Friday, July 18, 2008

Neural Code

An open question in neuroscience is: what is the neural code? By that it is meant, how is information represented and processed in the brain. I would say that the majority of neuroscientists, especially experimentalists, don't worry too much about this problem and implicitly assume what is called a rate code, which I will describe below. There is then a small but active group of experimentalists and theorists who are keenly interested in this question and there is a yearly conference, usually at a ski resort, devoted to the topic. I would venture to say that within this group - who use tools from statistics, Bayesian analysis, machine learning and information theory to analyze data obtained from in vivo multi-electrode recordings of neural activity in awake or sedated animals given various stimuli - there is a larger amount of skepticism towards a basic rate code than the general neural community.

For the beneficiary of the uninitiated, I will first give a very brief and elementary review of neural signaling. The brain consists of 10^11 or so neurons, which are intricately connected to one another. Each neuron has a body, called the soma, an output cable, called the axon, and input cables, called the dendrites. Axons "connect" to dendrites through synapses. Neurons signal each other with a hybrid electro-chemical scheme. The electrical part involves voltage pulses called action potentials or spikes. The spikes propagate down axons through the movement of ions across the cell membrane. When the spikes reach a synapse, they trigger a release of neurotransmitters, which diffuse across the synaptic cleft, bind to receptors on the receiving end of the synapses and induce either a depolarizing voltage pulse (excitatory signal) or a hyperpolarizing voltage pulse (inhibitory signal). In that way, spikes from a given neuron can either increase or decrease the probability of spikes in a connected neuron.

The neuroscience community is basically all in agreement that neural information is carried by the spikes. So the question of the neural code becomes: how is information coded into spikes? For example, if you look at an apple, something in the spiking pattern of the neurons in the brain is representing the apple. Does this change involve just a single neuron? This is called the grandmother cell code, from the joke that there is a single neuron in the brain that represents your grandmother. Or does it involve a population of neurons, known not surprisingly, as a population code. How did the spiking pattern change? Neurons have some background spiking rate, so do they simply spike faster when they are coding for something, or does the precise spiking pattern matter. If it is just a matter of spiking faster then this is called a rate code, since it is just the spiking rate of the neuron that contains information. If the pattern of the spikes matter then it is called a timing code.

The majority of neuroscientists, especially experimentalists, implicitly assume that the brain uses a population rate code. The main reason they believe this is because in most systems neuroscience experiments, an animal will be given a stimulus, and then neurons in some brain region are recorded to see if any respond to that particular stimulus. To measure the response they often count the number of spikes in some time window, say 500 ms, and see if it exceeds some background level. What seems to be true from almost all of these experiments is that no matter how complicated a stimulus you want to try, a group of neurons can usually be found that respond to that stimulus. So, the code must involve some population of neurons and the spiking rate must increase. What is not known is which and how many neurons are involved and whether or not the timing of the spikes matter.

My sense is that the neural code is a population rate code but the population and time window change and adapt depending on context. Thus understanding the neural code is no simpler than understanding how the brain computes. In molecular biology, deciphering the genetic code ultimately led to understanding the mechanisms behind gene transcription but I think in neuroscience it may be the other way around.


Mehrdad said...

I just got acquainted to your blog by Bard’s post in neurocomp mailing list! The weblog looks like the one I have been looking for...
When people talking about coding I am always looking for answer to the Big Problem in my mind, but soon get disappointed like this time!
The problem which some people point out in their books or review articles and give the most irrelevant answers to it easily and go on... Can you separate coding and decoding? and if you do so, is it not equivalent to dualism?
To show more how the people are far from the real unbreakable connection between these two, you can search the two terms of “neural coding” and “neural decoding” in to see that the proportion of articles is 1 to 100!!!
Don’t you think it is funny to “discover” a coding scheme by changing the stimuli and focusing on a certain brain area, and if you are very devoted to decoding use an statistical method like maximum likelihood to show how the “rest of the brain” is going read out?
Even in the case of extreme localised activity, no feedback connection and extinct of any spontaneous activity, are we the one who should understand the code –usually by the methods introduced by communication engineers(!)-as a whole or the certain connected regions should find it out part by part. What do you think about this example:
Suppose we are detecting the sound of people from another planet talking to each other. Those people are also using different languages due to their geographical situation, etc. and the language is quite irrelevant to in terms of nature to what human beings speak(Their nature selected a completely different way of evolution!) and again suppose that you can have a good control on what you can show them as stimuli and hear exactly what ever any individual says. Can you think of any strategy to decode their language?
My position is still quite agnostic about the possibility of such a seperation but I am quite sure that the present direction wouldn’t work out! What do you think?

Carson C. Chow said...

Hi Mehrdad,

I think you are asking deep questions and I don't know if I have anything more to add. You are absolutely correct that many people are not addressing the question of what the brain is actually doing with a code. That is why I don't think the question of the neural code is independent of the question of how the brain works.

As for your example, I think there is no way to decode a language without some kind of prior. You have to share something in common with what that person is interested in to decode their language. This is a very interesting problem in the philosophy of language. I was actually going to blog about this topic soon.