At the beginning of a book, there is usually an Acknowledgments section where the author thanks the many people who helped them in writing the book. I certainly have lots of people to thank who have made An Internet in Your Head possible. But acknowledgments have a special meaning in the infrastructure of the internet. Any time you send information over the Internet, like sending an email, the receiver acknowledges receipt of your data with a tiny return message called an ack.

The internet succeeds in part because of trust among routers, but acks ensure that messages arrive intact. The internet operates according to Ronald Reagan’s maxim for international relations: trust but verify.

If a receiving router doesn’t send back an ack for some message parts in a timely manner, the sender knows to resend them because those parts probably got lost. It’s a spectacularly effective solution, one that allows any sender to pass messages to almost any receiver with very high reliability.

Under the internet metaphor, acks should exist in some form in the brain. Individual parts of the brain send messages to many possible destinations, and do so flexibly. How can a given brain part know that its message was received? I hypothesize that the brain could provide acknowledgments through its many looping connections.

One brain area that is famous for looping connections is the thalamus. This area gets connections originating in all sensory organs (except smell); for example, everything you will ever see travels from the eye to the thalamus, then on to the cerebral cortex.

But the cortex sends many more connections back down to the thalamus. Each part of the thalamus specializes in a particular sensory function (also motor functions), and each one receives a great deal of feedback from the cortex. Information traveling over these neural wires (axons) goes in a loop. The thalamus’ connection architecture seems ideally suited to providing acknowledgments: higher areas in the cortex–ones involved in detecting motion, for example–can talk back to the thalamus to say that relevant visual information coming from the eye made it to the right place, and was processed successfully (or not). It is known that signals can traverse loops from the thalamus to the cortex and back very quickly, with round trips taking as little as 9 milliseconds.

Seeing the looping connections between the thalamus and cortex as supporting reliable message delivery is a change from the computer metaphor explanation of this architecture. Under the computer metaphor, the thalamus serves to “adjust the weights” of signals as they move around the brain, making some stronger and some weaker. But no one has explained why loops are necessary to do this. The long axons required to build a loop are difficult to build during development. They are also metabolically expensive. We can only fit so much wiring in our heads, so the brain sharply economizes on long axons. Building loops with longer axons also costs precious milliseconds of signal travel time. If the thalamus’ job is to adjust weights, why can’t it do this locally, on the first pass?

One reason the thalamus hasn’t been considered in terms of communication engineering–and why the possibility of acks being passed in corticothalamic networks hasn’t been raised before–is that the thalamus’ job has traditionally been seen as computation. In other words, the signals sent over thalamic loops have been assumed to be undergoing computational operations, like making signals bigger or smaller. You would only notice that thalamic signals were something like an ack if you thought of those signals as messages, and if you saw the system as a whole as a communication network. This is the change in perspective afforded by the internet metaphor for the brain.

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