Anticipation, Synaptic Plasticity, and Introspection



A Poem

Neurons are united together.
to mediate a complex behavior.
Connected by synapses,
like the roads between blocks.
A city of neurons has emerged.
A brain as it is called. 1

Anticipation is an intention:
when stimuli of a kind is received,
a specific behavior is followed. 2
Stimulating a neuron might activate
many pathways that succeeds.
As many paths you can choose
to go from a block in the city.
But anticipation ensures
only one pathway is to be activated,
that from the stimuli to the behavior. 3
Its synapses have to be facilitated,
as the roads are broadened,
so the pathway can beat the others,
whenever the stimuli are received.
Facilitation by chemical modulators.
They are produced in the synapses,
at each moment it is anticipated.
Persist for a while, soon metabolized. 4

When continually anticipate
an anticipation without a break,
produced more than metabolized,
the chemical modulators accumulate.
Concentrations rush high and
plasticity is triggered.
In a short period it forms
long-lasting change in the synapses. 5
By simply sustaining it,
anticipation becomes a reality.
As reality is so plastic.

But mind desires to wander around.
It must be carefully watched,
for an anticipation can be sustained.
An introspection that is persistent.
For plasticity it is important.

Example

Here is an example illustrating what happens when we try to sustained the anticipation of being present. The stimuli come from the six sensory organs 6 and the behavior is noticing the stimuli at each moment. Usually, we are not in the moment, but lost in the thoughts that are far away from the present. The anticipation can be represented by meaningful inner sounds or images, and evoked by reciting the sounds or imagining the images. For example, it can be an instruction like “fetching the newest senses”. These inner sounds or images may be different for each individual. But, no matter what representation you tend to choose, it has to be ensured that, immediately after anticipating, you can notice the senses constantly arising moment by moment. This is because the synapses along the pathway from the stimuli of senses to the behavior of noticing are facilitated, so it is ensured that, when senses arise, it is this pathway, rather than any other that succeeds to the stimuli of senses, that is activated. Even though the senses are still unclear at first, it has indicated that the pathway from the stimuli to the behavior does exist and be modulable. This modulation is mediated by the chemical modulators in the synapses along the modulated pathway. If the anticipation can be sustained by, for example, constantly reciting “fetching the newest senses” and thinking of its meaning without a break, your noticing will become sharper and sharper, and increasingly consistent. Details reveal themselves. This is the effect of accumulation of the chemical modulators. But, it will not last long. The mind is constantly searching for something attractive, something new. When the mind slips away, if it is detected in time, we bring the mind back to anticipation again. Then, nothing is really interrupted and the accumulation goes on. Otherwise, the mind goes astray and soon the chemical modulators lose their concentrations. When the anticipation can be sustained, nothing stops the chemical modulators accumulating. Synaptic plasticity is then triggered in an extreme way. It is like forming highly emotional memory: long-lasting changes are made in a period that is short.

The process illustrated here holds not for the anticipation of being present alone, but for any anticipation to which a modulable pathway corresponds, since the principle behind is the same.

References

  1. Principles of Neural Science, by E. Kandel, et al., the 6th edition. ISBN: 1259642232.
  2. Attention enhances synaptic efficacy and signal-to-noise in neural circuits, by Briggs, F., Mangun, G. R., and Usrey, W. M. (2013). DOI:10.1038/nature12276

  1. Neurons are the cells used for processing electrical signals. A neuron accepts signals from some others by its branch-like input terminals. It then integrates the signals, propagates the result along a long channel called axon, and transmits to other neurons at the ends. For most neurons, at the output terminals of axon, the electrical signals have to be converted to chemical signals, called neurotransmitters, released from the output terminals to the input terminals of the successive neurons. These terminals are called synapses.

    A neuron is too simple to mediate behavior. Neurons must work together. Connected by synapses, they form a network. It is like a city. Neurons are tightly connected locally, like a lot of small towns. Between the towns are long distance connections, the highways. A complex behavior is then mediated by activating a cascade of neurons, a pathway in the network, like a path in the city, along which information is propagated and processed.

    For details, see chapter 3 of Principles of Neural Science. ↩

  2. Throughout this note, we use the word “anticipation” in this narrow sense. When we are looking for something red, we anticipate finding red object, where the stimuli come from seeing an object and the behavior is identifying if it is red. Before sleep, we anticipate feeling more and more relaxed as the clock ticks, where the stimuli come from hearing the clock ticks and the behavior is active relaxation. When practicing guitar, the stimuli come from playing a chord and the behavior is playing the successive one. Notice that anticipation is independent of the appearance of the stimuli; it is an intention, a mental activity. ↩

  3. As an example, when we are cued to detect red objects, the pathway that responsible for detecting red color is facilitated, so that whenever we see an object, this pathway has much higher probability of being activated, so that we can detect red object correctly but missing the objects with other colors. For details, see the paper Attention enhances synaptic efficacy and signal-to-noise in neural circuits.

    The famous “invisible gorilla” experiment made by Daniel Simons is another example. ↩

  4. Neurotransmitters are received by receptors on the membranes of synapses. There are two kinds of receptors. One is ionotropic receptor, which mediates the propagation of electrical signals. It opens ion channels which in turn changes the membrane potential, converting chemical signals back to electrical signals. The other is metabotropic receptor, which modulates the propagation by modifying the strength of synapses. When specific molecules bind to a metabotropic receptor, it changes its shape and reveals some sites. These sites will activate specific enzyme-like substances, which then diffuse in the synapse to catalyze a series of chemical reactions that facilitate or inhibit the release or the reception of neurotransmitters. An activated metabotropic receptor is like a coffee machine, and inactive enzyme-like substances are exhausted workers. They drift to the coffee machine, drink espresso, and become energetic and capable of work. They will keep energetic for a while, from hundreds of milliseconds to minutes, then becomes exhausted again. The energetic workers are called modulators. Any time when a metabotropic receptor is activated, certain amount of the modulators are produced in the synapse.

    For the types of receptor, see chapter 11 of Principles of Neural Science. And for the details of metabotropic receptors, see chapter 14 of the same book. ↩

  5. The chemical modulators in the synapses along the modulated pathway are called also second messengers, such as protein kinase A. They will accumulate within the synapses. When their concentrations become sufficient, it is possible that the modulators diffuse into nucleus. Over there, with a series chemical reactions, they trigger the expression of DNA that produces the proteins used for building new synapses. This leads to an anatomical change in the synapses, a change that is long-lasting.

    It should be noted that not all classes of neuron are equally modifiable. For instance, the neurons for primary physiological functions should not be plastic.

    For details, see chapter 53 of Principles of Neural Science. ↩

  6. Aristotle defined five senses: vision, hearing, smell, taste and touch. Intuition is usually recognized as the sixth sense. These six senses are consist with the Buddhist cognition of sensory system, vividly named as “six sensory doors”: eyes that mediate vision, ears that mediate hearing, nose that mediates smell, tongue that mediates taste, body that mediates bodily sensations, and mind that mediates emotions and thoughts. ↩