How does intelligence or general cognitive ability vary with level of CO₂? The cognitive ability of humans is varies with the concentration of CO₂ in breathing air. But how does it vary?

A very high concentration causes sleepiness, and sleepiness is a state of reduced cognitive ability.

That means it does vary, but I think there is no sudden onset of the reduction.

I would be interested in, ideally, a function of some experimental correlate of cognitive ability by level of CO₂, but any theoretical or experimental information about the shape of the function is interesting.

I'm aware that the signal to noise ratio in an experimental measurement is high, but qualitative results are the most important.
According to Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderate CO2 Concentrations on Human Decision-Making Performance Environ Health Perspect. 2012 Dec; 120(12): 1671–1677: 

of 9 different cognitive tests performed at 600ppm (green), 1000ppm (blue) and 2500ppm (red) several show very marked decrease in scores at higher C02 concentration.  

 

Authors summarized findings as: 


  Relative to 600 ppm, at 1,000 ppm CO2, moderate and statistically significant decrements occurred in six of nine scales of decision-making performance. At 2,500 ppm, large and statistically significant reductions occurred in seven scales of decision-making performance 


Four years after the above was published, the negative effects of carbon dioxide were confirmed in Associations of cognitive function scores with carbon dioxide, ventilation, and volatile organic compound exposures in office workers: a controlled exposure study of green and conventional office environments. Environ Health Perspect (June 2016) 124:805–812, which was mentioned on Earth SE by Ken Fabian, although there is one common author Usha Satish.  

On the other hand, a very recent article Effects of acute exposures to carbon dioxide on decision making and cognition in astronaut-like subjects NPJ Microgravity. (June 2019) 5: 17, which also has Usha Satish as an author, tends to contradict the above: