Consciousness

Chapter

Four

Neuroscience

those relating to neighbouring areas of skin or muscles, or
the visual field. Sensory areas are organised in a roughly
hierarchical manner: levels of processing build on each
other and neurons doing similar jobs are close together;
but no area is isolated from the rest and everywhere there
are long cortico-cortical connections and cortico-thalamic
loops providing a massively interlinked system with no
ultimate top.

The two hemispheres of the cortex are linked by the white
matter of the smaller anterior commissure and much larger
corpus callosum, which is a wide band of about 200 million
fibres beneath the cortex. Each hemisphere has four lobes
(see Figure 4.1). Although these were originally labelled by
appearance, they turn out to be roughly divided by function: the occipital lobe
deals with vision; the parietal lobe includes sensory association areas as well as
somatosensory cortex and the dorsal stream; the temporal lobes include auditory
areas and memory functions as well as the ventral stream; and the frontal lobes,
which are especially large in humans, deal with forward planning and executive
functions.

To understand the neural basis of consciousness, far more detail is needed (see
e.g. Baars and Gage, 2010), but this superficial overview should be enough of a
guide for those whose primary interest is psychological or philosophical. We can
now begin that look inside the brain. But what do we look for?

The most popular method of evading the mystery (or tackling the ‘easy problems’)
is to look for ‘the neural correlates of consciousness’ (NCCs).

CORRELATIONS BEFORE CAUSE

The idea behind studying the NCCs is to measure some aspect of neural function-
ing and then correlate it with reports of conscious experience (Metzinger, 2000).
The ‘contrastive method’ involves comparing measurements of neural function-
ing when a given action or perception is conscious with when it is not conscious
(Baars, 1997a, 1997b; Aru et al., 2012). But which aspect of neural functioning?
Measurements have been made, and theories proposed, at every scale from sin-
gle molecules to large-scale assemblies of neurons, for it is not yet obvious what
we should be looking for.

The classic method for looking for the NCCs is to focus on vision, and in particu-
lar the phenomenon of rivalry, in which perception alternates between different
options.

One form of rivalry involves ambiguous figures. For example, look at the Necker
cube in Figure 4.3. Keep your eyes fixated on the central dot and watch what hap-
pens. This simple figure can be interpreted in depth in two mutually incompatible
ways. Even though you are keeping your eyes still, you should find that the cube
flips back and forth between the two different interpretations. It is impossible to
see both at once, or to combine the views into one, so you experience alternation
or rivalry between the two.

Bipolar

(interneuron)

Unipolar

(sensory neuron)

Multipolar

(motoneuron)

Pyramidal

cell

Basic neuron types

FIGURE 4.2 • Some of the basic types of neuron