ScienceWeek

NEUROSCIENCE: QUANTUM MECHANICS AND THE BRAIN

The following points are made by C. Koch and K. Hepp (Nature 2006 440:611):

1) The relation between quantum mechanics and higher brain functions, including consciousness, is often discussed, but is far from being understood. Physicists, ignorant of modern neurobiology, are tempted to assume a formal or even dualistic view of the mind brain problem. Meanwhile, cognitive neuroscientists and neurobiologists consider the quantum world to be irrelevant to their concerns and therefore do not attempt to understand its concepts. What can we confidently state about the current relationship between these two fields of scientific inquiry?

2) All biological organisms must obey the laws of physics, both classical and quantum. In contrast to classical physics, quantum mechanics is fundamentally indeterministic. It explains a range of phenomena that cannot be understood within a classical context: the fact that light or any small particle can behave like a wave or particle depending on the experimental setup (wave particle duality); the inability to simultaneously determine, with perfect accuracy, both the position and momentum of an object (Heisenberg's uncertainty principle); and the fact that the quantum states of multiple objects, such as two coupled electrons, may be highly correlated even though the objects are spatially separated, thus violating our intuitions about locality (entanglement).

3) Major philosophical and conceptual problems surround the process of making measurements in quantum mechanics. To illuminate the paradoxical nature of superposition -- that is, the fact that particles or quantum bits (qubits) are allowed to exist in a superposition of states -- Schröedinger proposed a celebrated thought experiment: a sealed box containing the quantum superposition of both a dead and a live cat. When an observer peers inside the box, measuring its content, the wave function, which describes the probability that the system will be found in any one particular state, is said to collapse, and the system will be found in one or the other state with known probability.

4) The role of the conscious observer in this measuring process has been hotly debated since the early days of quantum mechanics. It is fair to say, however, that consciousness has been only a place holder in a chain of mathematical formulae, without much relevance to the study of neural circuits in intact organisms. Most quantum physicists view the brain as a classical instrument. A thought experiment involving an observer looking at a superimposed quantum system with one eye, and at a succession of faces with the other, challenges the idea that a quantum framework is needed to explain consciousness. The critical question is whether any components of the nervous system -- a 300-degrees Kelvin tissue strongly coupled to its environment --display macroscopic quantum behaviors, such as quantum entanglement, that are key to the brain's function.[1-5]

References:

1. Hepp, K. in Quantum Future: Lecture Notes in Physics (eds Blanchard, P. & Jadczyk, A.) 517, 92 104 (1998)

2. Koch, C. Biophysics of Computation: Information Processing in Single Neurons (Oxford Univ. Press, New York, 1999)

3. Koch, C. The Quest for Consciousness: A Neurobiological Approach (Roberts, Colorado, 2004)

4. Nielsen, M. & Chuang, I. Quantum Computation and Quantum Information (Cambridge Univ. Press, Cambridge, 2002)

5. Penrose, R. The Emperor's New Mind (Oxford Univ. Press, Oxford 1989)

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