Why ask here? This isn't an electronics question, in my opinion. And you can look up a variety of papers on the topic. Just search the literature. A quick hit I got on NCBI tells me that there are a variety of systems with varying responses. For example,
visual phosphenes are different from the Aplysia pacemaker neurons that synchronize with each other and drive the heart. Add hormonal impacts and you already have three different systems with three differing response, already. I was quickly able to find that under 200 kV/m is one thing, but over that figure is another thing.