Jonathan Tennenbaum series on the human brain and artificial intelligence

Jonathan Tennenbaum writes an interesting series about attempts to model artificial intelligence on the human brain: In the fourth and fifth installments, in particular he points to the astonishing complexity of the human brain, and the fallacy of modeling artificial intelligence solely on an understanding of the function of neurons as "based on elementary “all-or-nothing” processes of the sort that can easily be imitated by digital electronic circuits."

Here are 5 myths Tennenbaum discusses that suggests substantially more complexity than we discuss on page 144 where we simplify neuron behavior as "The neurons fire in digital-like pulses, either on or off, nothing in between."

"In this context, discoveries in neurobiology have overturned, one-by-one, nearly all the mechanistic dogmas which prevailed at the time AI was born. Here are some of them:

Dogma 1. The human brain is “hard-wired”: from a certain age onward the “circuits” formed by the neurons and their interconnections remain fixed.

No. Today it is well-known that in the adult brain new connections are constantly being formed (synaptogenesis) as well as removed (“pruned”). Neuroplasticity, which includes not only synaptogenesis but also constant changes in the morphology of existing synapses and the dendritic trees to which they are attached, play a central role in learning and other cognitive processes.

Dogma 2. In the adult brain neurons can die, but no new neurons are born.

No. In the hippocampus in particular – a cortical region identified as essential to learning and memory as well as emotional processes – new neurons are constantly being born (neurogenesis). These new neurons move around, migrating through the tissue before settling down into some suitable location and forming connections with other neurons. Neurogenesis is appears necessary for the healthy functioning of this part of the brain.

Dogma 3. Neurons communicate in a strictly “all-or-nothing” fashion, via the generation and propagation of discrete voltage spikes.

No. Neurons possess so-called “subthreshold membrane oscillations.” These are complex oscillations of the electrical potential of their membranes, which are too weak to trigger spikes, but which modify the spiking behavior of the neuron and can be communicated to other neurons without spikes. Among other things, subthreshold membrane oscillations appear to play an important role in synchronization of neuron activity. This discovery has revolutionary implications. The continuous variability of these oscillations, and their propagation from neuron to neuron, contradicts the notion that the brain operates like a digital system.

Dogma 4. All communication between neurons occurs via the network of axons and synapses.

No. It is now well-established that neurons also communicate via release of specialized molecules into the extracellular space, and their action on so-called extrasynaptic receptors carried by other neurons. This so-called “volume transmission”constitutes a second system of communication, alongside the so-called “wired transmission” via axons and synapses.

Dogma 5. The brain activity underlying cognition is based entirely on the interactions between neurons.

No. It has been established, that in addition to the neurons, the glial cells (astrocytes) in the brain play an active role in perception, memory, learning and control of concious activity. Glial cells outnumber neurons in the brain by a ratio of about 3:2. This discovery of role of glial cells in cognition, marks a revolution in neuroscience. The totality of these cells is sometimes referred to as a “second brain,” although the glial cells are so intimately connected to neurons metabolically and electrically that one can hardly separate the two.

For documentation, the reader can consult the following sources:

1. The Impact of Studying Brain Plasticity

2. What Do New Neurons in the Brains of Adults Actually Do?, Ashley Yeager, The Scientist, May 1, 2020

3. Generation and Propagation of Subthreshold Waves in a Network of Inferior Olivary Neurons

4. Extracellular-vesicle type of volume transmission and tunnelling-nanotube type of wiring transmission add a new dimension to brain neuro-glial networksand Extrasynaptic exocytosis and its mechanisms: a source of molecules mediating volume transmission in the nervous system (The latter two are rather technical publications, but give a good impression of the nearly unimaginable complexity of the cell-to-cell interactions which underlie brain activity.)

5. “The Other Brain” by R. Douglas Fields (Simon and Schuster 2009), and Astrocytes and human cognition: Modeling information integration and modulation of neuronal activity

His arguments don't necessarily undermine the particular function we (actually Geraud and Poppel) describe, but they do remind us of the extraordinary complexity of the human brain--especially when we get to chapter 10, and how we are so very early in our full understanding of how the brain--and the mind--actually work!

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