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As a butterfly flaps its wings in Tokyo, a neuron in your head veers slightly heavenward…

July 23, 2014

When you look at the edge of a table, there is a neuron in your head that goes from silence to pop pop pop. As you extend your arm, a nerve commanding the muscle does the same thing. Your retina has neurons whose firing rate goes up or down depending on whether it detects a light spot or a dark spot. The traditional view of the nervous system descends from experiments that have supported this view of neural activity. And perhaps it is true at the outer edges of the nervous system, near the sensory inputs and the motor outputs. But things get murkier once you get inside.

Historically, people began thinking about the brain in terms of how single neurons represent the physical world. The framework they settled on had neurons responding to a specific set of things out in the world, with the activity of those neurons increasing when they saw those specific things and decreasing when they saw their opposite. As time flowed by, this neural picture became jumbled up with questions about whether overall activity level or specific timing of an individual spike was what was important.

When it comes to multiple neurons, a similar view has generally prevailed: activity levels go up or down. Perhaps each neuron has some (noisy) preference for something in the world; now just think of the population as the conjunction of each of their activity. Then the combination of all of the neurons is less noisy than any individual. But still: it’s all about activity going up or down. Our current generation of tools for manipulating neural activity unconsciously echoes this idea of how the nervous system functions. Optogenetics cranks the activity of cells – though often specific subpopulations of cells – to move their activity up or down in aggregate.

An alternate view which I has been pushed primarily by Krishna Shenoy and Mark Churchland takes a dynamic perspective of neural activity, and I think comes from taking a premotor view of the nervous system. Generally, nervous  activity is designed to control our physical behavior: moving, shouting, breathing, looking, remaining silent. But that is a lot to have to control, and selection of the correct set of behaviors has to take a huge numbers of factors into account and has a lot to prepare for. What have I seen? How much do I like that? What am I afraid of? How hungry am I? This means that premotor cortical activity is probably representing many things simultaneously in order to choose among them.

The problem can be approached by looking at the population of activity and asking how many different things it could represent, without necessarily knowing what those are. Perhaps the population is considering six different things at the same time (a noted mark of genius)! Now that’s a slightly different perspective: it’s not about the up or down of overall activity, but how that activity flows through possibilities on the level of the whole population.

These streams of possible action must converge into a river somewhere. There are many possible options for how this could happen. They could be lying in wait, just below threshold, building up until they overcome the dam holding their behavior at bay. They could also be gated off, allowed to turn on when some other part of the system decides to allow movement.

But when we stop and consider the dynamics required in movement, in behavior, another possibility emerges. Perhaps there is just a dynamical system churning away, evolving to produce some reaching or jumping. Then these streams of preparatory activity could be pushing the state of the dynamical system in one direction or another to guide its later evolution. Its movement, its decision.

Churchland and Shenoy have worked on providing evidence for this happening in motor cortex as well as prefrontal cortex: neurons there may be tuned to move their activity in some large space, where only the joint activity of all the neurons is meaningful. In this context, we cannot think usefully about the individual neuron but instead must consider the whole population simultaneously. It is not the cog that matters, but the machine.

References

Kaufman MT, Churchland MM, Ryu SI, & Shenoy KV (2014). Cortical activity in the null space: permitting preparation without movement. Nature neuroscience, 17 (3), 440-8 PMID: 24487233

Mante V, Sussillo D, Shenoy KV, & Newsome WT (2013). Context-dependent computation by recurrent dynamics in prefrontal cortex. Nature, 503 (7474), 78-84 PMID: 24201281

Churchland, M., Cunningham, J., Kaufman, M., Foster, J., Nuyujukian, P., Ryu, S., & Shenoy, K. (2012). Neural population dynamics during reaching Nature DOI: 10.1038/nature11129

Shenoy KV, Sahani M, & Churchland MM (2013). Cortical control of arm movements: a dynamical systems perspective. Annual review of neuroscience, 36, 337-59 PMID: 23725001

Ames KC, Ryu SI, & Shenoy KV (2014). Neural dynamics of reaching following incorrect or absent motor preparation. Neuron, 81 (2), 438-51 PMID: 24462104

Churchland, M., Cunningham, J., Kaufman, M., Ryu, S., & Shenoy, K. (2010). Cortical Preparatory Activity: Representation of Movement or First Cog in a Dynamical Machine? Neuron, 68 (3), 387-400 DOI: 10.1016/j.neuron.2010.09.015

What should we be allowed to forget?

July 22, 2014

Should we be dampening the emotional aspect of memory?

Two decades ago, scientists began to wonder if they could weaken traumatic memories by suppressing the hormonal rush that accompanies their formation. They turned to propranolol, which was already on the market as a treatment for hypertension and blocks the activity of hormones like epinephrine and norepinephrine….Next, in 2002, neuroscientists reported that emergency room patients who took propranolol within 6 hours of a traumatic event were less likely to experience the heightened emotions and arousal associated with PTSD one month later, compared with people who took placebos.

The hitch was that in order to interfere with memory consolidation, propranolol needed to be given within hours of a trauma, long before doctors knew whether someone would go on to develop PTSD. But around the same time, studies began to show that memories can once again become fragile when they are recalled…Perhaps, researchers hypothesized, propranolol could weaken emotional memories if PTSD patients took the drug after they conjured up the details of a painful experience. By blocking the effects of norepinephrine and epinephrine upon recall, propranolol might dampen down activity in the amygdala and disrupt reconsolidation.

I liked this comment someone left on the article:

If the memory of my trauma were to be removed, I would make no sense to myself.

If we could edit our memories, what is important to who we are? Is there a threshold of pain beyond which we should not be forced to endure our entire lives? What was adaptive one hundred thousand years ago may not be adaptive in modern society.

Do “I” exist?

July 21, 2014

I think therefore I am; or rather, I am currently thinking, therefore I currently am. But where does the “I” come from?

Much has been made of clinical cases where the self seems to malfunction spectacularly: like Cotard syndrome, whose victims believe they do not exist, even though they admit to having a life history; or “dissociative identity disorder,” where a single body seems to harbour multiple selves, each with its own name, memory, and voice. Most of us are not afflicted by such exotic disorders. When we are told that both science and philosophy have revealed the self to be more fragile and fragmentary than we thought, we take the news in our stride and go on with our lives…

The basic question about the self is: what, in essence, am I? Is my identity rooted in something physical (my body/brain) or something psychological (my memories/personality)? Normally, physical and mental go together, so we are not compelled to think of ourselves as primarily one or the other. But thought experiments can vex our intuitions about personal identity. In An Essay Concerning Human Understanding (1689), John Locke imagined a prince and a cobbler miraculously having their memories switched while they sleep: the prince is shocked to find himself waking up in the body of the cobbler, and the cobbler in the body of the prince. To Locke, it seemed clear the prince and the cobbler had in effect undergone a body swap, so psychological criteria must be paramount in personal identity.

What is critical to your identity, Dainton claims, has nothing to do with your psychological make-up. It is your stream of consciousness that matters, regardless of its contents. That’s what makes you you. As long as “your consciousness flows on without interruption, you will go on existing”

So as long as you don’t fall asleep, then?

Something else that caught my eye:

Yet even the humble roundworm C elegans, with its paltry 302 neurons and 2,462 synaptic connections (which scientists have exhaustively mapped), has a single neuron devoted to distinguishing its body from the rest of the world. “I think it’s fair to say that C elegans has a very primitive self-representation” comments the philosopher-neuroscientist Patricia Churchland—indeed, she adds, “a self.”

Now, I don’t know which neuron she is referring to so I can’t refer to the primary research. However, one strength of C. elegans is that it is so simplified it promotes very clear thinking about complex topics. Consider this: there must be multiple neurons whose activity is affected by the worm’s own internal state; and there are definitely multiple neurons devoted to getting sensory information in from the rest of the world. So does sensing external input + sensing internal state = sense of self? Or does it require intentional interrogation of the internal computations that are detecting internal state? Just because the information is there does not mean the ‘sense’ is there.

Why would robots have heads?

July 18, 2014

Or conversely, why is your head near your brain? Sensory organs came before or after cephalization? In other words, do we have a head because it is advantageous to be able to respond quickly to quickly changing incoming sensations (vision, audition)?

This is interesting:

However, flatworms differ from more advanced animals in that their mouths are in the center of their bodies, not at the anterior end.

 

Naked mole rats, star-nosed moles, and tentacled snakes: the research of Ken Catania

July 17, 2014

mole rate somatosensory cortex

A classic paper about Naked Mole Rats was passed around on twitter recently and I thought that it would be a good time to revisit some of the greatest hits of Ken Catania, wonder neuroethologist.

There is tons of interesting neuroscience questions that pertain to the strange animals you’ll find in the wild but very few people able to do that research – I suspect because of the lack of funding. But look at the list of animals that Ken Catania studies according to his web page: star-nosed moles, tentacled snakes, water shrews, crocodiles, worm-grunting, and more. Who else will show us videos of moles attempting to smell in stereo? Or of tentacled snakes in action? It’s worth your time to watch this lecture of some of his fascinating neuroethology research:

 

A search for the science of the mind

July 13, 2014

More history of the scientists who wanted to understand the mind. Turns out, there was a lot of racism in early 20th century science – what a surprise.

He modelled the brain’s structure as though it was an archaeological site, the different levels supposedly reflecting evolutionary advances. The neocortex, shared by all mammals, controlled basic functions while the prefrontal area was the seat of more advanced abilities.

Investigating the perception of pain, Head had two cutaneous nerves on his left forearm severed. Every Friday for the next four years, he visited Rivers in his college rooms to chart the process of regeneration and the areas of acute sensitivity. Echoing Elliot Smith’s ideas about the evolutionary levels of the brain, Rivers and Head decided that the nervous system contained two layers: one older and more primitive; the other more subtle and localized. They speculated that the two systems “owed their origin to the developmental history of the nervous system. They reveal the means by which an imperfect organism has struggled towards improved functions and physical unity”. And this “could be seen as a metaphor for the triumph of civilization over savagery in human history”. Frederic Bartlett, a student of Rivers who went on to become a leading psychologist in the next generation, noted that this metaphor informed all Rivers’s later theories in physiology, psychology and anthropology. The structure of every human organ, every social institution, revealed cumulative layers of progressive development.

Psychology was looked down on by the Cambridge establishment, but Ludwig Wittgenstein was intrigued and regularly came to Mill Lane to work with Myers. “I had a discussion with Myers about the relations between Logic and Philosophy”, he wrote to Bertrand Russell. “I was very candid and I am sure he thinks that I am the most arrogant devil who ever lived . . . . I think he was a bit less confused after the discussion than before.” When the laboratory was opened to the public in 1913, Wittgenstein exhibited an apparatus for investigating the perception of rhythm. Perhaps influenced by Wittgenstein, Myers was moving away from biological determinism. The physiologists, he complained, “in their attempts to penetrate the reality of the known, were deliberately ignoring the knower”

And some more on the history of the word ‘scientist’ (see previously):

Carrington had noticed the spread of a particular term related to scientific research [it was "scientist"]. He himself felt the word was “not satisfactory,” and he wrote to eight prominent writers and men of science to ask if they considered it legitimate. Seven responded. Huxley and Argyll joined a five-to-two majority when they denounced the term. “I regard it with great dislike,” proclaimed Argyll. Huxley, exhibiting his usual gift for witty dismissals, said that the word in question “must be about as pleasing a word as ‘Electrocution.’”

…The English academic William Whewell first put the word “scientist” into print in 1834 in a review of Mary Somerville’s On the Connexion of the Physical Sciences.Whewell’s review argued that science was becoming fragmented, that chemists and mathematicians and physicists had less and less to do with one another. “A curious illustration of this result,” he wrote, “may be observed in the want of any name by which we can designate the students of the knowledge of the material world collectively.” He then proposed “scientist,” an analogue to “artist,” as the term that could provide linguistic unity to those studying the various branches of the sciences.

…“Scientist” met with a friendlier reception across the Atlantic. By the 1870s, “scientist” had replaced “man of science” in the United States. Interestingly, the term was embraced partly in order to distinguish the American “scientist,” a figure devoted to “pure” research, from the “professional,” who used scientific knowledge to pursue commercial gains…For most British readers, however, the popularity of the word in America was, if anything, evidence that the term was illegitimate and barbarous.

The story of stress

July 12, 2014

A history of the science behind stress:

“He would subject them to extreme temperatures, make them go hungry for long periods, or make them exercise a lot,” the medical historian Mark Jackson says. “Then what he would do is kill the rats and look at their organs.”

What was interesting to Selye was that no matter how different the tortures he devised for the rats were — from icy winds to painful injections — when he cut them open to examine their guts it appeared that the physical effects of his different tortures were always the same.

“Almost universally these rats showed a particular set of signs,” Jackson says. “There would be changes particularly in the adrenal gland. So Selye began to suggest that subjecting an animal to prolonged stress led to tissue changes and physiological changes with the release of certain hormones, that would then cause disease and ultimately the death of the animal.”

And so the idea of stress — and its potential costs to the body — was born.

But here’s the thing: The idea of stress wasn’t born to just any parent. It was born to Selye, a scientist absolutely determined to make the concept of stress an international sensation.

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