  Both local geometry and collective, extended excitations drive the moves of a
particle in the cage of its neighbours in dense liquids. The strength of their
influence is investigated by Molecular Dynamics simulations of a supercooled
liquid of fully-flexible trimers with semirigid or rigid bonds. The rattling in
the cage is investigated on different length scales. First, the rattling
anisotropy due to local order is characterized by two order parameters sensing
the monomers succeeding or failing to escape from the cage. Then, the
collective response of the surroundings excited by the monomer-monomer
collisions is considered. The collective response is initially restricted to
the nearest neighbours of the colliding particle by a Voronoi analysis
revealing elastic contributions. Then, the long-range excitation of the
farthest neighbours is scrutinised by searching spatially-extended correlations
between the simultaneous fast displacements of the caged particle and the
surroundings. It is found that the longitudinal component has stronger spatial
modulation than the transverse one with wavelength of about one particle
diameter, in close resemblance with experimental findings on colloids. It is
concluded that the cage rattling is largely affected by solid-like extended
modes.
