4 ms·

Yep! At these fields, the effects of shaking your head around an MRI machine are: - Dizziness/vertigo/headache: AC: typical thresholds is 2T/s for 1 s. DC me

Yep! At these fields, the effects of shaking your head around an MRI machine are: - Dizziness/vertigo/headache: AC: typical thresholds is 2T/s for 1 s. DC mechanism: differential susceptibility of vestibular structures (46 T2/m  perceived 1% g) - Acidic/metallic taste: provoked presumably by electrolytic reactions due to eddy currents in the tongue, effect felt by 15% of test subjects @3T. Threshold dB/dt2.3 T/s (e.g. shaking head @ 0.6 Hz in a 0.5 T fringe field). May be contrasted by opening the mouth. - Magnetophosphenes: perceived flickering lights caused by electrical stimulation of the retina/optical nerve. (0.2 mV). Threshold dB/dt 2T/s for 50 mS (max. sensitivity 20 Hz) (source in my other comment)

mattkrause 5y ago
Another way to think about this is via the effects achievable with transcranial magnetic stimulation. A TMS machine generates fields on the order of 1T at the magnet, and rather less in the brain. From this, you can get all kinds of interesting phenomena: phosphenes, temporary “lesions”, and even facilitation of some behaviors.
- xwdv 5y ago
  What behaviors?
  - kyshoc 5y ago
    Not sure this is what GP was referencing, but TMS is approved[0] for treatment-resistant OCD, depression, and migraine headaches. [0]: https://www.fda.gov/news-events/press-announcements/fda-permits-marketing-transcranial-magnetic-stimulation-treatment-obsessive-compulsive-disorder https://www.fda.gov/news-events/press-announcements/fda-perm...
    - amelius 5y ago
      There are also promising studies into the use of TMS for treating substance addiction. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4206564/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4206564/
  - mattkrause 5y ago
    Tough question! TMS is fairly common tool for neuroscience research and its effects depend on where, when, and how it is applied, as well as the quirks of each person’s own anatomy. This paper has a table of some studies reporting “beneficial” effects, mostly on perception and memory: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4083569/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4083569/ There are many more studies showing that TMS (transiently) impairs performance too. We don’t totally understand how it works—-or what’s normally required for many of these behaviors—-so it’s hard to give a grand unified theory of how it acts, but it definitely does something to brain circuits. That said, the FDA approved TMS treatments for depression and a few other indications, and it’s also used as a tool for neurosurgical planning.
  - murrayh 5y ago
    I was a test subject for someone's PHD on this. AFAIR it was about identifying the part of the brain that is responsible for aborting actions. Like if you decide to pickup a pan, and then notice the handle is hot, some people can't help but pickup the pan anyways because they lack this abort function. They took an MRI of my brain. They found where one part of my brain resided in the MRI images. I performed reaction tests -- press this button for green circle, press that button for blue circle, abort pressing button if the icon gets crossed out (the crossing out was delayed) Then they strapped the magnet to my head (not touching but very close AFAIR) Then do all the tests again AFAIR they showed that part of the brain did affect your ability to abort an action. I think they knew this anyway because of behavior of people with brain injuries. So I guessed they learned the magnet scrambled that part of the brain? It was extremely boring doing these tests. I don't remember much about it except that the magnet made unreasonably loud popping sounds.
    - lifeisstillgood 5y ago
      Absolutely fascinating :-) I am assured that the human brain is so incredibly complex that it is almost impossible to understand it ... but experiments like this make me worry that Evolution just knocked most of it up in Perl over a weekend... An inability to abort an action when facts change ? What if there was a test for that. Would we stop people standing for office? Be set free from some crimes? I demand to see the source code for human brains ! It needs a proper security audit. (relevant XKCD reference to be looked up later)
      - trombonechamp 5y ago
        My PhD was (partially) on this. The inability to inhibit actions when facts change on a short timescale (sub-second) is thought to be biological. According to the most widely accepted theory, think of the brain as having a slow system and a fast system. The slow system is good for complex processing, but of course it is slow. There is also a fast system for quickly responding to things. If something changes while the slow system is working, usually the fast system is pretty good at stopping the slow system from acting, allowing the brain time to incorporate the new information into its plans. But if you are already planning on doing something and getting ready to do it, there is a limit to how quickly the fast system can interrupt the slow system. It tends to be on the order of 1/10 to 1/5 of a second. On a longer timescale (minutes to days), there is a clinical symptom called "perseveration" whereby people can't let go of previously held beliefs in the face of changing information. It is common in, e.g., patients with schizophrenia.
- weinzierl 5y ago
  "Sensory Substitution Devices" by Tom Wright was an interesting talk at CodeKen 2011[1] touching on the subject of TMS. It was the first time I had heard about this and it fascinated me deeply. I remember that the talk got a lot of votes in the feedback round, so I was certainly not alone with this experience. [1] https://m.youtube.com/watch?v=81cN6XFIPSg https://m.youtube.com/watch?v=81cN6XFIPSg
junon 5y ago
Dumb question potentially, but this sounds incredibly dangerous to the layperson. Is it?
- heisenzombie 5y ago
  Not really, these are huge fields and quite mild effects. The vertigo one is the most dangerous because you could fall and hit your head on something!
amelius 5y ago
> AC: typical thresholds is 2T/s for 1 s. Does this depend on frequency?
- heisenzombie 5y ago
  Yeah, 2, tesla per second is a ramp rate — equivalent to roughly half a Hz for a 1T magnitude field.
lazyjones 5y ago
I've done a few MRI scans around the head (both 3T and 1.5T machines) and I tasted this metallic taste every time, despite staying still as much as possible (I'm not so bad at it).
- heisenzombie 5y ago
  Interesting - if it happened during the scan it would be due to the gradient coils, not the main solenoid field. The gradient coils are much weaker but turn on and off during the scan (the main solenoidal field is static). https://radiopaedia.org/articles/slew-rate https://radiopaedia.org/articles/slew-rate

mattkrause 5y ago

Another way to think about this is via the effects achievable with transcranial magnetic stimulation. A TMS machine generates fields on the order of 1T at the magnet, and rather less in the brain. From this, you can get all kinds of interesting phenomena: phosphenes, temporary “lesions”, and even facilitation of some behaviors.

xwdv 5y ago
What behaviors?
- kyshoc 5y ago
  Not sure this is what GP was referencing, but TMS is approved[0] for treatment-resistant OCD, depression, and migraine headaches. [0]: https://www.fda.gov/news-events/press-announcements/fda-permits-marketing-transcranial-magnetic-stimulation-treatment-obsessive-compulsive-disorder https://www.fda.gov/news-events/press-announcements/fda-perm...
  - amelius 5y ago
    There are also promising studies into the use of TMS for treating substance addiction. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4206564/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4206564/
- mattkrause 5y ago
  Tough question! TMS is fairly common tool for neuroscience research and its effects depend on where, when, and how it is applied, as well as the quirks of each person’s own anatomy. This paper has a table of some studies reporting “beneficial” effects, mostly on perception and memory: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4083569/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4083569/ There are many more studies showing that TMS (transiently) impairs performance too. We don’t totally understand how it works—-or what’s normally required for many of these behaviors—-so it’s hard to give a grand unified theory of how it acts, but it definitely does something to brain circuits. That said, the FDA approved TMS treatments for depression and a few other indications, and it’s also used as a tool for neurosurgical planning.
- murrayh 5y ago
  I was a test subject for someone's PHD on this. AFAIR it was about identifying the part of the brain that is responsible for aborting actions. Like if you decide to pickup a pan, and then notice the handle is hot, some people can't help but pickup the pan anyways because they lack this abort function. They took an MRI of my brain. They found where one part of my brain resided in the MRI images. I performed reaction tests -- press this button for green circle, press that button for blue circle, abort pressing button if the icon gets crossed out (the crossing out was delayed) Then they strapped the magnet to my head (not touching but very close AFAIR) Then do all the tests again AFAIR they showed that part of the brain did affect your ability to abort an action. I think they knew this anyway because of behavior of people with brain injuries. So I guessed they learned the magnet scrambled that part of the brain? It was extremely boring doing these tests. I don't remember much about it except that the magnet made unreasonably loud popping sounds.
  - lifeisstillgood 5y ago
    Absolutely fascinating :-) I am assured that the human brain is so incredibly complex that it is almost impossible to understand it ... but experiments like this make me worry that Evolution just knocked most of it up in Perl over a weekend... An inability to abort an action when facts change ? What if there was a test for that. Would we stop people standing for office? Be set free from some crimes? I demand to see the source code for human brains ! It needs a proper security audit. (relevant XKCD reference to be looked up later)
    - trombonechamp 5y ago
      My PhD was (partially) on this. The inability to inhibit actions when facts change on a short timescale (sub-second) is thought to be biological. According to the most widely accepted theory, think of the brain as having a slow system and a fast system. The slow system is good for complex processing, but of course it is slow. There is also a fast system for quickly responding to things. If something changes while the slow system is working, usually the fast system is pretty good at stopping the slow system from acting, allowing the brain time to incorporate the new information into its plans. But if you are already planning on doing something and getting ready to do it, there is a limit to how quickly the fast system can interrupt the slow system. It tends to be on the order of 1/10 to 1/5 of a second. On a longer timescale (minutes to days), there is a clinical symptom called "perseveration" whereby people can't let go of previously held beliefs in the face of changing information. It is common in, e.g., patients with schizophrenia.
weinzierl 5y ago
"Sensory Substitution Devices" by Tom Wright was an interesting talk at CodeKen 2011[1] touching on the subject of TMS. It was the first time I had heard about this and it fascinated me deeply. I remember that the talk got a lot of votes in the feedback round, so I was certainly not alone with this experience. [1] https://m.youtube.com/watch?v=81cN6XFIPSg https://m.youtube.com/watch?v=81cN6XFIPSg

junon 5y ago

Dumb question potentially, but this sounds incredibly dangerous to the layperson. Is it?

heisenzombie 5y ago
Not really, these are huge fields and quite mild effects. The vertigo one is the most dangerous because you could fall and hit your head on something!

amelius 5y ago

> AC: typical thresholds is 2T/s for 1 s. Does this depend on frequency?

heisenzombie 5y ago
Yeah, 2, tesla per second is a ramp rate — equivalent to roughly half a Hz for a 1T magnitude field.

lazyjones 5y ago

I've done a few MRI scans around the head (both 3T and 1.5T machines) and I tasted this metallic taste every time, despite staying still as much as possible (I'm not so bad at it).

heisenzombie 5y ago
Interesting - if it happened during the scan it would be due to the gradient coils, not the main solenoid field. The gradient coils are much weaker but turn on and off during the scan (the main solenoidal field is static). https://radiopaedia.org/articles/slew-rate https://radiopaedia.org/articles/slew-rate