  In the mid-1990s it was proposed that quantum effects in proteins known as
microtubules play a role in the nature of consciousness. The theory was largely
dismissed due to the fact that quantum effects were thought unlikely to occur
in biological systems, which are warm and wet and subject to decoherence.
However, the development of quantum biology now suggests otherwise. Quantum
effects have been implicated in photosynthesis, a process fundamental to life
on earth. They are also possibly at play in other biological processes such as
avian migration and olfaction. The microtubule mechanism of quantum
consciousness has been joined by other theories of quantum cognition. It has
been proposed that general anaesthetic, which switches off consciousness, does
this through quantum means, measured by changes in electron spin. The
tunnelling hypothesis developed in the context of olfaction has been applied to
the action of neurotransmitters. A recent theory outlines how quantum
entanglement between phosphorus nuclei might influence the firing of neurons.
These, and other theories, have contributed to a growing field of research that
investigates whether quantum effects might contribute to neural processing.
This review aims to investigate the current state of this research and how
fully the theory is supported by convincing experimental evidence. It also aims
to clarify the biological sites of these proposed quantum effects and how
progress made in the wider field of quantum biology might be relevant to the
specific case of the brain.
