Consciousness (12): What the brain thinks it knows

Number twelve in my series of posts on consciousness. Table of Contents is here.
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In this continuation of the previous two posts, let’s finish examining the parallels between linguistic interpretation and visual perception. Recall the first three similarities were: 1) applicability of the content/vehicle distinction, 2) the possibility of ambiguity, and 3) sensitivity to context. In this post we'll add one final point to the list.

4. Use of background knowledge/assumptions
As we discussed previously (post eight), interpreting a sentence draws on your background knowledge about the world. A sentence whose meaning is transparent to you (e.g., ‘George Washington was the first President of the US’) will be perplexing to someone who doesn’t share your knowledge of American history.

For another example, if someone were to utter ‘John was told what to do by the river,’ we (often implicitly) use our knowledge that rivers cannot talk to home in on the correct interpretation. If we were to replace 'river' with 'teacher' we would not be able to use our knowledge to determine the correct interpretation.

Even fallible assumptions about the structure of the world can bias us toward interpreting a sentence a certain way. For instance, we can modify a semantically "bistable" sentence from post eleven ('I saw the man with the telescope') in ways that will make folks much more likely to settle on one of the interpretations:

[1] The astronomer stared at the girls with his telescope.

While [1] is technically still ambiguous, the modifications make us much more likely to interpret the sentence as describing someone using his telescope to spy on some girls.

Our brain also seems to use background knowledge and assumptions about the world when generating its perceptual response to a stimulus. As an example, let’s consider the brain’s apparent knowledge of perspective.

Perspective refers to the depth cues that result when a scene is projected onto a flat sheet located at a particular location (such as a retina, as described in post nine). One such cue is linear perspective, which refers to the fact that a projection of two parallel lines that recede into the distance will converge to a common point rather than remain parallel. This can be seen in any stock image of railroad tracks, as in Figure 1. While we know the tracks are actually parallel to one another, when they project to a flat surface (such as a retina or the plate of sensors in your digital camera), they appear to converge.

Figure 1: Railroad tracks illustrating linear perspective.


The brain seems to "understand" linear perspective, as evidenced by the powerful experience of depth that results when artists recreate its effects on a two-dimensional sheet of paper or canvas. There are artists that, armed with nothing more than wits and a few pieces of chalk, are able to create a vivid impression of a rich three-dimensional scene on a sidewalk (Figure 2) or even an entire street (Figure 3). You can see the convergence of lines in these works, the use of linear perspective that helps create a striking impression of depth.

Figure 2: Taking the Plunge, a sidewalk chalk drawing by Julian Beever.


Figure 3:Lava Burst, as painted on the streets of Gelden, Germany.


Because our brain evolved partly to help us navigate three-dimensional terrain, it isn’t too surprising that it is so keyed in to such cues. Even when we know at a cognitive level that we are looking at a two-dimensional surface, our brains reflexively construct an experience of depth. The great psychologist Gordon Shepard, in his inimitable style, explains things as follows:

[W]e cannot choose to see a drawing merely as what it is—a pattern of lines on a flat, two dimensional surface. To the extent that that pattern of lines conforms with the rules of linear perspective, for example, that pattern automatically triggers the circuits in the brain that make the three-dimensional interpretation appropriate to such a perspective display. Any consciously adopted intentions to ignore such an interpretation are largely powerless against the swift deliverances of this underlying machinery. This should not surprise us. We have inherited this machinery from individuals who, long before the advent of picture making, interpreted—by virtue of this machinery--what was going on in the three-dimensional world around them with sufficient efficiency to survive and to continue our ancestral line.

In the normal world of perception in a three-dimensional world, such perspective cues are quite reliable indicators of the spatial structure of the world. Clever artists (including 3D cinematographers) and psychologists merely exploit the neuronal circuits which unquestionably believe such cues even in ethologically peculiar contexts.

Let’s finish by looking at a couple of rather stunning visual illusions psychologists have created to exploit the brain’s knowledge of perspective. First, consider the two tables in Figure 4. The tops of the two tables are actually the exact same size! If you were to rotate the left table top by 90 degrees and move it to the right, the two tabletops would overlap perfectly. Especially surprising is that the long edge of the table on the left (the edge that appears to be receding away from the viewer) is the same length as the front edge of the table on the right (the edge roughly parallel to the viewer).

Figure 4: Turning the tables.


It’s as if an understanding of the physics of projection is hard-wired into our visual system, and our brain automatically makes adjustments when it "thinks" an object’s size has been contracted because of viewing angle. Shepard says, "The fact that the retinal images of the two quadrilaterals interpreted as table tops are identical in length then implies that the real length of the table going back in depth must be greater than the real length of the crosswise table." In other words, because the table on the left is (apparently) receding into the distance, it must actually be longer than the table on the right, which runs parallel to the viewer.

Such compensation by our brain for such perspective-dependent distortions can also explain why an object such as a coin appears circular even when presented at an angle so it actually projects an oval to the retina.

A related illusion is shown in Figure 5, which appears to be a sort of display case for four pieces of plumbing. Each piece consists of two tubes connected in the middle by a ball. While the angles between the tubes looks quite different, they are actually all identical, as demonstrated in the bottom panel of the figure.

Figure 5: Purves’ plumbing.

Figure 5 contains strong cues about the spatial arrangement of the pieces relative to the viewer. For instance, the tubes in the red structure span from the back of the scene to the front, which would only be possible if the two tubes subtended an obtuse angle. While the structure happens to be projecting a right angle to the retina, this is because of its contingent orientation with respect to the viewer. Similarly, the angle between the tubes in the green piece clearly seems to be acute, even though it projects an identical 90 degree angle to our retina.

Again we find that the brain seems to compensate for the distorting filter of perspective, generating an experience to conform more closely with the actual angle than the stimulus (i.e., the angle projected to the retina).

Where next?
That's the last parallel I'll explore between language and perception. With that, we are ready to more closely evaluate the hypothesis that perception is a form of stimulus interpretation. We'll do that in the next post.

Sources of examples
‘John was told what to do by the river’ is from Norvig (1988), an article which also inspired sentence [1]. 'Taking the Plunge' was created by Julian Beever (http://users.skynet.be/J.Beever/pave.htm). The street carnage 'Lava Burst' was drawn by Edgar Mueller (http://www.metanamorph.com/). 'Turning the Tables' is from Shepard (1991). Purves' Plumbing is from Purves and Lotto (2003).

References
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.

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Table of Contents of posts on consciousness.

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

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

References
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.

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Table of Contents of posts on consciousness.

Consciousness (10): Contents and Vehicles

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

References
Dennett, D (1978) Where am I? Chapter 17 in Brainstorms: Philosophical Essays on Mind and Psychology, Montgomery, VT: Bradford Books


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Table of Contents of posts on consciousness.

Consciousness (9): From texts to the grotesque cinema

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


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Table of Contents of posts on consciousness.

I'm still stalking Mr B

I haven't forgotten that I owe further posts in the consciousness series. I've been working at them, but have so far held off on posting until their organization was clear in my mind.

The material is finally coming together in a way I like. Upshot? I have at least six posts in the queue, but am still organizing them and polishing them. I expect to have the first out next week and then they will come fairly quickly.

Admittedly, the material is exploding in my hands, so I plan to turn this entire project of stalking Mr B into a book proposal. Frankly, though, his name may need to be changed, as there is almost as much psychology as biology in the stuff I'm writing.

Film of recent meeting about reviewer comments

Bistable Anatomy

In the rat thalamus, each whisker is represented by a chunk of tissue known as a 'barreloid'. The following image is Figure 12c from Haidarliu and Ahissar (2001) Size Gradients of Barreloids in the Rat Thalamus. It is a drawing of a large chunk of a rat's brain, with the location of the barreloids indicated by the cuboid shape:


Where, exactly, are the barreloids? Perceptually the cube is bistable, a Necker cube the solid lines either representing the cube's front face or back face. It could drive an anatomist crazy! (Note: 'L' stands for 'lateral' and 'R' stands for 'rostral').

While the figure is perceptually ambiguous, it is clear from the paper that they follow the convention that solid lines are to be interpreted as in the front. Also, based on an informal poll of people in my lab, it seems most people lock in on the "correct" perceptual interpretation initially.

Barrel Cortex Overview

The barrel cortex roughly corresponds to the primary somatosensory cortex of rodents. It has become a standard model system for the study of cortical structure, function, and development. It's the system where I spend most of my life as a postdoc.

Fox's book Barrel Cortex is the best overview that I have seen. It is unfortunately much too costly for out-of-pocket purchase (140 bucks), but your library may have a copy.

Consciousness (8): From perception to interpretation

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

References
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.

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Table of Contents of posts on consciousness.

Consciousness (7): More Ambiguous Figures

The seventh in my series of posts on consciousness. All the posts are indexed here.
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In this post we'll finish the tour, started in the previous post, of ambiguous figures.

Motion
Some of the most compelling illusions include things that move. Indeed, every time we watch a movie we succumb to the illusion of apparent motion. As we saw in the previous post, a rotating Necker Cube evokes vivid bistability. The present group of ambiguous figures include moving parts that are essential for the illusion.

Ambiguous structure from motion
The following video looks like a cylinder rotating either clockwise or counterclockwise (its direction is bistable). There is no cylinder drawn in the video, just a bunch of randomly placed spots. The spots' motion is set to match the velocity they would have if painted on the surface of a cylinder, and this motion signal alone is enough to give the impression of a particular shape.



It sometimes takes more than 30 seconds for the percept to switch, so you might need to watch the movie more than once. I am able to make it reverse faster by rotating my finger around the bottom of the imaginary cylinder as if I were pushing it in a new direction.

Bistable See-Saw
In the following animation, the barbell-shaped object should look like it is flipping back and forth like a see-saw. Sometimes the see-saw crosses through the horizontal axis, and other times the vertical axis. You might even see the barbell rotating 'round and 'round in a circle, though in my experience this is rare.



I lock in fairly strongly to the horizontal see-saw, but when I cover up the bottom half of the image for a few seconds, this brings out the other percept.


One Plaid or Two Gratings?
Stare at the black dot in the following animation. While initially you probably see a plaid pattern moving upward, you will eventually see two translucent sinusoidal patterns (often described as 'gratings') sliding past one another. It took me almost 30 seconds the first time before the percept switched, so stick with it.





The Spinning Girl
One of my favorite illusions. This beautiful ballerina was created by Nobuyuki Kayahara. In which direction is the ballerina doing her pirouette? Most people see her rotating clockwise initially, but the stimulus is actually ambiguous, so you can also see her rotating counterclockwise.




If you have trouble getting her to switch directions, cover her body and look only at the shadows at the bottom of the image. With the number of cues reduced, you should be able to see the shadow change direction. Once that happens, slowly lift your hand while maintaining the new direction of rotation to reveal the new pirouette direction.

This illusion has been misinterpreted as a test of handedness, or a test of whether you are right-brained or left-brained. There is no evidence for these claims, and I'm not sure where the rumors originated.


Binocular rivalry
Binocular rivalry has been a workhorse for the study of consciousness. This is partly because, in addition to the extensive psychological studies of binocular rivalry, neuroscientists have locked onto rivalry as a model for the study of the neural basis of consciousness. While we'll look more deeply at rivalry in future posts, for now we'll treat it as just another cool bistable percept.

To experience rivalry in the following image, put a piece of paper perpendicular to the screen between the two images, so your left eye sees the face and your right eye sees the house (your face should be about six inches from the screen). Be sure to fuse the checkered circles in the center of each figure. Once you obtain fusion, hold it for a while and you will experience rivalry.


Most people do not see a simple fusion of the house and face, but rather the patterns alternate. For instance, you might see the house for a few seconds, and then the face will dominate for a while, and so on. That is binocular rivalry. During transitions, the new percept will spread across the old in a kind of traveling wave, in which case you might see a dynamic quilt-like pattern.


Ambiguous forms
In this class of ambiguous figures, perception alternates between often drastically different types of objects (e.g., face and vase). These illusions are probably better known than all of the others. They are used in advertisements and art, and there are so many on the internet that I can only show a tiny sample. I won't say much about them, as the titles suggest what the two objects are supposed to be, and most of them aren't very difficult to see.

Vase versus Face
The old standby in every introductory psychology textbook.



Duck versus Rabbit
Another classic. I like the following version (from Torrey (1970)) because the two interpretations seem equally likely.



Wife/Mother-in-law and Husband/Father-in-law
On the top is a beautiful young socialite and a nasty witch-like banshee. Below is a handsome gadabout and a wretchedly distasteful lecher.



Chef versus Dog
Tilt your head to the left to see the dog, and to the right to see the goofy French chef.



Nude woman versus Reagan face



Kissing Jesters
It alternates between a single jester facing you, and two jesters facing each other, their lips lightly touching.

Gypsy versus Narcissist
The top of the image shows the ambiguous version, while the bottom shows disambiguated versions (gypsy on the left and narcissistic woman looking into the mirror on the right).

Man's face or Woman Reading?
Our last figure. I would be remiss, in a tour of ambiguous forms, if I didn't pay homage to the great surrealist Salvador Dali. His paintings are filled with beautiful and sometimes hauntingly plastic forms. The following painting, 'The Image Disappears,' was painted by Dali in 1938.


It seems somehow appropriate to let Salvador Dali be the last stop in our tour of ambiguous figures. If you have any favorites that I haven't included, please let me know in the comments or via email.

Where we are headed
While ambiguous images are intrinsically cool, they also provide a window into the nature of visual consciousness. Based on these illusions, in the next post I'll make some general hypotheses about the nature of (visual) perception. These hypotheses will give us a target for the neuronal data, to which we will then turn.


Sources of Illusions
The structure-from-motion demo is supplementary material in Krug et al. (2008). The Bistable See-Saw is adapted from the ambiguous quartet illusion, which was described by Ramachandran and Antsis (1985) (a tactile version is described in Carter et al. (2008)). The plaid/grating illusion is from Stoner et al (1990). The Spinning Girl was created by Nobuyuki Kayahara, who works in digital design. The house-face image used for binocular rivalry is from Tong et al. (1998). Vase/face goes back to Rubin (1915), but the one here is from Fischer (1967). The duck-rabbit was published originally by Jastrow (1899), but the one here is from Torrey (1970). The mother-in-law/wife image was originally published by Hill (1915), and the husband/father-in-law was originally published in Botwinick (1961). Kissing Jesters is from Fisher (1967). Chef/Dog is from Wallach and Austin (1954). Nude/Reagan is from Fisher (1968), a paper that shows 30 ambiguous forms from the history of psychology. Gypsy/Narcissist is from Fisher (1967).


References
Botwinick (1961) Husband and father-in-law: A reversible figure. American Journal of Psychology, 74: 312-313.

Carter, O, Konkle, T, Wang, Q, Hayward, V, and Moore C (2008) Tactile Rivalry Demonstrated with an Ambiguous Apparent-Motion Quartet. Current Biology 18: 1050-1054.

Fisher, G (1967), Measuring Ambiguity, American Journal of Psychology 80: 541-557.

Fischer, G (1968) Ambiguity of form: Old and new. Perception and Psychophysics 4: 189-192.

Hill, We (1915) My wife and my mother-in-law. Puck November 6.

Jastrow, J. (1899) The Mind's Eye. Popular Sci. Monthly, 54: 299-312.

Kristine Krug, Emma Brunskill, Antonina Scarna, Guy M Goodwin, Andrew J Parker (2008) Perceptual switch rates with ambiguous structure-from-motion figures in bipolar disorder. Proc. R. Soc. B, 275: 1839-1848.

Ramachandran, V.S., and Anstis, S.M. (1985). Perceptual organization in multistable apparent motion. Perception 14, 135-143.

Rubin, EJ (1915) Synsopleved Figurer: Studier i psykologisk Analyse. [If anyone has the full reference please let me know]

Stoner, GR, Albright TD, and Ramachandran VS (1990) Transparency and coherence in human motion perception. Nature 344: 153-5.

Tong, Nakayama, Vaughan, and Kanwisher (1998) Binocular rivalry and visual awareness in human extrastriate cortex, Neuron 21: 753–759

Torrey, CC (1970) Trace Localization and the Recognition of Visual Form. The American Journal of Psychology, 83: 591-600.

Wallach, H, and Austin, P (1954) Recognition and the localization of visual traces. Am J Psychol, 67:338-40.

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Table of Contents of posts on consciousness.

Consciousness (6): Reversible Figures

The sixth in my series of posts on consciousness. All the posts are indexed here.

Background: Why do we need psychology?
Instead of just diving into the neural data, let's take some time to examine our target, consciousness. Clarifying the features of conscious awareness will provide a more precise target for our neuronal theories.

The ultimate goal is to develop an understanding of consciousness at both the neuronal and psychological levels. The two approaches should coevolve until they fit together as nicely as our ideas about trait inheritance and DNA, as well as our ideas about action potential generation and single channel biophysics.

To flesh out the analogy with inheritance/DNA, let's consider the work of Gregor Mendel. By controlling the reproduction of different strains of pea plants, Mendel was able to measure many features of inheritance well before anybody had heard of DNA. His work provided an explanatory target for molecular biologists who unraveled the mechanisms only much later. Similarly, psychologists have gained quite a bit of knowledge of consciousness by simply studying consciousness, knowledge gained without focusing much at all on the specific neuronal mechanisms involved.

There are literally thousands of psychological experiments and clinical studies that reveal interesting features of consciousness. Obviously, we'll only be able to look at a tiny subset of these data. This means I'll need to curate the data with some caution: I must be wary of cherry picking, or focusing only on data that lets me push some pet theory. The right approach toward any pet hypothesis is to try to kill it with data. We should actively seek out falsifying evidence, not data that confirms what we already believe.

We have to start somewhere in this galaxy of data, so let's begin with a set of illusions that has entertained and puzzled psychologists, and the general public, for nearly 200 years: ambiguous stimuli. I start with them partly for their cocktail party value, but also because they are an excellent gateway into the psychology of conscious perception.


Ambiguous Visual Stimuli and Bistable Perception
A stimulus is ambiguous when it can evoke different percepts. That is, even though the stimulus is unchanging, our experience of the stimulus oscillates between two "interpretations." The alternating perceptual experiences are known as bistable percepts. (If this is confusing, hold on: many examples are coming up).

In this post I'll focus on cases in which the percepts switch back and forth between two identical objects (e.g., two cubes) that are seen from two different perspectives. They are often called 'reversible figures.'


Necker cube
The Necker Cube is probably the most famous reversible figure. It was first discussed in print in 1832 by a Professor of Minerology, LA Necker (Necker, 1832). The Necker Cube is at the top of the following figure. Looking at the line drawing tends to evoke alternating experiences of two cubes. These cubes are shown, in an unambiguous form, below the Necker Cube.


Perceptually, one of the cubes appears to come out of the page pointing down toward the left (the pink cube on the left), while the other cube appears to come out of the page pointing up to the right (the pink cube on the right).

While most people's visual system automatically generates a percept of one of the cubes (an amazing fact in itself), some people tend to stay locked into one interpretation. That is, they don't spontaneously experience bistability. If this is you, just keep staring and your percept will eventually switch. The longer you look at a reversible figure, the more frequently the perceptual alternation will occur.


Reversible Steeple
It is relatively easy to generate ambiguous drawings similar to the Necker Cube: make a line drawing of an arbitrary 3-D polygon and it is likely to generate bistable percepts. For instance, here is a five-sided solid, the Reversible Steeple:

One of the steeples points toward you (with the rectangular base further away), while the other points away (its base will be closer to you).


Schröder's Staircase
The following figure should appear as a set of steps with either the blue or the pink "wall" closer to you.


When the red wall is closest, it seems you are looking at a staircase from above (the more standard perspective such as when you are approaching a set of stairs to climb). When the blue wall appears closest, it will seem as if you are looking up at a staircase from underneath, or an upside-down staircase, or an overhanging unfinished brick wall (the latter two descriptions are from Wallin's book).


Plush Chair
You can imagine the following is one of those plush velvet chairs with brass buttons on the front and back.



One percept is of a chair facing you: you see the chair from above with the backrest facing you and the seat of the chair is coming out toward you. The other percept is of a chair facing away from you: you see the chair from below, with the back of the backrest facing you and the seat is going away from you.


Inverting Hairbrush
It appears to be a hair brush. It can appear either with the bristles facing you, or the bristles facing away.


Scripture's Blocks
This is one of my favorite reversible figure in this post, one of the more vivid cases of bistability. The image should appear as a set of long rectangular blocks stacked upon each another.


In one percept, each block is oriented down to the left, capped on the bottom by a white face. The hatched shading is the top surface of each block. In the other percept, each block is oriented up to the right with its white face at the top. In this case, the hatched shading coats the front surface of each block.


The scope of perceptual reorganization
I'll finish by illustrating the deep and sometimes startling nature of the perceptual reorganization during alternation. We'll look at two modifications of the Necker Cube.


Arrowhead Cube
I've placed two arrows on the "surface" of the Necker Cube below. Consider two questions. Are the arrows on the inside or outside surface of the cube? In what direction are the arrows pointing? As you probably guess, the answer depends on which cube you see!



When you see the down-left cube, then the arrows appear on the outside of the cube, and seem to point toward you. However, when you see the top-right cube, they appear to be painted on the inside surface of the cube, and to point backwards away from you.

Somehow, when the brain alternates between cubes, it takes note of additional features of the cube and integrates them into the percept in an appropriate way. It does this without you having to think about it, without you consciously knowing how you do it.


Rotating Necker Cube
The final bistable percept is my favorite of the bunch, the Rotating Necker Cube. It is a picture of a cube that is rotated by the same amount (in the same direction) with each time step. You should see a rotating cube. Does the cube still show bistability even when rotating?



Not only does the Rotating Necker Cube still alternate, but when it alternates it reverses its apparent direction of rotation! Once the percept switches, our visual system interprets the exact same movement as rotation in the opposite direction.

I will be devoting a future post to the Necker cube, as it is such a rich source of ideas and data.


Where we are headed
In the next post (maybe even two) we'll continue looking at ambiguous stimuli. This post has been a quick list of reversible figures, without much theory or discussion of consciousness. We will ultimately use these illusions to brainstorm about the nature of visual consciousness. Then we'll have something more precise that we can target from a neuronal perspective.


Philosophical dessert
Just as Mendel's laws were consistent with many possible molecular mechanisms, these visual illusions are consistent with any number of lower-level neuronal explanations. For that matter, the illusions considered in isolation are consistent with dualism (roughly speaking, dualists believe that the mind is not part of nature, that it is a different kind of thing altogether such as a soul). Illusions provide useful data that all people (not just neurophiles like myself) interested in consciousness should struggle to explain. Dualists of the world, get off of your armchairs!

Original Sources of Illusions
The Reversible Steeple is adapted from John Wallin's wonderful monograph Optical Illusions of Reversible Perspective published in 1905 (it is available free at Google Books). Schröder's staircase was first published in Schröder (1858). I got the idea for coloring the two walls of the staircase from planetpurplex.com, a site full of optical illusions. The Inverting Hairbrush and Plush Chair are both adapted from Wallin (1905). Scripture's blocks were introduced by Scripture (1897), though the figure used above is taken from Wallin (1905). The Arrowhead Cube is adapted from Mason et al. (1973).

I am not sure who first noticed bistability in the Rotating Necker Cube. If anyone knows the background, please let me know. Wallin (pages 46-47) says Wheatstone looked at moving Necker Cubes, but it seems Wheatstone just held wire cubes in his hand and contemplated them while he moved them about (see Wheatstone (1838)). Neither Wheatstone nor Wallin remarked on the apparent reversal of rotation, so the first observation was likely after the publication of Wallin's monograph in 1905.

References
Mason, J, Kaszor, P, and Bourassa, C.M. (1973) Perceptual structure of the Necker cube. Nature 244: 54-56.

Necker, LA (1832) Observations on some remarkable Optical Phænomena seen in Switzerland; and on an Optical Phænomenon which occurs on viewing a Figure of a Crystal or geometric Solid. The London and Edinburgh Philosophical Magazine and Journal of Science (3^rd Series) 1, No 5, 329-337.

Schröder, H (1858) Über eine optische Inversion bei Betrachtung verkehrter, durch optische Vorrichtung entworfener physischer Bilder. Annalen der Physik und Chemie 181: 298-311. [Note last name sometimes spelled 'Schroeder' or 'Schroder']

Scripture, E.W. (1897) The New Psychology. Walter Scott Ltd, London.

Wallin, J.E.W. (1905) Optical Illusions of Reversible Perspective: A volume of historical and experimental researches. Stanton Call Press, Stanton IA.

Wheatstone, C (1838) Contributions to the Physiology of Vision.—Part the First. On some remarkable, and hitherto unobserved, Phenomena of Binocular Vision. Philosophical Transactions of the Royal Society of London, 128: 371 - 394.

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Table of Contents of posts on consciousness.

The Necker Prism

To prepare the next installment in my series on consciousness research, I've been studying the Necker Cube. Little did I know that the cube was actually a prism that displays the main threads of perception research since 1833. An entire book could easily be written about this simple little line drawing. Despite all the research, there are still basic questions about the cube that haven't been addressed experimentally. I'll post what I've learned in the next couple of weeks.

Consciousness: Table of Contents

This post will be a permanent placeholder for links to all of my consciousness posts. It will expand until I finish posting on the topic.

(1) Creationists take aim at consciousness: It begins by mentioning that the Creationists have finally discovered consciousness, but mostly focuses on why real scientists should take consciousness seriously as an object of (scientific) study.

(2) Introducing Mr B: A look at the methods of your garden-variety biologist, and what distinguishes a biological approach from other perspectives (like that of the physicist).

(3) Mr B's first take on consciousness: In his first look at the evidence, Mr B concludes that neuronal processes are necessary and sufficient for consciousness in humans.

(4) Leveling with Mr B: We examine the different spatial and temporal levels of organization in the nervous system, and pinpoint the levels most relevant for consciousness.

(5) Switching Voices: Discusses the reasons I will begin referring to Mr B in the first person.

(6) Reversible Figures: Shows and discusses several reversible figures to illustrate perceptual ambiguity and bistable perception.

(7) More Ambiguous Figures: finishing our tour of ambiguous visual stimuli.

(8) From perception to interpretation: starts to explore the claim that perception involves stimulus interpretation. Focus is on determining just what interpretation is.

(9) From texts to the grotesque cinema: to help make precise the claim that perception is stimulus interpretation, we examine the question, 'What is a stimulus?' in some detail.

(10) Contents and Vehicles: starts exploring the analogies between linguistic interpretation and visual perception. First up: there is a content/vehicle distinction.

(11) Ambiguity and context: a continuation of post 10. I examine ambiguity and contextual influences in perception and language interpretation.

(12) What the brain thinks it knows: continuing the previous two posts. I examine the effects of background knowledge and assumptions in perception and language interpretation.

(13) The interpreter versus the scribe: summary of the view that perception is stimulus interpretation.

(14) Interpretation mechanics: Discussion of the view that perception involves unconscious inference. Representations introduced.

(15) Opening the time capsule: quotations, from some great thinkers, that tie together the previous nine posts.

Consciousness (5): Switching Voices

An intermezzo in my series of consciousness posts. It's been a while since I posted, so I needed to oil the chain.

In conversations about consciousness, the voice of the garden-variety biologist (Mr B) often gets drowned out. This is typically due to blithely confident philosopher-types who act as if the armchair provides just as much authority as the lab bench when it comes to consciousness. Equally perplexing is that some folk's confidence actually becomes bolstered by the paucity of theoretically significant experimental results about consciousness. A lack of data makes good scientists less confident, not more confident, about a topic. (There are interesting parallels with creationism here.)

I am happy to start by inverting this antiscientific bias about consciousness. In that spirit, I will grant Mr B sole control of the lectern until he has finished saying what he has to say. Only then will I entertain questions from those armchair pilots who believe they have deadly objections to Mr B's project. At that point we will be better posed to see if they are right.

While I am not Mr B, for the above reasons I do consider myself his advocate. Because it is becoming a distraction to talk about him in the third-person, I will simply speak in his voice for a bit. Because confusion is likely to follow such a grammatical shift, this post will serve as a handy reference (especially for those tempted to bemoan my ignorance of what the philosophers have (putatively) contributed to our understanding of consciousness).

Next up, we'll get back to the science.

Nature Trifecta

A big day for systems neuroscience in Nature yesterday: three papers! Each paper investigates a different question about synaptic organization in the cortex. Not one paper created a new word ending in '-omics,' an auspicious sign.

I superficially describe the main results from each paper below, with some figures.

First, Brown and Hestrin bring us Intracortical circuits of pyramidal neurons reflect their long-range axonal targets. After fluorescent labeling of corticocortical (CC), corticostriatal (CS), and corticotectal (CT) pyramidal cells in cortex, they sliced the mouse brain and patched onto as many as four of the cells in layer five of V1 to measure the probability of cells synapsing onto cells of the same type (and later in the paper, different types).

They found distinct patterns of connectivity for the different cell types (see Figure). For instance, while 20 percent of the CS cells were connected monosynaptically, CT pyramidal cells only hooked up with one another about five percent of the time. They further showed this wasn't merely because some cells are more promiscuous than others (though they didn't show this for CS neurons).

Next up was a paper from Murayama and others in Larkum's group titled Dendritic encoding of sensory stimuli controlled by deep cortical interneurons. They loaded layer V cells in rat somatosensory cortex with calcium indicator, and then imaged layer 1-3 calcium activity during hindlimb stimulation. The supragranular signals represent activity solely in the apical dendrites from the loaded Layer 5 pyramidal cells. Via various pharmacological manipulations (often involve injecting more boluses into layer V), as well as in-vitro patch clamps, they support the claim that a particular type of inhibitory interneuron in layer V suppresses dendritic calcium levels. Then, using triple patch clamp (two neighboring layer V pyramidal cells, and one of their dendrites), they showed that stimulating one of the cells produced dendritic inhibition in the other cell via a disynaptic connection.

Of the three papers, this would be the best one to present in a journal club because it is fairly complicated and hard to understand on a quick once-through. A journal club audience would appreciate you doing the work for them. Frankly, I still haven't thought through the logic of all their experimental manipulations.

Third, from Petreanu and others in Svoboda's group is The subcellular organization of neocortical excitatory connections. It was only a matter of time before the channel rhodopsins spawned acronyms. In this quite elegant paper they described their application of sCRACM [subcellular ChR2-assisted circuit mapping] to determine the spatial organization of axodendritic synapses onto neurons in somatosensory cortex of mice. They did this in slices in which particular areas expressed channel rhodopsin ChR2. For instance, they expressed ChR2 in the VPM nucleus of the thalamus, which carries information to the whisker barrels in S1. Then they could stimulate the axons of the VPM neurons with a laser to find its postsynaptic targets.

They patched onto a cell in S1, and would then laser-stimulate the channelrhodopsin-expressing axons in the vicinity of the patched neuron. When the HH channels were blocked, a laser pulse on a ChR2-expressing axon would still generate PSPs in the postsynaptic cell. The spatial resolution of the mapping was approximately 60 um², so they were able to map the distribution of synapses with decent precision. The figure accompanying this paragraph is an activation map of a layer 5B pyramidal cell in which the ChRs were expressed in the VPM nucleus of the thalamus. Most of the VPM->S1 synapses occur in Layers IV and VB, though there is some activity in supragranular layers.

The second and third papers use very cool methods to achieve fairly unsurprising results. The first paper used more common methods, but the results were a bit more interesting (i.e., less predictable). All in all, a good week for synapses.