History’s Top Brain Computation Insights: Day 6

6) Neural networks consist of excitatory and inhibitory neurons connected by synapses (Sherrington – 1906)

Based on his observations in the spinal cord, Sherrington theorized that the brain consists of complex networks of excitatory and inhibitory cells he was the first to term 'neurons', with connection points he was the first to term 'synapses'. His theories turned out to be correct.

Sherrington's insight into the network nature of neural interactions is still at the cutting edge of neuroscience. It's ramifications are vast and complex, and we will still be working to understand the implications of this insight for decades to come.

Sherrington himself was able to prove that opposing muscles on the limbs inhibited each other via network interaction in the spinal cord. Also, it was found that timing of muscle movements is controlled by negative feedback via local neuronal networks. These spinal cord networks were an essential proof of concept for Sherrington, but they pale in comparison to the complexity of network interactions recently discovered in cortex, basal ganglia, and cerebellum.

Implication: The mind is implemented in an electric organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via synaptic connections.

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description)]

-MC

History’s Top Brain Computation Insights: Day 4

Primary sensory-motor cortical regions 4) Functions can be localized in the brain (Bouillaud – 1825, Broca – 1861, Fritsch & Hitzig – 1870)

Bouillaud and Broca discovered patients with frontal cortex lesions who had speech problems. Fritsch & Hitzig discovered primary motor cortex; a specialized chunk of cortex specifically for motor control. Broca believed that all brain functions would be localized eventually, and held the brain area he discovered (now termed Broca's area) as an example. It was found that all sensory modalities have dedicated chunks of cortex, called primary sensory areas.

This localized function insight immediately clashed with the distributed function insight (see yesterday's post). Flourens and his followers were adamantly against the localizationist view championed by Broca and others.

The origin of the concept of localizing brain function, phrenology, added discredit to the practice. Phrenology claimed that differences in localized brain function were reflected in bumps on individuals' skulls. Once it was clear that phrenological findings were not replicated between individuals the practice was labeled a pseudoscience. Unfortunately, localization of function applied without the fallacies of phrenology was not spared the renewed skepticism of many scientists.

Today localization of function is well established, especially in primary sensory-motor regions. Functions have also been localized within association cortex, though much work remains in understanding how such localization arises. Insights within the last 20 years have led to a more sophisticated view of how functions arise in association cortex involving network interactions (see future insight posts).

Implication: The mind is implemented in an electric organ with distributed and modular function.

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description)]
-MC

History’s Top Brain Computation Insights: Day 3

3)  Functions are distributed in the brain (Flourens – 1824, Lashley – 1929)

Flourens found that it did not matter where he lesioned inside cortex; what mattered was how much he lesioned. This suggested that functions were equally distributed (the law of equipotentiality) and widely distributed (the law of mass action) across cortex. Lashley updated this research by acknowledging localized functions in primary sensory-motor cortex (see tomorrow's entry), labeling the rest of cortex 'association cortex'.

Modern research has shown that Flourens and Lashley just weren't looking hard enough. Equipotentiality does not hold true (i.e, there is specialization within association cortex), but there is a great deal of mass action in the form of distributed network interaction across association cortex.

Implication: The mind is implemented in an electric organ with distributed function.

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description)]

-MC

History’s Top Brain Computation Insights: Day 2

This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description).

2)  Brain signals are electrical (Galvani – 1791, Rolando – 1809)

Galvani (whose name inspired the word 'galvanize') discovered that electrically shocking frog nerves made their muscles move. Rolando used this same method to stimulate cortex in the brain.

As has often occurred in history, the latest and most mysterious new technology was used to explain brain function. Here, at the turn of the 19th century the brain was "explained" as a mysterious electrical device. Later it will be "explained" as a telegraph with its many communicators (neurons) and wires (axons). Then it was described as a telephone network, with switchboards acting to control the flow of information. Eventually, with the cognitive revolution, it was compared to a computer. Finally, today, its complex and distributed network architecture is compared to the internet.

Though each new comparison is always premature and inadequate, new insights nonetheless seem to derive from each analogy. What new technology of tomorrow will be used to describe brain function?

Implication: The mind is implemented in an electrical machine-like organ.

-MC