Speech clock trained on 2,928 Spanish speakers rates cognitively impaired people as older

Ageing clocks already exist for brains and for DNA. Brain clocks read signatures from neuroimaging and epigenetic clocks read patterns of methyl tags on DNA, but until this study nobody had built one from speech, according to Nature's report ^[11]. Ibáñez's reason for trying is that speaking involves a "huge amount of brain work", as he put it ^[12].

The healthy participants are the control, and they came out about where they should: their speech age generally matched their chronological age ^[4]. "We can see a huge predictive value, just with a very simple four minutes of speech recordings," Ibáñez said ^[8].

The denominator is 2,928 people recruited in Argentina, Chile, Colombia, Mexico and Peru ^[5]. Against more than 700 speech features ^[7], that is at most about four participants per feature ^[13]. With a ratio like that, the first figure I would want is accuracy on people held out of training. The report does not give it, nor the split between healthy and impaired participants, nor the size of the gap in each group.

Separating people with a known diagnosis from healthy volunteers ^[6] is the first test a screening tool has to pass, and the easiest one. In people who are already impaired, the gap is a correlate of that impairment. A screen has to find the healthy-seeming person who will decline among many who will not. When most of the people tested are well, even a small false-positive rate sends many of them for follow-up they did not need.

In my view the speech gap has earned a prospective trial. Jed Meltzer, a University of Toronto cognitive neuroscientist who specializes in language and was not involved in the work, went further. "It is a very impressive piece of work," he said ^[10].