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Comment by heisenzombie | Hacker News Reader

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)

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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.

What behaviors?

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-perm...

There are also promising studies into the use of TMS for treating substance addiction.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4206564/

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/ 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.

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.

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)

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.

https://en.wikipedia.org/wiki/Perseveration

oh - the highly repetitive response common in autism.

Are there gradients of perseveration? This is absolutely fascinating - a brain based answer to why people do many behaviours - from sitting in the corner mumbling one word over and over, to various forms of self sabotage ("always picking the wrong man")

(And I might say heartening - as the father of an ASD child, it can seem hopeless, but just being armed with some knowledge of where the behaviours come from might allow some hope)

There are ways we can measure perseveration in the lab, and so yes, it can be higher or lower in different people. If you want to look into it more, a popular way to measure it is the Wisconsin Card Sort Test (WCST), or the version for children, the Dimension Change Card Sort task (DCCS). However, like all cognitive tests, these are imperfect measurements of perseveration - they may be indicators of higher or lower perseveration, but scores may be influenced by a multitude of other factors as well. Likewise, there may be aspects of perseveration not captured by these tests - like you mention, there are different ways that one can be perseverative, and one single number can't ever represent the complexity of human behavior.

thank you

I was once using an old but high end HP DC power supply to test a repaired 12V microcontroller circuit that someone else mysteriously blew up "with a spark". Before I hooked up the fraying and grimy alligator clips coming off the supply, I had the thought to check the actual voltage with a multimeter, in case the supply was out of calibration. Before I consciously perceived what my multimeter read, my hands rapidly dropped the frayed wires. I connected up the multimeter again, and it read 120V! Someone had mistakenly missed a decimal point, and the archaic LED display made it look like 12.0V.

A low level part of my brain definitely took over control of my hands, based on interpretation of visual signals. That was 6 years ago and I still think about it every few days.

https://xkcd.com/224/

> I think they knew this anyway because of behaviour of people with brain injuries.

Tests like this are intended to prove or refine information derived from clues from damaged subjects. You can learn a lot of things about complex systems like the brain by studying their failure modes, but you have to be careful of inferring causation from correlation and other such fallacies — for instance here they could have been trying to rule out the behaviour being a secondary symptom (the correct response actually being controlled elsewhere normally, but that is blocked by the damage rather than the damage having affected it more directly), or testing to see if multiple areas are directly involved in the behaviour rather than it being as simple as that one area seeming to control the veto, or just ruling out a pre-existing condition in the initial subjects unrelated to the subsequent damage.

"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

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

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!

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).

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

> AC: typical thresholds is 2T/s for 1 s.

Does this depend on frequency?

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

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