Ask a Neuroscientist! - What is the synaptic firing rate of the human brain?

A couple of days ago, we received an email from a high school student named Joseph. Joseph, having spent some time trawling the net and his library, found himself with no answer to the question, "How many synaptic fires [sic] are there (in a human brain) per second?"

An edited version of my response to the question appears below. In my response, I break down the neural complexity that makes answering Joseph's question extremely difficult. I then totally ignore that complexity in order to produce two firing rate ranges: neuronal and synaptic.

Do you have an additional mathematical solution to Joseph's question? A philosophical objection to the idea of quantifying the human brain in terms of synaptic firing rates? The comments section is where it usually is. 

What is the synaptic firing rate of the human brain?

I'd like to be able to provide a single number, but in reality, the brain is pretty complex, so it's difficult to come up with a single number to describe the firing rate of the entire brain.

But let's explore the question a bit.

First, I'm going to simplify the question by looking at neuronal firing rates, rather than synaptic firing rates. So from that starting point, if we wanted to be really simplistic about the whole thing, we could estimate a firing rate. Such a calculation might go something like this:

Current estimates of the number of neurons in the human brain are around 86 billion (see Bradley Voytek's discussion of how scientists extrapolate this number, also this article on the estimate). Multiply 86 billion by the "average firing rate" of an individual neuron.

Now, the firing rate of an individual neuron can vary quite a bit, and the firing rates of different types of neurons also is extremely variable. This variance in part depends on the intrinsic properties of the neurons (some a tuned to fire very, very quickly - 200+ Hz, whereas some neuron types prefer to fire more slowly, below the 10Hz range). A lot of the variance also depends on what the brain is doing. For example, neurons in the visual system may be practically silent in the dark (or during sleep), but will be firing very fast when visual information is coursing into the nervous system from the eyes. And the exact rate of firing in visual neurons is going to depend on the properties of the visual stimulus (how bright, how fast it moves, what color). Similarly, neurons in your hippocampus, a brain structure important for memory and spatial navigation, may fire quickly as you walk around your room, but may be relatively quiet as you sit in front of a computer reading this email.

All this variability is what makes it so hard to estimate the firing rate of a human brain at any given second.

But, if you press me for a back-of-the envelope calculation, I'd say the best way to estimate the firing rate of a neuron is to come up with a potential range. Now, there's probably been a bunch of research on the distribution of firing rates within various cell populations, and quite frankly, I'd only really believe that rate in the context of a particular activity you are interested (rates can change dramatically between passive sitting and active participation in a task). But generally, the range for a "typical" neuron is probably from <1 Hz (1 spike per second) to ~200 Hz (200 spikes per second).

To ruthlessly simplify, treating all 86 billion neurons in the human brain as copies of that a single "typical" neuron, ignoring all of the glorious cellular specificity that characterizes the brain, we're left with a range of 86 billion to 17.2 trillion action potentials per second.

Let's go back to the question of synaptic firing rates. Even though an action potential produced in a neuron is not guaranteed to produce release of neurotransmitter at a synapse, let's ignore that point and assume the opposite. I've seen people quote a minimum number of synapses as 100 trillion (although I'm not clear where that number came from). So, let's do our math again. 100 trillion synapses, each with an independent firing rate range of < 1Hz to ~200 Hz. So a range of 100 trillion to 20 quadrillion.

Again, and I really cannot stress this enough, these numbers doing reflect what actually goes on in a human brain in any given second. The actual firing rate depends so much on what the brain is doing at that moment, that back-of-the-envelope calculations such as the ones I just wrote down are (in my opinion) absolutely meaningless. But for what its worth, there they are. And if these numbers at least give us a range, you can imagine the sheer computational power that will be required to record all the neurons the human brain.

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If you have a question for one of our neuroscientist contributors, email Astra Bryant at stanfordneuro@gmail.com, or leave your question in the comment box below.

Ask a Neuroscientist: How to Train Your Brain

In this edition of Ask a Neuroscientist, we’ll answer two questions that address a similar principle: Can you train to have a better brain?

The first question comes from Allyson Thomley, who writes:

“I am an elementary science teacher seeking to reach a better understanding of how the brain works. As a novice, it has been difficult to sort out the pseudoscience from valid, data-supported information. Sadly, there is a great deal of misinformation circulating amongst teachers who are genuinely trying to incorporate brain research into their practice.

One such claim that I have come across more frequently has to do with exercises that 'cross the midline.' It is suggested that by engaging in activities in which the right arm or leg is crossed over to the left side, connections between the right and left hemispheres of the brain are strengthened. Any grains of truth here?”

This idea appears to have originated (or is at least most heavily propagated) by Paul and Gail Dennison and their commercial learning program called Brain Gym. They call their program “educational kinesiology,” and claim that engaging in activities that “recall the movements naturally done during the first years of life when learning to coordinate the eyes, ears, hands, and whole body” can dramatically improve concentration and focus, memory, academics, physical coordination, relationships, self-responsibility, organization skills, and attitude.

Those are quite extraordinary claims, and as the saying goes, extraordinary claims require extraordinary evidence, of which they provide little to none. In fact, there are no peer-reviewed, controlled studies testing whether or not these exercises do anything at all. All of the papers they use to support their claims are self-published in the journal The Brain Gym Global Observer. On their website, they address why there are no peer-reviewed articles supporting their claims, explaining that because a scientific study would require that some students receive the Brain Gym training (the experimental group), and some receive no training or a different kind of training (control group), it would be unethical to deprive some student’s of the Brain Gym training.

Any study like this would only last a few weeks or a few months at most, so this excuse is pretty weak, and is a huge red flag with regard to the validity of their claims. That being said, we can’t completely rule out the general idea that engaging in crossing the midline exercises has a positive effect on learning because this idea has not been rigorously tested.

The underlying science – that performing an activity that simultaneously engages both cerebral hemispheres can improve cognition – does appear to be true. The best studied example of this is musicians who began training during early childhood. Neurons on either side of the cortex send axons across the midline, which then make synapses with neurons on the other side. The axons are covered in a white substance called myelin, which acts as an insulator, protecting the electrical communication between neurons from leakage, and increasing the speed at which the signal can travel down the axon. This collection of axons between the midline is called the corpus callosum, and research has shown that the corpus callosum is larger in early-trained musicians compared to late-trained musicians and nonmusicians, especially if the training began before the age of 7.

The hypothesis is that because musical training involves the coordination of multiple modalities – i.e. taking visual and auditory input (reading and listening to music, respectively) and coordinating it with motor output (playing the instrument) – the connections between these brain areas become stronger and more tightly connected, resulting in better sensorimotor integration. And indeed, early-trained musicians have better spatial and verbal memory, attention, mathematics skills, and perform better on other tasks involving the integration of multiple sensory and motor inputs. You can find a nice review on the topic here: The Musician's Brain. 

So, while the Brain Gym technique does not seem like a good candidate, encouraging your students to learn an instrument could go a long way in improving their cognitive functions. Unfortunately, adults who learn an instrument do not see the same improvements.

Our second question comes from Kelly Bertei, who asks:

“Does playing games to improve working memory work? If so, since my brain is only so big, would other parts of my brain reduce in functioning to accommodate for increases in working memory?”

The literature on this is very mixed – some reports show that these games can lead to increased working memory and other measures of cognitive function, whereas other studies show no difference in performance.

For example, in a paper published earlier this year in the online journal PLOSone, researcher Rui Nouchi and colleagues asked 34 volunteers to play either the brain training game Brain Age (which the authors created and profit from, it should be noted) or Tetris. They played for 15 minutes a day, 5 days a week for 4 weeks. The participants were then tested on cognitive performance before and after the training period. Interestingly, both groups performed better after the training than before, and the Brain Age group showed greater improvements on executive functions, working memory, and processing speed compared to the Tetris group, while the Tetris group showed greater improvements on attention and visuo-spatial ability.

So these results seem to support the idea that brain training exercises can improve some aspects of cognitive function. However, another paper published in 2013 in the journal Computers in Human Behavior (which is a real journal, and actually looks pretty awesome) showed no improvement in cognitive function after 3 weeks of training. In this study, volunteers were asked to play either Brain Age, Dr. Kawashima’s Brain Training (a game they designed themselves), Phage Wars (an online strategy game), or no game at all. They were tested on cognitive performance before training, immediately after training, and a week after training had ceased. Most of the groups showed no significant difference in performance, positive or negative, across all time points, the one exception being the Phage Wars group, who performed significantly worse in the follow-up test than they did immediately after the training period.

That is only two papers, there are many more out there, some showing that these brain training games do improve cognition, and some showing that they do not. Basically, science still hasn’t figured this one out yet.

Lest you think there is nothing you can do to make your brain work better, there is one activity that has been shown to improve working and long-term memory, improves mood, staves off dementia in old age, and in general, makes your brain and body happy – cardiovascular exercise. Exercise triggers a molecular cascade in the brain that ultimately results in an increase in synaptic plasticity, that is, the ability of the synapse to strengthen or weaken in response to stimuli. This, in turn, is believed to improve learning, memory, and other forms of cognition.

Exercise also results in an increase in the birth of new neurons in a part of the brain important for learning and memory called the hippocampus. Which brings me to the second part of your question, whether improving memory would result in a decrease in function of another brain area. Cardiovascular exercise does in fact result in an increase in the volume of the hippocampus by about 2%, and it is a reasonable assumption to think that would draw resources away from another brain area. But as we saw with the early-trained musicians, increasing a brain structure could result in better functioning of neighboring regions as the new neurons make more connections. It’s unknown what the limits of this is, though, and as far as I could tell, no one has gone looking for deficits in other brain regions following the increase in hippocampus size, so it’s definitely possible.

Now let’s all go for a run!

If you have a question for one of our neuroscientsist contributors, email Astra Bryant at stanfordneuro@gmail.com, or leave your question in the comment box below.