Is it OK for your boss to humiliate you in front of co-workers by poking you in the forehead repeatedly and telling you to use your little pea brain?

There is no question but that your boss's behavior is unprofessional (not to mention personally hurtful). However, there may not be anything that you can do about it. Workplace harassment (also referred to as a "hostile work environment") has to do with actions and/or words made by a superior and/or co-worker that create an environment that is so hostile that it prevents the employee from reasonably performing their job duties.

Tha having been said, this conduct must be "discriminatory" and not merely the result of boorish behavior. Specifically, discrimination is action taken against an employee because he/she is a member of a "protected class". In other words, their treatment must not be based on a person's religion, age, race, disability, gender, sexual orientation, national origin, etc). A hostile work environment can also include retaliatory action and sexual harassment.

Unfortunately, based on the facts presented, your employer's remarks do not appear to rise to the level of an actionable claim. That is unless (as outlined above) discrimination is involved, there exisits company policy covering such a situation or you are protected by an employment contract or union agreement to the contrary.