement of an auditory cue in a virtual 3D space .The sound localization mechanisms of the...
- Long-term potentiationLong-term potentiationIn neuroscience, long-term potentiation is a long-lasting enhancement in signal transmission between two neurons that results from stimulating them synchronously. It is one of several phenomena underlying synaptic plasticity, the ability of chemical synapses to change their strength...
- Long-term depressionLong-term depressionLong-term depression , in neurophysiology, is an activity-dependent reduction in the efficacy of neuronal synapses lasting hours or longer. LTD occurs in many areas of the CNS with varying mechanisms depending upon brain region and developmental progress...
- Hebbian theoryHebbian theoryHebbian theory describes a basic mechanism for synaptic plasticity wherein an increase in synaptic efficacy arises from the presynaptic cell's repeated and persistent stimulation of the postsynaptic cell...
- Coincidence circuitCoincidence circuitIn physics, a coincidence circuit is an electronic device with one output and two inputs. The output is activated only when signals are received within a time window accepted as at the same time and in parallel at both inputs...
- NeuroethologyNeuroethologyNeuroethology is the evolutionary and comparative approach to the study of animal behavior and its underlying mechanistic control by the nervous system...
Further reading
- http://www.neurophys.wisc.edu/yin/jorissmithyin98.pdf
- http://jp.physoc.org/cgi/content/abstract/518/1/109
- http://bbsonline.cup.cam.ac.uk/Preprints/OldArchive/bbs.neur4.crepel.html
- http://www.jneurosci.org/cgi/content/full/26/16/4166