Friday, January 15, 2021

The Serpentine Illusion: A Visual Motion Illusion Induced by Phase-Shifted Line Gratings

The Serpentine Illusion: A Visual Motion Illusion Induced by Phase-Shifted Line Gratings. Junxiang Luo et al. Front. Neurosci., December 7 2020. https://doi.org/10.3389/fnins.2020.612153

In a pattern of horizontal lines containing ± 45° zigzagging phase-shifted strips, vivid illusory motion is perceived when the pattern is translated up or down at a moderate speed. Two forms of illusory motion are seen: [i] a motion “racing” along the diagonal interface between the strips and [ii] lateral (sideways) motion of the strip sections. We found the relative salience of these two illusory motions to be strongly influenced by the vertical spacing and length of the line gratings, and the period length of the zigzag strips. Both illusory motions are abolished when the abutting strips are interleaved, separated by a gap or when a real line is superimposed at the interface. Illusory motion is also severely weakened when equiluminant colored grating lines are used. Illusory motion perception is fully restored at < 20% luminance contrast. Using adaptation, we find that line-ends alone are insufficient for illusory motion perception, and that both physical carrier motion and line orientation are required. We finally test a classical spatiotemporal energy model of V1 cells that exhibit direction tuning changes that are consistent with the direction of illusory motion. Taking this data together, we constructed a new visual illusion and surmise its origin to interactions of spatial and temporal energy of the lines and line-ends preferentially driving the magnocellular pathway.

General Discussion

In this paper, we describe a visual motion illusion, which we have called the Serpentine Illusion. It is elicited by a pattern of phase-shifted grating strips, abutting each other along a zigzagging interface. When the stimulus pattern is moved up or down, the intersections formed by the offset line gratings are seen to move in an undulating snake-like fashion. In addition to this motion along the diagonals, lateral motion of the sections is also seen. The strength of both illusory motions depends on the stimulus parameters. The illusion is luminance-contrast dependent, suggesting that magnocellular pathway signals have a predominant impact on the Serpentine Illusion. Results from selective adaptation show that both line gratings and physical motion are necessary for the full perception of the illusion, and modeling suggests the illusory motion can partly be explained by linear spatiotemporal receptive fields of motion sensitive V1 cells.

Visual Features Contributing to the Serpentine Illusion

We used an adaptation paradigm to test the contributions of orientation and motion mechanisms. We used moving and flashed grating-lines or grating-endpoints, and random dots, to adapt out the orientation, motion and end-stopping signals driven by the illusion inducing pattern. According to Figure 10, it was the moving line gratings that had the largest effect of weakening the perception of the Serpentine Illusion. To further test the contribution of line gratings for inducing the illusory motions, we introduced a pattern in which the line gratings of the original pattern were occluded by zigzag masks of varying thickness (Supplementary Figure 6). We found that illusory motion was abolished in the pattern with only endpoints visible (Supplementary Figure 6A), and only slightly restored when 1/2 or 1/3 of the grating lines were masked (Supplementary Figures 6B,C); illusory motion was weakened even with a very thin occluding mask (Supplementary Figure 6D). These observations suggest that continuous line gratings are crucial for the generation of both diagonal and lateral illusory motions.

Differences and Similarities Between Lateral and Diagonal Motion

Diagonal motion occurs primarily when the local contrast differences driven by the endpoints follow each other at close range (i.e., high density). This is the case when the line gratings are narrowly spaced, when the distance between the end points of a line is long and when the period length of the zigzags is short. This will make a grating strip look like an undulating column. On the other hand, lateral motion is favored by low density chains of end points as found with widely spaced line gratings, short horizontal distances between pairs of end points, and a long period of zigzags; these features favor a percept of horizontally arranged rows of end points moving sideward together. Apart from the parameters tested in Experiment 2, we further varied the angle of the zigzagging abutting interface. When the angle is changed to ± 30°, illusory motion along the diagonals predominates (Supplementary Figure 7A), whereas when the angle is changed to ± 60°, strong lateral motion is perceived (Supplementary Figure 7B). In the former case, the zigzags emphasize the vertical columnar structure, whereas at more acute angles the columns are less salient and the horizontal structure predominates. These parametric conditions for seeing diagonal and lateral illusory motion produce groupings that are consistent with the Gestalt principles of proximity and common fate. Whether these two illusory motion patterns drive the same or different underlying neural mechanisms remains unknown. The fact that either of the motion patterns can be almost eliminated when optimizing for the other condition (Supplementary Figures 24), suggests the neural mechanisms may be dissociable. Future studies will need to systemically explore the neural origins of both illusory motions using theoretical, psychophysical and physiological methods.

Illusory Motions Are Luminance-Contrast Dependent

Motion and color signals were classically thought to be encoded differentially (Ramachandran and Gregory, 1978Zeki, 1978Livingstone and Hubel, 1988). This parallel division receives some support from psychophysical studies in which chromatic gratings without luminance contrast can effectively weaken the ability of a subject to discriminate motion direction/speed (Cavanagh et al., 1984Troscianko and Fahle, 1988Cavanagh and Anstis, 1991Kooi and Devalois, 1992Mullen and Boulton, 1992abHenning and Derrington, 1994). Other studies, however, show that equiluminant color contrast can also provide weak cues for motion perception (Cavanagh and Favreau, 1985Saito et al., 1989Hawken et al., 1994Gegenfurtner and Hawken, 19951996a,bBurr et al., 1998Dougherty et al., 1999Lu et al., 1999Yoshizawa et al., 2000Willis and Anderson, 2002Cropper and Wuerger, 2005Burton and McKeefry, 2007). Cortical areas like MT (Saito et al., 1989Seidemann et al., 1999Thiele et al., 1999Wandell et al., 1999Barberini et al., 2005) and V3A (McKeefry et al., 2010) are able to encode motion signals derived from chromatically defined stimuli. In addition it is well known that area V4 encodes motion information and contains mixed parvocellular and magnocellular inputs (Desimone and Schein, 1987Mountcastle et al., 1987Ferrera and Maunsell, 2005Tolias et al., 2005Mysore et al., 2008An et al., 2012Li et al., 2013Yin et al., 2015Birman and Gardner, 2018). This physiological and anatomical data is consistent with the psychophysical data suggesting physical motion signals are processed through mixed pathways (Willis and Anderson, 2002Takeuchi et al., 2003McKeefry and Burton, 2009). Although the exact balance of interactions across form, color and motion signaling circuits is still a matter of some debate, there is nevertheless psychophysical evidence showing that motion illusions are minimized when presented under chromatic equiluminant conditions (Khang and Essock, 1997Hamburger, 2012).

The original Serpentine Illusion stimulus pattern has high luminance contrast between line gratings and background. When we reduced the contrast to physical equiluminance, illusory motion was greatly weakened (Figure 7). This suggests that luminance contrast is an important factor in generating the Serpentine Illusion, an observation consistent with several other motion illusions, which are found to be luminance-contrast dependent (Hamburger, 2012). Examples include the Pinna-Brelstaff illusion (Pinna and Brelstaff, 2000), the Rotating Snakes illusion (Kitaoka and Ashida, 2003), the Rotating-Tilted-Lines illusion (Gori and Hamburger, 2006), the Boogie-Woogie illusion (Cavanagh and Anstis, 2002), and the Dotted Lines illusion (Ito et al., 2009). This indicates that unlike physical motion, illusory motions in the Serpentine and other motion illusions are largely mediated by the magnocellular pathway. Additionally, previous psychophysical studies found that subjects underestimate the speed of moving gratings with relatively low luminance contrast (Campbell and Maffei, 1981Thompson, 1982Stone and Thompson, 1992Blakemore and Snowden, 1999Johnston et al., 1999Anstis, 2003). Analogously, the saliency of illusory motion can also be controlled through changing the strength of luminance contrast in the physical stimulus (Cavanagh and Anstis, 2002Anstis, 2004Backus and Oruc, 2005Howe et al., 2006Hamburger, 2012). It has been hypothesized that a moderate luminance contrast is the main factor in generating some motion illusions (Foster and Altschuler, 2001Cavanagh and Anstis, 2002Conway et al., 2005Ito et al., 2009Hamburger, 2012). However, for the Serpentine Illusion, only the extreme low luminance contrast (10% in Figure 8) can reduce the strength of the perceptual illusory motions.

The observations in Experiment 2 point toward a role for local contrast differences between the abutting line endings. Each pair of juxtaposed, phase-shifted line ends produces a dark patch that stands out when the stimulus pattern is moved up or down. One possible explanation of the illusory motion seen is due to the larger contrast of the dark patch relative to the grating strips. It is known that high-contrast gratings appear to move faster than low-contrast gratings (Thompson, 1982Stone and Thompson, 1992) and this might explain the faster rate at which the dark patches are seen to be moving relative to the flanking line gratings.

Alternatives to the Energy Model

Without directly recording from the nervous system, theoretical models are one of the best ways to infer neural mechanisms that underlie visual perceptual phenomenon. Here, we used the classical motion energy model (Adelson and Bergen, 1985) to show that primary visual cortex neurons respond with directional biases consistent with perception to the illusory motions in Serpentine Illusion stimuli. The motion energy model is a simplified linear summation model (Baker and Issa, 2005Mante and Carandini, 2005), best at predicting response properties in the earliest motion processing stages (Reid et al., 1987Carandini et al., 1997). Complex cells in primary visual cortex and neurons in downstream visual areas like MT and MST have more non-linear response properties (Emerson et al., 1992Simoncelli and Heeger, 1998Pack et al., 2006), and they contribute significantly to the neural representation and perception of illusory motions (Luo et al., 2019). Future studies should explore compare both linear and non-linear coding components and hierarchical population responses (mirroring the hierarchical spatiotemporal integration of motion information), as these may better predict the cortical responses to Serpentine and other illusory motion patterns.

Similarities and Differences Between Serpentine and Other Visual Illusions

The diagonal illusory motion along the zigzagging contour of the interface is reminiscent of the apparent motion in other two well-known motion illusions: the Boogie-Woogie illusion (Cavanagh and Anstis, 2002) and the dotted lines illusion (Ito et al., 2009). The Boogie-Woogie illusion is constructed by a grid of horizontal and vertical bars made up from alternating dark and bright squares placed on a gray background. When this grid is moved diagonally from the lower left to the upper right, the small squares making up the bars appear to “race” up the verticals and to the right along the horizontals. The authors attribute their illusion to different strengths of the motion signals elicited by the vertical and horizontal bars. Apparent upward motion resulting from the alternating light and dark edges of the squares moving “along” the bar (first-order motion) is said to constitute a more efficient motion signal than the “across” motion by the textured edge of the horizontal bar which therefore would appear to move more slowly and be overtaken by the vertically moving squares. The dotted lines illusion contains obliquely aligned white and black dots on a median gray background. Horizontal movement of the patten produce relative motion along the row of dots. The authors suggest that illusory motion is produced by the stronger luminance contrast between adjacent dots along the length of the line, compared to with the luminance difference between dotted lines and background.

Interestingly, another illusory motion effect can be perceived when the Serpentine Illusion picture is moved horizontally along the abutting line gratings (or by tracking the cursor as it moves leftward or rightward). The tilted zigzag illusory contours appears to swell out- or inward to each other. Such illusory motions are inconsistent with the aperture effect (Nakayama and Silverman, 1988), and appear related to the dotted-line illusion. Despite similarities, the lateral motion is not observed in either Boogie-Woogie illusion or dotted lines illusion. Moreover, the boogie-woogie illusion elicits the strongest motion at low contrast while the Serpentine Illusion becomes invisible at low contrast. Compared with the Boogie-Woogie and dotted lines illusion, the Serpentine Illusion does not contain luminance contrast along line gratings. Nevertheless, we cannot rule out the possibility that these three illusions may share similar underlying coding mechanisms, since luminance contrast is critical for producing illusory motion in all three patterns, and under equiluminant conditions, illusory motion is largely eliminated in all three patterns. Further psychophysical and physiological experiments are therefore needed to reveal the neuronal mechanisms underlying the Serpentine Illusion. 

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