Performance for a large variety of perceptual tasks is superior for stimuli aligned in horizontal or vertical orientations, as compared to stimuli in oblique orientations. This phenomenon appears in the human adult and child, and throughout the animal kingdom. Neurophysiological mechanisms for orientation analysis have been found in the higher visual pathways of many animals, and the suggestive evidence is compelling that these mechanisms underly the orientation preferences reported behaviorally. This paper reviews both the behavioral and neurophysiological studies of orientation preferences, and suggests additional methods for determining the cause of these effects. Stimulus orientation has received increased attention in the past decade from neurophysiological studies of visual pathways. These studies have located mechanisms for orientation analysis in single cells of mammalian visual systems. The study of stimulus orientation, however, has a long history in the behavioral and psychophysical literature. A persistent feature of these latter studies has been a small but consistent superiority in performance when visual stimuli are horizontal or vertical, as opposed to oblique. (For convenience, this phenomenon is subsequently referred to as the oblique effect.) The origin of the oblique effect has not been precisely determined, but a relationship between the behavioral results and the neurophysiological data is suggestive. This paper deals with the oblique effect in two parts. First, it reviews the behavioral and psychophysical studies of orientation preferences. Second, it discusses the neurophysiological substrates of orientation perception. The first section helps define the oblique effect by elaborating on the many forms in which it appears. The second section amplifies the first by evaluating the neurophysiological findings in respect to these orientation preferences.

Although the oblique effect has been conceptualized as a purely visual phenomenon, recent studies report its occurrence in a haptic matching task and present the hypothesis that differences in haptic orientational sensitivity might be responsible for the results. The possibility that procedural variables could be responsible was investigated. Specifically, the effect of prior knowledge of the stimulus orientation standards and of use of bilateral haptic exploration of standard and comparison orientations was examined. The results indicate that the reported oblique effect is reduced either when subjects are not informed which orientations will be tested, or when a unilateral matching procedure instead of a bilateral one is used. When both conditions are combined, the haptic oblique effect is eliminated. It is concluded that this particular manifestation of the oblique effect is not related to haptic sensitivity, but stems from the use of well-established imagery as referent for a match (imagery for oblique stimulus orientations is inferior) and the inherently different scanning patterns required in bilateral exploration of obliques (percepts of standard and comparison obliques will be necessarily different).

Although the 'oblique effect' (poorer performance on oblique orientations as compared to performance on vertical and horizontal orientations) is generally understood as a strictly visual phenomenon, a haptic oblique effect occurs for blindfolded subjects required to set a stimulus rod by hand. Because oblique effects are often attributed to the observer's experience with a predominantly horizontal and vertical environment, we assessed the effect of visual and haptic experience by providing subjects with modality-specific inspection periods to familiarize them with the more poorly judged obliques. Oblique error was significantly reduced in magnitude for judgments made by the modality of experience, and for judgments made across modalities. Rate of improvement, consistency of transfer, and the subjective reports of subjects indicate that this haptic oblique effect is more strongly influenced by visual experience and imagery than by haptic experience. It need not be interpreted as an effect based on factors intrinsic to the haptic modality.