An experimental measure of the Coulomb interaction between quasi-two-dimensional electron disks embedded in the positive background charge of a laterally structured heterojunction is achieved by studying the plasma resonances of circular disks in a rectangular lattice. In contrast to noninteracting disks, or disks in a square lattice, we observe two modes at zero magnetic field, depending on the polarization of the incident field. This anisotropy is caused by local-field corrections and reflects the symmetry of the lattice. The coupling strength is well described by a model of interacting dipoles.