>>531524351
It's been stated in every dead space game that those smart enough see symbols and codes of a puzzle from the markers where those who aren't just end up going insane from the information overload, like Ross in dead space 2. Ellie is also shown to have a similar resistance like Isaac. However smart people aren't exactly immune they can still end up just losing it and killing themselves they just have a higher tolerance, Isaac can end up killing himself during a cutscene but if you succeed it's shown that he's the one holding the needle he nearly stabbed himself in the eye with. Isaac also 'built' the marker in 2 with the copy inside his brain, and is shown understanding the symbols and writing in 3. He ends up being the one understanding the marker's master plan for convergence and understanding the alien's machine and how they basically froze the planet to keep the moons from finding them via the convergence event.

tldr: Issac's intelligence played a big part but his willpower and tenacity helped him keep going. Fuck EA.