The standard Frenkel exciton Hamiltonian for molecular aggregates only contains quadratic (intraband) couplings between chromophores. However, interband interactions between chromophores whose excitation frequences are well separated are dominated by quartic couplings. We show that the latter are responsible for spectral line shifts of the Carotenoids observed in pump-probe experiments of LH2 photosynthetic antenna following excitation of the chrolophylls.