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Related Concept Videos

Free Energy01:21

Free Energy

Free energy—abbreviated as G for the scientist Gibbs who discovered it—is a measurement of useful energy that can be extracted from a reaction to do work. It is the energy in a chemical reaction that is available after entropy is accounted for. Reactions that take in energy are considered endergonic and reactions that release energy are exergonic. Plants carry out endergonic reactions by taking in sunlight and carbon dioxide to produce glucose and oxygen. Animals, in turn, break down the...
An Introduction to Free Energy01:05

An Introduction to Free Energy

How can we compare the energy that releases from one reaction to that of another reaction? We use a measurement of free energy to quantitate these energy transfers. Scientists call this free energy Gibbs free energy (abbreviated with the letter G) after Josiah Willard Gibbs, the scientist who developed the measurement. According to the second law of thermodynamics, all energy transfers involve losing some energy in an unusable form such as heat, resulting in entropy. Gibbs free energy...
Gibbs Free Energy02:39

Gibbs Free Energy

One of the challenges of using the second law of thermodynamics to determine if a process is spontaneous is that it requires measurements of the entropy change for the system and the entropy change for the surroundings. An alternative approach involving a new thermodynamic property defined in terms of system properties only was introduced in the late nineteenth century by American mathematician Josiah Willard Gibbs. This new property is called the Gibbs free energy (G) (or simply the free...
Endergonic and Exergonic Reactions in the Cell01:27

Endergonic and Exergonic Reactions in the Cell

If energy releases during a chemical reaction, then the resulting value will be a negative number. In other words, reactions that release energy have a ∆G < 0. A negative ∆G also means that the reaction's products have less free energy than the reactants because they gave off some free energy during the reaction. Scientists call reactions with a negative ∆G, and which consequently release free energy, exergonic reactions. Exergonic means energy is exiting the system. We also refer to these...
Entropy within the Cell01:22

Entropy within the Cell

A living cell's primary tasks of obtaining, transforming, and using energy to do work may seem simple. However, the second law of thermodynamics explains why these tasks are harder than they appear. None of the energy transfers in the universe are completely efficient. In every energy transfer, some amount of energy is lost in a form that is unusable. In most cases, this form is heat energy. Thermodynamically, heat energy is defined as the energy transferred from one system to another that is...
Gibbs Free Energy and Thermodynamic Favorability02:23

Gibbs Free Energy and Thermodynamic Favorability

The spontaneity of a process depends upon the temperature of the system. Phase transitions, for example, will proceed spontaneously in one direction or the other depending upon the temperature of the substance in question. Likewise, some chemical reactions can also exhibit temperature-dependent spontaneities. To illustrate this concept, the equation relating free energy change to the enthalpy and entropy changes for the process is considered:

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Related Experiment Video

Updated: Jun 24, 2026

Visualizing Monocarboxylates and Other Relevant Metabolites in the Ex Vivo Drosophila Larval Brain Using Genetically Encoded Sensors
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Visualizing Monocarboxylates and Other Relevant Metabolites in the Ex Vivo Drosophila Larval Brain Using Genetically Encoded Sensors

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Free-energy and the brain.

Karl J Friston1, Klaas E Stephan

  • 1Wellcome Trust Centre for Neuroimaging, University College London, United Kingdom.

Synthese
|March 28, 2009
PubMed
Summary

This study presents a unified model of perception and learning based on statistical physics and hierarchical Bayesian inference. It explains neurobiological facts by showing how minimizing free-energy enables adaptive environmental interactions through perception and action.

Area of Science:

  • Neuroscience
  • Cognitive Science
  • Statistical Physics

Background:

  • Helmholtz's theories on perception can be updated with modern computational approaches.
  • Perceptual inference and learning are key to understanding brain function.

Purpose of the Study:

  • To present a unified model of perception and learning based on modern theories.
  • To explain neurobiological facts using principles from statistical physics.
  • To demonstrate how minimizing free-energy can explain brain dynamics and structure.

Main Methods:

  • Formulating Helmholtz's ideas within modern theories of perceptual inference and learning.
  • Applying constructs from statistical physics to model inference and learning.
  • Utilizing Empirical Bayes and hierarchical models for sensory information generation.

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  • Framing perception and action as mechanisms for minimizing free-energy.

    • Main Results:

    • A model explaining a wide range of neurobiological facts through perceptual inference and learning.
    • Demonstration that inferring causes of sensory input and learning causal regularities can be achieved using unified principles.
    • Perception and learning can proceed in a biologically plausible manner.
    • Hierarchical models enable dynamic and context-sensitive prior expectations.

    Conclusions:

    • Perceptual processes are an emergent property of systems conforming to a free-energy principle.
    • Minimizing free-energy drives adaptive exchanges with the environment via action and perception.
    • Brain dynamics and structure can be explained by the minimization of free-energy, implying an implicit probabilistic model of the environment.