Tuesday, March 16, 2010

Consciousness (10): Contents and Vehicles

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

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:
[1]Rattlesnake bites are poisonous.
Imagine if a child asked an adult the meaning of [1], 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 [1] 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, [1] 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 [1] 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.

Sunday, March 07, 2010

Consciousness (9): From texts to the grotesque cinema

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

We are examining the popular view that visual perception is a form of interpretation, specifically the interpretation of a stimulus. We should start by determining what, exactly, is a stimulus?

So as not to keep you waiting, the answer is roughly that a visual stimulus is visible light that is projected from the world to the retina. If that is unclear, or if you are interested in the biology, then keep reading.

In general, a stimulus is anything that can activate our sensory transducers, the cells that convert external signals into internal electrochemical signals that can be used by the rest of the nervous system. In the case of vision the transducers are in the eye, so let’s consider the general relationship between the world, the eye, and visual stimuli.

Our world is filled with objects that project light into our eyes. Let’s denote the set of such objects at a given time the scene. For example, the eye in Figure 1 is inspecting a scene that consists of two objects: a cube and a house. The eye seems to be looking directly at the cube, while the house is up to the left of the hovering orb.

Figure 1: An eye viewing a scene.
The light from the scene is projected to the retina by the eye's lens (Figure 2, top). The retina is a thin red sheet of tissue that coats the inside of the eye, biology’s grotesque movie screen.

Photoreceptors are probably the most important cells in the retina: they convert light into chemical signals that the rest of the nervous system can understand. However, the retina is much more than a sheet of photoreceptors. It is an extremely complicated neuronal processor in its own right. The retina contains multiple layers of neurons (Figure 2, bottom), and it is only the axons of the final layer of neurons that make their way through the optic nerve toward the brain.

Figure 2: Gross anatomy of the eye (top), and
cross-section of the retina (bottom).
The retina includes a special region, the fovea, which has an extremely high density of photoreceptors. When we look about the world, we typically direct our eye so the fovea aims at the most interesting parts of the scene, and this lets us take in more information from those regions. It’s like having a video camera with extremely high definition in the middle of the screen, but as you move away from the middle the picture gets quite fuzzy. Note that in Figure 1, the retina is painted onto the back of the eye in red, but the high-def fovea is represented by a small yellow region in the center of the retina.

Imagine gently peeling the retina from the inside of the eye and laying it flat on the page. Figure 3 is a graphical depiction of such a flattened retina, drawn so that the spacing of the grid lines represents the density of photoreceptors. The fovea is in the center of the graph, with finely-spaced grid lines that tell us we are in the high-def region. The density of photoreceptors quickly decreases as you move away from the fovea, as indicated by the more coarsely-spaced lines.

Figure 3: The retina from a functional
point of view.
You can directly experience this drop in resolution by trying to read this text while looking off to the side of the page. Even though you can still make out the coarse features of the page (e.g., there are words and pictures), it is extremely difficult to discriminate the finer-grained spatial properties such as individual letters and words.

There is one more notable feature of Figure 3. Namely, the top half of the retina is labeled ‘Bottom’ while the bottom half is labeled ‘Top.’ ‘Left’ and ‘Right’ also seem to be reversed. These labels are not mistakes, but highlight that an inverted image of the scene projects to the retina. For instance, an object located above the eye’s fixation point, such as the house in Figure 1, is projected to the bottom half of the retina.

I included the geometrical rationale for such image inversion in Figure 1 above. The light from the house travels from above, down to the eye, and the rays of light continue traveling down within the eye until they hit the lower half of the retina. For similar reasons, objects to the left of fixation project to the right-hand side of the retina.

All of this inversion business is illustrated in Figure 4, which shows the house/cube scene projected onto the retina. As we would expect, the fixated cube-face is projected squarely to the fovea. The image of the house, which arrives from the top-left part of the world, is projected to the lower-right quadrant of the retina. That is, the house projects to the quadrant labeled ‘Top Left,’ so-called because the image is projected from the top-left hand region of the world. Note that even the image of the house is inverted, so an upside-down house is projected onto the retina.

Figure 4: The scene from Figure 1 projected
onto the retinal graph from Figure 3. Note how
the projections are inverted.
Finally I think we have arrived at a respectable-enough understanding of visual stimuli. A visual stimulus consists of the images projected from the scene onto the retina. We should stipulate that only light that is able to activate the retina’s photoreceptors counts as a visual stimulus. Not all electromagnetic radiation (i.e., light) can activate photoreceptors. Photoreceptors are tiny light detectors that are sensitive to light within a certain narrow band of wavelengths, as shown in Figure 5. So, absorbing this wrinkle into our account of visual stimuli, and using the plural of ‘retina’ because we have two eyes, yields:
A visual stimulus is light, within the visible part of the electromagnetic spectrum, that is projected from the scene to the retinae.
So, with that definition, I think we have a respectable characterization of visual stimuli.

There is one practical detail I should add before closing this discussion. Because vision is a distal sense that involves the perception of things far away from our bodies, in practice there is quite a bit of flexibility in what researchers count as a stimulus. They will sometimes identify the stimulus as the object itself out in the world (e.g., the Necker Cube drawing), the light emitted from the object, or the light from the object that hits the surface of the eye. In practice the things that researchers count as visual stimuli depend on the question being asked and the experimental setup. In the future I may call any of the above ‘stimuli’ when it seems appropriate, though typically I use the word to refer to the image projected to the retina.

Figure 5: The electromagnetic spectrum,
with the visible spectrum as a subset.

Table of Contents of posts on consciousness.