Behaviour and the brain part 2: Cognitive Neuroscience
Introduction: explaining the brain, modes of explanation
Neuroscience regards itself with answering the question: can we find one to one mappings between
mechanisms in the brain and psychology (mental stuff). There are many modules / processes /
mechanisms involved in the relationship between thoughts and the brain. Descartes was the first
person to introduce the mind-body problem. The mind-body problem can be divided into 2:
- Dualism (cartesian duality): mind - matter
- Monism
o Physicalism: matter > mind
o Idealism: matter < mind
o Neutral monism: 3rd substance > matter & mind
The consensus is that the mind is what the brain does (physicalism). Phineas Gage is one of the most
famous neuroscience cases. He worked in railroad construction, and after suffering from brain injury,
he survived but had a personality change.
There still is the question whether the brain is a homogenous general-purpose machine or if distinct
regions execute distinct functions. Phrenology leads us to believe that there is modularity in the brain.
According to Fodor, there are 3 features of a module:
1. Modules are domain-specific
2. Their operation is mandatory
3. They are informationally encapsulated
Empiricists have long stated that the mind at birth is a “tabula rasa” (empty slate), which is a key topic
in the nature vs nurture debate. The idea of modularity is closely linked to that of innate knowledge.
There are many examples that support the idea of modularity: language, vision, maps, pathways etc.
These examples help us study the brain, and hopefully the organization of the brain reveals something
about the architecture of the mind. The interesting question, however, is not where modules are, but
what they do and how they map onto mind and behaviour.
Different explanations of a function or a mental phenomenon:
- Box-arrow cognition models (functional): e.g., the explanation of the ‘stages’ of memory
o Behaviourist model: can only study observable / external behaviour
o Cognitive model: can scientifically study internal behaviour
They apply some sort of functional analysis (asking why), often use mental terms in the
explanatory boxen. ‘How possibly?’, but not ‘how actually?’. Box-arrow models are often
underdetermined (multiple different models can explain the observed phenomenon)
- Reductionist explanations: identity statements between higher and lower levels (e.g., X
reduces to Y, or, X is the sum of the parts of Y). These models assume that descriptions at
lower levels can fully replace descriptions at higher level. Bridge-laws make the higher-level
descriptions superfluous. However, in practice, this does not seem to work. The whole is the
same as the sum of its parts (= reductive).
- Mechanistic explanations: a mechanism for a phenomenon consists of entities and activities
organized in such a way that they are responsible for the phenomenon. The whole is more
than the sum of its parts (= nonreductive). Emergent properties lead to the statement that the
whole is more than the sum of parts, but there is no ‘spooky emergence’. A phenomenon can
consist of multiple levels. Phenomena may not have a ‘fundamental’ level of explanation but
require a multi-level understanding. Some levels can be so far removed from the phenomenon
that it is hard to see how they are related, despite being essential.
Etiological causal relevance does not identify a mechanism. To identify a mechanism, we need to
establish constitutive relevance. When we want to identify a function, there are two types of errors
that may take place:
, - Lumping errors: describing two or more mechanisms in the brain as a single cognitive
function. We may also describe this as ‘one-to-many mapping’. For example, memory can be
explained by different functions from different brain regions (e.g., declarative vs working
memory). Memory is not one mechanism/area.
- Splitting errors: describing a single mechanism in the brain as multiple cognitive functions.
Attention may be constituted by the same mechanism as imagery as working memory etc. We
may also describe this as ‘many-to-one mapping’.
History, neurons, and action potentials
The Edwin Smith Papyrus is an ancient Egyptian medical text, treatise on trauma. It contains the first
known anatomical descriptions of the brain that are rational. The word ‘brain’ appeared for the first
time in any language. The ancient Greeks were the first to try explaining natural phenomena through
theories and hypothesis without reference to mythology and the first to use deductive reasoning
(applied to geometry). Pythagoras made the first distinction between mind and soul, the first
conceptualization of dualism. Aristotle wrote about motivation; urges, desires, impulses and used the
mind/soul as reason for the existence of the body.
Early neuroscience was called phrenology, founded by Franz Joseph Gall and George Combe. Luigi
Galvani was the father of electrophysiology, which is the study of electrical properties of biological
cells and tissues. Galvani discovered in the late 1780s that stimulating the nerves of a dead frog with
electricity resulted in muscle movement. He coined the term ‘animal electricity’ to describe the force
that activated these movements. In 1924, Hans Berger succeeded in recording an EEG in a human
subject for the first time.
Santiago Cajal was a histologist who used staining to identify neurons. The brain is composed of
discrete units called neurons. The brain consists of 86 billion neurons. The first neurons probably
developed over 600 million years ago and the first brain-like structures probably evolved in the first
50-100 million years after the first neuron. There are 1 quadrillion synapses in the brain of a 3-year
old. Cajal was also the first to discover the synaptic cleft. Otto Loewi discovered the mechanism of
chemical transmission between neurons. Then, the Hodgkin-Huxley model of the action potential was
developed.
Neurons are protected by glia cells, of which there are different types:
- Oligodendrocyte: in CNS, myelinate many axons
- Schwann cells: in CNS, myeline one single axon
- Astrocytes: structural support, maintain chemical balance
- Microglia: immune defence, move around the CNS to destroy intruders
Neurotransmitter release: Action potential invades the axon terminal, opening the Ca 2+ channels -> the
entry of the Ca2+ causes the vesicles to fuse with the membrane, allowing neurotransmitters to be
released.
1 neuron has 4 functions:
1. Dendrites: collecting signals -> neurotransmitter uptake
2. Soma: integrating (signal summation) -> action potential
3. Axon: conducting signals
4. Axon terminal: outputting signals -> neurotransmitter release
Neurons perform logical calculus. If neuron Z needs 2 inputs to fire, it will only fire by 2 excitatory
signals, not from 1 excitation + 1 inhibition (since they cancel each other out).
There are different types of neurons:
- Sensory neurons = input
- Motor neurons = output
- Interneurons = between sensory input and motor output. Nearly all neurons in the brain are
interneurons.
There is huge variation in dendritic spread.
