Sunday, November 21, 2010

The Binding Problem

The binding problem is an issue in theoretical neuroscience that arises because of the lack of clarity about how two features of the same object are bound together in the brain leading to a single, coherent perception. Most neuroscientists in this area assume that individual features of sensory stimuli correspond to tiny assemblies of cells in the brain. These neural assemblies specialize in representing specific features and will become active if you perceive this feature, or even if you simply imagine it. For example, if one sees a blue, wrinkled glove, neural assemblies that correspond to blue, wrinkled and glove would all be activated simultaneously. The coactivation of these assemblies, and all of the other assemblies responsible for other features of the glove, would create a unified picture of a single object. The question is though, how do these assemblies take discrete, individual features and combine them to make one perception?

It is thought that when assemblies that correspond to the same perception are coactivated they send electric messages back and forth. They also recruit other areas that get involved in these informational oscillations. When these assemblies oscillate in synchrony they are thought to bind features such as contour, shape, motion, color, depth and other aspects into a composite image. These electrical oscillations alone cannot adequately explain how conscious perception arises in the brain but they give clues about how we perceive multiple objects. 

Another very similar problem in feature binding is the superposition catastrophe. This is where you have multiple objects, each with their own features, and you are able to keep track of the features of each object. Imagine that you are looking at a green raccoon and a yellow hippo. If all of the assemblies that correspond to these animals and colors must fire simultaneously, then how can you discern that the raccoon is green and the hippo is yellow? How can you tell that the raccoon is not yellow? One way of solving this problem is to assume that we have dedicated assemblies for green raccoons and yellow hippos and that these can become activated at the same time without confusing their contents. This is an unreasonable hypothesis though because most of us have never seen or imagined these animals. If we had dedicated assemblies for every combination of features in our memory the total number of necessary assemblies would be astronomical, and would require far more neurons than we can hold in our heads. Neuroscientists are starting to think that perhaps the assemblies for green and raccoon are activated simultaneously, the assemblies yellow and hippo as well, but that the two pairs are activated out of sync with each other. In other words, the assemblies for green, those for raccoon, and all of the other assemblies and networks associated with these, but not with yellow hippos, fire neural messages back and forth with each other at their own tempo that is distinct from the timing of other activities that are going on in the brain. Such amalgams of associated assemblies are often called modules in the literature. The ability of the brain to compartmentalize features into modules allows the segregation of information which in turn allows the perception of multiple objects, each with their own features. 

Modules of neural assemblies are thought to contribute to conscious perceptions and are thought to interact with other modules to create behavior. If they could be visualized these modules would not appear as neat, uniform structures in the brain, but would probably be messy, wiry structures that criss-cross between a variety of brain areas. It is not as easy to visualize the neural assemblies. Some may involve neurons that are clustered very close together yet others may be more distributed. It is thought that assemblies have to fire back and forth very fast (around 40 Hz or 40 times per second) to bind together to form a module. Experiments have shown that when someone is experiencing an optical illusion, that illusorily associated features bind together. Interestingly, it has also been shown that when someone cannot perceive a hidden image, that the features necessary to see the image are either not activated or not bound. In other words, in order to notice something you have to bind its features together and when we are fooled we bind the wrong features.

A Partial Solution to the Binding Problem:

I don't believe that there is a superposition catastrophe. To posit one you must assume that in higher brain centers, the physical location and orientation of the two, differently colored objects are lost. I think that the higher-order associative areas may not code this information but that the primary and secondary visual areas (which the association areas continually interact with) do because they are organized retinotopically and would be able to keep the objects, and their respective features (such as color), consistent without mixing things up. This is akin to having a TV in your head, as long as you are watching the TV, you cannot forget the colors of different objects. Certainly, this occipital "TV" is erased every 250 ms due to the fact that its outputs amount to transient "sensory memory" but if the higher association areas bind to the right elements, they can keep them active on this TV for as long as they are either visually rehearsed or committed to memory.

Read the full article that I wrote on this topic here:

Friday, November 12, 2010

Don't Miss These Great Pictures

The website has a bunch of great pictures from the new book "Portraits of the Mind" by Carl Schoonover. The book has received good reviews but I especially recommend the 7 vivid "portraits" that you can find at:

The picture above shows growing axons and their scaffoldings reaching out to make connections with other cells. The picture is false-colored yellow using an immunohistochemistry technique that uses dyed antibodies to tag molecules revealing cellular structure.

Grandmother Cells


The term "grandmother cell" was coined by Jerry Lettvin in 1969 to describe a hypothetical neuron that can be shown to represent a specific psychological concept. This cell would become active every time a person thinks about a complex thing, such as his or her grandmother. The question was: Are there any cells, anywhere in the brain, that are dedicated specificlly to processing information about one's grandmother? It seems that this may be the case despite the fact that neuroscientists were convinced, for decades, that this was a gross oversimplification.

It is known that millions of cells work together to help us visualize even the most simplistic visual objects. The retinas of the eyes relay information about what we look at to early visual processing areas in the back of the head. This data passes through a series of neural areas before objects are recognized. These areas, which have been fine-tuned by life experience, act as filters that allow the visual data to matchup against the brain's best existing representation of what is being seen. In this sense, when we see, we are not really looking at what is out there, but instead piecing together a collage of things that we know to try to recreate the scene. When our brain does this, large numbers of neurons in the simple visual areas that correspond to the lines and contours of what we are seeing send their information to a smaller number of more complex neurons that deal with shape and form. These neurons, in turn, converge on even smaller populations of neurons that code for recognizable objects like people, cars and animals. It is thought that populations of cells, in these high-order processing regions, can be dedicated to processing very specific objects. Grandmother cells are the theoretical limit to this convergence where the activities of a large interconnected structure of networks meet together to activate a single neuron that in some senses, holds much of the information of the entire network (because much of the network must be activated for it to fire).

Some cells that come close to meeting the requirements of a grandmother cell have been found. One study by Rodrigo Quiroga and colleagues used patients undergoing treatment for epilepsy where 100 tiny electrodes were implanted in their brains. Each subject saw around 100 images of famous people, places and things. Overall, almost 1,000 neurons were sampled and 132 of these reacted to at least one of the images. The objects that elicited increased activity were used in another round of recognition except different images of these objects were shown. For example, if a head-on photo of a truck elicited a response, then a profile picture might be shown next. The researchers were able to find grandmother-like cells in some of the participants. One female participant had a neuron that only responded to the actress Jennifer Aniston. The neuron did not respond to other pictures, even of similar-looking female actresses, with one exception. This neuron responded to some pictures of Lisa Kudrow, Jennifer’s costar on the show Friends.

Another female participant had a neuron that responded only to Halle Berry. Pictures of the actress, a line drawing, a profile, even the words of her name, all made this neuron fire. These neurons were generally in convergence areas – such as the hippocampus- where a lot of processing, in areas such as vision and audition, meet up. This neuron must have been highly tuned, not only to the physical aspects of Halle, but to the abstract representation of her overall persona because even a picture of Halle’s catwoman character made the neuron fire despite the fact that she was masked.

Early critics of the idea pointed out that there are not enough neurons in the brain to account for every possible sensory object in one-to-one correspondence. Grandmother cells don’t necessarily work this way though. It seems that sensory objects are represented by networks of neurons many of which overlap and intersect. The individual neurons that make up these networks usually have the capacity to contribute toward the perception and recognition of several different objects of sensation. The further down the processing stream these neurons lie - the closer in the brain they are to the retinal inputs - the more fundamental they are in the process of recognition and the more objects they contribute to. The contribution of an individual neuron is actually very weak. One neuron usually does not have the capacity to send a global message that can be perceived consciously. In fact, many, many neurons would have to be removed to abolish the ability to recognize your grandmothers. Even more (tens of thousands or perhaps millions), would have to be removed to ensure that you could not remember anything about your grandmothers.

Experience fine-tunes neurons and the inputs that they are receptive to. The brain will even tune some neurons so narrowly that they become dedicated processing specialists for things that you recognize frequently, especially things that you see over and over again. They may not become active to every representation of a grandmother, they may become active to totally unrelated representations and there may be a number of them in each person’s brain, that vary in their responsivity to grandmothers. Even so, grandmother cells, at least loosely defined, do seem to exist. It is probably a safe bet that the cartoon drawing of a grandmother at the beginning of this entry activated some of the same cells in your brain that respond uniquely to images of your parent’s mothers.

Quiroga, R. Q., Reddy, L., Kreiman, G., Koch, C. & Fried, I. 2005. Invariant visual representation by single neurons in the human brain. Nature 435: 1102-1107.

Read the full article that I wrote on this topic here:

Monday, November 8, 2010

Realizing I Was Made of Cells Made My Skin Crawl

I remember being in my 8th grade biology class and coming to a mind-shattering revelation. I sat there as I did everyday that semester, bored and completely mentally checked out (it is ironic that today the only thing I get really excited about is biology). The teacher began to show some videos of cells on the overhead. I was amazed by the way they looked and by the way they buzzed and jittered about. It was fascinating how their microscopic movements, propelled by flagella, were so different from movements I was used to seeing on a macroscopic scale. I was thinking about the cells we had cultured in petri dishes and how they spread and multiplied. I began to wonder about how the cells that I am composed of move when it hit me...

Somehow, up until this point, I had never really appreciated how I was composed - from head to foot - of individual cellular organisms. I was made of cells that, if cultured properly, could each live on their own. These cells that combined to make me, could not all self-replicate, but they each descended from ancestors, like bacteria and protists, that could. All of the sudden I was no longer myself. I was simply a blob, a colony of organisms, each of which wanted me to feed them. I was revolted and enthralled at the same time. Somehow this insight was extremely emotionally powerful, as if someone had proven to me that I was made up of millions of tiny, disgusting insects.

I had previously come to grips with the fact that I was composed of individual atom and molecules, none of which were alive but each of which contributed to who I was. However, something about being composed of individual forms of life - that could live on their own in a petri dish with a food substrate - hurt me badly. It felt like having my privacy invaded by foreign beings, but I wasn't invaded by them, I was them.

It was immediately clear to me that all of my thoughts, dreams, memories and aspirations were somehow held together by connections between living cells. I was interested to know how these cells work together to create my thoughts and behaviors, but I was too overcome with, what felt like, true grief to think much about this. I felt like I had completely lost my selfhood and my sentience. I remember sitting in the cafeteria as a mass of wet, slimy cells, surrounded by other moving, talking but oblivious masses. I felt like I was the only cluster of gunk in the whole junior high that understood it was a cluster of gunk. I was certainly the only one worried about it. Everything that was familiar and comforting, from my voice to my appearance to my language, was now hideously derived.

What caused me to feel this way was the sentiment that, like the people of a country, the parts of a whole are often what is real and important and the whole itself is nominal and inconsequential. Since, I have changed my mind about this. :) I have since come to know more about how colonies of cells began to build animals and how animals became progressively more intelligent and individualistic. This allowed my focus to shift enough so that I still sometimes see the cells as individual organisms, but I see the animal as a "superorganism." Both viewpoints are valid. I look back at this experience as taking a "cells eye view" of the world. It was eye-opening to me and has caused me to look for other perspectives on myself that could change my worldview.