Homeostatic and Hebbian plasticity are related in adult humans

Introduction: Homeostatic and Hebbian plasticity co-operate during the critical period, refining neuronal circuits, however the interaction between these two forms of plasticity is still unclear, especially in adulthood. Here we investigate directly this issue in adult humans using two consolidated paradigms to elicit each form of plasticity: the LTP-like change of the visual evoked potential (Kirk et al 2021) and the shift of ocular dominance induced by short-term monocular deprivation (Baroncelli & Lunghi 2021). Methods: We tested three different conditions in a group (N=17) of adult volunteers (counterbalanced order). In the homeostatic condition, we measured ocular dominance (binocular rivalry between gratings, 2°, 2cpd, 64% contrast) before and after 1h of monocular deprivation (MD). In the Hebbian condition, we measured visual evoked potentials (VEP) in response to flashed (mean rate 1Hz) visual stimuli (checkerboard: 4°, 2 cpd, 64% contrast) presented binocularly in the upper visual field, before and after a 2-min high-frequency visual stimulation (HFS, checkerboard flickering at 9Hz). The mixed condition was similar to the homeostatic condition, except that HFS was delivered just before MD. Results: In the homeostatic and hebbian conditions we confirmed that the two experimental paradigms successfully induced each form of plasticity: ocular dominance shifted in favor of the deprived eye after MD (t(16)=8.23, p<0.001), and the amplitude of the N1b component of the VEP was reduced after HFS (t(16)=-2.569, p=0.021). Importantly, across participants, the effect of MD and the effect of HFS were correlated (r=-0.538, p=0.026). In the mixed condition, the effect of MD was comparable to the homeostatic condition (t(16)=-0.138, p=0.892). Conclusion: Homeostatic and Hebbian plasticity correlate in amplitude in adult humans, but they do not seem to interact when induced simultaneously. The relationship between these two forms of plasticity can explain the success of homeostatic plasticity based paradigms for the treatment of adult amblyopia.