The distribution of oriented contours in the real world

RESULTS

Fig. 3 shows examples of typical scenes from each setting. The distribution of orientations projected from objects in indoor scenes was strongly biased toward the cardinal axes (Fig. 3A). Indeed, simply looking at the original projections makes plain that the contours of corners, the edges of walls, windows, and doors dominate many of these images, thus biasing the distribution toward vertical and horizontal orientations. The near vertical and horizontal (±22.5°) orientation magnitudes were more than twice as strongly represented as the near oblique orientations (Fig. 4A; Table 1).

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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.

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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.

A similar result was obtained for outdoor environments on the Duke campus (Fig. 3B). The projections from such scenes also were biased toward the cardinal axes, the magnitude of vertical and horizontal orientations being nearly 50% greater than the values determined for near oblique contours (Fig. 4B; Table 1). As expected, entirely natural scenes acquired in the various terrains encountered in the Duke Forest (Fig. 3C) showed a more uniform distribution of orientations. Even in this circumstance, however, near vertical and horizontal contours predominated (Fig. 4C; Table 1). Thus, the summed magnitudes of contour projections near the cardinal axes in natural scenes were ≈10% greater than the magnitudes of the projections near the right and left obliques. Because of the relatively greater complexity of natural scenes, the overall values of oriented contours weighted by their magnitude were substantially greater than in the indoor or outdoor settings. The greater summed magnitudes in outdoor and natural scenes compared with indoor scenes presumably derive from the prevalence of uniform surfaces (walls, ceilings, floors) in the latter setting. Because indoor scenes have more expanses such as walls or floors that have relatively few oriented contours (i.e., regions of the scene in which each pixel is surrounded by neighbors of similar or identical gray-scale value), there is a relative paucity of contours in this setting compared with outdoor and natural scenes.

A potential limitation of this sample of natural scenes is its provenance from a particular geographic region (the North Carolina piedmont; we assume that the indoor and outdoor scenes would be generally similar in other locations). Accordingly, an additional set of scenes was analyzed from a coastal region of North Carolina that featured marshes, dunes, beachscapes, and the maritime flora typical of the Outer Banks. The results were consistent with our original sample of natural scenes, showing a significant prevalence of vertical and horizontal contours relative to oblique angles.

The reason for a bias toward the cardinal axes in such different natural settings is presumably an omnipresent horizon dictated by the earth’s surface (which guarantees horizontal components in most scenes) and an abundance of plants that use vertical supports to counter the force of gravity and horizontal extension to capture sunlight with maximum efficiency.

DISCUSSION

Taken together, these results indicate that the oblique effect, i.e., the greater sensitivity of the visual system in humans and other animals to information oriented near the cardinal axes, accords with the biased distribution of contours projected onto the retinas from objects in the real world. What then is the link, if indeed there is one, between this aspect of visual behavior and the structure of the world with respect to oriented contours?

Several studies have addressed the anatomical and physiological basis of the oblique effect. Psychophysical tests of orientation processing using interference fringes, a technique for presenting an oriented stimulus to human subjects that essentially nullifies the optics of the eye, have shown that the cause of the oblique effect does not lie in the eye itself (12, 13). It has therefore been proposed that this asymmetrical visual behavior has its origin more centrally in the distribution and tuning properties of orientation-selective cells in primary visual cortex (14). Indeed, surveys of single-unit responses in the primary visual cortex of cat (14–17) and monkey (18, 19) have established that more neurons respond to orientations near the cardinal axes than to obliquely oriented stimuli. Moreover, vertical and horizontal stimuli evoke larger cortical potentials measured with surface electrodes than do obliquely oriented stimuli (20–23). In accord with these electrophysiological results, we recently have found that more primary visual cortex in ferrets responds to stimuli in the cardinal axes than to obliquely oriented stimuli (24). In the light of these several lines of evidence, it seems likely that the oblique effect is based on a greater amount of neural machinery devoted to the analysis of orientations near the cardinal axes.

A disproportionate allocation of cortical circuitry devoted to analyzing contours near the cardinal axes could be instantiated during phylogeny, ontogeny, or both. Although the prevalence of vertical and horizontal contours in real world scenes is consistent with either of these possibilities, the results we report here provide some encouragement to consider anew the role of normal experience in the establishment of the mature visual system. That orientation selectivity can be influenced by early experience is indicated by the phenomenon of meridional amblyopia in individuals who suffered from uncorrected astigmatism in early life (25). In such patients, some orientations are much better seen during development than others. As a result, these subjects develop a permanent inability to adequately resolve specific orientations, even when the astigmatism is fully corrected. If the quality of experience with oriented contours can affect the neural circuitry dedicated to analyzing specific orientations under these pathological circumstances, it is reasonable to imagine that a real world bias in the prevalence of oriented contours also influences the structure of the maturing brain. Further support for this view comes from recent work showing that, in the somatic sensory system at least, regions of cortex that are most active during development grow to a greater extent than less active cortical regions (26–29).

Despite this and other evidence that the environment can influence the circuitry concerned with orientation selectivity (30, 31), the influence of visual experience on the organization of cortical orientation domains is still debated. Because the development of ocular dominance columns is modified readily by early experience in cats, monkeys, and a variety of other species (32–34), it seemed likely that orientation columns would be similarly affected. However, the arrangement of orientation domains examined by optical imaging remains unchanged when mapped after vision through one eye and then the other for a prolonged period (ref. 35; see also ref. 36). Moreover, recent experiments indicate that vertical and horizontal orientation columns are detectable very soon after eye opening, implying a largely intrinsic mechanism for their formation (37, 38). The resolution of this apparently conflicting evidence may simply be that, whereas visual experience has little or no influence on the overall arrangement of cortical modules like orientation columns (or ocular dominance columns, for that matter; see ref. 39), it nonetheless has an important effect on the size of columns devoted to processing different orientations.

In summary, the results of digital scene analysis show a prevalence of contours near the cardinal axes in a wide variety of visual environments. Based on recent evidence about the effects of neural activity on cortical development, this biased experience may explain the greater ability of the adult visual system to process information about vertical and horizontal contours.