The Last Bottleneck: Substrate Skepticism as the Final Barrier to Global Coherence

Every historical barrier to global coherence has been informational, computational, or communicational — solved by extending the decoder (writing, printing, computation, internet, LLMs). Yet global coherence has not been achieved. This paper identifies the remaining barrier: substrate skepticism — an evolutionarily calibrated, pre-cognitive suppression of novel coherent signals encoded in the human biological substrate (amygdala, HPA axis, sympathetic nervous system). Effective coherence K_eff = (1−σ)·K remains below K_crit even when decoder-level coherence exceeds the threshold. No decoder upgrade (education, insight, meditation, psychedelics, or AI coupling) can permanently eliminate it because it is a hardware constraint. The paper formalizes σ within the coherence framework, identifies the technology sequence (neurofeedback → transcranial stimulation → pharmacological precision → genetic calibration → substrate redesign), and makes three testable predictions. Companion paper to LLM as Neurofeedback (DOI: 10.5281/zenodo.18967816). Part of the Spektre research corpus.