  In this work, we report an atomic scale study on the ferromagnetic insulator
manganite LaMnO$_{3.12}$ using $\gamma-\gamma$ PAC spectroscopy. Data analysis
reveals a nanoscopic transition from an undistorted to a Jahn-Teller-distorted
local environment upon cooling. The percolation thresholds of the two local
environments enclose a macroscopic structural transition
(Rhombohedric-Orthorhombic). Two distinct regimes of JT-distortions were found:
a high temperature regime where uncorrelated polaron clusters with severe
distortions of the Mn$^{3+}$O$_{6}$ octahedra survive up to $T \approx 800 K$
and a low temperature regime where correlated regions have a weaker
JT-distorted symmetry.
