Wednesday, September 23, 2009

Consciousness (8): From perception to interpretation

Number eight in my series of posts on consciousness. All the posts are indexed here.

The previous two posts were a tour of ambiguous visual stimuli. Let's use these data to generate ideas about consciousness. Ideally, these ideas will lead to prediction-generating hypotheses about consciousness and clarify the explanatory target for neuroscience.

Interpreting Necker
Let's jump-start our thinking with a familiar example: your experience of the Necker Cube (shown on the right). This time, try looking at it with one eye. You should still experience perceptual bistability.

People seem to naturally gravitate toward describing bistable perception as an alternation between two different interpretations of a stimulus. The psychologists that study bistable perception do the same. For instance, Suzuki and Peterson (2000) say:
Bistable displays are displays that afford at least two potential interpretations even though the physical displays remain unchanged. [...] At any given moment, only one interpretation of a bistable display is seen; over time, the two perceptual interpretations spontaneously and stochastically alternate.
How are we to interpret the view that the brain interprets a stimulus? Is it a metaphor? If so, is it useful? Let's start by considering the nature of interpretation more generally, independently of the issue of conscious perception.

What is interpretation?
In general, to interpret something is to determine what it means. We are probably most aware of the need for an interpretation when we encounter difficult bits of writing. What fan of JRR Tolkien hasn't struggled to interpret Bilbo's pronouncement to his fellow Hobbits, "I like less than half of you half as well as you deserve"? What the heck does that mean?

Interpreting a complicated text can be a painstaking process that often requires a good deal of specialized knowledge. People build careers on their ability to interpret confusing legalese, complex poems, or arcane works of philosophy. Some philosophers are infamous for the patience and charity required to construct an intelligible interpretation of their work. For example, the oft-revered philosopher Ludwig Wittgenstein (1922) wrote, 'The thing is independent, in so far as it can occur in all possible circumstances, but this form of independence is a form of connexion with the atomic fact, a form of dependence.' Most readers will probably agree that it is hard to interpret Wittgenstein's sentence, that the meaning is not transparent.

While our need to interpret text is most obvious when we encounter tortured prose, technically speaking we interpret even the clearest expressions. The meaning of the sentence, 'George Washington was the first President of the United States,' is fairly transparent to most Americans. That is, interpreting the sentence is effortless, given our background knowledge. For someone just learning English, or someone with no knowledge of the United States, the sentence's meaning will not be so clear. For some philosophers, the meaning of the above quote from Wittgenstein might seem transparent. Transparency of meaning is not an intrinsic feature of a chunk of text, but depends on the background knowledge we bring to the text.

We have to be careful, as some texts might not mean anything, or if they do it might not be worth the effort to decipher them. Chomsky (1957) produced the famous sentence, 'Colorless green ideas sleep furiously' as an example of grammatically well-formed nonsense. Of course, we could generate grammatically ill-formed nonsense too: 'Gorp dilettante achieve on.' Whether such strings are literally meaningless is an interesting philosophical question that we won't explore. I include this discussion partly to highlight that I have been throwing around the term 'meaning' without defining it, a point we will revisit in the next post.

So far I've focused on interpreting expressions in natural language. However, people also interpret paintings, dreams, medical test results, pretty much anything. Psychologists used to be quite fond of asking people to interpret random smears of ink on sheets of paper (the Rorschach test). While these cases are interesting, to keep the discussion more manageable, in the next post I'll focus on the analogies between perception and interpretation of expressions in natural language.

With this rudimentary understanding of interpretation in hand, in the next post we will consider ways in which perception and interpretation are similar. While I will ultimately eschew thinking of perception as literally identical to interpretation, it is an analogy worth mining for ideas about conscious visual perception.

Chomsky (1957) Syntactic Structures Mouton, The Hague/Paris.

Suzuki and Peterson (2000) Multiplicative effects of intention on the perception of bistable apparent motion, Psychological Science 11: 202–209.

Wittgenstein, L (1922) Tractatus Logico-Philosophicus (Ogden translation) Cosimo Classics.

Table of Contents of posts on consciousness.


Anonymous said...

When I look at various bistable drawings, I find that I like to see them switch at will. This makes my experience of them quite different than the spontaneous and stochastic alternations that are usually described.
I wonder if others experience it more like me or as described.
(It has bearing on some basic propositions about consciousness, no?)


Eric Thomson said...

Sonic: good point that we can (to some degree) exert voluntary control over the percept seen. This is most obvious for me with the spinning girl illusion, which for me literally never spontaneously switches. I have to do all sorts of tricks to get her to spin in the other direction. But even Necker I am able to "push" it toward one or the other perspective.

Another major factors I didn't mention from the literature is eye movements. There was a great deal of controversy about whether all of these bistable phenomena could be explained by fixations and saccades. I didn't include that discussion because even though it seems clear that eye movements can influence things, we also experience bistability even without eye movements (at least for the Necker cube this seems pretty clear).

ben nelson said...

Interesting. Bishop Berkeley was a proponent of the perception = interpretation analogy. Unfortunately, it had the severe handicap of coming from the mouth of Bishop Berkeley.

Joking aside, he is somewhat noted and respected by people who did optics in his day, and it's sort of interesting to read his theory of vision. He might not have come to the right answer, but he did popularize a critique of the Ancient Principle (concerning reflective surfaces).