Oblique Stimuli Research Articles

A flexible Python-based touchscreen chamber for operant conditioning reveals improved visual perception of cardinal orientations in mice.

Natural scenes are composed of a wide range of edge angles and spatial frequencies, with a strong overrepresentation of vertical and horizontal edges. Correspondingly, many mammalian species are much better at discriminating these cardinal orientations compared to obliques. A potential reason for this increased performance could be an increased number of neurons in the visual cortex that are tuned to cardinal orientations, which is likely to be an adaptation to the natural scene statistics. Such biased angular tuning has recently been shown in the mouse primary visual cortex. However, it is still unknown if mice also show a perceptual dominance of cardinal orientations. Here, we describe the design of a novel custom-built touchscreen chamber that allows testing natural scene perception and orientation discrimination performance by applying different task designs. Using this chamber, we applied an iterative convergence towards orientation discrimination thresholds for cardinal or oblique orientations in different cohorts of mice. Surprisingly, the expert discrimination performance was similar for both groups but showed large inter-individual differences in performance and training time. To study the discrimination of cardinal and oblique stimuli in the same mice, we, therefore, applied, a different training regime where mice learned to discriminate cardinal and oblique gratings in parallel. Parallel training revealed a higher task performance for cardinal orientations in an early phase of the training. The performance for both orientations became similar after prolonged training, suggesting that learning permits equally high perceptual tuning towards oblique stimuli. In summary, our custom-built touchscreen chamber offers a flexible tool to test natural visual perception in rodents and revealed a training-induced increase in the perception of oblique gratings. The touchscreen chamber is entirely open-source, easy to build, and freely available to the scientific community to conduct visual or multimodal behavioral studies. It is also based on the FAIR principles for data management and sharing and could therefore serve as a catalyst for testing the perception of complex and natural visual stimuli across behavioral labs.

Decoding the orientation of contrast edges from MEG evoked and induced responses

Visual gamma oscillations have been proposed to subserve perceptual binding, but their strong modulation by diverse stimulus features confounds interpretations of their precise functional role. Overcoming this challenge necessitates a comprehensive account of the relationship between gamma responses and stimulus features. Here we used multivariate pattern analyses on human MEG data to characterize the relationships between gamma responses and one basic stimulus feature, the orientation of contrast edges. Our findings confirmed we could decode orientation information from induced responses in two dominant frequency bands at 24–32 Hz and 50–58 Hz. Decoding was higher for cardinal than oblique orientations, with similar results also obtained for evoked MEG responses. In contrast to multivariate analyses, orientation information was mostly absent in univariate signals: evoked and induced responses in early visual cortex were similar in all orientations, with only exception an inverse oblique effect observed in induced responses, such that cardinal orientations produced weaker oscillatory signals than oblique orientations. Taken together, our results showed multivariate methods are well suited for the analysis of gamma oscillations, with multivariate patterns robustly encoding orientation information and predominantly discriminating cardinal from oblique stimuli.

The Effects of tDCS Across the Spatial Frequencies and Orientations that Comprise the Contrast Sensitivity Function.

Transcranial Direct Current Stimulation (tDCS) has recently been employed in traditional psychophysical paradigms in an effort to measure direct manipulations on spatial frequency channel operations in the early visual system. However, the effects of tDCS on contrast sensitivity have only been measured at a single spatial frequency and orientation. Since contrast sensitivity is known to depend on spatial frequency and orientation, we ask how the effects of anodal and cathodal tDCS may vary according to these dimensions. We measured contrast sensitivity with sinusoidal gratings at four different spatial frequencies (0.5, 4, 8, and 12 cycles/°), two orientations (45° Oblique and Horizontal), and for two stimulus size conditions [fixed size (3°) and fixed period (1.5 cycles)]. Only contrast sensitivity measured with a 45° oblique grating with a spatial frequency of 8 cycles/° (period = 1.5 cycles) demonstrated clear polarity specific effects of tDCS, whereby cathodal tDCS increased and anodal tDCS decreased contrast sensitivity. Overall, effects of tDCS were largest for oblique stimuli presented at high spatial frequencies (i.e., 8 and 12 cycles/°), and were small or absent at lower spatial frequencies, other orientations and stimulus size. Thus, the impact of tDCS on contrast sensitivity, and therefore on spatial frequency channel operations, is opposite in direction to other behavioral effects of tDCS, and only measurable in stimuli that generally elicit lower contrast sensitivity (e.g., oblique gratings with period of 1.5 cycles at spatial frequencies above the peak of the contrast sensitivity function).

Perceptual comparison of features within and between objects: A new look

The integration of spatially distinct elements into coherent objects is a fundamental process of vision. Yet notwithstanding an extensive literature on perceptual grouping, we still lack a clear understanding of the representational consequences of grouping disparate visual locations. We investigated this question in a feature comparison task; subjects identified matching features that belonged either to the same apparent object (within-object condition) or to different apparent objects (between-object condition). The stimulus was backward-masked at a variable SOA, to examine the consequences of changes in the perceptual organization of the segments over time. Critical to our aims, the two objects composing our stimulus were occluded to a variable extent, so that differences in within-object and between-object performance could be unequivocally related to the formation of objects. For certain stimulus arrangements, we found superior performance for within-object matches. The pattern of performance was, however, highly dependent on the stimulus orientation and was not related to the strength of the object percept. Using an oblique stimulus arrangement, we observed superior between-object comparisons that did vary with the object percept. We conclude that performance in our feature comparison task is strongly influenced by spatial relations between features that are independent of object properties. Indeed, this dominating effect may hide an underlying mechanism whereby formation of a visual object suppresses comparison of distinct features within the object.

Weakened feedback abolishes neural oblique effect evoked by pseudo-natural visual stimuli in area 17 of the cat

The psychological oblique effect, a well-known phenomenon that humans and some mammals are more visually sensitive to cardinal (vertical and horizontal) contours than to oblique ones, has commonly been associated with the overrepresentation of cardinal orientations in the visual cortex. In contrast to the oblique effect, however, Essock et al. [E.A. Essock, J.K. DeFord, B.C. Hansen, M.J. Sinai, Oblique stimuli are seen best (not worst!) broad-band stimuli: a horizontal effect, Vision Res. 43 (2003) 1329-1335] reported a psychological 'horizontal effect', in which visual stimuli dominated by oblique orientations were best perceived by human subjects when tested with unique natural broad-band stimuli. In this study, using optical imaging and the similar visual stimuli, we found an overrepresentation of cardinal orientations, i.e. the neural oblique effect, but not 'horizontal effect', in area 17 of the cat. In addition, the oblique effect was abolished by GABA administration in area 21a due to the preferred orientation shifting (6.0%) and decrease of orientation selectivity strength of neurons (26.9%) in area 17. These results indicate a neuronal basis of the oblique effect when animals watch a more natural scene, whereas no evidence was found for the 'horizontal effect'.

Effects of Unilateral Ocular Motor Nerve Palsies on Smooth Pursuit Eye Movements in Adult Patients

Aim: The aim of this study was to investigate the influence of single ocular muscle weakness on smooth pursuit eye movements. Methods: Infrared video recordings of horizontal and vertical eye movements were obtained from 14 adult patients with either unilateral abducens nerve palsy or trochlear nerve palsy. During the recordings, subsequent series of horizontal, vertical and oblique ramp stimuli of 10°/s constant target velocity and ±10° amplitude were presented under monocular viewing conditions. Results: In both forms of ocular nerve palsies, similar changes of pursuit eye movements were observed in the pulling plane of the paretic muscles. The movements of the covered paretic eye showed the lowest amplitude and gain values as well as the lowest numbers of catch-up saccades. The highest amplitude and gain values were calculated from the movements of the covered sound eye. The highest numbers of saccades, however, were produced by the fixating paretic eye. Conclusions: We conclude that the fixating paretic eye compensates for the paresis by raising the pursuit gain and the number of catch-up saccades. In the covered paretic eye, however, monocular adaptation is connected with a symmetric low pursuit gain and a reduced number of saccades in the pulling plane of the paretic muscle.

The origin of the oblique effect examined with pattern adaptation and masking.

The decreased visibility of obliquely oriented patterns as compared to horizontal or vertical ones is termed the oblique effect. The origin of the oblique effect in the chain of visual processing was examined by comparing the potency of oblique adapting gratings to the potency of horizontal ones. Oblique gratings (which were less visible but of equal physical contrast) were as powerful or more powerful than horizontal gratings as adapting stimuli. Obliquely oriented stimuli also produced a slightly stronger tilt aftereffect than stimuli near the cardinal axes. These results suggest that the diminished neural representation of oblique stimuli arises in the human cortex, rather than from impairments of sensitivity or resolution in the initial geniculo-cortical projection.

Oblique stimuli are seen best (not worst!) in naturalistic broad-band stimuli: a horizontal effect

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.

Early involvement of the temporal area in attentional selection of grating orientation: an ERP study

The aim of the present study was to investigate the neural mechanisms of stimulus orientation selection in humans by recording event-related potentials (ERPs) of the brain with a 32-channel montage. Stimuli were isoluminant black-and-white gratings (3 cpd) having an orientation of 50°, 70°, 90°, 110° and 130°, randomly presented in the foveal portion (2° of visual angle) of the central visual field. The task consisted in selectively attending and responding to one of the five grating orientations, while ignoring the others. ERP results showed that orientation selection affected neural processing starting already at an early post-stimulus latency. The P1 component (80–140 ms) measured at temporal area, which might well be reflecting the activity of the ventral stream (i.e. ‘WHAT’ system) of the visual pathways, showed an enhanced amplitude for target orientations. These effects increased with progressive neural processing over time as reflected by selection negativity (SN) and P300 components. In addition, both reaction times (RTs) and ERPs showed a strong ‘oblique’ effect, very probably reflecting the perceptual predominance of orthogonal versus oblique stimulus orientation in the human visual system: RTs were much faster, and SN and P300 components much larger, to gratings presented vertically than in other orientations.

An oblique effect in human primary visual cortex.

Visual perception critically depends on orientation-specific signals that arise early in visual processing. Humans show greater behavioral sensitivity to gratings with horizontal or vertical (0 degrees /90 degrees; 'cardinal') orientations than to other, 'oblique' orientations. Here we used functional magnetic resonance imaging (fMRI) to measure an asymmetry in the responses of human primary visual cortex (V1) to oriented stimuli. We found that neural responses in V1 were larger for cardinal stimuli than for oblique (45 degrees /135 degrees ) stimuli. Thus the fMRI pattern in V1 closely resembled subjects' behavioral judgments; responses in V1 were greater for those orientations that yielded better perceptual performance.

Effects of stimulus orientation on spatial frequency function of the visual evoked potential.

Visual performance is better in response to vertical and horizontal stimuli than oblique ones in many visual tasks; this is called the orientation effect. In order to elucidate the electrophysiological basis of this psychophysical effect, we studied the effects of stimulus orientation on the amplitudes and latencies of visual evoked potentials (VEPs) over different spatial frequencies of the visual stimulation. VEPs to sinusoidal gratings at four orientations (vertical, horizontal, and oblique at 45 degrees and 135 degrees) with eight spatial frequencies (0.5-10.7 cycles/deg) at reversal rates of 1 Hz and 4 Hz were recorded in nine subjects. At 1-Hz stimulation, the amplitude and latency of P100 were measured. At 4-Hz stimulation, VEPs were Fourier-analyzed to obtain phase and amplitude of the second harmonic response (2F). At 1-Hz stimulation, P100 latencies were decreased for oblique stimuli compared with those for horizontal and vertical stimuli at lower spatial frequencies. Conversely, those for oblique stimuli were increased compared with those for horizontal and vertical stimuli at higher spatial frequencies. At 4-Hz stimulation, spatial tuning observed in 2F amplitude of the oblique gratings shifted to lower spatial frequencies when compared with those of vertical stimulation. The alteration of the VEP spatial frequency function caused by the oblique stimuli was in good agreement with the orientation effect observed in psychophysical studies. Our study may have a clinical implication in that VEP testing with stimuli in more than one orientation at slow and fast temporal modulations can be useful in evaluating neurological disease affecting the visual system.

Orientation anisotropy in vernier acuity

An oblique effect is evident in vernier acuity for abutting lines. In Experiment 1 we show that the oblique effect in vernier acuity exists over a range of contrast levels, and is evident even when the horizontal and oblique lines are equally detectable or discriminable. Since this oblique effect cannot be explained by the lower visibility of oblique vernier lines or of the “dipole” cue, it is unlikely to be a consequence of lower neuronal sensitivity. In Experiment 2 we measured the orientation and spatial frequency tuning characteristics of vernier acuity for horizontal and oblique (45 deg) stimuli using a simultaneous masking paradigm. Our results showed no significant differences between either the orientation or spatial frequency tuning for horizontal and oblique stimuli; thus the oblique effect is unlikely to result from differences in the tuning of neurons sensitive to the oblique meridians. Finally, in Experiment 3, we tested the notion that the oblique effect for vernier judgments might reflect limitations imposed beyond the initial filtering operation by measuring vernier acuity for horizontal and oblique lines with either the observer or the frame tilted at an angle of 45 deg. The oblique effect for vernier followed retinal (rather than gravitational) coordinates, and was unaffected by the orientation of the frame, suggesting a relatively low level cause. By exclusion, we suggest that the dependence of vernier acuity on orientation may result from increased positional uncertainty at obliquer orientations, perhaps as a consequence of lower cortical neuronal density and/or increased topographic noise.

Orientation anisotropy in vernier acuity

An oblique effect is evident in vernier acuity for abutting lines. In Experiment 1 we show that the oblique effect in vernier acuity exists over a range of contrast levels, and is evident even when the horizontal and oblique lines are equally detectable or discriminable. Since this oblique effect cannot be explained by the lower visibility of oblique vernier lines or of the “dipole” cue, it is unlikely to be a consequence of lower neuronal sensitivity. In Experiment 2 we measured the orientation and spatial frequency tuning characteristics of vernier acuity for horizontal and oblique (45 deg) stimuli using a simultaneous masking paradigm. Our results showed no significant differences between either the orientation or spatial frequency tuning for horizontal and oblique stimuli; thus the oblique effect is unlikely to result from differences in the tuning of neurons sensitive to the oblique meridians. Finally, in Experiment 3, we tested the notion that the oblique effect for vernier judgments might reflect limitations imposed beyond the initial filtering operation by measuring vernier acuity for horizontal and oblique lines with either the observer or the frame tilted at an angle of 45 deg. The oblique effect for vernier followed retinal (rather than gravitational) coordinates, and was unaffected by the orientation of the frame, suggesting a relatively low level cause. By exclusion, we suggest that the dependence of vernier acuity on orientation may result from increased positional uncertainty at obliquer orientations, perhaps as a consequence of lower cortical neuronal density and/or increased topographic noise.

Occlusion and the distortion of alignment in three-dimensional space.

When observers are asked to align two rectangular stimuli oriented at 45 degrees to the visual axis there is a slight tendency to set the more distant stimulus closer to the eye than its true coplanar position. However, when a large rectangular surface is interpolated between the two oblique stimuli and the observer, errors of alignment become relatively much larger. The displacement caused by the interpolated stimulus occurs both when the display is viewed monocularly and when it is viewed binocularly. Reducing the obliquity of the rectangles results in smaller judgment errors and increasing obliquity increases errors; this is true with and without occlusion. The addition of texture elements to the surfaces of the rectangles reduces judgment errors significantly, but only under conditions of occlusion. It is possible that misalignments recorded for three-dimensional displays have something in common with the two-dimensional Poggendorff illusion.

Seeing versus imagining movement in depth.

Subjects judged the quality of rigid motion between pairs of three-dimensional drawings that differed by a rotation in depth. The figures were aligned with, and rotated around, either the vertical axis or an axis that was oblique with respect to the XYZ co-ordinate system. Rated quality of motion decreased with increasing angular disparity between the figures and with decreasing stimulus duration, regardless of whether the figures were vertical or oblique. The same subjects then participated in a mental rotation task using the same stimuli and angular disparities. An effect of principal axis emerged, such that subjects took longer to make decisions about obliquely aligned stimuli than about vertically aligned stimuli, especially if they received the oblique stimuli first. These data imply that perceived versus imagined movement through the same trajectory involves different processes. Whereas the apparent motion system performs its computations relatively automatically, the processes involved in mental rotation are more strategic in nature.

Evidence for a general category of oblique orientations in four-month-old infants

Recent work on orientation perception and memory in infants suggests that oblique stimulus orientations are treated as members of a category. The two studies in this report support this hypothesis and extend previous findings by demonstrating that this category includes obliques on either side of vertical even when infants are previously exposed to obliques on only one side.

Eliminating the haptic oblique effect: influence of scanning incongruity and prior knowledge of the standards.

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).

Internal frame of reference as a determinant of the oblique effect.

Theorists in perception have included peripheral or central factors in their explanations of the oblique effect. Studies investigating the central factors have shown that stimuli oriented vertically or horizontally have better encoding properties than do stimuli oriented obliquely and that oblique stimuli are more confusable. In the present study, the oblique effect was studied using single dots as stimuli. These dots were surrounded by either a square or a circle frame that provided information about their position in the visual field. The results of classification, focusing, and discrimination tasks showed that the processing of these dots followed the same pattern of results as it did with line stimuli: Dots located on the main axes (vertical and horizontal) were easier to classify than dots located on the oblique axes. The same pattern of results was found in both the square- and circle-frame experiments. The results suggest that an internal frame of reference, aligned with vertical and horizontal, facilitates processing of stimuli located on those axes.

The influence of body position on perception of orientation in infants

The present experiment was conducted in order to assess the relative contribution of gravitational and retinal factors in the perception of orientation in infants by studying the effect of body position on the so-called ‘oblique effect’. Looking times at vertical, horizontal and oblique patterns were compared in vertical and tilted body positions in 18 infants ranging in age from 3 to 7 months. The results clearly demonstrate the influence of body position on the oblique effect: infants tested in the vertical position showed the classical oblique effect, looking longer at the principal orthogonals than at oblique patterns, while tilted infants spent more time looking at oblique stimuli congruent with body orientation. These results strongly suggest that, in infants, the oblique effect is related to retinal coordinates as determined by body position.

Scalp potential fields evoked by grating stimuli: Effects of spatial frequency and orientation

Visually evoked potentials were recorded from an array of 16 electrodes over the occipital cortex in 12 subjects. Grating stimuli of different spatial frequencies were presented on a circular test field, and potential fields evoked by vertical, horizontal and oblique stimuli were compared. Component latencies were determined by computing a reference-independent measure of field power. Scalp field maps were constructed at component latencies and compared over all conditions. Significant influences of both spatial frequency and orientation on component latencies were found. Component latencies were shortest for 2.3 c/deg gratings, and for vertically oriented stimuli. Latencies increased for horizontal, and further for oblique, gratings. The effect of spatial frequency or orientation on the location of the major components on the scalp was less pronounced. On the other hand, there were significant differences both in component latency and scalp distribution when upper and lower hemiretinal stimuli were compared. Upper hemiretinal stimulation yielded shorter latencies and a more anteriorly located potential peak on the scalp.