Consciousness

Between the midbrain and the cortex is the thala-

mus, which contains relay areas for sensory inputs

including vision, hearing, and touch, as well as

motor functions. These ‘relays’ do not just pass sig-

nals on but form crucial parts of complex loops to

and from the cortex which lies above and around it.

These thalamocortical loops have been implicated

in consciousness (e.g. Llinas and Ribary, 2001; Trapp,

Schroll and Hamker, 2012) and are especially well

developed in humans.

Finally, there is the cortex, or outermost layer of the

brain. Its oldest and innermost part, the limbic sys-

tem, is common to many other animals and is some-

times referred to as the reptilian brain. This includes

many structures implicated in consciousness: the

hippocampus, essential for laying down long-term

memories and forming cognitive maps; the amyg-

dala, with roles in rewards and emotions; the hypo-

thalamus, which regulates the autonomic system

including blood pressure, heart rate, and sexual

arousal; and the cingulate gyrus, which is involved

in emotion, pain, and motivational responses. These

are all hidden underneath the neocortex.

A man who attempts to ease the miseries

of failing minds by repairing brains is

bound to respect the material world,

its limits, and what it can sustain –

consciousness, no less. It isn’t an article of

faith with him, he knows it for a quotidian

fact, the mind is what the brain, mere

matter, performs. If that’s worthy of awe, it

also deserves curiosity; the actual, not the

magical, should be the challenge.

(Ian McEwan, Saturday, 2005, p. 67)

The neocortex has expanded more than any other

part during human evolution, becoming deeply

folded to give a large surface area of grey matter

(neuron cell bodies and unmyelinated, or electri-

cally uninsulated, fibres) on top of the white matter

(myelinated axons). Its two main types of neurons

are excitatory pyramidal cells and inhibitory inter-

neurons. Most of the cortex is arranged in six layers,

with layer 1 on the outside. There are also vertical

columns showing functional organisation, such as

nuclear magnetic Resonance (mRI)

mRI measures the radio signals emitted by some atomic
nuclei (e.g.^1 H,^13 C, and^31 P) when placed in a magnetic
field and excited by radio frequency energy. the radiation
emitted provides information about the chemical envi-
ronment of the nuclei. In the 1970s, the idea of using
hydrogen atoms in the body for imaging was developed
into fmRI (functional mRI), which can provide extremely
detailed images of living brains. early methods required
injections of a paramagnetic substance, but in the 1990s
totally non-invasive methods followed, including the use
of BoLD (blood oxygen level-dependent) contrast, which
allows measurement of local brain metabolism. fmRI
measures neuronal activity only indirectly, depending on
metabolic and haemodynamic responses to neural activity,
which limits its temporal resolution. For brain scanning,
the head has to be placed inside the scanner and kept
very still. the results are displayed using false colour
to produce the familiar coloured images of the brain in
action. Although they may look like direct representa-
tions of brain activity, the published images have gone
through many stages of processing and statistical analysis
and must be interpreted with care: the readings are sub-
ject to noise at every stage of the process; false positives
are extremely easy to generate when a set of ‘standard
assumptions’ are not met (eklund, nichols, and Knutsson,
2016) – so easy that a dead Arctic salmon can appear to
be engaged in a perspective-taking task (Bennett, miller,
and Wolford, 2009); and basic variables like breathing
may be serious confounds (Birn et al., 2006; Huijbers et
al., 2014). there are now numerous open-science initia-
tives for enhancing validity and reproducibility in fmRI
research, but we must bear in mind its limitations.

transcranial magnetic stimulation (tms)

In tms, or repetitive tms (rtms), a coil held over the brain
generates a pulsed magnetic field which stimulates neu-
rons in a focused area by inducing small local currents.
stimulating motor areas induces involuntary movements,
and if the precise area stimulated is located by scanning,
this allows motor cortex to be accurately mapped. simi-
larly, visual or speech areas can be mapped because tms
suppresses function in the area stimulated. tms can also
be used to induce particular experiences or altered states
of consciousness (Chapter 13).