Connectome (52 page)

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Eric Drexler:
Drexler 1986.

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Charles Olson:
Olson 1988.

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fixing them in place:
Formaldehyde and glutaraldehyde are used to link protein molecules together. An even more toxic fixative, osmium tetroxide, has the dual function of binding together fat molecules and staining the membranes to which they belong.

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Figure 53, left:
The tissue is embedded in Epon, an epoxy resin, and appears black because of the osmium staining.

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Lenin was embalmed:
Modern embalming methods began to develop in the seventeenth and eighteenth centuries. Most notoriously, the eccentric London dentist Martin van Butchell embalmed his dead wife in 1775 and displayed her in the window of his home office. See Dobson 1953.

 

15. Save As . . .

 

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“mind uploading”:
In his 1955 story “The Tunnel under the World,” Frederik Pohl wrote: “Each machine was controlled by a sort of computer which reproduced, in its electronic snarl, the actual memory and mind of a human being. . . . It was only a matter . . . of transferring a man's habit patterns from brain cells to vacuum-tube cells” (Pohl 1956). The first mention in the scientific literature may have been in Martin 1971: “We shall assume that developments in neurobiology, bioengineering and related disciplines . . . will ultimately provide suitable techniques of ‘read-out' of the stored information from cryobiologically preserved brains into
n
th generation computers capable of vastly outdoing the dynamic patterning of operation of our cerebral neurones.”

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requires dying first:
After being resurrected, Jesus is said to have ascended to heaven without dying again. Shoemaker 2002 describes how Christians have argued for millennia over whether the Virgin Mary also entered heaven without dying first. Being carried up to heaven by God is called “Assumption,” to distinguish it from the “Ascension” of Jesus, which happened by his own power. In 1950, Pope Pius XII promulgated
Munificentissimus Deus,
which decreed that Mary, “having completed the course of her earthly life, was assumed body and soul into heavenly glory.” This dogma recognized the importance of the Assumption but didn't really settle the debates, because its wording was ambiguous. Christians have also long argued over whether the Old Testament figures of Elijah and Enoch were assumed into heaven without dying first.

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“brain in a vat”:
The story “Where Am I?” in Dennett 1978 is a wonderful example. For an actual attempt to keep an isolated guinea pig brain alive and functioning, see Llinas, Yarom, and Sugimori 1981.

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pyramidal tract contains:
Lassek and Rasmussen 1940. For another way of tallying the numbers, let's categorize the neurons of a nervous system by their connection to the outside world. Sensory neurons convert external stimuli into neural signals. For example, the photoreceptors of the retina produce electrical signals when stimulated by light. Motor neurons make synapses onto muscles and convert neural signals into movements. The remainder are called interneurons, because they are interposed between sensory and motor neurons. In the
C. elegans
nervous system, sensory neurons, motor neurons, and interneurons are found in comparable numbers. But sensory and motor neurons make up a vanishingly small fraction of our nervous system. Saying that a neuron is an interneuron is no great distinction, because almost all are. Very few of the neurons in our brains “talk” with the outside world. They mostly talk with each other.

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running on a gigantic computer:
Bostrom 2003; Lloyd 2006.

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Alan Turing:
Turing 1950.

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successful example of AI:
There are some slight differences in Turing's original setup of the test. The interested reader should consult Turing's paper, which is very readable.

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a proper Turing test:
Natalie Zemon Davis has argued that Guerre's wife knew very well that the new Guerre was fake, but fell in love and conspired with him (Davis 1983, 1988). But no historians question that some of Guerre's sisters and friends were genuinely fooled.

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The more accurate the simulation:
Then again, self-models are often not very accurate. Researchers have shown that most people have inflated opinions of their own abilities. This is called the Lake Wobegon Effect, after the humorist Garrison Keillor's fictional town in which “all the women are strong, all the men are good-looking, and all the children are above average.”

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Markram was one of the first:
He also showed that the strength of a cortical synapse can fluctuate from spike to spike. In collaboration with theoretical colleagues, he introduced mathematical models describing this phenomenon, known as short-term synaptic plasticity.

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simulation of a cat brain: Ananthanarayanan et al. 2009.
Ananthanarayanan et al. 2009.

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“Cat Fight Brews Over Cat Brain”:
Adee 2009 also prints the full text of the letter.

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neurons of the same type:
For example, when neuroscientists inject electrical current into an inhibitory neuron of the neocortex, it can generate spikes for a long time without faltering (Connors and Gutnick 1990). But when they stimulate a pyramidal neuron, it slows down after the first few spikes, as if it were becoming “fatigued.”

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Once all neuron types:
It will also be necessary to classify synapses into types. Here I've taken the view that neuron types already include all information about synapse types. According to Dale's Principle, a neuron secretes the same neurotransmitter (or set of neurotransmitters) at all of the synapses it makes onto other neurons. That's why all the outgoing synapses of a pyramidal neuron secrete glutamate. There are many variants of glutamate receptor molecules. The particular variant that occurs at a synapse may be a property of the neuron type of the receiving neuron. In other words, the type of a synapse may be determined by the types of the neurons that it connects. If this turns out not to be true, then connectomes will have to include separate information about synapse types as well as neuron types.

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millions of ion channels:
This numerical estimate is courtesy of Michael Hausser and Arnd Roth. The multicompartmental models are based on the aggregate behaviors of large populations of channels. This has some similarity to the way in which pollsters keep track of the percentage of voters who support a candidate. Each compartment represents some part of the neuronal membrane. It contains multiple populations of ion channels, one population for each channel type. Therefore, if a neuron is divided into one hundred compartments, and there are ten types of ion channel, then the model contains a thousand variables for specifying the states of the ion channels. That may sound like a lot of variables, but it's still much less than the total number of ion channels in the neuron.

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multicompartmental model neurons:
Multicompartmental models are essential when different parts of a neuron function independently. The dendrites of a single starburst amacrine cell of the retina, for example, detect multiple directions of visual motion and send different signals to other neurons (Euler, Detwiler, and Denk 2002).

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Peters' Rule:
This was first stated in its general form by Braitenberg and Schüz 1998, and named in honor of Alan Peters for formulating a specific case of the rule.

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more difficult for C. elegans:
Lockery and Goodman 2009.

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The only information unique:
More realistically, the properties of each neuron type might vary slightly across normal people. These variations might be predictable from their genomes. If so, we'd have to say “You are your connectome plus models of neuron types plus your genome.” But again, a genome contains much less information than a connectome, so “You are your connectome” would still be a good approximation.

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about one hundred types: White et al. 1986.
White et al. 1986.

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diffusion of neurotransmitter:
Electronic circuits sometimes behave differently from their simulations, in which components can interact only if they are connected by wires. A real circuit can contain interactions mediated by “thin air” rather than wires. For example, one wire can set up an electric field that is felt by a nearby wire, a phenomenon known as “stray capacitance” that is analogous to extrasynaptic interactions in the brain. This type of deviation from the model can be extremely difficult to identify and troubleshoot.

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almost beyond imagining:
If you're up to the mind-bending task of thinking about such a simulation, you can consult Tipler 1994, which proves that it should be possible in this universe.

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all the positions and velocities:
I'm avoiding the issue of whether quantum physics is important for the functioning of the brain. Tegmark 2000 provides some insight into the subject.

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Ralph Merkle:
Merkle 1992. Some of the earliest writings about connectomics were penned by proponents of cryonics and uploading, although the term
connectome
was not coined until later. In his 1989 technical report, “The Large Scale Analysis of Neural Structures,” Ralph Merkle reviewed the state of the art in serial electron microscopy. He knew that the
C. elegans
connectome had been mapped, and speculated about scaling up to the human brain.

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