There is a natural tendency to describe bistable perception as the experience of competing interpretations of a stimulus. This tendency can be partly explained by the many similarities between linguistic interpretation and visual perception. Let’s consider three such similarities in this and the next post.
1. Applicability of a content/vehicle distinction
The first similarity is that a content/vehicle distinction (to be defined shortly) applies to language and to conscious experience. Let's take a look at each case.
Content and vehicle in language
As we discussed previously, to interpret a sentence is to determine what it means. Consider the following sentence:
Rattlesnake bites are poisonous.Imagine if a child asked an adult the meaning of , and the adult responded with ‘It is a four-word sentence written in black 10-point Times New Roman font.’ The child would be right to get annoyed.
What would a more appropriate answer look like? Minimally, interpreting  requires determining what the sentence expresses about the world outside of language. For instance, it tells us that a certain type of snake’s bite will harm humans. Similarly, the sentence ‘Fred got married’ tells you that something happened out in the real world, that some guy named Fred got married. To focus on the font size or color of the writing used to communicate the information would be to miss the point.
This distinction between the physical/structural features of a sentence, and the meaning it expresses, is well-known to everyone. It is often described by philosophers as the distinction between an expression’s content or meaning and the vehicle or medium of expression.
In general, the vehicle-properties of a sentence are different from its content-properties. Its vehicle-properties include physical features of the individual letters (e.g., their shape, properties of the ink, and the material on which they are presented) as well as structural features of the sentence as a whole (e.g., how many words it contains). The content-properties of a sentence, on the other hand, include those extralinguistic facts that the sentence tells you about. For instance,  tells you that rattlesnakes can bite, and that such bites are dangerous to people. When interpreting a sentence, we focus on such content-level properties, what is expressed about the world, and it is usually confused to focus on the vehicle properties.
The content/vehicle distinction also applies to individual words. The word ‘ice’ has certain vehicle-properties (it is three letters long, is written in a certain font, etc), while at the level of content it refers to the solid phase of water, out in the real world. The differences between the vehicle-properties and content-properties of a word are legend. The word ‘monosyllabic’ is not monosyllabic (also consider the word ‘palindrome’). See also Figure 1.
Figure 1: Sample words in which
content and vehicle disagree.
While the content/vehicle distinction isn’t the topic of polite conversation, it is blithely exploited by everyone that uses language. To talk with one another, we must be able to see past the vehicles of communication and respond to the content being expressed. If someone says ‘I got a puppy,’ we respond by asking more questions about their dog such as where they got it; we don’t focus on the vehicle-properties of the sentence. Indeed, if you were to systematically focus on the vehicle-properties of what people said, they would quickly lose all interest in talking to you (which is the fate of those deranged enough to focus incessantly on other people’s grammar or spelling). Such behavior indicates your interpreter is malfunctioning.
Every time we assess the truth or falsity of a declarative statement, we are implicitly examining whether the content of what someone says matches up to reality. For instance, ‘Fred did his math homework’ is true if Fred in fact did his math homework. It is false if he did not. Sentence  is true because it expresses something that indeed holds of rattlesnakes in the real world. We don’t determine if a claim is true or false by measuring the color of the font used to express it or the mean intensity of the sound waves when it was uttered. ‘Whales are fish’ is false not because it is written in Times New Roman font, but because whales aren’t fish.
Even the childhood rhyme ‘Sticks and stones’ plays on the content/vehicle distinction in a fun if oppressive manner. Sure, linguistic vehicles don’t harm your body (billboards notwithstanding), but obviously it’s the content of what is said that inflicts psychic pain when you are insulted.
Content and vehicle in experience
It seems a content/vehicle distinction also applies to conscious experience. While the brain is the organ (i.e., the vehicle) of conscious experience, what we actually experience (i.e., the contents of our experience) doesn’t seem neuronal at all. Indeed, we constantly experience things going on outside of our brains, things like seeing an ice cube out there, three feet in front of us; feeling a sharp pain in our toe, way down at our feet; hearing that song we like on the radio. All the while, the brain doing the experiencing, the vehicle that mediates such experiences, is locked up inside our skull.
These examples indicate that, as in language, the contents and vehicles of consciousness generally have quite different properties. When we experience an ice cube three feet in front of us, we don’t expect someone to find a literal cube of ice in our brains.
Daniel Dennett’s delightful essay Where am I? served to brand into my brain the stark divergence between the contents and vehicles of experience. Therein, Dennett describes how, through the wonders of neuroengineering, his brain was extracted from his skull and kept alive in a tank of cerebrospinal fluid (see Figure 2). Dennett’s brain was connected to his body through various transmitters and receivers (note the router attached to his brain in Figure 2), so his disembrained body could still get about normally. The signals from his optic nerves were transmitted to the brain so he could still see the world. His body could still move around because the motor commands produced by his brain were transmitted to microstimulators in his spinal cord.
Dennett then described what it was like the first time he visited his own brain (Figure 2):
I peered through the glass. There, floating in what looked like ginger ale, was undeniably a human brain, though it was almost covered with printed circuit chips, plastic tubules, electrodes, and other paraphernalia…I thought to myself: “Well, here I am sitting on a folding chair, staring through a piece of plate glass at my own brain . . . But wait,” I said to myself, “shouldn't I have thought, ‘Here I am, suspended in a bubbling fluid, being stared at by my own eyes’?” I tried to think this latter thought. I tried to project it into the tank, offering it hopefully to my brain, but I failed to carry off the exercise with any conviction…[W]hen I thought “Here I am,” where the thought occurred to me was here, outside the vat, where I, Dennett, was standing staring at my brain.Dennett’s thought experiment vividly illustrates the extent to which contents and vehicles of experience can diverge. While his brain is still in a tank at some undisclosed location, Dennett enjoys a rich and varied mental life to this day. Of course, we are in a similar predicament every time we dream.
Figure 2: Where is Dennett?
Just as the content of a sentence involves reference to things in the extralinguistic world, the content of our visual experience involves reference to things outside of our eyes and brains. The visual stimulus, a projection of the scene onto the retinal movie screen, triggers an avalanche of neuronal processes that ultimately produces an experience of what is happening out there beyond the brain.
Using a little poetic license, we can say that once a scene is projected onto the retina, our brain then projects a scene back out into the world. The contents of this outwardly projected scene are the contents of conscious experience.
Caveats and such
It would be contentious to claim that the content/vehicle distinction at play in language is identical to the distinction in experience. They may simply be two species in the same genus. Hence, without a lot more argument, we should be clear to distinguish perceptual content/vehicles and linguistic content/vehicles. Regardless of this caveat, the parallels between linguistic meaning and the contents of experience probably provide the perception-as-interpretation view with a good deal of its traction.
Conceptually, the content/vehicle distinction has probably been around since humans started to think about language. The terminology is relatively new, however, and is likely due to Dan Dennett, as I discussed here.
The obvious question this discussion brings up is, “How a brain can be anything but a vehicle? How can a brain state have content?” Neuroscience has a lot to say about this question, but let’s not get too ahead of ourselves. We are going to revisit the content/vehicle distinction many times, but for now let’s continue delineating the analogies between linguistic interpretation and perception. We'll look at two more in the next post.
Dennett, D (1978) Where am I? Chapter 17 in Brainstorms: Philosophical Essays on Mind and Psychology, Montgomery, VT: Bradford Books
Table of Contents of posts on consciousness.