In 1973, psychophysicist Gunnar Johansson attached ten reflective patches to the body of an actor and filmed him moving around in a dark room. A group of healthy adults who watched the film were not only able to recognize human motion in the pattern of lights, but also to infer the sex of the walker¹.

More than two decades later, researchers used point-of-light displays to discover that, in people with autism, the ability to perceive biological motion, broadly defined as movement by humans or animals, is impaired².

Since then, many studies have confirmed this finding using point-of-light displays. But it’s been unclear whether this deficit is part of a larger problem visually processing motion generally, or is confined to the detection of biological motion.

Two new studies published in the past two months independently resolve this question by suggesting that people with autism don’t have a general difficulty processing visual information. What’s more, one of the studies found that only a subset of people with autism — those with lower intelligence quotients (IQs) — have trouble perceiving biological motion, and the other found that those with low IQs are more impaired than those with higher IQs.

However, the studies suggest that no matter their level of impairment, all individuals with autism struggle to pick up social cues from bodies in motion.

“Biological motion ability seems to fall within the same domain of social understanding as do other social understanding and mentalizing tasks,” says Tony Charman, professor of psychology and human development at the University of London and an investigator on one ofthe studies.

Surprisingly, both studies reported wide differences in the ability to perceive biological motion among both people with autism and healthy controls. But in all cases, those who have trouble identifying a walking man in a pattern of dots also have trouble interpreting social cues.

Social motion:

Charman’s study, published in August in Autism Research³, found that people who have difficulty perceiving biological motion also show impairments in a classic theory of mind task. Theory of mind is the ability to interpret the beliefs, desires and intentions of other people.

In their study, Charman and his colleagues found that 89 adolescents with autism were just as adept as 52 controls at perceiving coherent patterns and identifying shapes in patterns of dots displayed on a screen.

Only 18 percent of the individuals with autism struggled to distinguish a walking man from a scrambled pattern of dots, and all of them had IQs below 70.

These same participants also had trouble interpreting social interactions between a pair of moving triangles in the Frith-Happé animations, a classic test of theory of mind.

“Both tasks involve some level of social understanding, though they are rather far apart in terms of what the participants are asked to do: ‘Point to the man walking’ versus ‘What are the triangles doing?’” Charman says.

Poor performance on both tasks points to the critical role played by the superior temporal sulcus, a brain region robustly implicated in both biological motion perception and theory of mind.

The other study, published in September in Developmental Science, used functional magnetic resonance imaging to show that two separate networks are involved in processing visual information and biological motion⁴.

In the study, 16 adolescents with autism and 16 controls varied in their ability to identify a walking figure in a point-of-light display. But participants with autism did just as well as controls when asked to find coherent patterns of motion in moving dots.

Participants with autism show normal patterns in the visual processing circuit when looking at the dots, according to the study. But they showed much lower activity than controls in the posterior temporal sulcus and other brain regions known to be involved in processing biological motion when trying to identify the walking man in point-of-light displays.

“The [study] pretty clearly demonstrates that it’s not a basic visual motion processing deficit, but rather something specific to perceiving biological motion,” says Kevin Pelphrey, director of the Child Neuroscience Lab at Yale University. Pelphrey was not involved in either of the new studies.

Scrambled circuits:

In healthy people, these two systems typically work together, but a growing body of evidence suggests they are somehow disconnected in many people with autism, Pelphrey says.

Charman’s study is also consistent with that conclusion, Pelphrey says. “What they are suggesting is that it’s not necessarily a basic deficit in discriminating biological motion. It’s more about reading the intentions of the biological motion,” he says.

The two studies used slightly different methods to assess deficits in the perception of biological motion. Researchers in the U.K. group simply asked participants to point to the image of a walking man in one of two screens. The imaging study presented participants with a more difficult task by introducing ‘noise’ — dots used in the coherent motion task — into the image. They also asked participants to report which direction the walking figure was facing.

It’s no surprise that adolescents with normal to high IQs performed well on the U.K. study, which uses a much simpler metric, says Ami Klin, director of Emory University’s Marcus Autism Center in Atlanta. “It’s problem-solving,” says Klin, who was not involved with either study.

But Klin cautions against broadly interpreting the results from that study. The results don’t necessarily mean that the participants with autism are well attuned to biological motion in a natural context, or that they use the same circuits to identify biological motion as typical controls do, he says.

The imaging study clearly shows significant differences in the way that participants with autism process biological motion compared with controls, he says.

It’s also important not to confuse a tool — point-of-light displays — with the concept it seeks to measure, Klin notes. “A point-of-light display is not biological motion,” he says. “In this case, the method is so brilliant that it has come to define the concept.”