Monkeys perceive the orientation of objects relative to the vertical axis

Orientation processing is essential for segmenting contour from the background, which allows perception of the shape and stability of objects. However, little is known about how monkeys determine the degree and direction of orientation. In this study, to determine the reference axis for orientation perception in monkeys, post-discrimination generalization tests were conducted following discrimination training between the 67.5° and 112.5° orientations and between the 22.5° and 157.5° orientations. After discrimination training between the 67.5° and 112.5° orientations, the slope of the generalization gradient around the S⁺ orientation was broad, while the slope was steep after discrimination training between the 22.5° and 157.5° orientations. Comparing the shapes of the gradients indicated that the subjective distance between the 67.5° and 112.5° orientations was small, while the subjective distance between the 22.5° and 157.5° orientation was large. In other words, the monkeys recognized that the former and the latter distances were 45° and 135° across the vertical axis, rather than 135° and 45° across the horizontal axis, respectively. These findings indicate that the monkeys determined the degree and direction of the tilt using the vertical reference.     

Body tilt effect on the reproduction of orientations: studies on the visual oblique effect and subjective orientations

Body tilt effects on the visual reproduction of orientations and the Class 2 oblique effect (E. A. Essock, 1980) were examined. Body tilts indicate whether the oblique effect (i.e., lower performance in oblique orientations than in vertical-horizontal orientations) is defined in an egocentric or a gravitational reference frame. Results showed that the oblique effect observed in upright posture disappeared in tilted conditions, mainly due to a decrease in the precision of the vertical and horizontal settings. In tilted conditions, the subjective visual vertical proved to be the orientation reproduced the most precisely. Thus, the oblique effect seemed to be not purely gravitationally or egocentrically defined but, rather, to depend on a subjective gravitational reference frame tilted in the same direction as body tilts.     

Visual orientation during and after lateral head,body, and trunk tilt

Visual judgments of orientation were investigated during (effect) and after (aftereffect) different body postures. In Experiment 1 four trained Ss made apparent verticality (AV) judgments before and after 2 min in each of seven orientations: head tilt left and right, body tilt left and right, trunk tilt left and right and a control condition with head ’and body upright. The aftereffect was significant for all postures excepting trunk tilt left and the control. The aftereffect from head tilt was greater than that from the same degree of body tilt, and that in the trunk tilt condition was in the same direction as’ predicted from neck stimulation. In Experiment 2, 30 Ss made AV judgments during tilt in the same seven postures. The E-phenomenon resulted from both head and body tilts, and an effect was found for trunk tilt in the direction predicted from neck stimulation. The results are discussed in terms of the otolith, neck, and trunk receptor systems.     

Perceptual learning of orientation judgments in oblique meridians

Fourteen daily training sessions in orientation discrimination of foveal lines in the 45-deg meridian improved thresholds in the trained meridian by an average of 25 % in five observers. A substantial amount of training transferred to the other obliques, but none to the cardinal meridians, with a consequent reduction in the oblique effect. The data were interpreted as showing perceptual learning at two levels: performance facilitation specific to the trained orientation and improved proficiency globally. The failure of the cardinal orientations to share in the benefit is likely to have its origin in the fact that contour orientation in these meridians is so well established that it had already reached maximum hyperacuity thresholds. The judgment of obliques depends much more than the judgment of cardinals on whether the comparison and test stimuli are shown simultaneously or in succession, but this effect is not changed by perceptual training.     

The role of visuospatial and verbal working memory in perceptual category learning

The role of verbal and visuospatial working memory in rule-based and information-integration category learning was examined. Previously, Maddox, Ashby, Ing, and Pickering found that a sequentially presented verbal working memory task did not affect information-integration learning, but disrupted rule-based learning when the rule was on the spatial frequency of a Gabor stimulus. This pattern was replicated in Experiment 1, in which the same category structures were used, but in which the verbal working memory task was replaced with a visuospatial analog. Experiment 2A examined rule-based learning on an oblique orientation and also found both verbal and visuospatial working memory tasks disrupting learning. Experiment 2B examined rule-based learning on a cardinal orientation and found a minimal effect of the verbal working memory task, but a large effect of the visuospatial working memory task. The conceptual significance of cardinal orientations and the role of visuospatial and verbal working memory in category learning are discussed.     

Reference frames and haptic perception of orientation: Body and head tilt effects on the oblique effect

The aim of this study was to examine the effect of body and head tilts on the haptic oblique effect. This effect reflects the more accurate processing of vertical and horizontal orientations, relative to oblique orientations. Body or head tilts lead to a mismatch between egocentric and gravitational axes and indicate whether the haptic oblique effect is defined in an egocentric or a gravitational reference frame. The ability to reproduce principal (vertical and horizontal) and oblique orientations was studied in upright and tilted postures. Moreover, by controlling the deviation of the haptic subjective vertical provoked by postural tilt, the possible role of a subjective gravitational reference frame was tested. Results showed that the haptic reproduction of orientations was strongly affected by both the position of the body (Experiment 1) and the position of the head (Experiment 2). In particular, the classical haptic oblique effect observed in the upright posture disappeared in tilted conditions, mainly because of a decrease in the accuracy of the vertical and horizontal settings. The subjective vertical appeared to be the orientation reproduced the most accurately. These results suggest that the haptic oblique effect is not purely gravitationally or egocentrically defined but, rather, depends on a subjective gravitational reference frame that is tilted in a direction opposite to that of the head in tilted postures (Experiment 3).     

Reference frames and haptic perception of orientation: body and head tilt effects on the oblique effect.

The aim of this study was to examine the effect of body and head tilts on the haptic oblique effect. This effect reflects the more accurate processing of vertical and horizontal orientations, relative to oblique orientations. Body or head tilts lead to a mismatch between egocentric and gravitational axes and indicate whether the haptic oblique effect is defined in an egocentric or a gravitational reference frame. The ability to reproduce principal (vertical and horizontal) and oblique orientations was studied in upright and tilted postures. Moreover, by controlling the deviation of the haptic subjective vertical provoked by postural tilt, the possible role of a subjective gravitational reference frame was tested. Results showed that the haptic reproduction of orientations was strongly affected by both the position of the body (Experiment 1) and the position of the head (Experiment 2). In particular, the classical haptic oblique effect observed in the upright posture disappeared in tilted conditions, mainly because of a decrease in the accuracy of the vertical and horizontal settings. The subjective vertical appeared to be the orientation reproduced the most accurately. These results suggest that the haptic oblique effect is not purely gravitationally or egocentrically defined but, rather, depends on a subjective gravitational reference frame that is tilted in a direction opposite to that of the head in tilted postures (Experiment 3).     

Why do children confuse mirror-image obliques?

Thirty-five children aged from 4.17 to 6.58 years were given a delayed-matching task in which they had to choose on each trial which of two lines was the same as a previously displayed standard line. Their choices were no better than random when the lines differed in degree of slope but not in left-right orientation and were only marginally more accurate when the lines were left-right mirror images. Performance improved significantly when the lines differed both in degree of slope from the vertical and in left-right orientation and improved still further if at least one of the lines was horizontal or vertical. The results suggest that young children have extreme difficulty encoding in memory either the degree of slope or the left-right orientation of an oblique line.     

The beholder's share in the perception of orientation of 2-D shapes

A considerable amount of research demonstrates that people perceive cardinal orientations (horizontal and vertical) more accurately than other orientations; this is termed the oblique effect. We investigated the interaction of this effect with the degree of elongation of the stimulus. Our stimuli were ellipses with a wide range of aspect ratios, varying from a circle (aspect ratio = 1) to a line (aspect ratio = 123.5). The task was to set a probe line in the same orientation as the long axis of the ellipse. In our first experiment, we determined that performance is degraded as the aspect ratio decreases; furthermore, the bias and response variability are linearly related to a transformation of aspect ratio (roundness). We found significant individual differences; the results show high within-subjects correlations and low between-subjects correlations. In our second experiment, we had observers judge the orientation of circles randomly mixed in with ellipses of low aspect ratio. The observers demonstrated intrinsic preferences and generated reproducible distributions of orientation settings with idiosyncratic profiles. These distributions predict the influence on the response to ellipses with an aspect ratio higher than one and can be considered as the beholder's share in the perception of shape orientation.     

The reproduction of vertical and oblique orientations in the visual, haptic, and somatovestibular systems

This study investigates whether the vertical orientation may be predominantly used as an amodal reference norm by the visual, haptic, and somato-vestibular perceptual systems to define oblique orientations. We examined this question by asking the same sighted adult subjects to reproduce, in the frontal (roll) plane, the vertical (0°) and six oblique orientations in three tasks involving different perceptual systems. In the visual task, the subjects adjusted a moveable rod so that it reproduced the orientation of a visual rod seen previously in a dark room. In the haptic task, the blindfolded sighted subjects scanned an oriented rod with one hand and reproduced its orientation, with the same hand, on a moveable response rod. In the somato-vestibular task, the blindfolded sighted subjects, sitting in a rotating chair, adjusted this chair in order to reproduce the tested orientation of their own body. The results showed that similar oblique effects (unsigned angular error difference between six oblique orientations and vertical orientation) were observed across the three tasks. However, there were no positive correlations between the visual, haptic,     

Drawing with oblique coordinates: on a single case

We describe a patient with right hemisphere damage affected by mild left visuo-spatial neglect and constructional apraxia. During the rehabilitation, he failed to draw a draught-board using horizontal and vertical trajectories, but he performed it successfully using oblique trajectories. These observations suggested an impairment of vertical/horizontal spatial coordinates system. In copying tasks including figure elements in different orientations he drew more accurately components in oblique orientation, whereas failed to reproduce components in horizontal orientation. The patient performed visuospatial perceptual and perceptual-imaginative tasks successfully. From these findings, it is possible to suggest that the oblique coordinate system of reference operates independently of vertical and horizontal coordinate systems in building a complex figure and that, therefore, cardinal orientation do not constitute a reference norm to define oblique orientation, as previously suggested.     

The beholder’s share in the perception of orientation of 2-D shapes

A considerable amount of research demonstrates that people perceive cardinal orientations (horizontal and vertical) more accurately than other orientations; this is termed theoblique effect. We investigated the interaction of this effect with the degree of elongation of the stimulus. Our stimuli were ellipses with a wide range of aspect ratios, varying from a circle (aspect ratio = 1) to a line (aspect ratio = 123.5). The task was to set a probe line in the same orientation as the long axis of the ellipse. In our first experiment, we determined that performance is degraded as the aspect ratio decreases; furthermore, the bias and response variability are linearly related to a transformation of aspect ratio (roundness). We found significant individual differences; the results show high within-subjects correlations and low between-subjects correlations. In our second experiment, we had observers judge the orientation of circles randomly mixed in with ellipses of low aspect ratio. The observers demonstrated intrinsic preferences and generated reproducible distributions of orientation settings with idiosyncratic profiles. These distributions predict the influence on the response to ellipses with an aspect ratio higher than one and can be considered as thebeholder’s share in the perception of shape orientation.     

Recognizing rotated faces and Greebles: What properties drive the face inversion effect?

The fact that faces are strongly affected by picture-plane inversion has often been cited as evidence for face-specific mechanisms. It is unclear, however, whether this “face inversion effect” is driven by properties shared by faces or whether the effect is specific to faces as a category. To address this issue, we compared the recognition of faces and novel Greebles, which were specifically matched to faces along various stimulus dimensions. In two experiments, participants were required to name individual faces or Greebles following training at either single or multiple orientations. We found that performance systematically decreased with increasing misorientation from either the upright (Experiment 1) or nearest trained orientation (Experiment 2). Importantly, the magnitude of this orientation effect was similar for both faces and Greebles. Taken together, these results suggest that the face inversion effect may be a consequence of the visual homogeneity of the stimulus category, regardless of the category.     

Orientation Constancy in Neurons of Monkey Visual Cortex

We studied the effect of body tilt on the orientation selectivity of single neurons in the visual cortex of an alert monkey. The monkey performed a visual fixation task either in the upright position or with its whole body tilted about the naso-occipital (roll) axis by ±25° or ±30°. We determined the preferred stimulus orientation for 51 of 117 neurons in two or, if possible, three body positions (i.e. with the whole body upright, and tilted either left ear or right ear down). In striate cortex, most of the neurons were of a non-compensatory type, showing a change in the preferred orientation according to the body tilt and the estimated counterrolling of the eye. By contrast, about 40% of the neurons in prestriate cortex were of a compensatory type, preferring similar orientations in all body positions. This suggests that mechanisms which produce orientation constancy with respect to the direction of gravity are implemented at an early stage of cortical processing.     

Effect of orientation on visual and vibrotactile letter identification

The effect of stimulus orientation of letters presented either visually or vibrotactually was examined to obtain basic information on sensory substitution using the tactile sense. The reaction time (RT) to identify the letters F and R presented in normal or mirror-image form at four orientations each was measured. In addition, conditions of 0 degree and 270 degrees of head rotation from vertical and arm rotation from the midline axis were employed. Data from 5 trained subjects showed that vibrotactile RTs were always longer than visual RTs. Stimulus rotation away from normal orientation increased visual RTs significantly but not vibrotactile RTs. Visual orientation effect then seemed to be determined by the body-coordinate system but not the vibrotactile orientation. Although further studies are warranted, from the results of this experiment, any convenient and constant stimulus orientation could be used with a wearable vibrotactile display system to exploit passive touch.     

Effects of hand orientation and delay on the verbal judgment of haptically perceived orientation

We examined the haptic perception of orientations of a single bar throughout the horizontal plane using a verbal response: participants were to assign a number of minutes to the orientation of a bar defined with respect to the stimulus table. Performance was found to be systematically biased. Deviations were consistent with, yet much smaller than, those resulting from haptic motor matching tasks. The size and direction of the deviations were found to correlate with hand orientation, and not to depend on spatial location per se, suggesting a role for hand-centred reference frames in biasing performance. Delaying the response by 10 s led to a small improvement only of right-hand perceptions, indicating different hemispheric involvement in processes involved in retaining and/or recoding of haptic orientation information. Also the haptic oblique effect was found with the current verbal response. Importantly, it was affected neither by hand orientation nor by delay, suggesting that the oblique effect is independent of the aforementioned deviations in orientation perception.     

Tracking objects that move where they are headed

Previous work has demonstrated that the ability to keep track of moving objects is improved when the objects have unique visual features, such as color or shape. In the present study, we investigated how orientation information is used during the tracking of objects. Orientation is an interesting feature to explore in moving objects because it is directional and is often informative of the direction of motion. Most objects move forward, in the direction they are oriented. In the present experiments, participants tracked a subset of moving isosceles triangles whose orientations were constant, related, or unrelated to the direction of motion. In the standard multiple object tracking (MOT) task, tracking performance improved when orientations were unique and remained constant, but not when orientation and direction of motion were aligned. In the target recovery task, in which MOT was interrupted by a brief blanking of the display, performance did improve when orientation and direction were aligned. In the final experiment, results showed that orientation was not used before the blank to predict future target locations, but was instead used after the blank. We concluded that people use orientation to compare a stored representation to target position for recovery of lost targets.     

Direction repulsion in unfiltered and ring-filtered Julesz textures

Perceived directions of motion were measured for each of two superposed two-dimensional dynamic random patterns consisting of unfiltered or ring-filtered dense random-check (Julesz) textures. One pattern always moved in a cardinal direction (up, down, left, or right), and the other texture always moved in an oblique direction separated from the cardinal component by 20 degrees-80 degrees. Several cardinal/oblique speed ratios were tested. In Experiment 1, the textures were unfiltered. In Experiment 2, the textures were ring filtered and had the same center frequency (1, 2, or 4 cpd). In Experiment 3, a 1-cpd ring-filtered texture was paired with a 2-, 4-, or 8-cpd texture. Subjects consistently misperceived the directions of component motion in these experiments; the angular separation of movement of the two textures was perceptually exaggerated, a phenomenon referred to as direction repulsion (Marshak & Sekuler, 1979). The results show that (1) direction repulsion occurs across at least a fourfold range of spatial frequencies and a sixfold range of speed ratios, (2) direction repulsion varies systematically with speed ratio, and (3) across most conditions, direction repulsion is anisotropic--direction repulsion is more evident in the oblique directions than in the cardinal directions. These findings suggest that the spatiotemporal range of inhibitory interactions involved in motion transparency is much greater than previously appreciated.     

Recovery from adaptation as a function of stimulus orientation

These experiments examined the oblique effect in an adaptation paradigm. Reaction times (RT) to the presence of a grating test stimulus were obtained following adaptation to either a blank field or a grating of the same orientation as the test stimulus. Horizontal, vertical, and oblique test and adaptation orientations were employed. Test gratings were presented at several interstimulus intervals following offset of the adaptation stimulus. RTs following grating adaptation were elevated to a greater extent (relative to blank adaptation) for oblique then for horizontal or vertical stimuli, for two grating spatial frequencies. Differences in RT can be related to differences in sensitivity among channels responsible for detection of the various orientations.