The elements of the joke are as follows. I assumed that HN readers have a higher probability than the general population to associate words with scientific or engineering meanings. I further assumed that as this site has a high association with electronics, the electrical meaning of conductor would be most prevalent. This was reinforced by the parent post, who stated nearly exactly what I had been thinking, right down to the smirking overcleverness with which it described to motion of elecromagnetic charge. Before clicking on the link, I thought of other things that could be meant by "conductor". I came up with heat transfer and train tickets. I didn't even think of orchestras, probably because I had always been in bands, where the leaders were overwhelmingly directors, band leaders, and drum majors.

The second element of the joke is that I presumed that HN readers, having a variety of esoteric hobbies, might also have an interest in cognitive science and linguistics (particularly the AI folks), and might want to analyze the brain of an outlier. I then decided that, while less accessible to a home hobbyist than brain surgery or in-depth psychological profiling, fMRI imaging conveyed the point of the joke in shorter, more recognizable wording, without actually being a very probable outcome.

In reality, you would probably be wired up to a home-made sensor array and presented with a series of time-indexed sensory inputs via headphones and a computer screen, and perhaps asked to press certain button sequences in response.

So, in short, no, those people do not need a fMRI. You, however.... I would like to analyze your physiological response to traditional comedy and contrast that with nerd jokes....