Many encoding mechanisms and processing strategies in the visual system appear to have evolved to better process the prevalent content in the visual world. Here we examine the relationship between the prevalence of natural scene content at different orientations and visual ability for detecting oriented natural scene content. Whereas testing with isolated gratings shows best performance at horizontal and vertical (the oblique effect), we report that when tested with natural scene content, performance is best at obliques and worst at horizontal (the horizontal effect). The present analysis of typical natural scenes shows that the prevalence of natural scene content matches the inverse of this horizontal effect pattern with most scene content at horizontal, next most at vertical, and least at obliques. We suggest that encoding of orientation may have evolved to accommodate the anisotropy in natural scene content by perceptually discounting the most prevalent oriented content in a scene, thereby increasing the relative salience of objects and other content in a scene when viewed against a typical natural background.

Two classes of oblique effects are proposed. Oblique effects demonstrated in paradigms reflecting the basic functioning of the visual system are termed class 1, and those obtained in paradigms reflecting stimulus encoding and memory are termed class 2. The present experiments examine the class 2 oblique effect that has been obtained on reaction time (RT) tasks. Three RT tasks with different response requirements (identification, detection, and classification) were conducted to determine the basic conditions necessary for the production of the class 2 RT oblique effect. The results indicate that the source of the class 2 oblique effect obtained in RT paradigms is a greater confusability between the 45 degrees oblique lines than between the horizontal and vertical lines when identification of orientation is required.

People with normal eyesight typically see horizontal and vertical gratings better than oblique gratings (Psychological Bulletin 78 (1972) 266; Perception 9 (1980) 37). In the present study we investigated whether this oblique effect anisotropy is still observed when viewing more complex visual stimuli that better correspond to the content encountered in everyday viewing of the world. We show that the ability to see oriented structure in an image consisting of broadband spatial content is indeed anisotropic, but that the pattern of this orientation bias is completely different from that obtained with simpler stimuli. Horizontal stimuli are seen worst and oblique stimuli are seen best when tested with more realistic broadband stimuli. We suggest that this "horizontal effect" would be useful in an evolutionary capacity by serving to discount the horizon and other oriented content that tends to dominate natural scenes and thereby increase the salience of objects contained in typical outdoor scenes.

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Broadband oriented-noise masks were used to assess the orientation properties of spatial-context suppression in 'general' viewing conditions (i.e., a fixated, large field of 'naturalistic' noise). Suppression was orientation-tuned with a Gaussian shape and bandwidth of 40 degrees that was consistent across test orientation (0 degrees, 45 degrees, 90 degrees, and 135 degrees). Strength of suppression was highly anisotropic following a "horizontal effect" pattern (strongest suppression at horizontal and least suppression at oblique test orientations). Next, the time course of anisotropic masking was investigated by varying stimulus onset asynchrony (SOA). A standard "oblique effect" anisotropy is observed at long SOAs but becomes a "horizontal effect" when a noise mask is present within approximately 50 ms of the test onset. The orientation-tuned masking appears to result from an anisotropic gain-control mechanism that pools the weighted responses to the broadband mask, resulting in a changeover from oblique effect to horizontal effect. In addition, the relative magnitude of suppression at the orientations tested corresponds to the relative magnitudes of the content of typical natural scenes at the same orientations. We suggest that this anisotropic suppression may serve to equalize the visual system's response across orientation when viewing typical natural scenes, 'discounting' the anisotropy of typical natural scene content.

We investigated human perceptual performance allowed by relatively impoverished information conveyed in nighttime natural scenes. We used images of nighttime outdoor scenes rendered in image-intensified low-light visible (i2) sensors, thermal infrared (ir) sensors, and an i2/ir fusion technique with information added. We found that nighttime imagery provides adequate low-level image information for effective perceptual organization on a classification task, but that performance for exemplars within a given object category is dependent on the image type. Overall performance was best with the false-color fused images. This is consistent with the suggestion in the literature that color plays a predominate role in perceptual grouping and segmenting of objects in a scene and supports the suggestion that the addition of color in complex achromatic scenes aids the perceptual organization required for visual search. In the present study, we address the issue of assessment of perceptual performance with alternative night-vision sensors and fusion methods and begin to characterize perceptual organization abilities permitted by the information in relatively impoverished images of complex scenes. Applications of this research include improving night vision, medical, and other devices that use alternative sensors or degraded imagery.

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