This paper proposes a real time frequency method to detect 2D rigid rotations of pan or tilt of a moving head. We aim at interpreting head nods involved in the non verbal communication process in the same way as human being: direction of the rotation is estimated but not its precise amplitude. The idea of the method is to analyze the image spectrum in the log polar domain where global 2D head rotations are transformed into simple energy translations. In order to make the log polar spectrum easy to interpret, a prefiltering stage inspired from the biological model of the human retina is applied: mobile contours are enhanced and static contours are attenuated, high frequency noise is eliminated and variations of illumination are cancelled. Estimated rotations are integrated in a data fusion process able to detect and to interpret in real time head nods of approbation or negation.