UCLA

The waking brain demonstrates remarkable coordination across multiple scales and exhibits coherent global dynamics. However, in the transition to unconsciousness, the brain's cognitive processing capabilities diminish, accompanied by a reduction in the spatiotemporal complexity of global dynamics. The Critical Brain hypothesis is introduced as an explanatory framework, aiming to elucidate how the brain supports complex computations during wakefulness and the subsequent decline of these abilities in unconscious states. Neuroscientific evidence suggests that the brain operates slightly away from a critical point between percolating and non-percolating phases within the Mean Field Directed Percolation (MFDP) class while recent work suggests another critical point, the edge of chaos, may support the waking state. Experimental measures of proximity to the edge of chaos are difficult to find, but a new measure using concurrent transcranial magnetic stimulation and electroencephalography (TMS-EEG) has emerged as a potential proxy. Moreover, TMS-evoked responses can be leveraged to test MFDP-associated phenomena, allowing for the examination of proximity to either critical point within a single dataset. It remains unclear whether MFDP or the edge of chaos critical point sustains the wakeful conscious state or is associated with other cognitive functions. In the present dissertation, I first provide a general background on criticality and how TMS-EEG can be used to measure the proximity of the brain to MFDP or the edge of chaos. In the second chapter, I present work that confirms the hypothesis that a new measure using TMS-evoked responses can serve as an experimental indicator of proximity to the critical point at the edge of chaos . In the third chapter, experimental measures of the edge of chaos and MFDP are applied to TMS-evoked responses and compared across stimulation site and conscious states (wake and NREM sleep). In the fourth chapter, I investigate if these two experimental measures of criticality can be dissociated during wakefulness, using neuromodulation. Across the three presented experiments, complexity of the TMS-evoked response is more closely associated with changes in conscious state than MFDP-associated phenomena, providing further evidence to support the association between the edge of chaos critical point and consciousness.