As far a I can understand the study design, it can establish a correlation but bot a causal relationship.
So, instead of implying that using computer up to "2.4 h/day" decreases risk of dementia, but higher screen time increases it, we could as well conclude that dementia patients are either unable to use a PC (< 2.4 h/day) or unable to stop using a PC (> 2.4 h/day).
I'm not confident at all at my ability to interpret this study, since it is definitely not my area of expertise, so, please, correct me if I'm wrong.
Update #1, after reading the article more carefully:
> "First, as an observational cohort study, causal relationships cannot be inferred; observed associations may reflect residual or unmeasured confounding and reverse causation."
that's what I said
> Second, a single baseline self-report likely introduced measurement error, missed secular shifts in digital-media use, and lacked content/context; session fragmentation, breaks, concurrent media, timing (e.g., evening vs daytime), and device type were unavailable.
How much do we really trust a dementia patient's self-report on device usage? I wouldn't even trust a healthy person's self-report. I think that anyone who ever tried to track screen time on computer or on the phone can understand how unreliable are your intuitive guesstimates compared to actual measurements.
What the study could not exclude is the possibility that people who reported high screen use during the exposure period already had some kind of unmeasured brain damage, what the paper calls "prodromal" dementia.
The most likely explanation of these data is not that screen time causes dementia directly. Excessive screen time probably correlates with the sedentary and isolated lifestyles that are known to influence cardiovascular and mental health.
What is novel and interesting is that you wouldn't necessarily expect moderate computer use would have a protective effect.
Not worried: "Associations between television/computer use and dementia in socially inactive older adults remain unclear, and optimal limits are unknown."
