Additive effects in a rod-and-frame illusion estimated by response classification

Previous research has shown that not all segments of a square frame are necessary to produce the illusory tilt of an enclosed vertical line. Indeed, the presence of a single tilted line is often sufficient to induce the illusory tilt of a nearby vertical line (Carpenter & Blakemore, 1973). How do the four segments of a quadrilateral frame contribute to the illusory tilt if any one of them is sufficient to induce the illusion? Response classification (RC) was used in two experiments to examine the independent contributions of the four segments of a quadrilateral frame to judgments of the direction of tilt of an enclosed vertical line. Orientation perturbations were added independently to the four frame segments. The orientation of the top segment contributed most systematically to these judgments, whereas the orientation of the bottom segment contributed very little. Individual differences were observed with two of the four observers showing the largest apparent tilt of the test line for shear configurations of the quadrilateral in which the top and bottom segments were rotated in a direction opposite to the right and left segments. Logistic regression was used with a double-pass technique to estimate the relative importance of the four segments. Interactions between the segments were not systematically related to the observers' judgments. The results are discussed in terms of the utility of RC and logistic regression for studying perceptual phenomena whose mechanisms are thought to lie at levels such as orientation that are different from those typically examined with RC and pixel noise.     

The oblique effect depends on perceived, rather than physical, orientation and direction

Observers can better discriminate orientation or direction near the cardinal axes than near an oblique axis. We investigated whether this well-known oblique effect is determined by the physical or the perceived axis of the stimuli. Using the simultaneous tilt illusion, we generated perceptually different orientations for the same inner (target) grating by contrasting it with differently oriented outer gratings. Subjects compared the target orientation with a set of reference orientations. If orientation discriminability was determined by the physical orientations, the psychometric curves for the same target grating would be identical. Instead, all subjects produced steeper curves when perceiving target gratings near vertically as opposed to more obliquely. This result of orientation discrimination was confirmed by using adaptation-generated tilt aftereffect to manipulate the perceived orientation of a given physical orientation. Moreover, we obtained the same result in direction discrimination by using motion repulsion to alter the perceived direction of a given physical direction. We conclude that when the perceived orientation or direction differs from the physical orientation or direction, the oblique effect depends on perceived, rather than physical, orientation or direction. Finally, as a by-product of the study, we found that, around the vertical direction, motion repulsion is much stronger when the inducing direction is more clockwise to the test direction than when it is more counterclockwise.     

Feature-based integration of orientation signals in visual search

We have measured orientation discrimination in the presence of a variable number of neutral distracters for two distinct tasks: identification of the orientation of a tilted target and location of its position. Both tasks were performed in the presence of visual noise of variable contrasts. Under a range of conditions, subjects could identify the direction of target tilt at thresholds well below those necessary to locate its position. The location thresholds showed only weak dependency on set-size, consistent with a stimulus uncertainty of parallel search of the output of independent orientation analysers, while the identification thresholds showed a much stronger dependency, varying with the square root of set-size over a wide range noise contrasts. The square root relationship suggests perceptual summation of target and distracters. Manipulating the spread of visual noise suggests that the summation is feature-based, possibly operating on the outputs of first-stage orientation analysers. Pre-cueing the target eliminates the effects of set-size, showing that the summation is under rapid attentional control; the visual system can choose between high performance over a limited area and poorer performance over a much larger area.     

The subjective vertical and the sense of self orientation during active body tilt.

Previous testing of the ability to set a luminous line to the direction of gravity in passively-tilted subjects, in darkness, has revealed a remarkable pattern of systematic errors at tilts beyond 60 degrees, as if body tilt is undercompensated or underestimated (Aubert or A-effect). We investigated whether these consistent deviations from orientation constancy can be avoided during active body tilt, where more potential cues about body tilt (e.g. proprioception and efference copy) are available. The effects of active body tilt on the subjective vertical and on the perception of self tilt were studied in six subjects. After adopting a laterally-tilted posture, while standing in a dark room, they indicated the subjective vertical by adjusting a visual line and gave their verbal estimate of head orientation, expressed on a clock scale. Head roll tilts covered the range from -150 degrees to +150 degrees. The subjective vertical results showed no sign of improvement. Actively-tilted subjects still exhibited the same pattern of systematic errors that characterised their performance during passive tilt. Random errors in this task showed a steep monotonic increase with tilt angle, as in earlier passive tilt experiments. By contrast, verbal head-tilt estimates in the active experiments showed a clear improvement and were now almost devoid of systematic errors, but the noise level remained high. Various models are discussed in an attempt to clarify how these task-related differences and the selective improvement of the self-tilt estimates in the active experiments may have come about.     

Crowding and the tilt illusion: toward a unified account

Crowding, the difficult identification of peripherally viewed targets amidst similar distractors, has been explained as a compulsory pooling of target and distractor features. The tilt illusion, in which the difference between two adjacent gratings' orientations is exaggerated, has also been explained by pooling (of Mexican-hat-shaped population responses). In an attempt to establish both phenomena with the same stimuli--and account for them with the same model--we asked observers to identify (as clockwise or anticlockwise of vertical) slightly tilted targets surrounded by tilted distractors. Our results are inconsistent with the feature-pooling model: the ratio of assimilation (the tendency to perceive vertical targets as tilted in the same direction as slightly tilted distractors) to repulsion (the tendency to perceive vertical targets as tilted away from more oblique distractors) was too small. Instead, a general model of modulatory lateral interaction can better fit our results.     

The role of lateral modulation in orientation-specific adaptation effect

Center-surround modulation in visual processing reflects a normalization process of contrast gain control in the responsive neurons. Prior adaptation to a clockwise (CW) tilted grating, for example, leads to the percept of counterclockwise tilt in a vertical grating, referred to as the tilt-aftereffect (TAE). We previously reported that the magnitude of the TAE is modulated by adding a same-orientation annular surround to an adapter, suggesting inhibitory lateral modulation. To further examine the property of this lateral modulation effect on the perception of a central target, we here used center-surround sinusoidal patterns as adapters and varied the adapter surround and center orientations independently. The target had the same spatial extent as the adapter center with no physical overlap with the adapter surround. Participants were asked to judge the target orientation as tilted either CW or counterclockwise from vertical after adaptation. Results showed that, when the surround orientation was held constant, the TAE magnitude was determined by the adapter center, peaking between 10° and 20° of tilt. More important, the adapter surround orientation modulated the adaptation effect such that the TAE magnitude first decreased and then increased as the surround orientation became increasingly more different from that of the center, suggesting that the surround modulation effect was indeed orientation specific. Our data can be accounted for by a divisive inhibition model, in which (1) the adaptation effect is represented by increasing the normalizing constant and (2) the surround modulation is captured by two multiplicative sensitivity parameters determined by the adapter surround orientation.     

Postural adjustment as a function of scene orientation

Visual orientation plays an important role in postural control, but the specific characteristics of postural response to orientation remain unknown. In this study, we investigated the relationship between postural response and the subjective visual vertical (SVV) as a function of scene orientation. We presented a virtual room including everyday objects through a head-mounted display and measured head tilt around the naso-occipital axis. The room orientation varied from 165° counterclockwise to 180° clockwise around the center of display in 15° increments. In a separate session, we also conducted a rod adjustment task to record the participant''s SVV in the tilted room. We applied a weighted vector sum model to head tilt and SVV error and obtained the weight of three visual cues to orientation: frame, horizon, and polarity. We found significant contributions for all visual cues to head tilt and SVV error. For SVV error, frame cues made the largest contribution, whereas polarity contribution made the smallest. For head tilt, there was no clear difference across visual cue types, although the order of contribution was similar to the SVV. These findings suggest that multiple visual cues to orientation are involved in postural control and imply different representations of vertical orientation across postural control and perception.     

Stereoacuity testing: an illusion of tilt when viewing two parallel vertical rods or lines

When stereoacuity was measured, some subjects (but not all) saw the two vertical rods of the test to be tilted. The perceived tilt indicated that the top of a rod was nearer to or farther from the observer than the bottom. Sometimes one rod appeared tilted and sometimes both. It was rare for both to appear tilted in the same direction. There are 9 possible tilt combinations including both rods vertical. All were observed but only 1 of the 3 subjects observed them all. In some instances, the frequency with which a particular tilt combination was seen was influenced by which rod was nearer and by the binocular disparity presented by the rods. During a stereoacuity test, tilt can confuse the identification of the nearer rod. Uncertainty is avoided by regarding the middle of the rods. Lines drawn on paper were also seen to tilt by some subjects.     

Dynamics of the influence of segmentation cues on orientation perception

Contextual effects abound in vision. The tilt illusion (TI) is an example-a tilted surrounding annulus causes a vertical central pattern to appear rotated away from the surround. We investigate the dynamics of this effect by presenting components of the stimulus asynchronously. At equal contrast, the largest illusion occurs when centre and surround are presented simultaneously. We vary the spatial gap between centre and surround, the relative contrast and depth and find that these segmentation cues result in a reduced TI upon simultaneous presentation, but not all other times. This reveals the dynamics of orientation and other segmentation cue interactions.     

Strong tilt illusions always reduce orientation acuity

The apparent spatial orientation of an object can differ from its physical orientation when differently oriented objects surround it. This is the “tilt illusion”. Previously [Solomon, J. A., & Morgan, M. J. (2006). Stochastic re-calibration: Contextual effects on perceived tilt. Proceedings of the Royal Society of London. Series B, Biological Sciences, 273, 2681-2686], we reported a loss of orientation acuity whenever a large physical tilt was required to compensate for the tilt illusion and make a target appear horizontal. Since all of those targets appeared to be at least approximately horizontal, we concluded that orientation acuity was not wholly determined by the target’s apparent orientation. In the present study, we used oblique (i.e. neither horizontal nor vertical) reference orientations to more directly examine the effect of perceived orientation on orientation acuity. The results show that when surround and reference were parallel, there was no tilt illusion and acuity was high. Acuity suffered whenever the tilt illusion caused a large discrepancy between the target’s physical and perceived tilts. Since this was true even for tilted references, context-induced acuity loss cannot be simply an “oblique effect” of the target’s physical orientation.     

Inter-ocular transfer of the tilt illusion shows that monocular orientation mechanisms are colour selective

A vertical grating appears tilted when surrounded by a tilted inducer grating: the tilt illusion. We investigated the inter-ocular transfer of the tilt illusion for gratings modulated along parallel or orthogonal vectors in a L-M and L+M+S cone contrast space. We found that the monocular component of the tilt illusion is entirely colour selective and the binocular component shows only weak colour selectivity. These results suggest that colour and orientation processing interact at monocular stages of visual processing, whereas binocular visual mechanisms code for form in a manner that is largely insensitive to chromatic signature.     

Covert attention affects the psychometric function of contrast sensitivity

We examined the effect of transient covert attention on the psychometric function for contrast sensitivity in an orientation discrimination task when the target was presented alone in the absence of distracters and visual masks. Transient covert attention decreased both the threshold (consistent with a contrast gain mechanism) and, less consistently, the slope of the psychometric function. We assessed performance at 8 equidistant locations (4.5 degrees eccentricity) and found that threshold and slope depended on target location-both were higher on the vertical than the horizontal meridian, particularly directly above fixation. All effects were robust across a range of spatial frequencies, and the visual field asymmetries increased with spatial frequency. Notwithstanding the dependence of the psychometric function on target location, attention improved performance to a similar extent across the visual field.Given that, in this study, we excluded all sources of external noise, and that we showed experimentally that spatial uncertainty cannot explain the present results, we conclude that the observed attentional benefit is consistent with signal enhancement.     

Eye movements facilitate stereo-slant discrimination when horizontal disparity is noisy

Conditions in which saccadic gaze shifts within planar surfaces facilitate stereo-slant discrimination for slant about the horizontal and vertical axis were investigated. When horizontal disparity noise was added, large gaze shifts in the direction of the slant lowered stereo-slant discrimination thresholds compared to thresholds measured with steady central fixation, whereas eye movements orthogonal to the slant orientation did not lower slant-discrimination thresholds. When no horizontal noise was added, performance was the same with and without gaze shifts. These results suggest that slant is recovered from depth differences between target edges when horizontal disparity signals are variable and that foveal fixation improves the measures of disparity. Eye movements did not lower slant thresholds by providing multiple foveal samples of slant at different target locations that were averaged to reduce disparity noise levels, because eye movements only lowered the thresholds when there was a depth difference between the fixation points. To study which signals for azimuth are used when slant is recovered from the difference in depth between target edges, vertical disparity noise was added and stimulus height was reduced. Both methods elevated slant-discrimination thresholds when horizontal disparity noise was present, suggesting that vertical disparity is used as a cue for azimuth.     

Linear summation of tilt illusion and tilt aftereffect

A comparison of the tilt aftereffect (TAE) and the simultaneous tilt illusion (TI) revealed very similar angular functions. When an aftereffect and a simultaneous illusion of opposite signs were paired by first adapting to a clockwise orientation and then presenting the vertical test line together with a counterclockwise inducing line, the two effects summed linearly. The results suggest a unitary mechanism of TAE and TI. and are consistent with the hypothesis that they are manifestations of lateral inhibition between orientation detectors.     

The relationship between the apparent vertical and the vertical horopter

A luminous line subtending 31 at l m observation distance was set to appear vertical. For five observers with normal vision, in the absence of biasing factors, systematic errors were small. In a dichoptic condition (red-green separation) the line, at objective vertical, was seen as an x and had to be tilted, top away from the observer, for the red and the green images to be seen fully coincident: this “collinearity tilt”, taken to represent the tilt of the vertical horopter in the median plane, averaged 27 (S.E. 2.1) in the same five observers. Then, meridional magnification of 5.1% was introduced at 135° axis in one eye and at 45° axis in its fellow eye. The resulting inclinations of the apparent vertical and the vertical horopter were always in the direction predicted by the optical declination of the two retinal images, and the spatial locus of the apparent vertical always remained some 20 in front of the horopter, in the upper hemifield, and behind the horopter in the lower hemifield. The Helmholtzian declination of the main vertical retinal meridians is confirmed. A new scaling effect is postulated to account for veridicality of the stereoscopic vertical.     

Attention capture by eye of origin singletons even without awareness--a hallmark of a bottom-up saliency map in the primary visual cortex

Human observers are typically unaware of the eye of origin of visual inputs. This study shows that an eye of origin or ocular singleton, e.g., an item in the left eye among background items in the right eye, can nevertheless attract attention automatically. Observers searched for a uniquely oriented bar, i.e., an orientation singleton, in a background of horizontal bars. Their reports of the tilt direction of the search target in a brief (200 ms) display were more accurate in a dichoptic congruent (DC) condition, when the target was also an ocular singleton, than in a monocular (M) condition, when all bars were presented to the same single eye, or a dichoptic incongruent (DI) condition, when an ocular singleton was a background bar. The better performance in DC did not depend on the ability of the observers to report the presence of an ocular singleton by making forced choices in the same stimuli (though without the orientation singleton). This suggests that the ocular singleton exogenously cued attention to its location, facilitating the identification of the tilt singleton in the DC condition. When the search display persisted without being masked, observers' reaction times (RTs) for reporting the location of the search target were shorter in the DC, and longer in the DI, than the M condition, regardless of whether the observers were aware that different conditions existed. In an analogous design, similar RT patterns were observed for the task of finding an orientation contrast texture border. These results suggest that in typical trials, attention was more quickly attracted to or initially distracted from the target in the DC or DI condition, respectively. Hence, an ocular singleton, though elusive to awareness, can effectively compete for attention with an orientation singleton (tilted 20 or 50 degrees from background bars in the current study). Similarly, it can also make a difficult visual search easier by diminishing the set size effect. Since monocular neurons with the eye of origin information are abundant in the primary visual cortex (V1) and scarce in other cortical areas, and since visual awareness is believed to be absent or weaker in V1 than in other cortical areas, our results provide a hallmark of the role of V1 in creating a bottom-up saliency map to guide attentional selection.     

A Fraser illusion without local cues?

In the well-known Fraser illusion, a line composed of tilted elements itself appears tilted. The standard explanation of this illusion has been that the global orientation of the line is in some way influenced by the local orientation of the elements. The illusion was recreated using a texture composed of collinear Gabor stimuli, which were vertical. There was no local tilt. The illusory tilt was produced by gradually shifting the phase of the successive Gabors along each line. Although the 2D Fourier transform of this global pattern peaks at off-vertical orientations, the local energy of the patches is predominantly vertical. How does the visual system nevertheless pick up this global information? This can be explained by elongated linear filters, or a phase-tuned second-stage mechanism. We examined the first theory using a stereoscopic demonstration. When lines of opposite tilt are presented in the two eyes, they combine binocularly to produce stereoscopic slant. We tested whether the illusory tilts in the phase-shifted Gabors texture give stereoscopic slant, when opposite tilts are presented to the two eyes. They do not. Instead, stereoscopic depth is dominated by the local phase-disparity of the individual patches. This indicates that the illusion is not present at the stage of linear filters, which are input to stereo, but must involve second-stage interactions or collators.     

Detecting collinear dots in noise

We estimated the sensitivity for detecting a row of collinear target elements (usually dots) by measuring the maximum density of randomly positioned noise elements that allowed 75% correct detection of the orientation of alignment (binary choice: horizontal versus vertical) of the target elements. We varied the number of target elements, their mode of generation, and their accuracy of positioning. As reported previously (Moulden (1994) Higher-order processing in the visual system. Ciba Foundation Symposium 184. Chichester: Wiley), target detection improved rapidly until the number of target elements reached about seven, and then improved more slowly beyond this point. However, this break was reduced (and often removed entirely) when the target array was formed by repositioning pre-existing noise elements lying close to the target location, rather than by superimposition of additional target elements onto the noise array. This almost linear slope of improvement, coupled with the observation that target detection was disrupted more by random jitter of target elements at right angles to their axis of alignment than by jittering along this axis, argues against a two-stage process of perceptual grouping (Moulden, 1994) and supports instead an explanation based on the operation of a single mechanism. This single mechanism explanation is further supported by the observation that intrinsic positional uncertainty (estimated from the results of jitter experiments) was independent of target element number. Additional experiments showed that target detection is facilitated by aperiodic noise dots that fall close to the target axis. The results are discussed in relation to alternative explanations of perceptual grouping.     

A psychophysical investigation into the preview benefit in visual search

In preview search, half of the distracters are presented ahead of the remaining distracters and the target. Search under these conditions is more efficient than when all the items appear together (Watson & Humphreys, 1997). We investigated the mechanisms contributing to this preview benefit using an orientation discrimination task. In a display of vertical Gabors (all equidistant from fixation) one Gabor (chosen at random) was tilted (left or right). When half the non-tilted Gabors were previewed, thresholds increased less with the number of Gabors, relative to when all the Gabors appeared together (a preview benefit). In a further experiment, orientation noise was added to some of the Gabors. When all Gabors were presented simultaneously, orientation thresholds for the target increased. The effects of noise on thresholds was reduced, however, when the noisy Gabors were presented as a preview. Furthermore, there was less effect of noise in the preview condition than when observers were cued to a subset of Gabors (with a cue presented prior to the Gabors, adjacent to their positions). Visual information can be effectively excluded from the previewed locations to a greater degree than when attention is directed to a subset of display items. The implications for understanding the mechanisms involved in preview search are discussed.     

Orientation discrimination in 5-year-olds and adults tested with luminance-modulated and contrast-modulated gratings

We compared thresholds for discriminating orientation by 5-year-olds and adults for first-order (luminance-modulated) and second-order (contrast-modulated) gratings. To achieve equal visibility, we set the contrast for each age and condition at a fixed multiple of the contrast threshold for discriminating horizontal from vertical gratings. The minimum tilt that could be discriminated from vertical was four to five times larger in 5-year-olds than in adults, even when the noise was removed from the first-order stimuli and amplitude modulation increased to 0.90. Thresholds at both ages were significantly worse (1.2-1.5 times worse) for second-order modulation than for equally visible first-order modulation, and 5-year-olds were equally immature for both types of pattern. Together, the findings suggest that orientation discrimination is slow to develop and worse for second-order than first-order patterns in both children and adults.