I'm wondering how glutamate antagonists or gaba agonists (that lower monoamine and glutamate activity) can produce hallucinations.
If you remove your graphics processing unit from your PC while it is on then the screen won't show a computer game actively playing.
The most obvious explanation is that glutamate or gaba regulate brain activity such that glutamate antagonists increase brain activity and gaba agonists does so likewise, but this is contrary to what is known about the functioning of the brain.