At first glance, it seems that Magritte has “severed,” or interrupted, the front portion of the horse with what appears to be an opening to the distant background foliage. This leads to the most jarring, surreal element of the painting, because our perception can be drawn to see a single, complete horse completed behind some sort of (impossible?) occluder: empty-space-become-occluder. In these moments, our visual system has been “tricked” into perceiving distant background foliage in the gap between trees no. 1 and 2 as an occluding object (like an opaque, “foliage-patterned ribbon”) in the foreground, in front of the horse's torso paradoxically appearing to occlude part of the horse's reins and the rider's left hand. Above the level of the rider's hand and below the horse's torso, this same spatiochromatic texture is often perceived as merging with the distant background foliage. Six of 15 respondents (40%) saw it this way, but nine (60%) saw it as alternating between background and foreground compatible with the foliage pattern in the region of the horse's torso.
Magritte's construction of this region of the painting engages the “middle-vision” mechanisms that prioritize the parsing of scenes into surfaces of objects (e.g.,
Nakayama et al., 1995). Yet this striking, surreal effect is unstable: 14 of 15 (93%) interviewees perceived the textured pattern between trees no. 1 and 2 as foreground in front of the horse's torso or as alternating between foreground and background, and of those (3/15) 20% clearly saw it as foreground, in front of the horse. The similarity between the spatiochromatic texture in the region of the horse's torso and the regions above and below the horse biases us to see it all as background, which “severs” the horse. However, the amodal completion of the horse, reins, and rider's hand may act to draw our perception back to seeing that same texture as an opaque occluder (like a textured opaque ribbon). The latter details, as well as the disappearance of the end of the riding crop, also may bias our perception toward seeing an (impossible) occlusion by the spatiochromatic foliage texture in the space between trees no. 1 and no. 2. In this way, what would otherwise readily be seen as distant, background foliage converts into a foliage-patterned occluding object, like the surface of an occluding patterned ribbon.
An additional factor that may contribute to a perception of the foliage pattern appearing as an occluding object in the foreground could derive from artists’ “tricks of the trade,” namely that they will tend to paint objects in the foreground as significantly brighter than or darker than the background. However, Magritte appears to deliberately ambiguate this region of the painting by making the foliage pattern in the region of the torso approximately equal in “value” (the artists’ term for brightness) to the background foliage in the distance.
There are several pictorial T-junctions that would tend to bias us to perceive the foliage pattern in the region of the horse's torso as an occluder (
Nakayama & Shimojo, 1992;
Nakayama et al., 1995;
Rubin, 2001). They occur most notably at the right and left edges of the pattern where they exit the horse's body at the top and bottom. However, unlike T-junctions in real occlusions of real objects, or images of objects (real or abstract), these T junctions are unstable: they only act like “ordinary” T junctions during the moments when our perception has “flipped” to see the foliage pattern as an occluding object. In that state, the left and right vertical borders of this region are seen as crisp edges of an (occluding) object, not as open space between objects.
The “victory” of amodal completion, however transient, and its impact on our percept of the foliage in the region of the horse's torso highlights our visual system's prioritization of maintenance of object integrity and assignment of surfaces to objects.
Nakayama et al. (1995) discussed the importance of perceiving surfaces as part of the process of object identification and segregation in the overall process of scene understanding.
The perception of an “occluding” ribbon implies that, in the moments when that percept is experienced, the spatiochromatic texture has perceptually coalesced into a surface, an object with a distinct border. Research on primate cortex supports a working hypothesis that surface texture and border ownership may be coded in mid-level visual pathways (V2, and human analog to primate V4;
Zhou, Friedman, & von der Heydt, 2000;
Pasupathy & Connor, 2001;
Pasupathy, Kim, & Popovkina, 2019).
Area V2 in visual cortex has been proposed to play a role in figure-ground segregation in that it contains cells that are “side-of-figure selective” (e.g.,
Zhou et al., 2000;
Sugihara et al., 2011;
Qui et al., 2005). However, it seems that the perception of this region of the painting cannot be explained simply by such mechanisms. The spatiochromatic texture in this region of the painting can only “own the borders” during the moments when it is perceived as an occluding object, like a textured (opaque) surface. This percept is bi-stable to a greater or lesser degree from viewer to viewer and, across viewing time, for each viewer.
An additional surreal effect may emerge. In principle, the open-space-become-occluder could be subject to the same perceptual effects that caused tree no. 3 to appear somewhat convex. After viewing this painting many times, to my eye, when I perceived the spatiochromatic texture as an “occluding ribbon” in front of the horse, the whole texture-become-occluder between trees 1 and 2 began to appear convex, curved toward me, because just above and below the horse it appeared to be distant background. Fourteen of 15 (93%) interviewees saw this region of the painting closer to them in the horse's torso region some of the time (11/15 saw this region as alternating between foreground and background). Such a percept implies that the whole strip between trees must arc from distant background at the tree bases and near the tops of the trees, passing closer to us in the horse's mid-section.