On your subject of an online questionnaire for synesthesia, I'm almost certain there was a post on HN related to this, but I don't find it in my bookmark librar

On your subject of an online questionnaire for synesthesia, I'm almost certain there was a post on HN related to this, but I don't find it in my bookmark library. But I think I found the same article with a Google search. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0118996 https://journals.plos.org/plosone/article?id=10.1371/journal... is a paper describing an online survey of colors versus letters, the url for the survey is in the paper, is still registered, but doesn't seem to be responding to me at the moment. Over 6500 people had taken the survey at that time. A strange correlation appeared when they looked at the data. They accidentally tracked down that that a Fisher Price toy with colored letters matched the correlation. And people who took the survey but were not the age where they would have been exposed to this toy did not show the correlation. You can find lots of related posts with a Google search for Fisher Price synesthesia or perhaps other plausible key words. If you could use similar methods and look for correlations between notes and colors then there might be some toy that young children experienced for many many hours that might explain some of this.