Measuring Dimensional Collapse: A Two-Domain Framework for Quantifying Response Space Constriction

Trauma-related and chronic stress-related conditions are widely described as involving a narrowing of the responses available to an organism. This idea — recently formalised as _dimensional collapse of response space_ — has clinical face validity but no operational definition, and therefore cannot be tested, falsified, or compared across studies. This paper proposes a measurement framework that renders the construct computable. The central claim is that response space constriction is a property of _distributions_, not of _magnitudes_, and can therefore be quantified using a single family of diversity statistics — Hill numbers — applied in two separable domains: the PHYSIOLOGICAL DOMAIN, where the effective dimensionality of autonomic reorganisation across a standardised multi-state protocol is estimated; and the REPERTOIRE DOMAIN, where the effective number of regulatory strategies is estimated from the subscale profile of an established multidimensional self-report instrument. Both domains yield indices on the same scale — an _effective number of dimensions_ — permitting direct comparison, a coherence statistic, and a discrepancy statistic with potential clinical meaning. The framework is instrument-agnostic in the repertoire domain and requires only stated minimum sensor conditions in the physiological domain. We report a synthetic validation of the computational pipeline. Simulated organisms constructed to have IDENTICAL RESTING RMSSD AND IDENTICAL TOTAL EXCURSION but differing configuration geometry were separated by the physiological index (1.55 vs 1.08, non-overlapping 95% intervals), confirming that the index carries information unavailable to conventional resting-state HRV measures. The same simulations revealed two problems not anticipated in the framework’s initial specification: the originally proposed six-feature set is substantially redundant, and a whitening correction that appeared theoretically motivated in fact destroys discrimination. Both findings are reported and the specification revised accordingly. We derive a falsifiable double-dissociation prediction distinguishing post-traumatic stress disorder (phasic collapse) from myalgic encephalomyelitis/chronic fatigue syndrome (tonic collapse), and specify the empirical validation required. No efficacy claim of any kind is made or implied.