Cognition is not controlled by a single "brain chemical." It emerges from interacting networks involving neurotransmitters, receptors, ion channels, blood flow, sleep pressure, and stress hormones. Nootropics often work by shifting the probability that certain circuits will fire, rather than directly creating better thinking.

Many cognition-related systems follow an inverted-U relationship, where both too little and too much signaling can reduce performance. For example, modest catecholamine activity may support focus, while excessive stimulation can produce jitteriness, distractibility, or narrowed cognition.