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.

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.

The amplitude spectra of natural scenes are typically biased in terms of the amount of content at the cardinal orientations relative to the oblique orientations. This anisotropic distribution has been related to the 'oblique effect' (the greater visual sensitivity for simple line/grating stimuli at cardinal compared to oblique orientations). However, we have recently shown that with complex visual stimuli possessing broadband spatial content (i.e. random phase noise patterns), sensitivity for detecting oriented manipulations of amplitude is best for oblique orientations, and worst for horizontal orientations (the 'horizontal effect'). Here we investigated this effect with respect to the phase spectra of natural scenes. Oriented manipulations of both amplitude and phase were made on a set of natural scene images that were dominated by naturally occurring structure at one of four orientations in order to determine whether the presence of predominant scene content, carried by the Fourier phase spectra, altered the ability to detect an oriented increment of amplitude. The horizontal effect was observed regardless of any scene's content bias. In addition, a content-dependent effect was observed which could be related to the presence of spatial structure conveyed by the phase spectra of this set of natural scenes. Results are evaluated in the context of a divisive normalization model.

The amplitude spectra of natural scenes are typically biased in terms of the amount of content at the cardinal orientations relative to the oblique orientations. This anisotropic distribution has been related to the 'oblique effect' (the greater visual sensitivity for simple line/grating stimuli at cardinal compared to oblique orientations). However, we have recently shown that with complex visual stimuli possessing broadband spatial content (i.e. random phase noise patterns), sensitivity for detecting oriented manipulations of amplitude is best for oblique orientations, and worst for horizontal orientations (the 'horizontal effect'). Here we investigated this effect with respect to the phase spectra of natural scenes. Oriented manipulations of both amplitude and phase were made on a set of natural scene images that were dominated by naturally occurring structure at one of four orientations in order to determine whether the presence of predominant scene content, carried by the Fourier phase spectra, altered the ability to detect an oriented increment of amplitude. The horizontal effect was observed regardless of any scene's content bias. In addition, a content-dependent effect was observed which could be related to the presence of spatial structure conveyed by the phase spectra of this set of natural scenes. Results are evaluated in the context of a divisive normalization model.

In this study the authors address the issue of how the perceptual usefulness of nonliteral imagery should be evaluated. Perceptual performance with nonliteral imagery of natural scenes obtained at night from infrared and image-intensified sensors and from multisensor fusion methods was assessed to relate performance on 2 basic perceptual tasks to fundamental characteristics of the imagery. Specifically, single-sensor imagery and fused multisensor imagery (both achromatic and false color) were used to test performance on a region recognition task and a texture segmentation task. Results indicate that the use of color rendering and type of scene content play specific roles in determining perceptual performance allowed by nonliteral imagery. The authors argue that the usefulness of various image-rendering methods should be evaluated with respect to multiple perceptual tasks.

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We investigated the type of spatial structure present in nighttime imagery that is perceptually relevant for human observers to be able to perform texture-based segmentation of real world scenes. Three psychophysical tasks were developed to evaluate performance of the nighttime imagery. The test imagery consisted of scenes obtained via an image-intensified low-light CCD, a long-wave infrared sensor and monochrome sensor-fusion. For one task, performance was best with the fused imagery, but for two tasks, performance with fused imagery was not improved (compared to performance with ir imagery). Spatial filtering of the scenes and further testing revealed that the mid spatial frequencies (1-4 cpd) were more critical in determining performance than either the low or high frequencies. Fourier analysis of the scenes revealed a strong relationship between power and performance, where scenes with more power (especially at the middle frequencies) supported better performance. Implications of this research are that performance depends on power at the middle frequencies for these low-level visual tasks and that fusion algorithms may be improved if this is taken under consideration.

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