Gaze stability of observers watching Op Art pictures

It has been the matter of some debate why we can experience vivid dynamic illusions when looking at static pictures composed from simple black and white patterns. The impression of illusory motion is particularly strong when viewing some of the works of 'Op Artists, such as Bridget Riley's painting Fall. Explanations of the illusory motion have ranged from retinal to cortical mechanisms, and an important role has been attributed to eye movements. To assess the possible contribution of eye movements to the illusory-motion percept we studied the strength of the illusion under different viewing conditions, and analysed the gaze stability of observers viewing the Riley painting and control patterns that do not produce the illusion. Whereas the illusion was reduced, but not abolished, when watching the painting through a pinhole, which reduces the effects of accommodation, it was not perceived in flash afterimages, suggesting an important role for eye movements in generating the illusion for this image. Recordings of eye movements revealed an abundance of small involuntary saccades when looking at the Riley pattern, despite the fact that gaze was kept within the dedicated fixation region. The frequency and particular characteristics of these rapid eye movements can vary considerably between different observers, but, although there was a tendency for gaze stability to deteriorate while viewing a Riley painting, there was no significant difference in saccade frequency between the stimulus and control patterns. Theoretical considerations indicate that such small image displacements can generate patterns of motion signals in a motion-detector network, which may serve as a simple and sufficient, but not necessarily exclusive, explanation for the illusion. Why such image displacements lead to perceptual results with a group of Op Art and similar patterns, but remain invisible for other stimuli, is discussed.     

The tactual horizontal-vertical illusion depends on radial motion of the entire arm

Wesought to clarify the causes of the tactual horizontal-vertical illusion, where vertical lines are overestimated as compared with horizontals in Land inverted-T figures. Experiment 1 did not use L or inverted-T figures, but examined continuous or bisected horizontal and vertical lines. It was expected that bisected lines would be perceived as shorter than continuous lines, as in the inverted-T figure in the horizontal-vertical illusion. Experiment 1 showed that the illusion could not be explained solely by bisection, since illusory effects were similar for continuous and bisected vertical and horizontal lines. Experiments 2 and 3 showed that the illusory effects were dependent upon stimulus size and scanning strategy. Overestimation of the vertical was minimal or absent for the smallest patterns, where it was proposed that stimuli were explored by finger movement, with flexion at the wrist. Larger stimuli induce whole-arm motions, and illusory effects were found in conditions requiring radial arm motion. The illusion was weakened or eliminated in Experiment 4 when subjects were forced to examine stimuli with finger-and-hand motion alone, that is, their elbows were kept down on the table surface, and they were prevented from making radial arm motions. Whole-arm motion damaged performance and induced perceptual error. The experiments support the hypothesis that overestimation of the vertical in the tactual horizontal-vertical illusion derives from radial scanning by the entire arm.     

Illusory motion induced by blurred red-blue edges

Visual patterns consisting of a red-and-blue region with a blurry edge yield illusory motion. Eye movements over a static pattern induced illusory motion of the edge in the direction opposite to the eye movement. The illusion also takes place for patterns in motion without eye movement. The illusion suggests the effect of colour combination on the spatial perception of a blurry edge.     

Halt and recovery of illusory motion perception from peripherally viewed static images

We quantitatively investigated the halt and recovery of illusory motion perception in static images. With steady fixation, participants viewed images causing four different motion illusions. The results showed that the time courses of the Fraser-Wilcox illusion and the modified Fraser-Wilcox illusion (i.e., "Rotating Snakes") were very similar, while the Ouchi and Enigma illusions showed quite a different trend. When participants viewed images causing the Fraser-Wilcox illusion and the modified Fraser-Wilcox illusion, they typically experienced disappearance of the illusory motion within several seconds. After a variable interstimulus interval (ISI), the images were presented again in the same retinal position. The magnitude of the illusory motion from the second image presentation increased as the ISI became longer. This suggests that the same adaptation process either directly causes or attenuates both the Fraser-Wilcox illusion and the modified Fraser-Wilcox illusion.     

Illusory motion produced by dichoptic stimuli during saccades

A new motion illusion is reported in which saccadic eye movements can produce a perceived jump of a static stimulus presented dichoptically. In three experiments, observers made saccades while viewing a stationary stimulus consisting of a disk and random dots presented separately to the two eyes. In experiments 1 and 2, by measuring the strength of the perceived motion and the velocity of binocular eye movements, we found that (a) motion ratings were high for the stimulus that contained a large interocular difference in luminance, and (b) the saccadic strategy of the observer was virtually identical across different stimulus conditions. In experiment 3, by measuring the detectability of a short temporal gap introduced into the stimulus around saccades, we found that saccadic suppression was normal in the dichoptic presentation. We discuss possible mechanisms underlying the illusory motion.     

Efference,perceived movement,and illusory displacement

Implications of the efferent signal associated with voluntary eye movements as a mechanism of perceived motion are reviewed. This signal normally subserves motion perception during pursuit eye movements and is also present when the pursuit is activated to prevent loss of fixation. Such efferent signals are the basis of a number of illusory motion and displacement phenomena. The contribution of efferent motion mechanisms to the oculogyral and apparent concomitant motion illusions is discussed. The phenomenon of roll-induced tilt is analysed in terms of illusory motion and displacement associated with voluntary cyclotorsional eye movements.     

Effects of illusory stripes on the Helmholtz illusion

This study examined the Helmholtz illusion by using "illusory stripes." A square patch is perceived as wider when vertical lines are drawn on it and is perceived as taller when horizontal lines are drawn on it, i.e., Helmholtz illusion. With vertical lines curved sinusoidally, horizontal "illusory stripes" are perceived; and with horizontal lines curved sinusoidally, vertical "illusory stripes" are perceived. The purpose of the present study was to test whether the "illusory stripes" produce the Helmholtz illusion. We measured the apparent size of a square patch filled with sinusoidal lines. Our subjects (N=27) judged the patch with horizontal "illusory stripes" taller than the square patch filled with vertical straight lines. The subjects also judged the square patch with vertical "illusory stripes" wider than the square patch filled with horizontal straight lines. These results demonstrate that "illusory stripes" can produce the Helmholtz illusion.     

The misperception of rotary motion

Two principles for predicting the relative frequency of illusory reversals of rotating plane objects were derived and tested empirically. Ten objects, variously’ combining valid and confounding depth cues, were used. Predictions based on the principles were confirmed in every case. The results are offered as an improved explanation of the Ames trapezoid illusion and other illusions of rotary motion.     

Induced motion of a fixated target: influence of voluntary eye deviation.

Induced motion (IM) was observed in a fixated target in the direction opposite to the real motion of a moving background. Relative to a fixation target located straight ahead, IM decreased when fixation was deviated 10 degrees in the same direction as background motion and increased when fixation was deviated 10 degrees opposite background motion. These results are consistent with a "nystagmus-suppression" hypothesis for subjective motion of fixated targets: the magnitude of illusory motion is correlated with the amount of voluntary efference required to oppose involuntary eye movements that would occur in the absence of fixation. In addition to the form of IM studied, this explanation applies to autokinesis, apparent concomitant motion, and the oculogyral illusion. Accounts of IM that stress visual capture of vection, afferent mechanisms, egocenter deviations, or phenomenological principles, although they may explain some forms of IM, do not account for the present results.     

Induced motion of a fixated target: Influence of voluntary eye deviation

Induced motion (IM) was observed in a fixated target in the direction opposite to the real motion of a moving background. Relative to a fixation target located straight ahead, IM decreased when fixation was deviated 10° in the same direction as background motion and increased when fixation was deviated 10° opposite background motion. These results are consistent with a “nystagmus-suppression” hypothesis for subjective motion of fixated targets: the magnitude of illusory motion is correlated with the amount of voluntary efference required to oppose involuntary eye movements that would occur in the absence of fixation. In addition to the form of IM studied, this explanation applies to autokinesis, apparent concomitant motion, and the oculogyral illusion. Accounts of IM that stress visual capture of vection, afferent mechanisms, egocenter deviations, or phenomenological principles, although they may explain some forms of IM, do not account for the present results.     

Simulated angular head oscillation enhances vection in depth

Research has shown that adding simulated linear head oscillation to radial optic flow displays enhances the illusion of self-motion in depth (ie linear vection). We examined whether this oscillation advantage for vection was due to either the added motion parallax or retinal slip generated by insufficient compensatory eye movement during display oscillation. We constructed radial flow displays which simulated 1 Hz horizontal linear head oscillation (generates motion parallax) or angular head oscillation in yaw (generates no motion parallax). We found that adding simulated angular or linear head oscillation to radial flow increased the strength of linear vection in depth. Neither type of simulated head oscillation significantly reduced vection onset latencies relative to pure radial flow. Simultaneous eye-movement recordings showed that slow-phase ocular following responses (OFRs) were induced in both linear and angular viewpoint oscillation conditions. Vection strength was significantly reduced by active central fixation when viewing displays which simulated angular, but not linear, head oscillation. When these displays with angular oscillation were viewed without stable fixation, vection strength was found to increase with the velocity and regularity of the OFR. We conclude that vection improvements observed during central viewing of displays with angular viewpoint oscillation depend on the generation of eye movements.     

New motion illusion caused by pictorial motion lines

Motion lines (MLs) are a pictorial technique used to represent object movement in a still picture. This study explored how MLs contribute to motion perception. In Experiment 1, we reported the creation of a motion illusion caused by MLs: random displacements of objects with MLs on each frame were perceived as unidirectional global motion along the pictorial motion direction implied by MLs. In Experiment 2, we showed that the illusory global motion in the peripheral visual field captured the perceived motion direction of random displacement of objects without MLs in the central visual field, and confirmed that the results in Experiment 1 did not stem simply from response bias, but resulted from perceptual processing. In Experiment 3, we showed that the spatial arrangement of orientation information rather than ML length is important for the illusory global motion. Our results indicate that the ML effect is based on perceptual processing rather than response bias, and that comparison of neighboring orientation components may underlie the determination of pictorial motion direction with MLs.     

Transformation of the visual-line value in binocular vision: stimuli on corresponding points can be seen in two different directions

We examined Wheatstone's (1838 Philosophical Transactions of the Royal Society of London 128 371-394) claim that images falling on retinally corresponding points can be seen in two different directions, in violation of Hering's law of identical visual direction. Our analyses showed that random-dot stereograms contain stimulus elements that are conceptually equivalent to the line stimuli in the stereogram from which Wheatstone made his claim. Our experiment demonstrated that two lines embedded in a random-dot stereogram appeared in two different directions when they stimulated retinally corresponding points, if the disparity gradient value of the lines was infinity relative to adjacent elements. To ensure that the two lines stimulated corresponding points, observers made vergence eye movements while maintaining the perception of the two lines in two different directions.     

A new variant of the Ouchi illusion reveals Fourier-component-based processing

We report that anomalous motion illusion in a new variant of the Ouchi figure is well predicted by the strength of its Fourier fundamentals and harmonics. The original Ouchi figure consists of a rectangular checkerboard pattern surrounded by an orthogonal rectangular checkerboard pattern, in which illusory relative motion between the two regions is perceived. Although this illusion has been explained in terms of biases in integrating one-dimensional motion signals to determine the two-dimensional motion direction, the physiological mechanism has not been clarified. With our new stimuli, which consisted of thin lines instead of rectangles, we found that the perceived illusion is drastically reduced when the position of each line element is randomly shifted. This is not predicted by simple models of local motion integration along the visible edges. We demonstrate that the relative amplitude of the relevant Fourier fundamentals and harmonics leads to a quantitative prediction. Our analysis was successfully applied to other variants of the Ouchi figure (Khang and Essock 1997 Perception 26 585-597), closely predicting the reported rating. The results indicate that the underlying physiological mechanism is sensitive to the Fourier components of the stimuli rather than the visible edges.     

Retinal and extraretinal information in movement perception: how to invert the Filehne illusion

During a pursuit eye movement made in darkness across a small stationary stimulus, the stimulus is perceived as moving in the opposite direction to the eyes. This so-called Filehne illusion is usually explained by assuming that during pursuit eye movements the extraretinal signal (which informs the visual system about eye velocity so that retinal image motion can be interpreted) falls short. A study is reported in which the concept of an extraretinal signal is replaced by the concept of a reference signal, which serves to inform the visual system about the velocity of the retinae in space. Reference signals are evoked in response to eye movements, but also in response to any stimulation that may yield a sensation of self-motion, because during self-motion the retinae also move in space. Optokinetic stimulation should therefore affect reference signal size. To test this prediction the Filehne illusion was investigated with stimuli of different optokinetic potentials. As predicted, with briefly presented stimuli (no optokinetic potential) the usual illusion always occurred. With longer stimulus presentation times the magnitude of the illusion was reduced when the spatial frequency of the stimulus was reduced (increased optokinetic potential). At very low spatial frequencies (strongest optokinetic potential) the illusion was inverted. The significance of the conclusion, that reference signal size increases with increasing optokinetic stimulus potential, is discussed. It appears to explain many visual illusions, such as the movement aftereffect and center-surround induced motion, and it may bridge the gap between direct Gibsonian and indirect inferential theories of motion perception.     

A variant of the anomalous motion illusion based upon contrast and visual latency

We examined a variant of the anomalous motion illusion. In a series of experiments, we ascertained luminance contrast to be the critical factor. Low-contrast random dots showed longer latency than high-contrast ones, irrespective of whether they were dark or light (experiments 1 -3). We conjecture that this illusion may share the same mechanism with the Hess effect, which is characterised by visual delay of a low-contrast, dark stimulus in a moving situation. Since the Hess effect is known as the monocular version of the Pulfrich effect, we examined whether illusory motion in depth could be observed if a high-contrast pattern was projected to one eye and the same pattern of low-contrast was presented to the other eye, and they were binocularly fused and swayed horizontally. Observers then reported illusory motion in depth when the low-contrast pattern was dark, but they did not when it was bright (experiment 4). Possible explanations of this inconsistency are discussed.     

Straightness as the main factor of the hErmann grid illusion

The generally accepted explanation of the Hermann grid illusion is Baumgartner's hypothesis that the illusory effect is generated by the response of retinal ganglion cells with concentric ON-OFF or OFF-ON receptive fields. To challenge this explanation, we have introduced some simple distortions to the grid lines which make the illusion disappear totally, while all preconditions of Baumgartner's hypothesis remain unchanged. To analyse the behaviour of the new versions of the grid, we carried out psychophysical experiments, in which we measured the distortion tolerance: the level of distortion at which the illusion disappears at a given type of distortion for a given subject. Statistical analysis has shown that the distortion tolerance is independent of grid-line width within a wide range, and of the type of distortion, except when one side of each line remains straight. We conclude that the main cause of the Hermann grid illusion is the straightness of the edges of the grid lines, and we propose a theory which explains why the illusory spots occur in the original Hermann grid and why they disappear in curved grids.     

The line-motion illusion can be reversed by motion signals after the line disappears

In the line-motion illusion, a briefly flashed line appears to propagate from the locus of attention, despite being physically presented on the screen all at once. It has been proposed that the illusion reflects low-level visual information processing that occurs faster at the locus of attention (Hikosaka et al 1993 Vision Research 33 1219-1240; Perception 22 517-526). Such an explanation implicitly embeds the assumption that speeding or slowing of neural signals will map directly onto perceptual timing. This 'online' hypothesis presupposes that signals which arrive first are perceived first. However, other evidence suggests that events in a window of time after the disappearance of a visual stimulus can influence the brain's interpretation of that stimulus (Eagleman and Sejnowski 2000 Science 287 2036-2038; 289 1107a; 290 1051a; 2002 Trends in Neuroscience 25 293). If the online hypothesis were true, we should expect that events occurring after the flashing of the line would not change the illusion. Consistent with our hypothesis that awareness is an a posteriori reconstruction, we demonstrate that the perceived direction of illusory line-motion can be reversed by manipulating stimuli after the line has disappeared.     

Stereoillusory motion concomitant with lateral head movements

Stationary objects in a stereogram can appear to move when viewed with lateral head movements. This illusory motion can be explained by the motion-distance invariance hypothesis, which states that illusory motion covaries with perceived depth in accordance with the geometric relationship between the position of the stereo stimuli and the head. We examined two predictions based on the hypothesis. In Experiment 1, illusory motion was studied while varying the magnitude of binocular disparity and the magnitude of lateral head movement, holding viewing distance constant. In Experiment 2, illusory motion was studied while varying binocular disparity and viewing distance, holding magnitude of head movement constant. Ancillary measures of perceived depth, perceived viewing distance, and perceived magnitude of lateral head movement were also obtained. The results from the two experiments show that the extent of illusory motion varies as a function of perceived depth, supporting the motion-distance invariance hypothesis. The results also show that the extent of illusory motion is close to that predicted from the geometry in crossed disparity conditions, whereas it is greater than the predicted motion in uncrossed disparity conditions. Furthermore, predictions based on perceptual variables were no more accurate than predictions based on geometry.     

Motion versus position in the perception of head-centred movement

Abstract. Observers can recover motion with respect to the head during an eye movement by comparing signals encoding retinal motion and the velocity of pursuit. Evidently there is a mismatch between these signals because perceived head-centred motion is not always veridical. One example is the Filehne illusion, in which a stationary object appears to move in the opposite direction to pursuit. Like the motion aftereffect, the phenomenal experience of the Filehne illusion is one in which the stimulus moves but does not seem to go anywhere. This raises problems when measuring the illusion by motion nulling because the more traditional technique confounds perceived motion with changes in perceived position. We devised a new nulling technique using global-motion stimuli that degraded familiar position cues but preserved cues to motion. Stimuli consisted of random-dot patterns comprising signal and noise dots that moved at the same retinal 'base' speed. Noise moved in random directions. In an eye-stationary speed-matching experiment we found noise slowed perceived retinal speed as 'coherence strength' (ie percentage of signal) was reduced. The effect occurred over the two-octave range of base speeds studied and well above direction threshold. When the same stimuli were combined with pursuit, observers were able to null the Filehne illusion by adjusting coherence. A power law relating coherence to retinal base speed fit the data well with a negative exponent. Eye-movement recordings showed that pursuit was quite accurate. We then tested the hypothesis that the stimuli found at the null-points appeared to move at the same retinal speed. Two observers supported the hypothesis, a third partially, and a fourth showed a small linear trend. In addition, the retinal speed found by the traditional Filehne technique was similar to the matches obtained with the global-motion stimuli. The results provide support for the idea that speed is the critical cue in head-centred motion perception.