From Music and People to Brain Waves and Neurons
Links to Wikipedia and other sources are provided throughout if you would like to read further into subjects.
By Blake Porter
Music Rhythms and Coordinating Dancers
Music has the ability to coordinate the movement of all those with the ability to hear it. Large dance troops can coordinate highly complex routines with precise timing by utilizing the different elements of a song for timing and cues. The measure of a song can set the timing so that all the dancers are moving at the same pace. Changes in specific elements of the music can indicate what moves must soon follow so the dancers can prime their brains for the next set of complex instructions. Listening in for specific motifs can allow individual dancers to integrate their part seamlessly with their group who are all also listening for the same epochs that cue their upcoming unique moves. Without the coordination provided by a song to direct many participants or when participants have yet to learn a song, chaotic activity can ensue. Individuals may move at their own pace rather than the pace of the song, resulting in asynchronous activity as a whole. If certain routines require multiple members, some participants may be waiting for the others or some will complete their part to quickly. So what does this have to do with coordinating activity in brains?
Brain Rhythms and Coordinating Neurons
Brain waves are measured just like electromagnetic waves or waves of the ocean. They have a period (how long one wave is), an amplitude (how large a wave is), and frequency (how fast the waves are coming). Period is measured in time, and often taken from the peak of one wave to the peak of the the next wave. Amplitude of brain waves are recorded in voltage, as it reflects the movement of electrically charged particles in the brain, ions. Frequency is how many waves come within one second, expressed in Hertz (Hz).
The different types of music of the brain is broadly defined in frequency bands. Different species have different names for different ranges. Generally, Alpha waves are the slowest, followed by Delta waves getting a bit faster, then Theta, Beta, and Gamma being the fastest; oscillating upwards of 120Hz. Other frequency’s can be observed in unique situations, such as sharp wave ripples in the hippocampus and sleep spindles in the neocortex which can be upwards of 450Hz but happen in only very short bursts. There are also very slow waves that can be observed with periods (the time from one peak to the next) on a level of dozens of seconds. Different rhythms are often seen with different brain activities, such as learning, or exploring the world, or sleep.
Brain waves can be measured in a wide variety of ways, from non-invasive caps to metal wires inserted deep into the brain. The closer you can get to the source of the wave, the cleaner the signal will be. Using EEG caps is like looking through a dirty window, just in this case the dirt is the bone of the skull. Bone is non conductive and blocks some of the electrical activity. EEG’s can only read electrical activity on the surface of the brain. Electrodes however, can measure the electrical activity around where their tip is placed. This allows for reading brain waves from deep brain structures. This also allows for a very clean signal, there is nothing in the way of measuring the electrical activity. When a recording is done with an electrode in the brain, the signal is called the Local Field Potential. This local field potential is the voltage potential of the field locally around the tip of the electrode.
At the most fundamental level, these oscillatory bands are the product of underlying cellular activity. However, many very smart people think, and evidence is mounting, that these rhythms play a functional role in the brain and are not merely an epi-phenomenon. Different frequency bands can exert influence on each other in order to better coordinate the activity of different brain regions. However, the goal of this page is it show how rhythms can coordinate neuronal firing.
Neurons are one type of brain cells and they are the primary means of information analysis and transfer in the nervous system. Neurons are vital for everything from feeling heat on your finger tips to calculating calculus to the generation of words for a novel. Neurons utilize electricity to send information in the form of action potentials. A cell will fire an action potential only when it receives enough input. Action potentials are all or nothing, similar to binary 1’s and 0’s. Neurons either fire or they don’t. The input to neurons can be very diverse. In our sensory system, this can be heat from a stove, pain from a needle, the smell of roses, or light from a computer screen. Neurons in your brain talk to each other with chemicals rather than physical inputs. These chemicals are called neurotransmitters.These different inputs (physical energy or neurotransmitters) act on the dendrites of neurons which all have a diverse range of channels. The dendrites of a neuron are where inputs are received, similar to a microphone. The channels on dendrites are complex proteins with a central pore through the middle, a channel. The proteins that make up the channel are acted on by inputs. These inputs can then modulate the channels, opening or closing the pore to different degrees. When channels are open, ions can flow through them. Ions are small, electrically charged particles. Ions are usually individual molecules of an element. Common ions neurons and their channels utilize to convey information are Sodium (Na+ ), Potassium (K+), Chlorine (Cl-), and Calcium (Ca++).
When a neuron receives enough input and enough ions have been allowed into the cell by channels, the cell fires. The electrical voltage of a cell much reach a threshold, the excitation threshold, to fire an action potential. Every cell has a different excitation threshold, but in general its around -50 microvolts. Without any inputs, most neurons have a resting voltage, called the resting membrane potential, of -70 microvolts. This is mainly due to the negative polarity of proteins within the cell as well specialized channels that pump out positive ions to keep the resting membrane potential stable. Again, a neuron must receive enough inputs to raise its voltage to its excitation threshold. Once the threshold is reached, the cell depolarizes; that is to become more positive in voltage. To achieve this, many sodium channels at the axon hillock quickly open in unison. They are triggered to open by the internal voltage of the cell, which at this point has been raised to the excitation threshold. Sodium is a positively charged ion, so as these Sodium channels open, positive current rushes into the neuron. Due to the negative voltage of the cell, these ions rush in very quickly, like the attraction the positive and negative ends of a magnet. Once the voltage is positive enough, the Sodium channels close and Potassium channels open. The positively charged Sodium can no longer enter the cell. Now, Potassium is also positively charged. However, neurons actively keep a large concentration of Potassium ions in the cell. They do this by pumping in Potassium ions by specialized channels. So, when Potassium channels open, the positively charged Potassium ions rush out of the cells, following a concentration gradient. You can think of this like adding a drop of food coloring to water, the concentrated drop spreads out and diffuses so that the concentration throughout the volume of water is equal. As Potassium exists the cell, it becomes more negative. This point of time, when the voltage is becoming more negative, is the repolarizing phase. However, the neuron overshoots its resting membrane potential and goes even more negative, say -90 microvolts. This is referred to as hyperpolarization or the undershoot. Potassium channels then close and other channels actively work until the membrane potential is back to the resting membrane potential. All of this can happen as quickly as a millisecond. For scale, an eye blink is about 300ms. Finally, there are refractory periods during an action potential. The absolute refractory period is, despite even large inputs, the neuron is unable to fire another action potential. This is during the depolarizing and repolarizing phase. After the repolarizing phase, just prior to the hyperpolariztion phase, the relative refractory period begins. During the relative refractory period, if the neuron receives a large enough input, it can fire again. This input must be a very large amount of positive current in order to overcome the cell’s propensity during this period for a negative potential.
Once a cell has received enough input and fires an action potential, the action potential propagates down the axon. The axon of a neuron can be though of like a speaker. It sends out information to other neuron’s dendrites (microphones). However, neurons don’t often communicate to each other directly with electricity, they use chemicals, the neurotransmitters. This allows for a more diverse range of information to be exchanged, or more complex conversations. If neurons used only action potentials, all they could say to each other way nothing or screaming, and nothing in between; akin to a baby crying. By using neurotransmitters, a wide range of inputs can be sent between neurons. Some neurotransmitters, like Glutamate, is excitatory, or nudges its neighbors to fire. GABA on the other hand, shushes its neighbors. More complex neurotransmitters can be more modulatory; rather than making its neighbors fire, it can make it more easy for them to fire if their microphones hear some Glutamate.
Brains Waves and their Influence on Neurons
As states above, brains waves are the aggregate activity of neurons. These two things play off one another though. Just as dancers use the beat of music to pace themselves, neurons can be paced by the rhythmic activity around it. When the rhythm swings around and the amplitude increases, it is reflective of the voltage. Thus, the voltage around a neuron is now more positive, this can allow for cells to fire more easily. Remember, to fire an action potential, the membrane potential of a neuron must become positive enough to pass the excitation threshold. If the voltage outside a cell is more positive, ions will want to move into the negatively charged cell more. Small inputs to the dendrites that open Sodium channels for a fixed amount of time will now allow for the flow of more ions into the neuron if outside the neuron is more positive. The converse is also true; as the rhythm falls and voltage becomes more negative, ions will have less affinity to want to enter the already negative neuron. To summarize, as the voltage of a brain wave becomes more positive, neurons are more likely to fire an action potential . As a brain wave becomes more negative, neurons are less likely to fire.
The Importance of Synchronous Activity
When neurons coordinate their firing activity to be synchronous, learning can occur. It is important to note, brain rhythms are not necessary for coordinating the activity of neurons. Coordination take place in a few cells in a petri dish. Another example may be with your eyes. The neurons of your right eye are very coordinated due to the similar light they take in. Whereas the activity of the neurons in your left eye will be slightly different. It could be said neurons of the vision system or coordinated by light.Carla Shatz simplified Donald Hebb’s theory of learning to “Cells that fire together, wire together.” This theory has been tested in a wide variety of experiments and has held up to be true throughout the nervous system. To explain further, in a circuit, if neurons that fire action potentials at similar time points and that also synapse onto the same neuron and if their action potentials also cause the cell they synapse on to fire, this entire circuit will be strengthened; dendrites of the postsynaptic cell get bigger and axons of the presynaptic cells grows grow. This is what is referred to as plasticity. Presynaptic cells are the cells that send information to a common neuron, pictured below in blue, orange, and green. The red neuron below is the postsynaptic cell. A synapse the the space between the axon bouton of one neuron (presynaptic) and the dendrite of a second neuron (postsynaptic). The synapse is where neurotransmitters are released. . Brain circuits are plastic and are changing all the time as we are learning. With more and bigger dendrites, more Sodium can be let into the postsynaptic neuron by more channels. With more and larger presynaptic axons, more neurotransmitter can be released.
But they are also changing when we are forgetting. It any point fails, the circuit is weakened, and this can cause forgetting. Forgetting is a very important part of learning, especially if wrong information is to be forgotten. If cells fire at different time points onto a common cell, the cell they synapse onto will likely not have enough input to fire and the circuit is weakened.
If cells fire at similar time points but fire after the neuron they synapse onto fires (possibly from input from other neurons), the circuit is weakened. This is called spike dependent plasticity. You can see below on the right, the post synaptic red neurons axon with spikes overlaid, ordered by time. You can see the spikes coming in from the three presynaptic neurons are not very coordinated with the firing of the postsynaptic neuron.
As stated above, spike timing is vital for learning and strengthening synapses as well. Below you can see the spike times of all three presyanaptic neurons are very coordinated and happen just prior to the red presynaptic neuron. This results in a strengthened circuit.
To conclude, just as dancers follow the rhythm of music to coordinate their activity into art, our neurons synchronize their activity to the rhythms of our brains into us.
Music Rhythms and Brain Activity
As a side note, music can have a strong influence on our brains and even has the ability to synchronize the brain activity of those participating in it’s generation, such as in duets Sanger et al, 2012. Crowds listening to a concert performance almost appear as a fluid being conducted by the gravitational influence of beats just as the moon does to our oceans. And just as the rhythm of music can coordinate the activity of those participating in its experience, brain rhythms can exert similar effects on the brain. Lots of research has been done on the brain and music. Check out the Music in the Brain project I did to hear brain waves.