A neuron is a cell that receives electrical signals from other neurons and transmits a signal to neuron, gland, or muscle cells via synapse link sites (Figure 6.35). Each neuron has many input-output synapses, even thousands, while computer transistors need only one input and output because programs define their links. For example, moving a mouse left moves the screen cursor left because a program links them, but brains had no designer to pre-define their links so nature, as it always does, tries every option. Neurons in the embryo brain grow to connect in a dense mat, like plant roots, to explore every link (Figure 6.36). Synapses then stabilize with use or wither with disuse, to allow experience-driven brain plasticity.
To survive, animals respond to stimuli selectively, as brains activated by every change waste energy. Evolution then favors brains that ignore noise, defined as signal changes that carry no useful data. For neurons with thousands of input dendrites, random firing due noise is a significant problem that needs a solution.
The answer found is that pyramidal dendrites don’t spike if their inputs differ, even when either input alone gives a spike (Gidon, 2020). If nearby dendrites agree, they both fire but if not, neither does. In computing, this is an XOR gate (Note 1), a function that takes two steps in classical processing, not the expected AND/OR gate. Instead of a dumb transistor that just adds inputs, each nerve is a processing network whose dendrite layer purifies the data by inhibiting erratic input (Cepelwicz, 2020).
Yet dendrites grow outwards rather than interlace, giving no links between them, so how can adjacent dendrites compare results to fire only if they agree? One answer is that if cell structures vibrate to cohere photoreceptors, they can cohere adjacent dendrites into a unity that cancels opposite results. Quantum coherence lets nearby dendrites observe signals in a unified way, so they must fire together or not at all, in an XOR operation that reduces signal noise.. Nerves can then use quantum effects to enhance their function just as cells do:
“Physicists thought the bustle of living cells would blot out quantum phenomena. Now they find that cells can nurture these phenomena – and exploit them.” (Vedral, 2015).
These quantum phenomena can evolve gradually because even a small photosynthetic or dendritic contribution confers a benefit. Yet that cells evolved to be niches that allow local quantum effects doesn’t explain how nerves in distant regions combine their results into one observation.
Note 1. An eXclusive OR operation compares two input bits and generates zero if the bits are the same and one if the bits are different. The XOR logic is widely used in cryptography.