How does task difficulty schedule affect the rate and efficiency of perceptual learning? In perceptual learning (line length/orientation discrimination, visual target detection, tone frequency discrimination, etc), when training people or animals to perform a task better, is there an optimal schedule for controlling task difficulty, to acheive maximal improvement?

At least 3 reasonable schedules come to (my) mind:


Easy to difficult - In education it is the custom to begin with easy examples or problems, and gradually progress to more difficult ones. Maybe this should work in perceptual learning as well.
Interleave different difficulties
Always the most difficult possible


Has there been a rigorous comparison of such schedules and their effect on the rate of learning, and the maximal performance level achieved at the end of training?
General pedagogical ideas around optimal difficulty

Many theories of instruction suggest that learning is optimal when an appropriate level of challenge is maintained. If a task is too easy, there's little to learn. If a task is too difficult, the learner can be overwhelmed. The implication for practice is that task difficulty should increase in conjunction with the increased skill of the learner. Of course, we can think of exceptions to these ideas, such as the value of over-learning, and the general idea of exposing people to challenging problems to motivate further learning. This also still leaves open the question of precisely how fast difficulty should increase for a given learner.

Academically, I've seen these ideas expressed in several places:

Deliberate practice: This is a concept drawn from the expertise literature. It is posited that amount of deliberate practice (rather than amount of simple experience) is one of the key factors that discriminates experts from non-experts (see Ericsson et al 2006). 
A key aspect of task difficulty relates to the sequencing of task difficulty with practice. 
Ericsson et al (2006) writes that


  The core assumption of deliberate practice ... is that expert
  performance is acquired gradually and that effective improvement of
  performance requires the opportunity to find suitable training tasks
  that the performer can master sequentially... Deliberate practice
  presents performers with tasks that are initially outside their
  current realm of reliable performance, yet can be mastered within
  hours of practice by concentrating on critical aspects and by
  gradually refining performance through repetitions after feedback.


Challenge point in motor learning:
Guadagnoli and Lee present a framework for thinking about optimal difficulty in motor learning. Their framework suggests that optimal learning is achieved by maintaining task difficulty at the point where it provides optimal information (where learning is possible, but it is not too difficult as to be overwhelming).

Research on task difficulty and perceptual learning

In a review article Ahissar and Hochstein (2004) also suggest easy to difficult is the most effective regime for perceptual learning:


  As found in our studies, training is more effective if subjects start
  with easy conditions and gradually move to more difficult con-
  ditions. The importance of beginning training with easy conditions was
  first found by Pavlov. When Pavlov reinforced a dog’s salivation
  following its seeing an ellipse but not following its seeing a circle
  (Figure I, rightmost pair of stimuli), the dog could not avoid
  generalization and salivated at sight of the circle, as well. Only by
  using very elongated ellipses, and training along the continuum, from
  left to right, was it able to achieve good performance for small
  circle/ellipse differences. This phenomenon was subsequently termed
  ‘transfer along a continuum’ (of different degrees of difficulty).


References


Ahissar, M. and Hochstein, S. (2004). The reverse hierarchy theory of
visual perceptual learning. Trends in cognitive sciences, 8(10):457-464. FREE PDF
Ericsson, K. et al. (2006). The influence of experience and deliberate
practice on the development of superior expert performance. The
Cambridge handbook of expertise and expert performance, 10(3):683-703.
FREE PDF
Guadagnoli, M. and Lee, T. (2004). Challenge point: a framework for
conceptualizing the effects of various practice conditions in motor
learning. Journal of Motor Behavior, 36(2):212-224. FREE PDF