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 eff ect can be observed in both animals and humans. Recent neurophysiological d ata suggest that the basis of this effect lies in the structure of the prim ary visual cortex, where more cortical area is devoted to processing contou rs with angles at horizontal and vertical orientations (cardinal orientatio ns). It has been suggested that this cortical feature has developed accordi ng to the statistical properties of natural scenes. To examine this hypothe sis in more detail, we established six image classes and categorized the im ages with respect to their semantical contents. From the images the oriente d energy was calculated by using the corresponding power spectra. We define d simple measures for the degree (cardinal versus oblique energy ratio) and the skewness or anisotropy (aligned energy ratio) of the alignment of ener gy at horizontal and vertical orientations. Our results provide evidence th at (1) alignment depends strongly on the environment, (2) the degree of ali gnment drops off characteristically at higher frequencies, and (3) in natur al images there is on the average an anisotropy in the distribution of ener gy 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 phyloge ny or ontogeny. (C) 2000 Optical Society of America. [S0740-3232(00)01204-7 ]. OCIS codes: 0100.0100, 100.2000, 100.2960.