"Can nuclear spin — a purely quantum property — modulate the collective dynamics of a neural network? Recent evidence shows that lithium isotopes ⁶Li and ⁷Li, chemically near-identical but differing in nuclear spin, produce opposite synaptic and behavioral effects that resist classical explanation. Classical pharmacological targets (GSK-3β, inositol monophosphatase) show zero isotopic discrimination, and the 17% mass difference cannot account for sign reversal. This paper proposes the first experiment designed to bridge quantum biology and neural criticality: measuring branching ratio σ, avalanche distributions, and critical exponents in hippocampal slices treated with enriched ⁶LiCl and ⁷LiCl. The design includes a novel classical control (17% overdose with natural lithium), simultaneous ROS measurement as causal intermediate, and pre-committed falsification criteria for every possible outcome. A positive result would establish that microscopic quantum properties can influence mesoscopic neural organization. A negative result would close a speculative line with direct evidence. This proposal is motivated by the Quantum-Neurocritical Filter Hypothesis (HFQNC; Collados, 2026), but is designed to be valuable independently of any particular theoretical framework. Both Spanish and English versions are included."