The "pure consciousness" claim of TM is merely a thought free state, not introverted samadhi. "Pure con" would be a better term!

Cute.

Not referring to ambient noise, but the acquired ability by masters of meditative absorption to withdraw from the senses and remain unresponsive to random blasts of extremely loud noise. TM, unfortunately, comes nowhere close. Light forms of trance are easily perturbable.

The theory for which PC comes about during TM is that a fundamental change occurs in how the thalamus processing raw sensory data and how it handles thalamocortical feedback loops:

It doesn't, but at the same time, the part of the thalamus involved in regulating long-distance connectivity between parts of the thalamus continues with little or no change in functioning.

THis sets up a situation where no sense-related data gets into the cortex, and no feedback loops are occurring, which explains the traditional "mind withdraws from the senses like a turtoise withdraws its limbs into its shell" description. Since long-distance communication continues, there may still be brain activity processing existing info that was "in the mix" before the cutoff happened.

Since there are numerous circuits, mostly having to do with danger and extreme emotion, that can trigger reactivation of the thalamus from sleep or dreaming, it is possible that the residual activity in the cortex during PC can trigger those circuits and bring someone out of PC.

Likewise, an abrupt change in the external environment might do the same thing.

Why you think it is a good thing that some adept coudl somehow turnoff these natural survival-oriented mechanisms, is beyond me. The experience of long-term meditators, including those with regular episodes of PC, is that even loud noises don't disrupt TM OR PC -just the unusual noises do.

I have meditated on the flight-line with F-111's revving their engines just 50 yards away. THis is a loud enough sound that without ear protection your eardrums are destroyed. People with regular PC episodes report meditating during CAT scans and still having PC episodes. That's a pretty noisy environment as well, but in both cases, the noises need not trigger "danger" signals as they are chronic, and deemed "safe" before the PC episode (or meditation) starts.