I wonder whether they had a control group, to eliminate the possibility that their "red or green target" wasn't absolutely invisible to a colour-blind monkey bu

I wonder whether they had a control group, to eliminate the possibility that their "red or green target" wasn't absolutely invisible to a colour-blind monkey but was close enough that it took five months of practice for "Dalton" to work out what he was supposed to be looking for. (Doesn't seem likely -- I'd have thought that if he were doing that, he'd have learned much faster.)