Top-down influences on ambiguous perception: the role of stable and transient states of the observer

Abstract

The world as it appears to the viewer is the result of a complex process of inference performed by the brain. The validity of this apparently counter-intuitive assertion becomes evident whenever we face noisy, feeble or ambiguous visual stimulation: in these conditions, the state of the observer may play a decisive role in determining what is currently perceived. On this background, ambiguous perception and its amenability to top-down influences can be employed as an empirical paradigm to explore the principles of perception. Here we offer an overview of both classical and recent contributions on how stable and transient states of the observer can impact ambiguous perception. As to the influence of the stable states of the observer, we show that what is currently perceived can be influenced (1) by cognitive and affective aspects, such as meaning, prior knowledge, motivation, and emotional content and (2) by individual differences, such as gender, handedness, genetic inheritance, clinical conditions, and personality traits and by (3) learning and conditioning. As to the impact of transient states of the observer, we outline the effects of (4) attention and (5) voluntary control, which have attracted much empirical work along the history of ambiguous perception. In the huge literature on the topic we trace a difference between the observer's ability to control dominance (i.e., the maintenance of a specific percept in visual awareness) and reversal rate (i.e., the switching between two alternative percepts). Other transient states of the observer that have more recently drawn researchers' attention regard (6) the effects of imagery and visual working memory. (7) Furthermore, we describe the transient effects of prior history of perceptual dominance. (8) Finally, we address the currently available computational models of ambiguous perception and how they can take into account the crucial share played by the state of the observer in perceiving ambiguous displays.

Keywords: ambiguous perception; ambiguous structure-from-motion (SFM); binocular rivalry; bistability; modeling; reversible figures; top-down control.

Figure 1

Examples of ambiguous stimuli. (A) Hollow-face illusion: a concave mask looks like a convex face. (B) It is not possible to state which side of the Necker Cube is facing the observer: perspective-based reversals occur between two possible configurations of the cube. (C) Hill's Wife/Mother-in-law ambiguous figure: the image can be alternatively perceived as a young or as an old lady. (D) Binocular rivalry: two different monocular images are displayed to each eye, causing subjective perception to alternate between the two. (E) Shape from shading: under the implicit assumption that light comes from above in a scene, the shading pattern becomes responsible for the perception of convexity or concavity of a visual object. (F) The ambiguous point-light walker is a particular instance of structure-from-motion (see G): when motion information is added to the display, the dots are perceived as a walker facing toward or away from the observer. (G) Static depiction of an ambiguously rotating structure-from-motion sphere: when motion information is added to the display, the stimulus is seen as a three-dimensional sphere and opposite directions of motion alternate in perception. (H,I) Two instances of ambiguous apparent motion: in the bistable motion quartet (H), single dots displayed in succession at a fixed frame rate induce the impression of vertical or horizontal translation. In (I), rather than perceiving two static dots of different size being flashed at different positions on the screen, the viewer perceives two dots exchanging positions (translation) or two dots expanding and contracting at two separate locations. (J) When two drifting sinusoidal gratings are superimposed, they can be perceived as either a coherently moving plaid pattern or as two semi-transparent gratings moving on top of each other in different directions.

Figure 2

Summary of the timeframes in which a few top-down determinants of ambiguous perception can be observed. Phenomena such as adaptation and attention typically dissipate within a window of a few seconds whereas the effects of inheritable individual differences linger on beyond the lifespan of a single individual. Blue and red lines illustrate, respectively transient and stable influences of the state of the observer on ambiguous perception, violet lines designate effects that occur at an intermediate level. Dashed lines indicate timescales under which the effects are plausibly taking place, but that have not been investigated yet.

Figure 3

Sketch of the generic neuro-computational model.