Thinking about Thoughts

Someone brought the following question to my attention:

Where is the first time that we see thoughts associated with the brain or head?

We are so used to thinking of thoughts originating in the mind that we don't give it a second thought (ha), but in Tanach, thought is not associated with the brain but with the heart.  In this week's parsha, the term "chacham lev" is used.  The famous work, the Chovos Ha'Levavos is not about obligatory feelings but about what we should be thinking about!

What about in the secular world - when did thoughts become associated with the brain?