This is a commentary on the book by David Marr. Highly recommend to read if you are interested in computation and brain.

Theorisation in the pursuit of Neuro-Gnosis

3 levels of explanation proposed by David Marr, in his book Vision. Understanding the brain and a particular process of the brain are two different things. The genesis of the argument David proposes comes from the idea that Computers and Computations though closesly related are different entities. He proposes a 3 step process that can be pursued parallelly or sequentially which will definitely reveal some intricacies of the system, but for a wholistic understanding of the information-processing task, the investigator has to attach all the 3 steps.

The 3 Levels • In order to understand any entity or system it is necessary to describe its attributes as to what it accomplishes. This is level 1 and this stems primarily from philosophical logic. • Moving further to understand the algorithmic nature stems from the quest of computational logic which puts the question of ‘How to implement’ in the central theme. • The final step in the physical realization of the idea is to run or implement it in parts or whole within a physical (synthetic) system or provide proof of its implementation (in a natural system) by means of the above 2 steps.

Looking at a problem in Neuroscience through this lens, and how it can be analyzed at these 3 levels.

Let’s take the example of the olfactory system in rats.

• At level 1, the Olfaction system tries to ‘see’ the environment through chemicals. As chemicals are the characteristic of any physical entity, it provides reliable inputs for the organism to facilitate its existence.

• At level 2, Many theories have been proposed, one such theory is that olfaction is primarily a bayesian inference task and that different types of neurons compute different variables in order to implement the sensory as a whole.

• At Level 3, Anatomy, morphology, synaptic plasticity mechanisms, the role of neuro-modulation, dendritic compartmentalizations, excitability of cells, physiology of ion channels form the basis of level 3.

Do these levels still hold?

I will try to put the assumptions or ground on which his theory stands, and if we question these assumptions we get arguments for - For and Against. The idea that everything is intricately related to everything else is not considered here. So, at a higher level of understanding two systems remain mutually exclusive to each other. Examples: The influence of vision on olfaction or on motor performance is considered not present, which is not the case in real life. Can we say for sure that the effect of rotation of the earth has no effect on information- processing of the brain? The goal of computation is defined as absolute, and as an end to itself, which in reality is not, and in the least, is debatable. This subjective nature and the concomitant run-down flow of logic may have very different consequences.