What do you see in
Figure 1: An old or a young woman? Prolonged inspection suggests both—but not simultaneously. When we observe an ambiguous figure, like the “Old/Young Woman” (Boring,
1930), our percept is unstable, changing spontaneously between two or more possible interpretations. The information that enters via our eyes is very often ambiguous, not least due to the projection of the 3D world onto our 2D retinae. Visual perception can thus be viewed as an incessant attempt to solve the visual “inverse problem” of how several perceptual interpretations can result from one and the same retinal image. Given this uncertainty, how is the brain at all able to construct a stable percept of the world, reliable enough for us to act successfully in the world and survive in the long run? There is a general consensus regarding the earliest steps of visual perception, such as the visual areas' retinotopic and columnar organization (Holmes & Lister,
1916; Schira, Wade, & Tyler,
2007), but a sizable gap remains if we approach object representation and consciousness. It is vividly discussed whether an object is represented by a small sample of grandmother-like neurons (e.g., Barlow,
1972; Quiroga, Reddy, Kreiman, Koch, & Fried,
2005) or by population codes of distributed neural assemblies (Pouget, Dayan, & Zemel,
2000). Ambiguous figures, like Boring's Old/Young Woman (Figure 1, Boring,
1930) or the Necker cube (Figure 2b, Necker,
1832) are found in the chapters on conscious perception of nearly every neuroscience textbook. Many authors regard those figures as key stimuli with which to address the topics of object representation and conscious perception, since they enable us to separate neural activity related to the visual input from activity related to the perceptual outcome (e.g., Blake & Logothetis,
2002).