Dynamic Routing on a Static Structure: Neuromodulatory Gating of Cortical Wave Propagation

Stimulus-evoked cortical responses range from tightly localized activations to spatially extended traveling waves, yet the mechanisms through which these distinct regimes are flexibly recruited remain unclear. Based on neuroanatomical and neuroimaging evidence, we hypothesize that ascending neuromodulatory systems, which deliver modulatory (gain-changing) rather than driving inputs to cortex, dynamically shape spatiotemporal propagation under a fixed anatomical backbone. To test this hypothesis, we extend an empirically grounded corticothalamic neural field model to incorporate a diffusely projecting modulatory population, parameterized on the noradrenergic (NA) arousal system of the locus coeruleus (LC) as the best-characterized case. We then examine evoked responses under transient stimulus-locked versus sustained modulation - corresponding to phasic and tonic modulation - with geometry given by distance-dependent connectivity plus additional fast-conducting non-local projections (FNPs). Transient modulation produces spatiotemporally sustained traveling waves of cortical activity, whereas sustained modulation yields temporally prolonged but spatially localized responses; without modulation, waves are weak and dissipative. The model shows that the timing and spatial pattern of modulatory engagement gate wave-onset latency and impose anisotropy, restricting stimulus propagation to modulated territories and halting it across unmodulated gaps. In the presence of FNPs, which capture contributions of long-range white matter fiber tracts in the brain, modulation also enables the rapid non-local activation of distant cortical regions required for selective long-distance routing. Our results identify patterned gain control as a parsimonious mechanism for variability in evoked dynamics, and suggest that ascending modulatory systems such as the NA system could transiently privilege specific pathways over a largely static connectome.