“In subsequent experiments, the researchers challenged the trained bees by making it more difficult
for them to distinguish between reward targets and punishment targets.
They placed the targets closer and closer to the reference line, to the point where it even became impossible
for the bee to “guess” which target would contain the sweet sucrose solution and which one was the bitter quinine solution.
As this distinction became more difficult, an increasing number of bees simply chose to not make a decision at all
and instead opted out of the test by flying into another container via an “exit hole”
http://www.scilogs.com/next_regeneration/bee-or-not-to-bee-difficult-choices/