  Living systems break detailed balance at small scales, consuming energy and
producing entropy in the environment in order to perform molecular and cellular
functions. However, it remains unclear how broken detailed balance manifests at
macroscopic scales, and how such dynamics support higher-order biological
functions. Here we present a framework to quantify broken detailed balance by
measuring entropy production in macroscopic systems. We apply our method to the
human brain, an organ whose immense metabolic consumption drives a diverse
range of cognitive functions. Using whole-brain imaging data, we demonstrate
that the brain nearly obeys detailed balance when at rest, but strongly breaks
detailed balance when performing physically and cognitively demanding tasks.
Using a dynamic Ising model, we show that these large-scale violations of
detailed balance can emerge from fine-scale asymmetries in the interactions
between elements, a known feature of neural systems. Together, these results
suggest that violations of detailed balance are vital for cognition, and
provide a general tool for quantifying entropy production in macroscopic
systems.
