Separating the chaff from the wheat: possible origins of the oblique effect.

The oblique effect refers to a better perception of horizontal and vertical image features as compared with the perception at oblique angles. This effect can be observed in both animals and humans. Recent neurophysiological data suggest that the basis of this effect lies in the structure of the primary visual cortex, where more cortical area is devoted to processing contours with angles at horizontal and vertical orientations (cardinal orientations). It has been suggested that this cortical feature has developed according to the statistical properties of natural scenes. To examine this hypothesis in more detail, we established six image classes and categorized the images with respect to their semantical contents. From the images the oriented energy was calculated by using the corresponding power spectra. We defined simple measures for the degree (cardinal versus oblique energy ratio) and the skewness or anisotropy (aligned energy ratio) of the alignment of energy at horizontal and vertical orientations. Our results provide evidence that (1) alignment depends strongly on the environment, (2) the degree of alignment drops off characteristically at higher frequencies, and (3) in natural images there is on the average an anisotropy in the distribution of energy at the cardinal orientations (i.e., a difference between the amounts of vertical energy and horizontal energy). In light of our results, we further discuss whether the observed cortical anisotropy has its origin in phylogeny or ontogeny.