Friday, January 22, 2010

Mechanisms, Experiments and Memories (Part 2): The Levels of the LTP Mechanism

This post is part of my series on the work of the philosopher Carl Craver. The series deals with the nature of neuroscientific explanations. For an index, see here.

I am currently looking at an article by Craver entitled "Interlevel Experiments and Multilevel Mechanisms in the Neuroscience of Memory".

Part One sketched Craver's informal account of mechanistic theories and applied it to the LTP (long-term potentiation) mechanism in memory-neuroscience. This part continues by examining the levels in the LTP mechanism. This paves the way for a taxonomy of the interlevel experiments that are used in neuroscience.

Three Types of Level
One of the crucial features of the LTP-mechanism is that it occurs in a nested hierarchy of mechanisms. This description suggests that we can talk meaningfully of different ontological levels of organisation. If we do so, we must be cautious. Craver identifies three different uses of the term "level".

First, there is the aggregative use of "level". This is the idea that entities are mere aggregates of smaller (i.e. lower-level) entities. This brings with it the idea of aggregative decomposition. You can decompose a ball of wax by cutting it into smaller pieces; you can recompose it by glueing these pieces back together. The aggregative use of "level" is concerned with relations of size, and not with activities and functional organisation.

Second, there is the functional use of "level". This describes relationships between abstract roles. Functional decomposition arises when you break one task down into a number of sub-tasks or sub-routines. This is sometimes done in neuroscience but there is a tendency for it to float-free of ontological reality (they are how-possibly models).

Third, there is the mechanistic use of "level". This arises when one mechanism is broken down into sub-mechanisms, which are in turn broken down into sub-sub-mechanisms. This is the preferred use of "level" in neuroscience and we can see it at work in the case of LTP.

The Levels of the LTP Mechanism
As mentioned in Part One, the LTP mechanism strengthens synaptic connections and is implicated in spatial memory and learning. Viewing it from the perspective of Craver's philosophy, we can discern at least four mechanistic levels.

We begin at the behavioural-organismic level. Suppose we are trying to study spatial memory and learning in the mouse. We will usually do so by placing a mouse in a maze of some sort and then subjecting them to different behavioural tests. From these tests we work out the conditions under which spatial learning takes place.

We then move to the computational-hippocampal level. This is our first glimpse beneath the skin of our model organism. We look to the structure and connectivity of the hippocampus and the various computational processes it performs learning. We investigate this level with direct stimulation, surgical resection, EEG, MRI and computer-modeling.

We next move to the electrical-synpatic level. Here we are examining how neurons are stimulated, the volumes of glutamate they release, the new growth in dendritic spines and so on. We investigate this level with microelectrodes and pharmacological agonists and antagonists.

Finally, we reach the molecular-kinetic level. We now look at NMDA and AMPA receptors, diffusion rates of Mg2+ and Ca2+, glutamate binding and so on.

These four levels are illustrated in the diagram below.

With this understanding of nested hierarchies and mechanistic levels in place, we can proceed to consider experimental and integrative (non-reductive) strategies in contemporary neuroscience. This will be done in the next part.

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