Trees of the Brain, Roots of the Mind by Giorgio A. Ascoli

Author:Giorgio A. Ascoli
Language: eng
Format: epub
ISBN: 9780262028981
Publisher: The MIT Press
Published: 2015-02-23T11:00:00+00:00

Brain Mind Temporal Scale

Circuit spiking patterns Mental state 0.3–30 seconds

Synaptic strength change Thought likelihood 0.3–30 seconds

Synaptic formation/pruning Knowledge acquisition 3 seconds to 30 minutes

Axonal-dendritic overlap Immediate learning ability 3 minutes to 3 hours

Axonal/dendritic growth/retraction Learning by training 3 days to 3 years

Neurogenesis and neural migration Development of new capacity Entire lifespan

Such lines of communication allow circuits to chain together consecutive waves of activity representing longer-lasting mental states, typically up to several seconds or even minutes. In addition to forming the neural correlates of mental experience, extended firing sequences also alter the strength of the underlying synapses. These modifications take only seconds to occur but can last for an entire lifetime. They are mediated by complex molecular machineries that are much (more than ten times) larger than the ions underlying spikes but still much smaller (also more than tenfold) than the thickness of the smallest neuronal branch.11 The mental correlates of these events are changes of how prone we are to experiencing the mental content implicating those synapses.

The endless flow of activity in the nervous system also reestablishes the very presence and location of connections among neurons, creating some new synapses on appropriate axonal-dendritic overlaps and eliminating others. These crucial connectivity touch-ups may take from a few seconds to several dozen minutes and reflect the quintessential process of learning or acquiring the capacity to experience a new mental state. The key players in such a game of structural plasticity are dendritic spines and axonal varicosities, whose size is comparable to the thickness of neuronal branches. Last, at the slowest end of the scale, new neurons can appear, and existing neurons can rearrange their branches during the course of several months and over the entire arbor, invoking a spatial span thousands of times larger than the branch thickness. These changes correspond to slow alterations of expertise and require persistent repetition. By modifying the compendium of axonal-dendritic overlaps, these arbor rearrangements fundamentally affect the capacity to form new synapses and thus the ability to learn further information.12

Before closing this very central chapter, we should remember once more that the neural correlates of mental states are cell assembly activity patterns rather than spikes in individual neurons. The last chapter distinguished between knowledge as the capability of instantiating mental states (con­ceptually corresponding to synaptic connectivity) and the probability of eliciting a given mental state (determined by synaptic strengths in the circuit). If mental states were represented by single neurons, the above correspondences of synaptic connectivity as knowledge and of synaptic strength as probability of experience would be strictly literal. Instead, in the last chapter we showed that coding by cell assemblies somehow blurs the line between these two aspects. As a result, synaptic strength occasionally affects the capability of instantiating a mental state, and synaptic formation similarly influences the probability of experience.

Coding by cell assemblies could have the same effect on the correspondence between axonal-dendritic proximity and the ability to learn. In other words in many circumstances it may be fair to say that axonal and dendritic branching determines the probability, rather than possibility, of learning.

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