GABA fluid & phosphenes
The scramble suit was an invention of the Bell Laboratories, conjured up by accident by an employee named SA Powers. He had, a few years ago, been experimenting with disinhibiting substances affecting neural tissue, and one night, having administered to himself an IV injection considered safe and mildly euphoric, had experienced a disastrous drop in the GABA fluid of his brain. Subjectively, he had then witnessed lurid phosphene activity projected on the far wall of his bedroom, a frantically progressing montage of what, at the time, he imagined to be modern-day abstract paintings.from A Scanner Darkly by Philip K. Dick, p. 22
I freely admit that Dick is on the "doesn't care about science" end of the science fiction author continuum, and further that I have no idea what the level of knowledge was about some of these topics at the time. But nonetheless, I came across this passage and thought it might be fun to dissect it a bit.
GABA stands for Gamma-aminobutyric acid, one of the most common neurotransmitters. GABA is the primary ihibitory neurotransmitter: activation of GABA receptors generally leads to either an influx of chloride ion or an efflux of potassium ion; either of these has the end result of hyperpolarizing the cell and reducing the probablility of neuronal firing. It's important to note that isolated GABA is a powder at room temperature. Although it's fully dissolved in physiological conditions, it's certainly not a "fluid." One could generously allow a reference to it as a "solution", but really you'd be safest sticking with just calling it "GABA."
Phosphenes are one of the more common types of entopic phenomenon: visual experiences that originate within the eye. Phosphenes, specifically, are defined as perceived light caused by a source other than light. One of the most common sources is mechanical pressure: this can be experienced by closing your eye and looking at the inside of your eyelid while rubbing it with your finger. The colors and shapes you'll see are phosphenes. They are visible in a wide scenario of circumstances: after rubbing your eyes, you may be able to see them with your eyes open; you should also generally be able to see them with your eyes closed, especially in a lit room. If you start paying attention to them, you might start to realize that they--and other entopic phenomena--are present a lot of the time, but the brain is well adapted to ignoring their presence.
Phosphenes are frequently associated with drug-related phenomena, although I'm not sure how many of the connections have been demonstrated by research. For instance, I've seen it often proposed that many drug-induced visual experiences (particularly those associated with the phenethylamine-class psychedelics, such as mescaline) are caused in part by altered perception of phosphenes and other entoptic phenomena. I've also seen it proposed that some outright hallucinations may involve such altered visualization, such as the classic withdrawal complaint of bugs crawling all over everything (which is also featured in A Scanner Darkly).
Is it possible for fluctuation in GABA levels to cause the effect described in the passage? Well, maybe but probably not.
A "disastrous drop" in GABA levels throughout the brain would really be most likely to cause a seizure. In fact, although there are an army of drugs designed to stimulate GABA receptors or increase GABA levels, I couldn't name a single drug used outside of a lab whose effect involves an inhibition in GABA activity.
On the other hand, there are a veritable army of drugs whose effects involve an increase in GABA activity. For instance, barbiturates and benzodiazepines primarily act through direct stimulation of GABA receptors--as does one of my favorite drugs, ethanol. Although these aren't really drugs known for their visual effects, others--such as thujone, one of the active ingredients in absinthe, or muscimol, found in fly agaric mushrooms--demonstrate that the possibility is certainly there.
And as to a scientist experimenting on himself with his own synthetic drugs: although I'm fairly confident that no Institutional Review Board would ever approve of such a thing, it does place him in fine company. Scientifically speaking.