LSD-induced increase of Ising temperature and algorithmic complexity of brain dynamics

This paper (2022) uses the Ising model of brain phase transition to assess fMRI BOLD data from a study in which LSD was administered (n=15). Several different concepts from statistical physics were applied to the fMRI data to analyze individualized Ising temperature increases under the influence of LSD and placebo, showing that LSD ingestion shifts the system away from the critical point between paramagnetic and ferromagnetic phases to a more disordered state. Overall, findings suggest that LSD increases the complexity of brain dynamics.

Abstract

“A topic of growing interest in computational neuroscience is the discovery of fundamental principles underlying global dynamics and self-organization of the brain. Statistical physics offers powerful tools to analyze complex many-body systems and has delivered computational models such as the generalized Ising model, one of the simplest known systems displaying phase transitions. Prior work has shown that different brain states may be mapped into pairwise maximum entropy Ising models at varying distances from the critical temperature. Here, we use this framework to analyze resting state fMRI BOLD data collected in an earlier study from fifteen subjects in a control condition (placebo) and during psychedelic ingestion (LSD). We first parcellate the data in AAL space and, to address the limited quantity of data available from typical fMRI BOLD studies, we create an “archetype” pairwise maximum entropy or Ising model representative of the entire dataset. The archetype model is then personalized for each individual and condition through an adjustment of the system temperature. We analyze the resulting set of temperatures to show first that, at the group level and in both conditions, the model is near criticality but situated in the paramagnetic phase. Second, the individualized Ising temperature increases significantly under the effects of LSD compared with the placebo condition (p = 9 x 10^-5). That is, LSD ingestion shifts the system away from the critical point between paramagnetic and ferromagnetic phases to a more disordered state. Next, we estimate the Lempel-Ziv-Welch (LZW) complexity of the binarized BOLD data (flattened along the spatial dimension first) for each participant and condition and of the synthetic data generated with the individualized model using the Metropolis algorithm. We find that the LZW complexity computed directly from experimental data reveals a weak statistical relationship with the condition (p = 0.04 one-tailed Wilcoxon test), and none with Ising temperature (r(13) = 0.13, p = 0.65), presumably due to the short length of the time series and group size. However, the LZW complexity computed using synthetic data from the personalized archetype model correlates strongly with individualized temperature (p = 2.7 x 10^-6) and hence with condition (p = 9 x 10^-5, one-tailed Wilcoxon test). We then explored the estimation of algorithmic complexity using the block decomposition method (BDM). The BDM complexity of the experimental data displayed a robust correlation with Ising temperature (r(13) = 0.56, p = 0.03) and a weak but significant correlation with condition (p = 0.04, one-tailed Wilcoxon test). We also calculated the BDM complexity of the synthetic data generated by the model, and it correlates strongly with Ising temperature (r(13) = 0.97, p = 8.9 x 10^-10) and hence condition (p = 2 x 10^-4, one-tailed Wilcoxon test). This study suggests, together with prior ones, that the effects of LSD increase the complexity of brain dynamics. In agreement with earlier work using the Ising formalism, we find the brain state in the placebo condition is already above the critical point, with LSD resulting in a shift away from criticality into a more disordered state.”

Authors: Giulio Ruffini, Giada Damiani, Diego Lozano-Soldevilla, Nikolas Deco, Fernando Rosas, Narasis Kiani, Adrian Ponce-Alvarez, Morten L. Kringelback, Robin L. Carhart-Harris & Gustavo L. Deco