Background. Smooth pursuit eye movement (SPEM) dysfunction is considered to be a promising candidate for a biological marker for genetic vulnerability to schizophrenia. There are conflicting findings regarding the question of what is exactly dysfunctional in SPEM dysfunction and what component of eye movements is really specific to schizophrenia. The purpose of the current study was to help to clarify the nature of (SPEM) dysfunction and its specificity to schizophrenia.

Methods. Smooth pursuit eye movements of 43 schizophrenic patients, 34 patients with major depression and 42 normal controls were examined using high resolution infrared oculography. These groups were compared on several indices of oculomotor functioning (gain, different saccadic categories).

Results. Schizophrenics had a significantly higher catch-up saccade rate than depressed patients and normals. The percentage of subjects with an abnormally high catch-up saccade rate defined as beyond the mean plus 2 s.d. of the normal control group was significantly higher in schizophrenics (27·9%) than in depressed patients (8·8%) and normal controls (0%). Low gain and higher numbers of intrusive saccades tended to be more prevalent in both patient groups but did not distinguish schizophrenics from depressed patients.

Conclusions. Low gain and high rates of intrusive saccades contribute to SPEM dysfunction in major depression. Abnormally high rates of catch-up saccades seem to be the oculomotor component in smooth pursuit, that is specific to schizophrenia.