Abstract Previous work has shown that anomalies in the contrast sensitivity functions of amblyopes are often insufficient to explain the degree of visual deficit in more complex tasks. Our stimuli were compound gratings composed of a fundamental and its third harmonic, added in either square-wave or triangle-wave phase. At medium to high spatial frequencies we find that many amblyopes, unlike normal observers, are unable to distinguish between such gratings which have identical power spectra, but different phase spectra. In this frequency range they can, however, easily distinguish between a compound grating and its fundamental component alone. It seems that in amblyopia visual processing occurs over a more truncated frequency range than is implied by detection experiments. Various explanations of this observation are considered.