The distribution of oriented contours in the real world

Abstract

In both humans and experimental animals, the ability to perceive contours that are vertically or horizontally oriented is superior to the perception of oblique angles. There is, however, no consensus about the developmental origins or functional basis of this phenomenon. Here, we report the analysis of a large library of digitized scenes using image processing with orientation-sensitive filters. Our results show a prevalence of vertical and horizontal orientations in indoor, outdoor, and even entirely natural settings. Because visual experience is known to influence the development of visual cortical circuitry, we suggest that this real world anisotropy is related to the enhanced ability of humans and other animals to process contours in the cardinal axes, perhaps by stimulating the development of a greater amount of visual circuitry devoted to processing vertical and horizontal contours.

Figure 1

Analysis of oriented contours in real-world scenes. (A) Digital photograph of the Neurobiology Building and its immediate surroundings at Duke University. (B) Image in A after the application of a Sobel direction filter. (C) Image in A after the application of a Sobel magnitude filter. (D) Plot of the summed pixel magnitudes, grouped by orientation. The peaks and troughs evident in the plot provide a quantitative measure of the predominance of projected contours near the vertical (V; 0/270°) and horizontal (H; 0/180°) axes in such scenes. A full 360° are shown because white to black transitions occupy half of the gray-scale, and black to white transitions occupy the other half.

Figure 2

Control observations validating the techniques used here. (A) Same digital photograph as in Fig. 1 but tilted at a 45° angle. (B) Analysis showing that the magnitude of the orientation biases observed in Fig. 1D are similar but that their distribution is shifted by the expected amount. (C) Digital “scene” comprised of random noise. (D) Combined analysis of 10 such scenes. Note the absence of anisotropy in this control.

Figure 3

Representative examples from a digitized library of 150 scenes obtained from indoor (A), outdoor (B), and entirely natural (C) environments at Duke University. The complete library of 150 digitized scenes is available on request.

Figure 4

Distribution of oriented contours in indoor (A), outdoor (B), and entirely natural (piedmont forest) (C) environments. Each graph represents the average of the analyses of 50 representative scenes in the indicated setting. As in Fig. 1, vertical contours (V) are 90/270°, and horizontal contours (H) are 0/180°. The cardinal axes predominate, particularly in the indoor and outdoor scenes.