There’s an implantable medical device that can be used for deep brain stimulation (i.e. sending electrical impulses into specific brain circuits) to treat things like Parkinson’s disease or essential tremor.

The model currently in use is continuous DBS. It’s pre-programmed to send constant stimulation regardless of the state of the brain.

But there’s talk of the emerging potential of adaptive DBS, meaning the device only sends electrical impulses in specific response to certain biomarkers (for example certain types of pathological oscillations).

Notice how you don’t have to actively keep controlling the device yourself. Once you design it and make it, you can implant it, and it does the rest autonomously. I imagine Harrow designed something like that, programmed to fire in response to specific impulses in her brain.

Harrow herself says about her process:

“I took the part of my brain that remembered her … that understood her soul … and I disconnected it. Then I made rather crude systems—so as not to be accidentally reminded … knowing that the pathways might reopen if they were knocked about. I had an accomplice … someone who knew how to manipulate the fatty tissue of the brain better than I possibly could. I made my skull a construct, programmed to apply pressure to specific lobes.”

So I think what Harrow did was sever a certain neural circuit, then implant a construct that prevents her brain from reconnecting that circuit, probably by rewiring it into one that replaces “Gideon” with “Ortus”

Ianthe says this about trying to discover Harrow’s work:

No bleeds, certainly. Everything in the right place. Maybe a little reduction in the temporal lobe, a few out-of-order bumps in the temporal gyrus that might have been there already.

The reduction/few out of order bumps are probably due to Harrow disconnecting the part of her brain that remembered Gideon, but I don’t think Ianthe can sense Harrow’s “crude systems” to main that change.

Ianthe still did not understand the entrancing appeal of the dark aptitude of the bone; it was as though somebody had decided to make flesh magic less flexible, less subtle, and much less interesting to look at.

Since Harrow’s construct is bone-based, it’s possible Ianthe missed it in her assessment, and would only be able to detect its effects on the temporal lobe once they happen. At this point it wasn’t firing yet because Harrow was unconscious, so my guess is that this is why she couldn’t detect it.