[tarkin2]

I'm perhaps missing something but if there were a significant connection, wouldn't this mean those in sunnier climates would have better verbal and visual memory?

If there were a connection then I would wager that there are more significant factors, since I have seen or heard of no evidence to assume those in sunnier climates have better verbal and visual memory.

The results of this study seem to show no significant correlation, anyhow.

[HexPhantom]

The paper isn't saying vitamin D determines cognition in general, just that there was a modest signal in a couple of memory measures

[appplication]

Not necessarily, if the effect was due to exposure to levels beyond what you’d even get in sunnier climates. It also could be possible those in sunnier climates do experience this effect and it was unnoticed, due to political and socioeconomic issues associated with typical equatorial countries