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Chapter III. The Neurological Basis of Conscious Experience >> Neuronal synchronization

Neuronal synchronization

Nearly everyone has seen a visual simulation of a heart pumping blood. The atrial contraction is quickly followed by the ventricular contraction. The cycle is repeated every second or so. The heart consists of hundreds of thousands of small muscle cells. Cardiac muscle cells look like cylinders about 0.05 millimeters in diameter and 0.1 millimeters in length - just a bit bigger than a neuron. For a heart to effectively pump blood, all cardiac muscles in the ventricles have to contract nearly simultaneously. To ensure that the contraction starts at the same time, a wave of electrical excitation runs through the ventricles and stimulates each cardiac muscle cell to fire an action potential. An action potential is quickly followed by the contraction of muscle cells. In other words, muscle cells must fire an action potential and contract synchronously on the micro level for effective blood pumping on the macro level.

When muscle cells of the ventricles de-synchronize, the heart stops pumping blood. This condition is called ventricular fibrillation. The heart looks like a bag of worms with patches of ventricles contracting out-of-phase with each other. Ventricular fibrillation is a life threatening condition. The patient can die, unless the heart muscles are synchronized within five to ten minutes. A defibrillator is used to synchronize the muscle cells of the ventricles. The defibrillator passes a large electrical current through the heart, which synchronizes all cardiac muscles. Synchronized cardiac muscle cells start to effectively pump blood, thus saving the patient’s life.

Until recently, the brain was viewed as a collection of neurons that fire in asynchrony. The synchronous firing of cortical neurons was only associated with epileptic seizures: wavelike electrical activity of a large number of neurons, often associated with loss of consciousness and involuntary body movement. New evidence indicates that neurons often communicate by firing near synchronous action potentials. In fact, there is evidence that de-synchronization is associated with disorders such as schizophrenia and ADHD. The emerging picture of the brain is reminiscent of the synchronized firing of cardiac muscles necessary for effective blood pumping.

The synchronous firing of neurons may be controlled on both the local and global levels (see Fries, 2005, for review). On the local level, groups of neurons within one cortical area have to synchronize in order to vote on a particular issue. In our analogy of the brain as a company, a local group of neurons corresponds to a department within the company. Each local group of neurons undergoes low amplitude depolarization waves with a frequency of 30 to 100 Hz. These waves are called gamma-frequency waves. The depolarization wave does not make each neuron in the department fire an action potential. However, neurons are more likely to fire an action potential at the peak of the depolarization wave because at the peak, the threshold for action potential firing is reduced. This mechanism ensures that voting neurons fire action potentials at the same time. Recall that the communication delays in one cortical area (one department) are on the order of 1 to 3 milliseconds. A depolarization wave with a frequency of 30 to 100 Hz corresponds to a cycle duration of 10 to 30 milliseconds. Thus, there is an ample window of time for communication on the upswing of the depolarization wave within each cycle.

When a gamma-frequency wave is superimposed on the resting membrane potential, neurons are more likely to fire action potentials on the upswing of the depolarization wave.

Global attention rhythm

In addition to the local depolarization wave, a global wave with a frequency of 15 to 25 Hz ensures effective communication between neurons in different cortical areas (analogous to a company-wide meeting). This wave is called a beta-frequency wave. Both beta-frequency and gamma-frequency cortical neuron synchronizations are observed for consciously perceived stimuli (Meador, 2002; Gross, 2004; Nakatani, 2005; Palva, 2005) and for conscious perception in binocular rivalry (Fries, 1997; Srinivasan, 1999; Fries, 2002; Doesburg, 2005). There is strong evidence that conscious perception is associated with the global synchronization of neuronal assemblies in the beta-frequency. Accordingly, lets refer to the beta-frequency wave as the attention rhythm. Such a naming emphasizes that consciousness is not continuous in time but intermittent, with 15 to 25 frames per second. According to the theory put forth in this book, neuronal assemblies that intend to reach consciousness have to fire action potentials in synchrony with and on the upswing of the attention rhythm. The assemblies firing out-of-phase or on the downswing of the attention rhythm cannot be registered by the consciousness. These assemblies continue processing information and serving their voting decisions subconsciously. Their work is not registered by the mind.

As a result, any mental process can be brought into consciousness or out of consciousness by phase shifting the assembly voting into synchrony or out-of-synchrony with the attention rhythm. This is a very convenient feature that allows our mind to switch attention between assemblies of neurons without disrupting the signal processing inside these assemblies. In other words, I can switch my attention from processing a basketball game on TV to processing my wife’s voice without turning off my visual cortex. All I need to do is to shift the visual processing of the TV out-of-phase with my attention rhythm. Note that my visual cortex is still processing information from the TV (I am still looking at the screen), but this information does not reach my consciousness. In fact, the visual cortex is actively processing the eye’s input even in anesthetized people and animals. Note that in this case, the visual cortex is active in complete absence of both conscious visual percept and mental imagery. Clearly, animals and humans are not conscious of all neural activity in the cortex.

Another example involves driving. When driving on a new road, one often needs to concentrate on the processing of visual cues. The visual processing occurs in-phase with the attention rhythm. It is recorded into memory and one can later recall where one was driving. On the other hand, when driving on a familiar road, one can shift visual processing of the road out-of-phase with the attention rhythm and think of something else.

The most dramatic example involves reading a book aloud while thinking of something else, as was discussed in the previous chapter. On many occasions I have had the experience of reading a book to one of my children and suddenly noticing that my thoughts were elsewhere. Whenever that happened, I could never recall the content of the book that I had just read. I could not remember the words that I had just spoken; I only remembered my own thoughts. What was going on in my mind? Clearly, I was visually processing the printed words, interpreting those words into auditory phonemes, and speaking those words aloud. All these processes occurred in-phase with each other but out-of-phase with my attention rhythm. None of the spoken words were registered by my consciousness because the reading and speaking occurred in between the consciousness frames. What was my consciousness doing? It was busy thinking of something else. The flow of thoughts occurred in-phase with the attention rhythm.

How is the brain able to synchronize cortical areas located in different poles of the cortex with the attention rhythm? First of all, the attention rhythm frequency of 15 to 25 Hz corresponds to a cycle-duration of 40 to 60 milliseconds. Thus, even with conduction delays as long as 5 milliseconds, all neurons can still vote on the depolarizing upswing of the attention rhythm. In fact, during the 20 to 30 milliseconds of the upswing of the attention rhythm, neurons have time to fire a train of up to 10 action potentials.

In addition, it turns out that the brain can regulate the conduction velocity in axons, the fibers connecting neurons. By changing the properties of axons, such as the diameter and insulation (myelination), the brain adjusts conduction velocities in the different parts of the cortex to ensure a synchronous arrival of action potentials at all neurons receiving the information, independent of physical distance (Innocenti, 1994; Salami, 2003).

Another question: how can the brain operate on such a fast time scale? When thinking about this question we should consider a body’s ability to control movement. A tennis player needs to coordinate hand movement on the millisecond time scale in order to hit the ball in the correct direction. Other activities such as juggling and driving are no less demanding. The execution of such precisely coordinated movements indicates that the brain is able to accurately time the activity of groups of neurons.

Note that the movie industry knew about the attention rhythm a long time ago. Silent movies were shot at a rate that was between 16 frames per second and 23 frames per second. (When sound-film was introduced in the late 1920s, 24 frames per second was chosen because it was the slowest (and thus cheapest) speed which allowed for sufficient sound quality. TV currently shows 24 frames per second.) The frame-rate of 16 to 23 frames per second was chosen because the viewer could not see the flickering between frames due to an effect known as persistence of vision, whereby the eye cannot react faster than about 60 milliseconds. At a frequency below the 16 frames per second threshold, the eye registers a distracting flicker caused by the shutter of the film projector. Note that a 16 frame per second threshold is just 1Hz greater than the 15 Hz frequency of the attention rhythm.

One can argue that persistence of vision is a property of the eye and has nothing to do with the attention rhythm. Not so. The eye sensory properties developed during evolution to serve the full potential of the mind. We do not need an eye that can change its activity faster than the mind can register the change. The eye has a 16 Hz threshold because this was the simplest construction that served a mind with a 15 Hz attention rhythm. If our attention rhythm were faster, the eye structure would have evolved to register faster changes. In other words, the eye is using the least complicated (and therefore the slowest) mechanism that serves the conscious mind with its 15 frames per second attention rhythm. The visual system is optimized for the conscious experience.

Conscious experience

The conscious experience is not limited to the visual domain. A conscious experience can also include audio sensations, touch, smell, and taste. The long-range attention rhythm synchronization theory can account for all sensations. The theory claims that an ensemble of neurons that fire in synchrony with the attention rhythm reaches consciousness. The ensemble of neurons need not be limited to visual neurons; it could include neurons coding for any sensual modality.

In the previous section we discussed the evidence that visual perception and mental imagery share underlying neural processes. We also discussed that the synchronization of an ensemble of neurons coding a visual object with the attention rhythm is responsible for generating a conscious experience of the visual perception of that object. Now it is time to expand the definition of neuronal ensemble to include all senses. We can now make the claim that perception of physical sensual information of any modality shares underlying neural processes with imagining that sensual information.

Throughout life we accumulate a myriad of conscious experiences. They can include senses of any modality. Every conscious experience is represented by synchronized activity of certain cortical neurons (neuronal ensemble). The information about each neuronal ensemble in each time frame is stored in memory. To re-enact the conscious experience in our mind (imaginary experience), we re-activate and synchronize largely the same cortical neurons that were activated during the real experience [10]. The more accurately the mind is able to recreate the cortical activity pattern, the closer the imaginary experience is to the real experience. Note that, normally, the brain does not bother to activate all neurons necessary for recreating the real experience (our memories are often vague). However, sometimes, under the influence of drugs, strong emotions, or significant concentration, a very realistic experience can be re-enacted from memory. In this case, we would expect the cortical activity to be very similar to that which occurred during the real experience.

Both the animal mind and the human mind are able to recreate cortical activity patterns in a memory recall process. However, the human mind can imagine objects that it has never seen before (for example, a bicycle with wings). Mental synthesis allows humans to synchronize several ensembles of neurons with the attention rhythm to enact a new, unique conscious experience. In other words, both humans and animals are recording their conscious experiences on a roll of film. Both humans and animals can watch any frame of that film in their mind. The uniqueness of humans is in their ability to combine any number of frames or parts of those frames at will. Humans can mentally reposition the actors, reset the scenery, change the words spoken by the actors, and then record the imaginary film on a new roll.

This is the process that we refer to as mental synthesis. Throughout the book we have only discussed mental synthesis in the context of a visual experience. However, mental synthesis does not need to be unimodal. Any number of experiences can be manipulated in our mind. Our mind can picture a person with three heads (since you have never seen a person with three heads, you must have synthesized it in your mind), then add a soundtrack, and, finally, imagine that this three-headed person is stinky. Thus, the definition of mental synthesis can be expanded: mental synthesis is the process of creating any new imaginary experience in the mind by combining any number of previous experiences together.

It should also be mentioned that not every conscious experience is stored in long-term memory. Initially, the conscious experiences are encoded by a short-term modification of synapses. We refer to this type of memory as short-term memory. The short-term memory can hold only a few days worth of recording. Unless short-term memories are converted to long-term memories, they are written over. Only the most important, emotionally charged experiences are converted from short-term memory to long-term memory. This arrangement makes a lot of sense since at the time of a conscious experience it is often not clear if that experience is important or not. The importance of events is often determined on the following day or even several days after the event. For example, I remember every detail of the conversation with my father the night before he was killed in an accident. At the time, the conversation was not important. I would have never remembered it if my father had not been killed. The death of my father on the following day made the conversation important and I still remember it 25 years later. In all likelihood, I am re-enacting the conversation in my mind by re-activating and synchronizing the same neurons that were active and synchronized with the attention rhythm 25 years ago during the real conversation.