There’s No “The” in “Neural Code”
One of the central, and most misunderstood, goals of the BRAIN Initiative is to understand how brains represent information. But, when people talk about cracking “the neural code,” what do they mean? It’s a reasonable metaphor, since it’s clear that brain activity in people (and animals) somehow represents the outside world in some abstract and non-trivial way. But there is really no single neural code. Not for different animals, and not even for different neurons within a single nervous system.
To really understand how different neural codes can be, we have to take a step back and look at how neurons represent their inputs, starting with one of the simplest of codes. Imagine you’ve got that glowing radioactive ingot that Homer Simpson fumbles around with. It’s in a lead-lined room on a table and you stand with your Geiger counter several feet away. What do you hear? Let’s say you are far enough away that you don’t hear anything, though you imagine if you waited around long enough in that position a stray particle might hit the collector and you would hear a single pop. So you move the wand closer and the sensor’s speaker starts to come to life, like the beginning of a hail storm on a tin roof. At some point there is a steady, though not regular, chatter that seems to max out as you touch the wand to the ingot. If you measured the number of pops-per-second from the Geiger counter, and plotted it against the distance of the wand from the ingot, it would probably look something like this:
Now let’s say Lenny is in the next room (no windows) listening to your Geiger counter output. It’s his job to figure out how far away the rod is from the ingot at any given moment just from listening to the popping sounds on the speaker. You can imagine that if the radioactivity of the ingot is pretty constant, and with a little training, Lenny should be able to figure this out after a while. That is to say, given a certain rate of popping, Lenny can guess the distance reasonably accurately. So the pop-rate of the counter’s speaker is a code for the distance of the wand from the ingot. So the counter is transmitting information about an attribute of the world (the distance of the wand from the radioactive source). In this case, the information is contained only in the pop-rate, and not in the individual timing of pops (which are random), so that implies that Lenny has to listen to the speaker for a while to get an estimate of the rate (the presence or absence of the pop in a small time window doesn’t tell him much).
What this thought experiment describes is essentially one of the simplest and most common of neural codes, especially in sensory and motor systems. Imagine instead of a Geiger counter, you are listening to the activity of a neuron embedded in the skin that measures pressure. Like the Geiger counter, the information from most neurons is conveyed by discrete all-or-nothing events called action potentials (or spikes), so in this case you’d hear a pop pop pop on your audio monitor and that would tell you how much pressure was being put on the skin around the cell. With the proper experimentation, you’d be able to make a graph relating spike rate (say, in spikes per second) to pressure (maybe in pounds per square inch), and then you could estimate one from observing the other. That’s a neural code. This particular system is called a rate code, for obvious reasons, and because it’s so common in sensory systems and also for certain theoretical reasons, it’s essentially the default model of neural coding in many cases. Importantly, while it is easiest to grasp in terms of sensory coding, rate codes can also be used to describe the output of the nervous system, where motor neurons drive muscles in a process that converts spike rate into muscle contraction.
While rate codes are likely fundamental to most nervous systems, at least as a default hypothesis. When systems neuroscientists or BRAIN Initiative scientists suggest using techniques like calcium imaging, they are at least tentatively endorsing the centrality of rate codes. Because calcium imaging relies on relatively slow ion dynamics, it provides a reasonable readout for spike rate as a measure of neural activity, but it cannot resolve action potential timing in fine detail. Still, there are many neural subsystems in many different animals that appear to rely on more precise spike timing to relay information, and understanding these requires different techniques. I’ll describe some of these alternative coding strategies in future posts.
Photo Credit: The Simpsons opening sequence, under fair use guidelines.