No one has direct access to your mind like you do. That's why introspection remains a valid technique even after the invention of brain scanners.

The placebo effect has an evil twin known as the nocebo effect. This is when the expectation of harm or pain becomes a self-fulfilling prophesy, even in the absence of any known physical effect. One candidate for the nocebo effect is the discomfort some people report experiencing after using mobile phones. Scientists have failed to identify a physical cause, so it's possible the adverse effects are caused by negative beliefs about the technology.