Phase noise and the classiWcation of natural images

We measured the eVect of global phase manipulations on a rapid animal categorization task. The Fourier spectra of our images of natural scenes were manipulated by adding zero-mean random phase noise at all spatial frequencies. The phase noise was the independent variable, uniformly and symmetrically distributed between 0° and §180°. Subjects were remarkably resistant to phase noise. Even with §120° phase noise subjects were still performing at 75% correct. The high resistance of the subjects’ animal categorization rate to phase noise suggests that the visual system is highly robust to such random image changes. The proportion of correct answers closely followed the correlation between original and the phase noise-distorted images. Animal detection rate was higher when the same task was performed with contrast reduced versions of the same natural images, at contrasts where the contrast reduction mimicked that resulting from our phase randomization. Since the subjects’ categorization rate was better in the contrast experiment, reduction of local contrast alone cannot explain the performance in the phase noise experiment. This result obtained with natural images diVers from those obtained for simple sinusoidal stimuli were performance changes due to phase changes are attributed to local contrast changes only. Thus the global phase-change accompanying disruption of image structure such as edges and object boundaries at diVerent spatial scales reduces object classiWcation over and above the performance deWcit resulting from reducing contrast. Additional color information improves the categorization performance by 2%.  2005 Elsevier Ltd. All rights reserved.