Many scientists, even physical scientists,
assert that the Holy Grail of science is to understand human consciousness. This human state is even hard to define, but
is characterized by a state in which we know what we believe, know, and
imagine, know what we decide and plan, and feel what we feel. That explains
nothing.
The problem in understanding is not only that the mechanisms
must surely be complicated, but also that we don’t have good non-invasive
experimental tools. There are only two useful tools, a metabolic proxy of
neural electrical activity (functional fMRI) and scalp monitoring of electrical
activity (the electroencephalogram {EEG), or its magnetic field counterpart.
Among the problems with fMRI are that it is only an indirect measure of the
actual signaling within the brain that generates thought and feeling and
enables consciousness. Its time resolution is about one-second or more, whereas
signaling in the brain occurs on a millisecond scale. Although the EEG monitors
activity on the appropriate time scale, it has very poor spatial resolution,
inasmuch as voltage fields over various regions of cortex overlap, because the
voltage extends in progressively diminished amplitude throughout the conductive
medium of brain from its source of generation to other source generators.
Although the EEG does monitor the appropriate target (electrical activity),
that activity is an envelope of the algebraically summed signals from
heterogeneous neuronal ensembles, which are nerve impulses and their associated
postsynaptic potentials nearest the sensing electrodes.
By Davidboyashi - Own work, CC BY-SA 4.0
Nonetheless, we do know many useful things about brain
function that are surely involved in conscious functioning. Neuroscientists
have discovered much of this in lower animals from invasive procedures that are
not permissible in humans. In summary, we can list the following brain
functions that are relevant to consciousness:
- The brain is a network of richly inter-connected
networks.
- Functions are modular. Different networks have
different and shifting primary functions, and some may be selectively recruited
when their function is needed.
- Some networks can perform multiple functions,
depending on which other networks have recruited them into action.
- Some aspects of functional connectivity of
different networks differ in unconscious and conscious states.
- Wakefulness and consciousness are not the same.
Wakefulness is necessary but not sufficient for consciousness.
- A great deal has been learned about the neural
mechanisms causing wakefulness but that has not helped much in understanding
consciousness.
- The messaging signals of brain are nerve
impulses and their neurotransmitter postsynaptic effects.
- The summed voltages of the messaging have
electrostatic effects that alter the excitability of the neurons within the
voltage field.
- The frequency of bursts of impulses and their
EEG envelope impose important effects on gating and throughput of information
as it propagates and is modified throughout the global workspace of networks.
- There are multiple neural correlates of
consciousness, but we have not identified with certainty which ones are
necessary and sufficient for consciousness.
Oscillatory electrical activity is thought to have a key
role in selective routing of information in the brain. Oscillations seem to
modulate excitability, depending on phase relationships of linked neuronal
ensembles. Two prominent hypotheses have been advanced as crucial for
consciousness, and they are not mutually exclusive:
- Phase-locked
activity in two or more ensembles (coherence)
- Inhibitory
gating that directs pathways for propagation within networks.
The key to discovering mechanisms of consciousness is to
identify all the neural correlates and then winnow the list to those that are
both necessary and sufficient for consciousness. Sometimes, important
discoveries occur when you study the opposite of what you want to study. This
principle is manifest in studies on brain function during various states of
unconsciousness (like anesthesia, coma, or non-dream sleep). A recent review of
research compared the neural correlates of unconsciousness with those of
consciousness. The evaluation showed disrupted connectivity in the brain and
greater modularity during unconscious state, which inhibited the efficient
integration of information required during consciousness. Additionally, the
review made the key point that the neural correlates of consciousness that
matter are the ones that occur in consciousness but not in unconscious states.
Of particular relevance are the correlates related to functional connectivity
among networks, because multiple lines of evidence reveal that this
connectivity degrades during unconscious states and returns when consciousness
resumes.
In rodents, multi-array recordings in visual cortex indicate
that connectivity patterns are the same during anesthesia as in wakefulness.
Perhaps this indicates that rodents do not have the needed network architecture
to enable consciousness. They can be awake but not conscious. Being awake is
clearly necessary for consciousness, but not sufficient. In addition (if you
don’t believe me, see the classical U-tube basketball-game video on
inattentional blindness). At any given instant, we are only consciously aware
of the specific cognitive targets to which we attend.
Statistical
co-variation of activity in linked networks is a measure of functional
connectivity. The activity in linked networks may randomly jitter or be in
phase or locked at certain time lags. Operationally, the connectivity may enable
one group of neurons to mediate or modulate activity in another for past,
present, or future operations. The temporal dynamics of these processes differ
depending on the state of consciousness.
A very popular view on consciousness among neuroscientists
these days is that higher-order thinking, especially conscious thinking, is
mediated by extracellular voltage fields that oscillate in the range of 12 to
60 or more waves per second. Changes in
oscillatory frequency and coherent coupling of the oscillations among various
pools of neurons are thought to reflect the nature and intensity of thought.
The issue arises as to how these voltages, commonly called
field potentials, can influence the underlying nerve impulse activity that
causes the oscillation in the first place. The messages of thought are carried
in patterns of nerve impulses flowing in neural networks. Field potentials are
not signaling, at least not directly.
They may well indirectly influence messaging by electrostatically biasing networks
to be more or less able to generate and propagate nerve impulse traffic.
Neuroscientists attach much importance to the temporal
dynamics of EEG voltage frequencies. For example, at one time neuroscientists
believed that 40/sec synchrony was critical to consciousness, but later studies
revealed that this synchrony can be maintained and even enhanced during
anesthesia. Later, investigators thought they had found a crucial role for
higher frequency gamma synchrony, but that too is now called into question.
This gamma synchrony can be present or even enhanced during unconsciousness.
However, the spatial extent of synchrony may be the meaningful correlate of
consciousness. Widespread synchrony breaks down during unconsciousness, while
more localized synchrony remains intact or even enhanced.
Numerous studies show a breakdown of functional connectivity
during various states of unconsciousness. For example, fMRIs reveal
cortico-cortical and thalamocortical disconnections during sleep, general
anesthesia, and pathological states. EEG analysis shows similar connectivity
breakdowns. Additionally, the repertoire of possible connectivity
configurations that can be accessed diminishes during unconscious states and is
restored as consciousness resumes. This obviously limits the robustness of
information processing that can occur in unconsciousness. Conscious selective
attentiveness likely requires a different repertoire of connectivity than
inattentive consciousness.
Neuroscientists are also discovering the importance not only
of multi-area coherence at a given frequency band, but also that the phase
synchrony to two different frequencies can also modulate network communication.
Cross-frequency coupling of the alpha and beta oscillations with higher
frequency gamma oscillations can amplify, inhibit, or gate the flow of nerve
impulses throughout circuitry.
Future advancements will surely include more emphasis on
monitoring functional connectivity as the brain shifts into and out of various
states of consciousness and unconsciousness. I think, however, that we will not
make definitive progress in consciousness research until we make progress in
one area of theory and another of tactical methodology.
The theory deficiency lies in models of neural networks.
Computer models of man-made networks yield interesting results, but they are
probably not relevant. Brains do not
work with the same principles that computers do. Moreover, brain networks have
intrinsic plasticity that cannot yet be duplicated by computers.
The method deficiency
is that we have no non-invasive way to monitor the actually signaling in even a
significant fraction of all the neurons in all the networks. Moreover, even if
we had a way to monitor individual neurons noninvasively, it would likely be
necessary to selectively monitor neurons in defined circuits. Ultimately, we
may confirm that some things are just not knowable. Surely, however, we can
learn more than we do now.
Sources:
Bonnefond, Mathilde et al. (2017). Communication between
brain areas base on nested oscillators. eNeuro. 10 March. 4(2) ENEURO.0153-16.2017.
doi: https://doi.org/101523/ENEURO.0153-16.2017.
Mashour, George A., and Hudetz, Anthony G. (2018). Neural
correlates of unconsciousness in large-scale brain networks. Trends in
Neurosciences. 41(3), 150-160.