There are many experimental methods available for altering consciousness. Partial list: pharmacological (drugs), tissue destruction (strokes, surgery), electrical stimulation (implanted electrodes), and transcranial magnetic stimulation (TMS). In addition, some people are born with altered forms of consciousness that can be traced to genetic defects (example, some people do not experience normal color vision). All of these can produce altered consciousness that is traced to altered brain activity.
TMS is particularly useful because it can be targeted to specific brain regions and it provides a reversible alteration of consciousness. Recent article on TMS as a means of localizing in the brain the locus for a particular type of perceptual consciousness.
Not all parts of the brain contribute equally to consciousness. Particular regions of the cerebral cortex are particularly important for consciousness. The brain regions contain the types of neuronal circuits required to generate conscious experience. This is a representative article exploring the particular neuronal networks involved in visual consciousness.
Can experimentally controlled alterations in consciousness lead us to the ultimate source of consciousness? If you make the assumption that there is a “force of free will” that is non-deterministic and not based on the material components of the brain and that this “force of free will” cannot be studied by neurobiologists then you might conclude that neuroscience cannot hope to tell us anything useful about the ultimate source of consciousness. However, there is no objective evidence for this hypothetical “force of free will” or any other source of consciousness beyond the physical structure of the brain. At best, an objection to “consciousness as brain activity” that is based on belief in a “force of free will” is an argument about competing hypotheses. The existence of competing hypotheses should lead to evaluation of the evidence that supports the competing hypotheses. The objective scientific evidence is on the side of “consciousness as brain activity” and so neurobiologists continue to build upon this hypothesis.