Incubation processes during problem solving

Is it true that setting aside a difficult problem for a period of what is known as 'incubation' leads to solution ideas occurring either spontaneously or very quickly on return to the task? Although recent research supports the usefulness of incubation periods, how incubation works is unclear.
The two main current explanations are:

that unconscious work takes place
that withdrawal of attention from the problem permits a new start on returning to the problem.
Previous studies have found support for both explanations. However, the conflicting studies have differed in the nature of the problems used (ie single solution insight tasks v. multisolution or "divergent" tasks) and in when the incubation period was allowed (immediately after task was explained v. after a period of work).
The present proposal will use both types of tasks and will examine whether type of task and placement of incubation period explain these conflicting results. The possible effects of similarities and differences between the target tasks and the activities during the incubation periods will also be assessed. The effects of the similarity or otherwise between target and interpolated incubation tasks are predicted to be different by explanations based on unconscious work as against attention switching.