Abstract
Entropy is an important trait for life as well as the human brain. Characterizing brain entropy (BEN) may provide an informative tool to assess brain states and brain functions. Yet little is known about the distribution and regional organization of BEN in normal brain. The purpose of this study was to examine the whole brain entropy patterns using a large cohort of normal subjects. A series of experiments were first performed to validate an approximate entropy measure regarding its sensitivity, specificity, and reliability using synthetic data and fMRI data. Resting state fMRI data from a large cohort of normal subjects (n = 1049) from multi-sites were then used to derive a 3-dimensional BEN map, showing a sharp low-high entropy contrast between the neocortex and the rest of brain. The spatial heterogeneity of resting BEN was further studied using a data-driven clustering method, and the entire brain was found to be organized into 7 hierarchical regional BEN networks that are consistent with known structural and functional brain parcellations. These findings suggest BEN mapping as a physiologically and functionally meaningful measure for studying brain functions.
Highlights
Entropy indicates system irregularity [1], which remains relatively low in living systems but increases over time in any closed system like our universe as dictated by the second law of thermodynamics [1,2]
Simulations on sample entropy (SampEn) calculations showed that data length has only a minor effect on SampEn (Fig. S1B), which ensured including all the resting-state fMRI (rsfMRI) data even with different time points for brain entropy (BEN) mapping
Our data demonstrated that SampEn is a robust and sensitive entropy measure for differentiating generic and Functional MRI (fMRI) signals with different regularities and that SampEn-based fMRI-derived BEN is reliable brain activity measure, which differs from that of a non-living object and background noise, and is sensitive to task-induced regional brain activity alterations
Summary
Entropy indicates system irregularity [1], which remains relatively low in living systems but increases over time in any closed system like our universe as dictated by the second law of thermodynamics [1,2]. BEN has long been measured using electrophysiological data [6,7,8,9,10] with low spatial resolution. Functional MRI (fMRI) reflects regional changes in cerebral blood flow and metabolism [11] and provides time-resolved volumetric imaging of brain function at relatively high spatial resolution, making it an attractive approach for mapping BEN. Entropy of the entire brain has been assessed using fMRI [16,17,18], but little is known about BEN’s spatially distribution and its regional organizations. The purposes of this paper were to generate a whole brain entropy map using a large cohort of subjects (n = 1049) and to investigate the spatial distribution and local organization of BEN
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