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liquid neural network

nounAlso: liquid time-constant network, LTC network, closed-form continuous-time network, CfC, neural circuit policy

In plain wordsA small, brain-inspired network for time-varying data whose neurons change how quickly they react depending on their input, letting a few neurons handle tasks such as steering a car.

  1. A continuous-time recurrent neural networkrecurrent neural network, n.: A neural network that reads a sequence, such as a sentence or an audio clip, one piece at a time, carrying a running summary forward so that what came earlier can shape how later pieces are understood., described by Ramin Hasani and colleagues, whose neurons each follow a simple first-order differential equation with a time constant that is not fixed: a learned nonlinear function of the current input and state speeds each unit up or slows it down, so its responsiveness shifts with the data, hence 'liquid'. Running the network means integrating these equations numerically, as with a neural ordinary differential equationneural ordinary differential equation, n.: A neural network that, instead of passing data through a fixed stack of layers, describes how the data changes continuously over 'time' and uses an equation solver to follow that change.; the authors proved that the states stay bounded and reported better time-series predictions than earlier recurrent models. A closed-form approximation (CfC, 2022) avoids the solver and runs much faster. Compact 'neural circuit policies' built from such neurons, with only a few dozen units, were demonstrated steering vehicles and drones with inspectable dynamics. The name is also associated with Liquid AI, a company co-founded in 2023 by several of the method's authors, whose foundation models use related but different hybrid architectures.

    A liquid neural network with 19 control neurons kept the car in its lane from camera input.

Origin

Grew out of 'neuronal circuit policies' modeled on the nervous system of the nematode C. elegans, by Mathias Lechner, Ramin Hasani and Radu Grosu of TU Wien (arXiv, March 2018; developed further in Nature Machine Intelligence, 2020). Hasani, Lechner, Alexander Amini, Daniela Rus and Grosu of TU Wien and MIT described liquid time-constant recurrent networks in a November 2018 preprint and in 'Liquid Time-constant Networks' (arXiv, June 2020; AAAI 2021).

Related

See also
  • recurrent neural networkrecurrent neural network, n.: A neural network that reads a sequence, such as a sentence or an audio clip, one piece at a time, carrying a running summary forward so that what came earlier can shape how later pieces are understood.
  • neural ordinary differential equationneural ordinary differential equation, n.: A neural network that, instead of passing data through a fixed stack of layers, describes how the data changes continuously over 'time' and uses an equation solver to follow that change.
  • spiking neural networkspiking neural network, n.: A neural network that, like the brain, communicates with brief pulses, or spikes, sent only when a neuron's charge crosses a threshold. On special chips this can save a great deal of energy.
  • edge AIedge AI, n.: Doing AI's computing on or near the devices that collect the data, such as cameras, cars, factory machines, phones or local servers, instead of sending everything off to distant data centers.
  • state space modelstate space model, n.: A type of sequence model that reads text like a running tally, updating a fixed-size internal summary at each step, so its memory and cost do not balloon as texts get longer.

Sources

  1. Hasani, R., Lechner, M., Amini, A., Rus, D. & Grosu, R. (2020). Liquid Time-constant Networks. arXiv:2006.04439; AAAI 2021. arxiv.org/abs/2006.04439
  2. Hasani, R. M., Lechner, M., Amini, A., Rus, D. & Grosu, R. (2018). Liquid Time-constant Recurrent Neural Networks as Universal Approximators. arXiv:1811.00321. arxiv.org/abs/1811.00321