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'''Multielectrode arrays (MEAs)''' or [[microelectrode]] arrays are devices that contain multiple plates or shanks through which neural [[signal (electronics)|signals]] are obtained or delivered, essentially serving as neural interfaces that connect [[neuron]]s to [[electric circuit|electronic circuitry]]. There are two general classes of MEAs: implantable MEAs, used ''[[in vivo]]'', and non-implantable MEAs, used ''[[in vitro]]''.

==Theory==
Neurons and [[muscle]] cells create [[ion]] currents through their [[Cell membrane|membranes]] when excited, causing a change in [[voltage]] between the inside and the outside of the cell. When recording, the [[electrode]]s on an MEA [[Signal transduction|transduce]] the change in [[voltage]] from the environment carried by [[ions]] into currents carried by [[electrons]] (electronic currents). When stimulating, [[electrodes]] transduce electronic currents into ionic currents through the media. This triggers the [[voltage-gated ion channel]]s on the [[cell membrane|membranes]] of the excitable cells, causing the cell to [[depolarization|depolarize]] and trigger an [[action potential]] if it is a neuron or a twitch if it is a muscle cell.{{Citation needed|reason=It is highly unlikely that a low-voltage extracellular current directly affects voltage-gated channels that are known to respond to large changes in the 70,000 V/m electric field of the cell membrane|date=December 2008}}

The size and shape of a recorded signal depend upon several factors: the nature of the medium in which the cell or cells are located (e.g. the medium's [[electrical conductivity]], [[capacitance]], and [[Homogeneous (chemistry)|homogeneity]]); the nature of contact between the cells and the MEA electrode (e.g. area of contact and tightness); the nature of the MEA electrode itself (e.g. its geometry, [[Electrical impedance|impedance]], and noise); the [[analog signal processing]] (e.g. the system's [[gain]], [[Bandwidth (signal processing)|bandwidth]], and behavior outside of [[cutoff frequency|cutoff frequencies]]); and the data [[sampling (signal processing)|sampling]] properties (e.g. [[sampling rate]] and [[digital signal processing]]).<ref name="Boven">Boven K-H, Fejtl M, Möller A, Nisch W, Stett A. On Micro-Electrode Array Revival. In: Baudry M, Taketani M, eds. ''Advances in Network Electrophysiology Using Multi-Electrode Arrays.'' New York: Springer Press; 2006: 24-37.</ref> For the recording of a single cell that partially covers a planar electrode, the voltage at the [[contact pad]] is approximately equal to the voltage of the overlapping region of the cell and electrode multiplied by the ratio the [[surface area]] of the overlapping region to the area of the entire electrode, or:

<math>V_{pad}=V_{overlap}\times\frac{A_{overlap}}{A_{electrode}}</math>

assuming the area around an electrode is [[insulator (electrical)|well-insulated]] and has a very small capacitance associated with it.<ref name="Boven" /> The equation above, however, relies on modeling the electrode, cells, and their surroundings as an equivalent [[circuit diagram]]. An alternative means of predicting cell-electrode behavior is by modeling the system using a geometry-based [[finite element analysis]] in an attempt to circumvent the limitations of oversimplifying the system in a lumped circuit element diagram.<ref name="Buitenweg">Buitenweg JR, Rutten WL, and Marani E. 2003. Geometry-based finite element modeling of the electrical contact between a cultured neuron and a microelectrode. ''IEEE Trans Biomed Eng. 50'': 501-509.</ref>

An MEA can be used to perform [[electrophysiological]] experiments on tissue slices or [[dissociated cell cultures]]. With acute tissue slices, the connections between the cells within the tissue slices prior to extraction and plating are more or less preserved, while the intercellular connections in dissociated cultures are destroyed prior to plating. With dissociated neuronal cultures, the neurons spontaneously form [[biological neural network|networks]].<ref name="Potter2001">Potter SM. 2001. Distributed processing in cultured neuronal networks. ''Prog Brain Res 130'': 49-62.</ref>

It can be seen that the voltage [[amplitude]] an electrode experiences is [[inverse relation|inversely]] related to the distance from which a cell depolarizes.<ref name="PineBook">Pine J. A History of MEA Development. In: Baudry M, Taketani M, eds. ''Advances in Network Electrophysiology Using Multi-Electrode Arrays.'' New York: Springer Press; 2006:3-23.</ref> Thus, it may be necessary for the cells to be cultured or otherwise placed as close to the electrodes as possible. With tissue slices, a layer of electrically passive dead cells form around the site of incision due to [[edema]].<ref name="Heuschkel">Heuschkel MO, Wirth C, Steidl EM, Buisson B. Development of 3-D Multi-Electrode Arrays for Use with Acute Tissue Slices. In: Baudry M, Taketani M, eds. ''Advances in Network Electrophysiology Using Multi-Electrode Arrays.'' New York: Springer Press; 2006:69-111.</ref> A way to deal with this is by fabricating an MEA with three-dimensional electrodes fabricated by [[photomask|masking]] and [[etching (microfabrication)|chemical etching]]. These 3-D electrodes penetrate the dead cell layer of the slice tissue, decreasing the distance between live cells and the electrodes.<ref name="Thiebaud">Thiebaud P, deRooij NF, Koudelka-Hep M, Stoppini L. 1997. Microelectrode arrays for electrophysiological monitoring of hippocampal organotypic slice cultures. ''IEEE Trans Biomed Eng. 44'': 1159-63.</ref> In dissociated cultures, proper adherence of the cells to the MEA substrate is important for getting robust signals.

==History==
The first implantable arrays were microwire arrays developed in the 1950s.<ref name="Cheung" /> The first experiment involving the use of an array of planar electrodes to record from cultured cells was conducted in 1972 by C.A. Thomas, Jr. and his colleagues.<ref name="PineBook" /> The experimental setup used a 2 x 15 array of [[gold]] electrodes plated with [[platinum black]], each spaced 100 µm apart from each other. [[Myocytes]] harvested from [[embryo]]nic chicks were dissociated and cultured onto the MEAs, and signals up to 1 mV high in amplitude were recorded.<ref name = "Thomas">Thomas CA, Springer PA, Loeb GE, Berwald-Netter Y, Okun LM. 1972. A miniature microelectrode array to monitor the bioelectric activity of cultured cells. ''Exp Cell Res. 74'': 61-66.</ref> MEAs were constructed and used to explore the electrophysiology of snail [[ganglia]] independently by G. Gross and his colleagues in 1977 without prior knowledge of Thomas and his colleagues' work.<ref name="PineBook" /> In 1982, Gross observed spontaneous electrophysiological activity from dissociated [[spinal cord]] neurons, and found that activity was very dependent on temperature. Below about 30˚C signal amplitudes decrease rapidly to relatively small value at [[room temperature]].<ref name="PineBook" />

Before the 1990s, significant [[entry barrier]]s existed for new laboratories that sought to conduct MEA research due to the custom MEA fabrication and software they had to develop.<ref name="Potter2001" /> However, with the advent of affordable computing power<ref name="Boven" /> and commercial MEA hardware and software,<ref name="Potter2001" /> many other laboratories were able to undertake research using MEAs.

==Types==
Microelectrode arrays can be divided up into subcategories based on their potential use: ''in vitro'' and ''in vivo'' arrays.

===''In vitro'' arrays===
[[Image:MEAinHand.jpg|thumb|An ''in vitro'' MEA |300px|right|An ''in vitro'' MEA]]
The standard type of ''in vitro'' MEA comes in a pattern of 8 x 8 or 6 x 10 electrodes. Electrodes are typically composed of [[indium tin oxide]] or [[titanium]] and have diameters between 10 and 30 μm. These arrays are normally used for single-cell cultures or acute brain slices.<ref name="Boven" />

One challenge among ''in vitro'' MEAs has been imaging them with [[microscopes]] that use high power lenses, requiring low [[working distance]]s on the order of micrometers. In order to avoid this problem, "thin"-MEAs have been created using cover slip glass. These arrays are approximately 180 μm allowing them to be used with high-power lenses.<ref name="Boven" /><ref name="tenacity">Minerbi A, Kahana R, Goldfeld L, Kaufman M, Marom S, Ziv NE. 2009. Long-term relationships between synaptic tenacity, synaptic remodeling, and network activity. PLoS Biol. 7(6):e1000136.</ref>

In another special design, 60 electrodes are split into 6 x 5 arrays separated by 500 μm. Electrodes within a group are separated by 30 um with diameters of 10 μm. Arrays such as this are used to examine local responses of neurons while also studying functional connectivity of organotypic slices.<ref name="Boven" /><ref>Segev R, Berry II, MJ. 2003. Recording from all of the ganglion cells in the retina. ''Soc Neurosci Abstr. 264'': 11.</ref>

Spatial resolution is one of the key advantages of MEAs and allows signals sent over a long distance to be taken with higher precision when a high-density MEA is used. These arrays usually have a square grid pattern of 256 electrodes that cover an area of 2.8 by 2.8 mm.<ref name="Boven" />

Increased spatial resolution is provided by CMOS-based high-density microelectrode arrays featuring thousands of electrodes along with integrated readout and stimulation circuits on compact chips of the size of a thumbnail.<ref name="A1">A. Hierlemann, U. Frey, S. Hafizovic, F. Heer (2011). Growing Cells atop Microelectronic Chips: Interfacing Electrogenic Cells in Vitro with CMOS-based Microelectrode Arrays. Proceedings of the IEEE, Vol. 99, No. 2, pp. 252-284.</ref> Even the resolution of signals propagating along single axons has been demonstrated.<ref name="A2">D. J. Bakkum, U. Frey, M. Radivojevic, T. L. Russell, J. Müller, M. Fiscella, H. Takahashi, A. Hierlemann (2013). Tracking axonal action potential propagation on a high-density microelectrode array across hundreds of sites. Nature Communications 2013, 4:2181.</ref>

In order to obtain quality signals electrodes and tissue must be in close contact with one another. The perforated MEA design applies negative [[pressure]] to openings in the substrate so that tissue slices can be positioned on the electrodes to enhance contact and recorded signals.<ref name="Boven" />

A different approach to lower the electrode impedance is by modification of the interface material, for example by using [[carbon nanotubes]],<ref>Yu, Z. et al. 2007. Vertically Aligned Carbon Nanofiber Arrays Record Electrophysiological Signals from Hippocampal Slices. ''Nano Lett. 7(8)'': 2188.</ref><ref>Gabai, T. et al. 2007. Electro-chemical and biological properties of carbon nanotube based multi-electrode arrays ''Nanotechnology. 18(3)'': 035201.</ref> or by modification of the structure of the electrodes, with for example gold nanopillars<ref>Brüggemann, D. et al. 2011. Nanostructured gold microelectrodes for extracellular recording from electrogenic cells ''Nanotechnology 22(26)'': 265104.</ref> or nanocavities.<ref>Hoffmann, B. et al. 2011. Nanocavity electrode array for recording from electrogenic cells ''Lab Chip 11'': 1054.</ref>

===''In vivo'' arrays===
[[Image:Utah array pat5215088.jpg|thumb|300px|right|Schematic of the "Utah" in vivo electrode array]]
The three major categories of implantable MEAs are microwire, [[silicon]]- based,<ref>R. Bhandari, S. Negi, F. Solzbacher, "Wafer Scale Fabrication of Penetrating Neural Electrode Arrays" Biomedical Microdevices, Vol. 12(5), pp. 797-807, 2010.</ref> and flexible microelectrode arrays. Microwire MEAs are largely made of stainless [[steel]] or [[tungsten]] and they can be used to estimate the position of individual recorded neurons by triangulation. Silicon-based microelectrode arrays include two specific models: the Michigan and Utah arrays. Michigan arrays allow a higher density of sensors for implantation as well as a higher spatial resolution than microwire MEAs. They also allow signals to be obtained along the length of the shank, rather than just at the ends of the shanks. In contrast to Michigan arrays, Utah arrays are 3-D, consisting of 100 conductive silicon needles. However, in a Utah array signals are only received from the tips of each electrode, which limits the amount of information that can be obtained at one time. Furthermore, Utah arrays are manufactured with set dimensions and parameters while the Michigan array allows for more design freedom. Flexible arrays, made with [[polyimide]], [[parylene]], or [[benzocyclobutene]], provide an advantage over rigid microelectrode arrays because they provide a closer mechanical match, as the [[Young's modulus]] of silicon is much larger than that of brain tissue, contributing to shear-induced [[inflammation]].<ref name="Cheung">Cheung KC. 2007. Implantable microscale neural interfaces. ''Biomedical Microdevices 9'': 923-38</ref>

==Data processing methods==

The fundamental unit of communication of neurons is, electrically, at least, the action potential. This all-or-nothing phenomenon originates at the [[axon hillock]],<ref name="Angelides">Angelides KJ, Elmer LW, Loftus D, Elson E. 1988. Distribution and lateral mobility of voltage-dependent sodium channels in neurons. ''J Cell Biol. 106'': 1911-25.</ref> resulting in a depolarization of the intracellular environment which propagates down the [[axon]]. This ion flux through the cellular membrane generates a sharp change in voltage in the extracellular environment, which is what the MEA electrodes ultimately detect. Thus, voltage spike counting and sorting is often used in research to characterize network activity.

==Advantages==
In general, the major strengths of ''in vitro'' arrays when compared to more traditional methods such as [[patch clamp]]ing include:<ref>Whitson J, Kubota D, Shimono K, Jia Y, Taketani M. Multi-Electrode Arrays: Enhancing Traditional Methods and Enabling Network Physiology. In: Baudry M, Taketani M, eds. ''Advances in Network Electrophysiology Using Multi-Electrode Arrays''. New York: Spring Press; 2006: 38-68</ref>

*Allowing the placement of multiple electrodes at once rather than individually
*The ability to set up controls within the same experimental setup (by using one electrode as a control and others as experimental)
*The ability to select different recordings sites within the array
*The ability to simultaneously receive data from multiple sites

Furthermore, ''in vitro'' arrays are non-invasive when compared to patch clamping because they do not require breaching of the cell membrane.

With respect to ''in vivo'' arrays however, the major advantage over patch clamping is the high spatial resolution. Implantable arrays allow signals to be obtained from individual neurons enabling information such as position or [[velocity]] of motor movement that can be used to control a [[prosthetic]] device.

==Disadvantages==
''In vitro'' MEAs are less suited for recording and stimulating single cells due to their low spatial resolution compared to patch clamp and [[dynamic clamp]] systems. The complexity of signals an MEA electrode could effectively transmit to other cells is limited compared to the capabilities of dynamic clamps.

There are also several biological responses to implantation of a microelectrode array, particularly in regards to chronic implantation. Most notable among these effects are neuronal cell loss, [[glial scarring]], and a drop in the number of functioning electrodes.<ref>Biran R, Martin DC, Tresco PA. 2005. Neuronal cell loss accompanies the brain tissue response to chronically implanted silicon microelectrode arrays. ''Experimental Neurology 195'': 115-26</ref> The tissue response to implantation is dependent among many factors including size of the MEA shanks, distance between the shanks, MEA material composition, and time period of insertion. The tissue response is typically divided into short term and long term response. The short term response occurs within hours of implantation and begins with an increased population of [[astrocytes]] and [[glial cells]] surrounding the device. The recruited [[microglia]] then initiate inflammation and a process of [[phagocytosis]] of the foreign material begins. Over time, the astrocytes and microglia recruited to the device begin to accumulate, forming a sheath surrounding the array that extends tens of micrometres around the device. This not only increases the space between electrode probes, but also insulates the electrodes and increases impedance measurements. Problems with chronic implantation of arrays have been a driving force in the research of these devices. One novel study examined the [[neurodegenerative]] effects of inflammation caused by chronic implantation.<ref>McConnell GC, Rees HD, Levey AI, Gross RG, Bellamkonda RV. 2008. Chronic electrodes induce a local, neurodegenerative state: Implications for chronic recording reliability. ''Society for Neuroscience'', Washington, D.C</ref> [[Immunohistochemical]] markers showed a surprising presence of hyperphosphorylated tau, an indicator of [[Alzheimer's disease]], near the electrode recording site. The phagocytosis of electrode material also brings to question the issue of a biocompatibility response, which research suggests has been minor and becomes almost nonexistent after 12 weeks ''in vivo''. Research to minimize the negative effects of device insertion includes surface coating of the devices with proteins that encourage neuron attachment, such as [[laminin]], or drug [[elution|eluting]] substances.<ref>He W, McConnell GC, Bellamkonda RV. 2006. Nanoscale laminin coating modulates cortical scarring response around implanted silicon microelectrode arrays. ''Journal of Neural Engineering 3'': 316-26</ref>

==Applications==

===''In vitro''===

[[Image:Animat Drawing.jpg|thumb|Schematic for a neurally controlled animat|500px|right|Schematic for a neurally controlled animat]]
The nature of dissociated neuronal networks does not seem to change or diminish the character of its [[pharmacological]] response when compared to ''in vivo'' models, suggesting that MEAs can be used to study pharmacological effects on dissociated neuronal cultures in a more simple, controlled environment.<ref name="Gopal">Gopal KV, Gross GW. Emerging Histotypic Properties of Cultured Neuronal Networks. In: Baudry M, Taketani M, eds. ''Advances in Network Electrophysiology Using Multi-Electrode Arrays''. New York: Springer Press; 2006:193-214.</ref> A number of pharmacological studies using MEAs on dissociated neuronal networks, e.g. studies with [[ethanol]].<ref name="Xia">Xia Y and Gross GW. 2003. Histotypic electrophysiological responses of cultured neuronal networks to ethanol. ''Alcohol 30'': 167-74.</ref>

In addition, a substantial body of work on various biophysical aspects of network function was carried out by reducing phenomena usually studied at the behavioral level to the dissociated cortical network level. For example, the capacity of such networks to extract spatial<ref name="Order">Shahaf G, Eytan D, Gal A, Kermany E, Lyakhov V, Zrenner C, Marom S. 2008. Order-based representation in random networks of cortical neurons. PLoS Comput Biol. 4(11):e1000228.</ref> and temporal<ref>Eytan D, Brenner N, Marom S. 2003. Selective adaptation in networks of cortical neurons.J Neurosci. 23, 9349-9356.</ref> features of various input signals, dynamics of synchronization,<ref>Eytan D, Marom S. 2006. Dynamics and effective topology underlying synchronization in networks of cortical neurons. J Neurosci. 26, 8465-8476.</ref> sensitivity to [[Neuromodulation (biology)|neuromodulation]]<ref>Eytan D, Minerbi A, Ziv NE, Marom S. 2004. Dopamine-induced dispersion of correlations between action potentials in networks of cortical neurons. J Neurophysiol. 92,1817-1824.</ref><ref>Tateno T, Jimbo Y, Robinson HP. 2005. Spatio-temporal cholinergic modulation in cultured networks of rat cortical neurons: spontaneous activity. Neuroscience. 134, 425-437</ref><ref>Tateno T, Jimbo Y, Robinson HP. 2005. Spatio-temporal cholinergic modulation in cultured networks of rat cortical neurons: evoked activity. Neuroscience. 134, 439-448</ref> and kinetics of learning using closed loop regimes.<ref>Shahaf G, Marom S. 2001. Learning in networks of cortical neurons. J Neurosci. 21,8782-8788.</ref><ref>Stegenga J, Le Feber J, Marani E, Rutten WL. 2009. The effect of learning on bursting. IEEE Trans Biomed Eng. 56,1220-1227.</ref> Finally, combining MEA technology with [[confocal microscopy]] allows for studying relationships between network activity and synaptic remodeling.<ref name="tenacity" />

MEAs have been used to interface neuronal networks with non-biological systems as a controller. For example, a neural-computer interface can be created using MEAs. Dissociated rat [[Cerebral cortex|cortical]] neurons were integrated into a closed stimulus-response feedback loop to control an animat in a virtual environment.<ref name="Animat">DeMarse TB, Wagenaar DA, Blau AW, Potter SM. 2001. The Neurally Controlled Animat: Biological Brains Acting with Simulated Bodies. ''Autonomous Robots 11'': 305-10.</ref> A [[Feedback|closed-loop]] stimulus-response system has also been constructed using an MEA by Dr. Potter, Dr. Mandhavan, and Dr. DeMarse,<ref name="potterrobot">Potter, SM, Madhavan, R and DeMarse, TB. 2003. Long-term bidirectional neuron interfaces for robotic control, and in vitro learning studies. Proc. ''25th IEEE EMBS Annual Meeting''.</ref> and by Mark Hammond, [[Kevin Warwick]], and Ben Whalley in the [[University of Reading]]. About 300,000 dissociated rat neurons were plated on an MEA, which was connected to motors and [[ultrasound]] sensors on a robot, and was conditioned to avoid obstacles when sensed.<ref name="Marks">Marks P. 2008. Rise of the rat-brained robots. ''New Scientist 2669''.</ref> Along these lines, Shimon Marom and colleagues in the [[Technion – Israel Institute of Technology|Technion]] hooked dissociated neuronal networks growing on MEAs to a [[Lego MindStorms]] robot; the visual field of the robot was classified by the network, and commands were delivered to the robot wheels such that it completely avoids bumping into obstacles .<ref name="Order" /> [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2580731/bin/pcbi.1000228.s004.mov link to movie]. Interestingly, this [[Braitenberg vehicles|"Braitenberg Vehicle"]] was used to demonstrate the [[underdetermination|indeterminacy]] of reverse neuro-engineering <ref>Marom S, Meir R, Braun E, Gal A, Kermany E, Eytan D. 2009. On the precarious path of reverse neuro-engineering. Front Comput Neurosci. ;3:5.</ref> showing that even in a simple setup with practically unlimited access to every piece of relevant information, it was impossible to deduce with certainty the specific [[neural coding]] scheme that was used to drive the robots behavior.

MEAs have been used to observe network firing in [[hippocampal]] slices.<ref name="Colgin">Colgin, L.L., Kramar, E.A., Gall, C.M., and Lynch, G. (2003). Septal modulation of excitatory transmission in hippocampus. ''J Neurophysiol. 90:'' 2358-2366.</ref>

===''In vivo''===
There are several implantable interfaces that are currently available for consumer use including [[deep brain stimulation|deep brain stimulators]], [[cochlear implants]], and [[artificial pacemaker|cardiac pacemaker]]s. Deep brain stimulation (DBS) has been effective at treating movement disorders such as [[Parkinson's disease]],<ref>Breit S, Schulz JB, Benabid AL. 2004. Deep Brain Stimulation. ''Cell Tissue Research 318'': 275-288.</ref> and cochlear implants have helped many to improve their hearing by assisting stimulation of the [[auditory nerve]]. Because of their remarkable potential, MEAs are a prominent area of neuroscience research. Research suggests that MEAs may provide insight into processes such as memory formation and perception and may also hold therapeutic value for conditions such as [[epilepsy]], [[Major depressive disorder|depression]], and [[obsessive-compulsive disorder]]. Clinical trials using interface devices for restoring motor control after spinal cord injury or as treatment for [[Amyotrophic lateral sclerosis|ALS]] have been initiated in a project entitled BrainGate (see video demo: [http://www.cyberkineticsinc.com/video.htm BrainGate]). MEAs provide the high resolution necessary to record time varying signals, giving them the ability to be used to both control and obtain feedback from prosthetic devices, as was shown by [[Kevin Warwick]], [[Mark Gasson]] and [[Peter Kyberd]].<ref>Warwick, K, Gasson, M, Hutt, B, Goodhew, I, Kyberd, P, Andrews, B, Teddy, P and Shad, A:"The Application of Implant Technology for Cybernetic Systems", ''Archives of Neurology'', 60(10), pp1369-1373, 2003</ref><ref>Schwartz AB. 2004. Cortical Neural Prosthetics. ''Annual Review of Neuroscience 27'': 487-507.</ref> Research suggests that MEA use may be able to assist in the restoration of vision by stimulating the [[optic nerve|optic pathway]].<ref name="Cheung" />

==MEA user meetings==
A biannual scientific user meeting is held in Reutlingen, organized by the [http://www.nmi.de Natural and Medical Sciences Institute] (NMI) at the University of Tuebingen. The meetings offer a comprehensive overview of all aspects related to new developments and current applications of Microelectrode Arrays in basic and applied neuroscience as well as in industrial drug discovery, safety pharmacology and neurotechnology. The biannual conference has developed into an international venue for scientists developing and using MEAs from both industry and academia, and is recognized as an information-packed scientific forum of high quality. The meeting contributions are available as open access proceeding books.

==See also==
*[[Animat]]
*[[Artificial pacemaker]]
*[[Deep brain stimulation]]
*[[Patch clamp]]
*[[Bioelectronics]]

==References==
{{Reflist}}

[[Category:Neurophysiology]]
[[Category:Physiology]]
[[Category:Electrophysiology]]
[[Category:Laboratory techniques]]

Dynamic response index - Revision history

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