Thursday, April 01, 2010

Consciousness (11): Ambiguity and Context in Perception and Language

Eleventh in my series of posts on consciousness. Table of Contents is here.

In this sequel to the previous post, we continue to examine the similarities between linguistic interpretation and perception, and explore the hypothesis that perception is stimulus interpretation.

2. Bistability
Just as we can experience bistable visual percepts, sentences can exhibit a “semantic bistability” in which there exist two equally reasonable interpretations. For example:
[1] Adam asked him to use the bathroom.
What is the meaning of [1]? Did Adam request that someone else go to the bathroom, or did Adam wish to use the bathroom?
[2] I saw the man with the telescope.
Who has the telescope, me or the man? There isn't enough information in sentences [1] or [2] to determine which interpretation is correct.

Similarly, ambiguous visual stimuli (discussed here and here) such as the Necker Cube don’t provide enough disambiguating information for our visual system to settle on one perceptual interpretation. Koch (2004) says:
Consider the twelve lines making up the Necker Cube. Due to the inherent ambiguity of inferring its three-dimensional shape from a two-dimensional drawing, the lines of the cube can be interpreted two ways, differing only in their orientation in space. Without perspective and shading cues, you are as likely to see one as the other. The physical stimulus--the line drawing--doesn't change, yet conscious perception flips back and forth between these two interpretations, in what is a paradigmatic example of a bistable percept.
Koch is claiming that because the visual stimulus does not adequately disambiguate the source object (i.e., the line drawing is ambiguous), our conscious experience flips back and forth between two plausible interpretations of the image.

Both types of bistability (i.e., linguistic and stimulus) can easily lead to errors. For instance, someone may understandably interpret sentence [2] to mean you were spying on someone, when in fact you really meant to say you innocently observed a guy that was carrying a telescope around. In the case of vision, if you see an actual wire frame of a cube (a real-life Necker Cube) your visual system will lock in on one or the other percept with about equal probability, and half the time it will be wrong.

3. Context dependence
The context in which a sentence is uttered can have strong effects on how we will interpret it. For example, in a conversation about money, you will likely interpret ‘I went to the bank’ differently than in a conversation about a canoe trip down a river.

Likewise, the visual experience produced by a localized stimulus depends strongly on the context of concurrent surrounding stimuli. For example, the top panels of Figure 1 show two cubes that are tiled with colored squares (a Rubik’s Cube sort of situation). The cubes seem to be placed in different sources of illumination: the cube on the left seems to be in yellow light, and the right cube in blue light. Remarkably, the blue tiles on top of the left cube are identical to the yellow tiles on top of the right cube. If you look at them in isolation, these tiles actually appear gray, as shown in the bottom half of Figure 1. Changing the visual context of these gray squares radically changes how we experience them.

Figure 1: Context dramatically affects color perception.

Another example of contextual influences on visual perception is shown in Figure 2, which depicts one monster chasing another through a sewer. While the monster chasing the other looks appreciably bigger, the two monsters are actually identical copies of one another, and project the same image to the retina. The context of the receding tunnel makes the monster on top seem much further away, and our brain somehow magnifies its apparent size based on such depth cues.

Figure 2: Big meanie chasing terrified rascal.

These examples demonstrate that our experience of something at a localized region of space is based partly on what is happening at other locations in our visual field. Our brain is quite sensitive to contextual factors in its construction of a percept.

In the next post we will finish this foray comparing perception and interpretation.

Sources of examples
The analogy between perceptual bistability and sentence ambiguity has been noted before. Norvig (1988) points to Hockett (1954) as the first. Hockett wrote, "The hearer, confronted with The old men and women stayed at home, is in much the same position as the observer who sees a picture of a hollow cube and can, almost at will, see first one corner and then another as closer to him." The sentence mentioned by Hockett can either mean that the old men and old women stayed, or that the women and the old men stayed.

Sentence [1] is from John Limber (personal communication), whose article Syntax and sentence interpretation (1976) has clear influences on this post. Sentence [2] seems to be ubiquitous in the linguistics literature, and its origins are opaque to me. I saw a reference to it in a 1961 memorandum from the RAND corporation, and I am trying to track it down. Figure 1 is from Shepard (1991). Figure 2 is from Purves and Lotto (2003) (there are many exceptional illusions at Purves' web site).

Hockett, CF (1954) Two models of grammatical description, Word, 386-399.

Koch, C (2004) The Quest for Consciousness: A neurobiological approach Roberts & Company Publishers.

Limber, J. (1976) Syntax and sentence interpretation, In R. Wales & E. C. T. Walker (Eds.), New
approaches to language mechanisms
(pp. 151-181). Amsterdam: North Holland.

Norvig, P (1988) Multiple Simultaneous Interpretations of Ambiguous Sentences. Proceedings of the 10th Annual Conference of the Cognitive Science Society.

Purves, DP, and Lotto, RB (2003) Why we see what we do: An empirical theory of vision Sinauer Associates.

Shepard, RN (1991) Mind Sights, W.H.Freeman & Co Ltd.

Table of Contents of posts on consciousness.

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