A free-energy principle has been proposed recently that accounts for action, perception and learning. This Review looks at some key brain theories in the biological (for example, neural Darwinism) and physical (for example, information theory and optimal control theory) sciences from the free-energy perspective. Crucially, one key theme runs through each of these theories — optimization. Furthermore, if we look closely at what is optimized, the same quantity keeps emerging, namely value (expected reward, expected utility) or its complement, surprise (prediction error, expected cost). This is the quantity that is optimized under the free-energy principle, which suggests that several global brain theories might be unified within a free-energy framework.

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The free-energy principle: a unified brain theory?

Prelude. Cycle 1. Introduction. Cycle 2. Structure defines function. Cycle 3. Diversity of cortical functions is provided by inhibition. Cycle 4. Windows on the brain. Cycle 5. A system of rhythms: from simple to complex dynamics. Cycle 6. Synchronization by oscillation. Cycle 7. The brain's default state: self-organized oscillations in rest and sleep. Cycle 8. Perturbation of the default patterns by experience. Cycle 9. The gamma buzz: gluing by oscillations in the waking brain. Cycle 10. Perceptions and actions are brain state-dependent. Cycle 11. Oscillations in the "other cortex:" navigation in real and memory space. Cycle 12. Coupling of systems by oscillations. Cycle 13. The tough problem. References.

Rhythms of the brain

The ionotropic glutamate receptors are ligand-gated ion channels that mediate the vast majority of excitatory neurotransmission in the brain. The cloning of cDNAs encoding glutamate receptor subunits, which occurred mainly between 1989 and 1992 ([Hollmann and Heinemann, 1994][1]), stimulated this

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The glutamate receptor ion channels

1980 Preface * 1999 Preface * 1999 Acknowledgements * Introduction * 1 Circular Logic * 2 Phase Singularities (Screwy Results of Circular Logic) * 3 The Rules of the Ring * 4 Ring Populations * 5 Getting Off the Ring * 6 Attracting Cycles and Isochrons * 7 Measuring the Trajectories of a Circadian Clock * 8 Populations of Attractor Cycle Oscillators * 9 Excitable Kinetics and Excitable Media * 10 The Varieties of Phaseless Experience: In Which the Geometrical Orderliness of Rhythmic Organization Breaks Down in Diverse Ways * 11 The Firefly Machine 12 Energy Metabolism in Cells * 13 The Malonic Acid Reagent ('Sodium Geometrate') * 14 Electrical Rhythmicity and Excitability in Cell Membranes * 15 The Aggregation of Slime Mold Amoebae * 16 Numerical Organizing Centers * 17 Electrical Singular Filaments in the Heart Wall * 18 Pattern Formation in the Fungi * 19 Circadian Rhythms in General * 20 The Circadian Clocks of Insect Eclosion * 21 The Flower of Kalanchoe * 22 The Cell Mitotic Cycle * 23 The Female Cycle * References * Index of Names * Index of Subjects

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/596B270197FE27DE5FABB598B6210622/S0025557200150892a.pdf/div-class-title-span-class-italic-the-geometry-of-biological-time-span-by-a-t-winfree-pp-544-dm68-corrected-second-printing-1990-isbn-3-540-52528-9-springer-div.pdf

The geometry of biological time , by A. T. Winfree. Pp 544. DM68. Corrected Second Printing 1990. ISBN 3-540-52528-9 (Springer)

Neuronal activity in the brain gives rise to transmembrane currents that can be measured in the extracellular medium. Although the major contributor of the extracellular signal is the synaptic transmembrane current, other sources — including Na+ and Ca2+ spikes, ionic fluxes through voltage- and ligand-gated channels, and intrinsic membrane oscillations — can substantially shape the extracellular field. High-density recordings of field activity in animals and subdural grid recordings in humans, combined with recently developed data processing tools and computational modelling, can provide insight into the cooperative behaviour of neurons, their average synaptic input and their spiking output, and can increase our understanding of how these processes contribute to the extracellular signal.

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The origin of extracellular fields and currents — EEG, ECoG, LFP and spikes

Associative long-term potentiation (LTP) is the dominant model of memory related synaptic modifications in the mammalian brain (Bliss & Lomo, 1973; Bliss & Collingridge, 1993). It has been studied mainly in the hippocampus, a structure of importance for memory (Morris et al. 1983; Squire & Zola-Morgan, 1991). Induction of associative LTP requires activation of the N-methyl-D-aspartate (NMDA) receptor (Collingridge et al. 1983; Bliss & Collingridge, 1993), which serves as a molecular coincidence detector, requiring both presynaptic release of glutamate and postsynaptic depolarization for its activation (Nowak et al. 1984; Mayer et al. 1984). Thus associative LTP obeys Hebb's learning rule (Hebb, 1949), which suggests that when the pre- and postsynaptic elements are active at the same time then the synapse between them will be strengthened. Indeed, pairing of presynaptic and postsynaptic activity can, under some experimental conditions, lead to synaptic potentiation (Wigstrom et al. 1986; Magee & Johnston, 1997; Markram et al. 1997). However, the physiological activity that occurs during learning behaviours and which produces the critical activation of NMDA receptors, leading to synaptic potentiation in adult hippocampus, has not been determined.

According to a common interpretation of Hebb's learning rule, synaptic potentiation would be expected to occur following temporal coincidence of presynaptic activity and postsynaptic single action potentials. However, when a rat learns about spatial relations during active exploration of an environment, neurons with appropriate place fields, i.e. coding for the current location of the rat in space, and therefore those neurons that are likely to be involved in associative memories, typically show bursting activity repeated at theta frequency (5-12 Hz) (e.g. O'Keefe & Recce, 1993). Perhaps postsynaptic bursts bear a special significance for associative synaptic modification.

We wanted to test directly the common interpretation of Hebb's rule, by investigating whether coincident single pre- and postsynaptic action potentials are sufficient to induce LTP in hippocampal slices from adult rat. In order to investigate whether bursts have a special role in associative synaptic modification, we compared the efficacy of pairing pre- and postsynaptic single action potentials and pre- and postsynaptic bursts in inducing synaptic change using neuronal activity seen during exploratory learning.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-7793.1999.0571p.x

Postsynaptic bursting is essential for ‘Hebbian’ induction of associative long-term potentiation at excitatory synapses in rat hippocampus

N-Methyl-d-aspartate receptor (NMDAR)-dependent synaptic plasticity is a strong candidate to mediate learning and memory processes that require the hippocampus. This plasticity is bidirectional, and how the same receptor can mediate opposite changes in synaptic weights remains a conundrum. It has been suggested that the NMDAR subunit composition could be involved. Specifically, one subunit composition of NMDARs would be responsible for the induction of long-term potentiation (LTP), whereas NMDARs with a different subunit composition would be engaged in the induction of long-term depression (LTD). Unfortunately, the results from studies that have investigated this hypothesis are contradictory, particularly in relation to LTD. Nevertheless, current evidence does suggest that the GluN2B subunit might be particularly important for plasticity and may make a synapse bidirectionally malleable. In particular, we conclude that the presence of GluN2B subunit-containing NMDARs at the postsynaptic density might be a necessary, though not a sufficient, condition for the strengthening of individual synapses. This is owing to the interaction of GluN2B with calcium/calmodulin-dependent protein kinase II (CaMKII) and is distinct from its contribution as an ion channel.

GluN2A and GluN2B subunit-containing NMDA receptors in hippocampal plasticity

Dopamine Neuron-Specific Optogenetic Stimulation in Rhesus Macaques

Studies in brain slices have provided a wealth of data on the basic features of neurons and synapses. In the intact brain, these properties may be strongly influenced by ongoing network activity. Although physiologically realistic patterns of network activity have been successfully induced in brain slices maintained in interface-type recording chambers, they have been harder to obtain in submerged-type chambers, which offer significant experimental advantages, including fast exchange of pharmacological agents, visually guided patch-clamp recordings, and imaging techniques. Here, we investigated conditions for the emergence of network oscillations in submerged slices prepared from the hippocampus of rats and mice. We found that the local oxygen level is critical for generation and propagation of both spontaneously occurring sharp wave–ripple oscillations and cholinergically induced fast oscillations. We suggest three ways to improve the oxygen supply to slices under submerged conditions: (i) optimizing chamber design for laminar flow of superfusion fluid; (ii) increasing the flow rate of superfusion fluid; and (iii) superfusing both surfaces of the slice. These improvements to the recording conditions enable detailed studies of neurons under more realistic conditions of network activity, which are essential for a better understanding of neuronal network operation.

TECHNICAL SPOTLIGHT Maintaining network activity in submerged hippocampal slices: importance of oxygen supply

Cortical networks spontaneously fluctuate between persistently active Up states and quiescent Down states. The Up states are maintained by recurrent excitation within local circuits, and can be turned on and off by synaptic input. GABAergic inhibition is believed to be important for stabilizing such persistent activity by balancing the excitation, and could have an additional role in terminating the Up state. Here, we report that GABAA and GABAB receptor-mediated inhibition have distinct and complementary roles in balancing and terminating persistent activity. In a model of Up–Down states expressed in slices of rat entorhinal cortex, the GABAA receptor antagonist, gabazine (50–500 nm), concentration-dependently decreased Up state duration, eventually leading to epileptiform bursts. In contrast, the GABAB receptor antagonist, CGP55845 (50 nm to 1 μm), increased the duration of persistent network activity, and prevented stimulus-induced Down state transitions. These results suggest that while GABAA receptor-mediated inhibition is necessary for balancing persistent activity, activation of GABAB receptors contributes to terminating Up states.

https://www.jneurosci.org/content/jneuro/29/23/7513.full.pdf

Ole Paulsen

Papers

Postsynaptic bursting is essential for ‘Hebbian’ induction of associative long-term potentiation at excitatory synapses in rat hippocampus

GluN2A and GluN2B subunit-containing NMDA receptors in hippocampal plasticity

Dopamine Neuron-Specific Optogenetic Stimulation in Rhesus Macaques

TECHNICAL SPOTLIGHT Maintaining network activity in submerged hippocampal slices: importance of oxygen supply

Distinct Roles of GABAA and GABAB Receptors in Balancing and Terminating Persistent Cortical Activity