Brain Entropy During Aging Through a Free Energy Principle Approach

Spontaneous neural activity becomes less complex and less entropic during states of reduced consciousness, such as those induced by psychedelics, a pattern termed the 'entropic brain hypothesis.' This review extends that hypothesis to physiological and pathological aging, where brain entropy is also reduced. The authors treat the mind-brain as a complex nonlinear dynamic adaptive system governed by the free energy principle, aiming to maintain a balance between order and chaos in neural dynamics and functional connectivity. They review studies showing increased entropy in acute psychedelic states and chronic psychotic states like schizophrenia, then contrast these with aging, where entropy decreases. They propose a general trend of brain entropy across primary states and cognitive aging.

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Abstract

Neural complexity and brain entropy (BEN) have gained greater interest in recent years. The dynamics of neural signals and their relations with information processing continue to be investigated through different measures in a variety of noteworthy studies. The BEN of spontaneous neural activity decreases during states of reduced consciousness. This evidence has been showed in primary consciousness states, such as psychedelic states, under the name of “the entropic brain hypothesis.” In this manuscript we propose an extension of this hypothesis to physiological and pathological aging. We review this particular facet of the complexity of the brain, mentioning studies that have investigated BEN in primary consciousness states, and extending this view to the field of neuroaging with a focus on resting-state functional Magnetic Resonance Imaging. We first introduce historic and conceptual ideas about entropy and neural complexity, treating the mindbrain as a complex nonlinear dynamic adaptive system, in light of the free energy principle. Then, we review the studies in this field, analyzing the idea that the aim of the neurocognitive system is to maintain a dynamic state of balance between order and chaos, both in terms of dynamics of neural signals and functional connectivity. In our exploration we will review studies both on acute psychedelic states and more chronic psychotic states and traits, such as those in schizophrenia, in order to show the increase of entropy in those states. Then we extend our exploration to physiological and pathological aging, where BEN is reduced. Finally, we propose an interpretation of these results, defining a general trend of BEN in primary states and cognitive aging.