More than a century ago, scientists discovered something usual about how people with schizophrenia move their eyes. The men, psychologist and inventor Raymond Dodge and psychiatrist Allen Diefendorf, were trying out one of Dodge’s inventions: an early incarnation of the modern eye tracker. When they used it on psychiatric patients, they found that most of their subjects with schizophrenia had a funny way of following a moving object with their eyes.
When a healthy person watches a smoothly moving object (say, an airplane crossing the sky), she tracks the plane with a smooth, continuous eye movement to match its displacement. This action is called smooth pursuit. But smooth pursuit isn’t smooth for most patients with schizophrenia. Their eyes often fall behind and they make a series of quick, tiny jerks to catch up or even dart ahead of their target. For the better part of a century, this movement pattern would remain a mystery. But in recent decades, scientific discoveries have led to a better understanding of smooth pursuit eye movements – both in health and in disease.
Scientists now know that smooth pursuit involves a lot more than simply moving your eyes. To illustrate, let’s say a sexy jogger catches your eye on the street. When you first see the runner, your eyes are stationary and his or her image is moving across your retinas at some relatively constant rate. Your visual system (in particular, your visual motion-processing area MT) must first determine this rate. Then your eyes can move to catch up with the target and match its speed. If you do this well, the jogger’s image will no longer be moving relative to your retinas. From your visual system’s perspective, the jogger is running in place and his or her surroundings are moving instead. From both visual cues and signals about your eye movements, your brain can predict where the jogger is headed and keep moving your eyes at just the right speed to keep pace.
Although the smooth pursuit abnormalities in schizophrenia may sound like a movement problem, they appear to reflect a problem with perception. Sensitive visual tests show that motion perception is disrupted in many patients. They can’t tell the difference between the speeds of two objects or integrate complex motion information as well as healthy controls. A functional MRI study helped explain why. The study found that people with schizophrenia activated their motion-processing area MT less than controls while doing motion-processing tasks. The next logical question – why MT doesn’t work as well for patients – remains unanswered for now.
In my last two posts I wrote about how delusions can develop in healthy people who don’t suffer from psychosis. The same is true of not-so-smooth pursuit. In particular, healthy relatives of patients with schizophrenia tend to have jerkier pursuit movements than subjects without a family history of the illness. They are also impaired at some of the same motion-processing tests that stymie patients. This pattern, along with the results of twin studies, suggests that smooth pursuit dysfunction is inherited. Following up on this idea, two studies have compared subjects’ genotypes with the inheritance patterns of smooth pursuit problems within families. While they couldn’t identify exactly which gene was involved (a limitation of the technique), they both tracked the culprit gene to the same genetic neighborhood on the sixth chromosome.
Despite this progress, the tale of smooth pursuit in schizophrenia is more complex than it appears. For one, there’s evidence that smooth pursuit problems differ for patients with different forms of the disorder. Patients with negative symptoms (like social withdrawal or no outward signs of emotion) may have problems with the first step of smooth pursuit: judging the target’s speed and moving their eyes to catch up. Meanwhile, those with more positive symptoms (like delusions or hallucinations) may have more trouble with the second step: predicting the future movement of the target and keeping pace with their eyes.
It’s also unclear exactly how common these problems are among patients; depending on the study, as many as 95% or as few as 12% of patients may have disrupted smooth pursuit. The studies that found the highest rates of smooth pursuit dysfunction in patients also found rates as high as 19% for the problems among healthy controls. These differences may boil down to the details of how the eye movements were measured in the different experiments. Still, the studies all agreed that people with schizophrenia are far more likely to have smooth pursuit problems than healthy controls. What the studies don’t agree on is how specific these problems are to schizophrenia compared with other psychiatric illnesses. Some studies have found smooth pursuit abnormalities in patients with bipolar disorder and major depression as well as in their close relatives; other studies have not.
Despite these messy issues, a group of scientists at the University of Aberdeen in Scotland recently tried to tell whether subjects had schizophrenia based on their eye movements alone. In addition to smooth pursuit, they used two other measures: the subject’s ability to fix her gaze on a stable target and how she looked at pictures of complex scenes. Most patients have trouble holding their eyes still in the presence of distractors and, when shown a meaningful picture, they tend to look at fewer objects or features in the scene.
Taking the results from all three measures into account, the group could distinguish between a new set of patients with schizophrenia and new healthy controls with an accuracy of 87.8%. While this rate is high, keep in mind that the scientists removed real-world messiness by selecting controls without other psychiatric illnesses or close relatives with psychosis. This makes their demonstration a lot less impressive – and a lot less useful in the real world. I don’t think this method will ever become a viable alternative to diagnosing schizophrenia based on their clinical symptoms, but the approach may hold promise in a similar vein: identifying young people who are at risk for developing the illness. Finding these individuals and helping them sooner could truly mean the difference between life and death.
Benson PJ, Beedie SA, Shephard E, Giegling I, Rujescu D, & St Clair D (2012). Simple viewing tests can detect eye movement abnormalities that distinguish schizophrenia cases from controls with exceptional accuracy. Biological psychiatry, 72 (9), 716-24 PMID: 22621999