I don't have the ACG yet, but I do have the play test pdf. Unless they gave the Investigator a Discovery class feats, no. If they did do it (did they change the name of Investigator Talent to Discovery?) they would have to have Cognatogen in their Discovery list. Or be allowed to pick from the Alchemist's list (it would then have to explicitly say that they are allowed to take, either that one, or all Alchemist Discoveries).
I'm leaning towards "no", but it seemed like the only way to get them access to Cognatogen which, oddly, isn't on their discovery list.
Reading the ACG, it looks like Investigator's don't have access to all alchemist discoveries by design. An Investigator can't take Extra Discovery as they don't have the Discovery class feature. However, an investigator can take Extra Investigator Talent to take alchemist discovery through their Investigator talent class feature. This has a fixed list of discoveries it supports. Cognatogen is not on the list, so it seems like you can't take it.
For context, this is also how similar cross-class features work, like using Extra Arcana for a Hexcrafter Magus to access another hex through their Hex Arcana class feature instead of taking Extra Hex directly (which they can't do because they don't have the Hex class feature). Unlike the Hex Arcana class feature, the Investigator's Alchemist Discovery talent strictly limits which other class' feature that can be taken.
Extra Investigator Talent
Spoiler:
You learn a new way to use your training and inspiration.
Prerequisite: Investigator talent† class feature.
Benefit: You gain one additional investigator talent. You must meet the prerequisites for this investigator talent.
Special: You can take this feat multiple times. Each time you do, you gain another investigator talent.
Alchemist Discovery (Ex):
Spoiler:
The investigator can select one of the following alchemist discoveries as an investigator talent: combine extracts, concentrate poison, dilution, elixir of life, enhance potion, eternal potion, extend potion,
infusion, mutagen(UM), and poison conversion(UC). When selecting an alchemist discovery, he must be high enough level to qualify for that discovery, using his investigator level as his alchemist level to determine if he qualifies. This talent can be selected multiple times; each time grants a new alchemist discovery.
Yep, they're intentionally not allowed to do that. If you really felt the need you could dip a level (or two) of Mindchemist. That would make you the undisputed king of Knowledge skills at the expense of, well, a lot actually, but it's possible.
Personally, I don't feel like it'd be all that thematic. A good Investigator shouldn't need drugs to improve their mind, only for other things. Even Holmes only did cocaine when he was bored between cases, not as an intellectual aid during them.
