Feature

Eye Movements and Schizophrenia

Characteristic eye movements may provide a means of
assessing the extent of schizophrenia across cultures.

Dr John S. Allen

Schizophrenia is an illness affecting millions of people world-wide and thousands in New Zealand. It is a significant and expensive health problem. There is no cure for the disease, but there are many who cope well with it through a combination of medication, therapy, and family and community support.

Psychiatrists are responsible for diagnosing schizophrenia. There are no laboratory tests to help them do this -- the diagnosis is based solely on the behaviour of the individual patient. Much research has been done to standardise the diagnosis of schizophrenia and other mental disorders. One outcome of this research is the Diagnostic and Statistical Manual of Mental Disorders, published by the American Psychiatric Association. It lists the characteristic psychotic symptoms of schizophrenia:

• delusions (especially bizarre delusions, such as believing one is being controlled by a dead person)
• prominent hallucinations (especially auditory)
• incoherence or marked loosening of associations, in which ideas shift from one subject to a completely unrelated or only obliquely related subject, without the speaker displaying any awareness that the topics are unconnected
• catatonic behaviour
• flat or grossly inappropriate emotional responses

In contrast to the quaint refrain of 1960s anti-psychiatry that schizophrenia is a "sane response to an insane world", current research into the cause of schizophrenia focuses on it as a brain disease with a biological basis. Adoption studies indicate that genes have a role; more recently, some investigators using sophisticated imaging technology have begun to look for anomalies in the brains of living schizophrenics that may help explain the source of their unusual behavioural patterns [link-textBrains Wanted, July 1993].

For many years, scientists have been looking for a biological "marker" that correlates with schizophrenia. In many cases, the genetic marker itself has nothing to do with the disease but simply indicates the probability of the disease's presence or likelihood of developing. A marker is useful as an additional, less subjective, way of controlling studies involving a disease such as schizophrenia. By adding an element beyond the schizophrenic behaviour under study, markers help us avoid circularity when we draw our conclusions.

As yet, no conventional genetic marker has been discovered for schizophrenia. Brain imaging studies have yielded intriguing results, but the technology involved is hardly conducive to large scale studies in diverse locations. Currently, the most experimentally robust marker for schizophrenia is a neurocognitive one involving eye movements.

You Can Tell by the Eyes

We make thousands of eye movements every day. We need to make eye movements in order to keep the image we are trying to follow directed on the fovea, the most light-sensitive part of the retina. "Smooth pursuit" eye movements are used to continuously follow a target moving across the visual field. "Saccadic" eye movements are used when we jump from point-to-point with our eyes; vision is suppressed while the eyes move quickly across the visual field during a "saccade", or jump.

In the early 1970s, researchers discovered that impaired smooth pursuit was significantly more common among schizophrenics than the general population. This was actually a rediscovery: the earliest mention of smooth pursuit dysfunction (SPD) in schizophrenia was in 1908! In those days, measuring and recording eye movements required equipment that was both clever and cumbersome, thus there were few follow-up studies.

Over the past 20 years, SPD in schizophrenia has been studied by many researchers throughout the world. Although it can result from a variety of organic neurological conditions, in the psychiatric context SPD is highly specific to schizophrenia (as opposed to manic-depressive illness, for example). It is not due to any of the standard medications given to treat schizophrenia, which one would expect since it was observed for the first time long before there were antipsychotic medicines in use. It is a trait marker rather than a state marker: SPD is present in the individual during periods of both illness and remission. Depending on the study, it is found in 50-80% of schizophrenics. Even more interesting is that it is found in about 40% of the first-degree relatives (such as parents and siblings) of schizophrenics, the vast majority of whom do not have schizophrenia.

This has lead to the development of a single gene model for the transmission of schizophrenia and SPD, which are seen to be two possible independent expressions of a single gene (which may be modified by other genes as well). In the model, SPD is more likely to be expressed than schizophrenia, although schizophrenia may be present without SPD. The model was developed in part to account for the fact that in some pedigrees, the schizophrenic does not have SPD but his first-degree relatives do. The model is said to fit available pedigrees better than any previous, and is being used to direct the search for a molecular genetic marker for schizophrenia.

Cross-Cultural Studies

I have used SPD as a tool to look at schizophrenia cross-culturally. People in different cultures behave differently. It is therefore highly likely that different cultures will define "abnormal" behaviour in different ways. Controlled studies indicate that many cultures identify an illness or condition that converges on the constellation of behaviours that comprise schizophrenia. However, these studies have almost all been based on the overt behaviour of schizophrenics, which is no doubt also influenced by cultural norms.

By using a marker for schizophrenia such as SPD, we can establish if people in different cultures who behave "abnormally" also share a common biology. Smooth pursuit eye movements are a behaviour, but at such a basic level that they are not much influenced by cultural norms. Therefore, SPD is a proxy indicating the possible role of biology and genetics in the etiology of schizophrenia.

I am currently undertaking a study to see if the frequency of SPD in schizophrenia varies among different ethnic groups in New Zealand. The basic rationale for such a study is that it is very difficult to interpret complex cultural factors in the expression of mental illness until you have controlled for -- or understand -- the biology involved.

In the past, schizophrenia was apparently less common among Maori than Europeans. The last 20 to 30 years have seen the Maori rate increase although their lengths of hospitalisation seem to be shorter. Pacific Islanders often show a pattern of acute psychosis resembling schizophrenia followed by recovery; this is a common pattern observed in migrant populations throughout the world.

Tracking the eye movements of schizophrenics of different races shows differing responses, as indicated in the box. This preliminary data indicates that SPD is associated with schizophrenia in different ethnic groups in New Zealand. Although the frequencies of expression in the ethnic groups are still being assessed, this is further evidence for biological universality for at least one form of schizophrenia.

In a related research project, I recently found that the marker is also present in some schizophrenics in Papua New Guinea. Given that it has been many thousands of years since Polynesians, Melanesians and Europeans all shared a common ancestor, it is possible to speculate that the genetic basis of schizophrenia is something shared by all human populations. As such, schizophrenia represents a basic aspect of biological variation in our species.

As an anthropologist, I am concerned with the study of "others", with the underlying hope that with knowledge and understanding, fear will become less likely and compassion more forthcoming. People with schizophrenia are not particularly well understood by people without schizophrenia. Perhaps one way to introduce them to understanding is through something as innocuous and universal as eye movements.

Dr John S. Allen is a lecturer in biological anthropology at Auckland University.