Susceptibility to visual illusions in autism

Visual illusions are good fun [don't follow this link if you have epilepsy], but they can also provide important insights into the way that our visual perceptual system works. In particular, they show how our knowledge and expectations about the world influence perception. For example, in the Shepard illusion below, the tops of the two tables are actually the same shape. However, because we naturally think of them as being pictures of 3D objects, we can't help but perceive the left table as being much more elongated than that on the right.

In 1996, Francesca Happe published an intriguing paper in which she reported that children with autism were less susceptible to visual illusions than non-autistic kids, suggesting in turn that the way they perceive the world is fundamentally different. However, subsequent studies have provided somewhat mixed results. Some have also found immunity to illusions; others have found that people with autism are just as susceptible as those not on the spectrum. It's not clear why the results are so contradictory, but it may be down to differences in the way the tests have been administered in different studies, or the fact that different illusions have been tested, or a combination of both.

Why are people with autism good at visual search tasks?

Autism is defined in terms of deficits - a person must have social impairments and communication impairments and repetitive behaviour or restricted interests to be diagnosed 'autistic'. However, it's often argued that the key to understanding autism is to look at strengths rather than the weaknesses.
Because autism is a pervasive disorder and can affect so many different aspects of cognition, it's often very difficult to pin down the underlying cause of any particular deficit or weakness. If someone with autism struggles on a test they are given, it could potentially be for any number of reasons - memory difficulties, poor concentration, stress brought on by the test situation, failure to understand what it was they were supposed to be doing. And so on. All of these factors might be expected to lead to poor performance. But when people with autism perform exceptionally well on a task, we can rule out these possible alternative explanations. If we can understand the origin of these strengths, then we might also gain some insight into some of the difficulties faced by people with autism.