13
$\begingroup$

I think I understand how a signal is transferred from neuron to neuron (from How do the brain and nerves create electrical pulses?).

My question, however, is not about the standard textbook material on neuronal transduction. Instead, I am wondering how an electrical impulse starts in the first place in the brain. In other words, how can an action potential be generated de novo from a conscious thought? I am wondering primarily about a signal like a movement of an appendage or something similar.

So in short, my question is:

How is the 'first' action potential generated in the cortex when one consciously/voluntarily decides of moving a limb (or comparable action)?

$\endgroup$
  • 2
    $\begingroup$ You can have a look at this article. It is not accessible to me right now. When I get to read it then I'll post an answer. $\endgroup$ – WYSIWYG Feb 17 '15 at 5:43
  • $\begingroup$ @WYSIWYG interesting article! Waiting for your answer! $\endgroup$ – One Face Feb 17 '15 at 8:08
  • $\begingroup$ Where are my comments? Is this your doing @CRags lol? $\endgroup$ – AliceD Feb 17 '15 at 12:35
  • $\begingroup$ @ChrisStronks not mine. Some moderator cleaned up! $\endgroup$ – One Face Feb 17 '15 at 13:59
9
$\begingroup$

This paper addresses the question that you are asking. The study is based on conscious decision making in rats.

Performance of rats were assessed in the following task:

The rats have to poke their snout into a waiting port until a auditory signal (beep) is heard. After first beep the rats can get a small reward (small amount of water) in the reward port. However if the rat waits for a second beep it gets to have a bigger reward.

So there were three behavioral outcomes in the experiment done with 37 individuals:

  1. Short-poke trials: Those that didn't wait for the signal (8.3 ± 4.6%)
  2. Impatient trials: Those that waited for the first beep but did not wait for the second beep (58.8 ± 3.9%)
  3. Patient trials: Those who successfully got the bigger reward. (32.9 ± 1.4%)

The impatient ones did not usually quit immediately after the first beep. This was not a lethargic response to signal but a moment of conflict between two decisions. There was a distribution of waiting times. The patient ones immediately responded to the second beep and took their reward.

Neural activity was studied in the M2 (secondary motor) cortex of impatient rats during the tasks and two kinds of neural activity was observed:

  • Ramp-to-threshold: Firing rate kept increasing till the end of waiting time suggestive of a neural threshold to quit waiting. (Fig 1)
  • Steady-transient activity: Firing rate correlated with waiting time i.e. higher firing rate higher the waiting time. This could constitute the neurons that are making the rat wait. (Fig 2)

       enter image description here

Figure 1: Ramp-to-Threshold.
Reproduced from Murakami et al. (2014) Nature Neuroscience 17,1574–1582

    enter image description here

Figure 2: Transient activity.
Reproduced from Murakami et al. (2014) Nature Neuroscience 17,1574–1582

It was also observed that this activity is task specific. The neurons that showed a certain activity in the above tasks did not behave the same way in the same rats performing a slightly different task.

So action potential is generated in the brain by "thoughts". However these thoughts are not generated de-novo. Any conscious decision is based on memory of past events and similar events evoke those memories, which in cellular terms is generation of action potential. In this case the action potential is generated in response to the rats' sensory input in the form of a beep. Other kinds of responses are instinctive and the brain circuitry has evolved to generate these kinds of responses to certain stimuli such as fear. The above example is a study of conscious decision making based on an acquired memory.

$\endgroup$
  • $\begingroup$ Does neurons that fire on their own (like the reticular activating system) have any role in thought generation? $\endgroup$ – One Face Feb 17 '15 at 14:45
  • 1
    $\begingroup$ @CRags Thoughts are essentially memories that are developed out of conscious actions and perceptions. The neurons that "fire on their own" are either controlled by an inbuilt oscillator (look at FHN model) or regulated by visceral stimuli. Then there are reflexes and other instinct driven responses which are basically preformed circuits. I am not sure how the autonomous nervous processes can affect conscious response- perhaps in an indirect way but I guess they don't have a direct role. $\endgroup$ – WYSIWYG Feb 17 '15 at 14:55
  • $\begingroup$ What about say, you come across an interesting formula for the first time and you start to think on how that formula was derived. In this case, would the short term memory be responsible for the string of thoughts that follow? $\endgroup$ – One Face Feb 17 '15 at 15:17
  • $\begingroup$ Very interesting answer though. +1 for that $\endgroup$ – One Face Feb 17 '15 at 15:18
  • 1
    $\begingroup$ @CRags I think when you are thinking about the derivation of that formula then you are essentially recalling similar formulae and trying to work it out in a similar fashion. $\endgroup$ – WYSIWYG Feb 17 '15 at 15:24
6
$\begingroup$
  1. There's a philosophical issue here: if you believe "thought" originates outside of the physical brain, you are probably some sort of dualist. However, I believe it is more likely that the brain is the machine that enables (apparent) consciousness and thoughts. If you believe "thought" causes neurons to fire, you might not be able to find a scientific answer, unless you can measure that "thought" external to the brain...

  2. There are spontaneous activity in the brain. That is, neurons will fire by themselves with or without any external input at random times. See this question and my answer there in. Brain is surprisingly noisy. Perception and behavioral variability is often correlated with such noise, and studied in neuroscience. For example, see

    • Nienborg, H. and Cumming, B. G. (2009). Decision-related activity in sensory neurons reflects more than a neuron’s causal effect. Nature, 459(7243):89-92.
$\endgroup$
  • $\begingroup$ While I'm not a dualist myself, how do you address causal mechanics originating thought when quantum physics has ruled out local determinism? Global determinism could still exist technically, but that would be an infinitely more complicated model of neuroscience. $\endgroup$ – John Joe Feb 21 '18 at 23:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.