Complex sentence: Shusterman and Troy define a realistic state of baseline physiological activity, which represents activity of the brain in the state of relaxation, in which neurons receive some level of spontaneous, weak stimulation by small, naturally present concentrations of neurohormonal substances.
Split into simpler sentences: Shusterman and Troy define a realistic state of baseline physiological activity, which represents activity of the brain in the state of relaxation. In this state, neurons receive some level of spontaneous, weak stimulation by small, naturally present concentrations of neurohormonal substances.