I'm looking to alter linguistic and musical associations in my brain for the purpose of writing music. ... related to the way their brain processes things like pattern recognition and other associations of the sounds of words and music and the ideas behind them, meanings and many other factors.
What instrument(s) do you play and for how long? When it comes to musical ability, practice practice practice. You can't create unique and novel music w/o the ability to play it. Also can depend on how old you are. The older u get, the more your brain becomes set in its associations and established patterns. The ability to do what u have already learned increases and gets refined, but novelty, creativity, spontaneity diminish somewhat. This shouldn't be much of a concern tho until your getting upper 30s and 40s.
When it comes to writing, boredom is always a great way to get creativity out of myself. Also, take up drawing a little, and make sure to draw/write with your non dominant hand - engages the other 1/2 of the brain. Maybe try writing your dreams down and turn them in to stories and poetry. But most importantly, just create and write. Even if you do absolutely nothing with it, keep it around to check out later. It will help months/years down the road. You can keep the pieces you like and rework/recombine them, and it helps give you perspective.
Learning another language can help as well.
As you become more confident in your musical ability, and have some good ideas of things you'd like to compose lyrically, (and maybe you're there already); then you can start researching foods/herbs that have the potential to alter your perceptions in the kind of way your talking about. Be aware, most of the things that do this in a marked, profound, and immediate way are unfortunately illegal. But for information sake, I'll talk about a couple substances and how/what they can affect in relation to musical ability.
First a disclaimer, this is only for informative purposes, not to advocate illegal usage - this will depend on where you live.
Marijuana - Ability to discern pitch seems to degrade, but timing, rhythm, and dynamic seem to increase. Weed for some reason slows your perception of time, and makes more distinctive the spaces and volume between and of sound. Also, expect a slight disassociation of thoughts and a slight increase in ability to link 'distant' sounds/notes/chords in ways that are both unique and pleasing to the ear. When I speak of thought disassociation, I mean a degraded ability to process the patterns and associations which characterize normal perception; but with an accompanying increase in easily associating more distantly related objects. A simple example: sweet/sour as opposed to sweet/lemon.
Mushrooms - Pitch and harmonics seem to resonate exceedingly well. Expect a high level of thought disassociation far surpassing that of marijuana. This will occur both wrt timing/rhythm, as well as chords/notes. You see, all tones relate to each other based on harmonics and the tones which surround them - the same is true even for timing/rhythm. I think this is one reason we enjoy music in the first place - our brains enjoy the distinctive/repetitive harmonic/acoustic patterns inherent in music - otherwise its just random noise. Mushrooms seem to trigger a distinct ability to discern less obviously and more distantly related harmonies, scales, chords, and timing structures. The boundaries between your senses dissolve somewhat; in higher doses where you feel as if you can almost see sound.
No doubt some very interesting and unique music has been produced with the aid of psychedelics. The Beatles, Dave Matthews, Radiohead, Pink Floyd, The Mars Volta just to name a few.
While I can't endorse anything illegal, if you do decide to use psychedelics, take caution and respect them. This type of experience can be very enlightening, but also very intense and is not for everyone. I have by no means given an exhaustive list of all effects, just some key ones related to music. Make sure you are well researched on their full effects, and always start at a minimum threshold dose, and have someone you trust stay sober and watch over you for the duration. Consume them within the purview of raw paleo (humans have been consuming pschedelics for millenia, well into the paleolithic). Be aware of the laws in your area.
A lot of really good info on everything that has effects on the mind, from caffeine to LSD can be found on
www.erowid.org 
