Psychophysical isolation of a motion-processing deficit in schizophrenics and their relatives and its association with impaired smooth pursuit

Abstract

Schizophrenia patients and many of their relatives show impaired smooth pursuit eye tracking. The brain mechanisms underlying this impairment are not yet known, but because reduced open-loop acceleration and closed-loop gain accompany it, compromised perceptual processing of motion signals is implicated. A previous study showed that motion discrimination is impaired in schizophrenia patients. Motion discrimination can make use of position and contrast as well as velocity cues. Here, we report that the motion discrimination deficit, which occurs in both schizophrenic patients and in their first-degree relatives, involves a failure of velocity detection, which appears when judging intermediate target velocities. At slower and faster velocities, judgments of velocity discrimination seemed normal until we experimentally disentangled velocity cues from nonmotion cues. We further report that compromised velocity discrimination is associated with sluggish initiation of smooth pursuit. These findings point to specific central nervous system correlates of schizophrenic pathophysiology.Although schizophrenia is significantly influenced by genetic factors (1, 2), the clinical disorder occurs relatively infrequently; estimates vary from 3.5% to Ϸ8% in the nuclear families of schizophrenic probands (3). This low intrafamilial prevalence poses difficulties for studies of genetic linkage in schizophrenia. Certain physiological traits, however, although uncommon in the general population, occur more frequently than the clinical syndrome in first-degree relatives and suggest themselves as more penetrant alternative phenotypes for the study of the genetics and pathophysiology of schizophrenia (4). One of the more widely studied of these cofamilial traits is eye tracking dysfunction (5-7).Eye tracking dysfunction shows itself specifically in smooth pursuit eye movements, a tracking response to moving visual targets. Other kinds of eye movements, such as voluntary saccades (8, 9), vestibularly controlled eye movements (10), and the oculocephalic reflex (11), are normal in both patients and relatives. This set of findings indicates that the eye tracking dysfunction is specific to the smooth pursuit system and that its neuroanatomical substrate is above the brain stem.Several investigators have characterized the nature of the eye tracking dysfunction in schizophrenia as low closed-loop gain (the ratio of eye velocity to target velocity) with frequent compensatory catch-up saccades and low initial acceleration of the eye, both of which indicate that eye velocity lags behind target velocity (12-15). There may be two explanations for these findings: (i) Processing of velocity signals during target movement is deficient (16, 17); and (ii) general performance is inefficient when processing external signals (18).As evidence to support the hypothesis that deficient velocity processing is involved in eye tracking dysfunction, it is necessary to show that velocity discrimination is compromised in schizophrenic patients and that the sa...

Bipolar and schizophrenic patients differ in patterns of visual motion discrimination

“…Velocity discrimination thresholds were also not significantly correlated with dose of antipsychotic drugs in the schizophrenic group, results similar to those we reported in an independent sample (Chen et al, 1999c). Further underscoring the independence of deficient performance from medication effects or symptom severity, velocity discrimination is deficient in a significant proportion of clinically unaffected and never-medicated relatives of schizophrenic patients (Chen et al, 1999c). The isolated finding of a modest but significant correlation between severity of clinical symptoms and velocity discrimination threshold at the slowest target velocity only is difficult to interpret in that clinical state did not have a generalized effect on performance in the schizophrenic group.…”

“…The current study and our previous one (Chen et al, 1999c) yielded consistent results in showing that velocity discrimination is impaired when schizophrenia patients have to rely on motion signals at intermediate velocities. Schizophrenia patients in our earlier study showed normal velocity discrimination thresholds at low and high base velocities, whereas schizophrenic patients in the present sample showed elevated mean thresholds not only at the intermediate but also at the lowest and highest velocities.…”

“…Medication effects and clinical state are unlikely explanations for the increased velocity discrimination thresholds of the bipolar patients at the fast velocities in that these thresholds were not significantly correlated with dose of antipsychotic drugs or BPRS scores. Velocity discrimination thresholds were also not significantly correlated with dose of antipsychotic drugs in the schizophrenic group, results similar to those we reported in an independent sample (Chen et al, 1999c). Further underscoring the independence of deficient performance from medication effects or symptom severity, velocity discrimination is deficient in a significant proportion of clinically unaffected and never-medicated relatives of schizophrenic patients (Chen et al, 1999c).…”

“…We hypothesized that those patients with schizophrenia who show ETD process velocity signals poorly during target movement, leading to impairments in both smooth pursuit and velocity discrimination. In one of our earlier studies (Chen et al, 1999c), we showed that velocity discrimination thresholds were elevated in schizophrenic patients at intermediate target speeds of 10 to 16.2°/s, where velocity cues predominate. At slower and faster speeds, differences between normal participants and schizophrenic patients (and their relatives) were not significant.…”

Seymour S. Kety and the Genetics of Schizophrenia

“…Elegant studies have shown that lesions to these areas impair motion detection and produce the same kind of eye tracking abnormalities that we see in schizophrenia (e.g., Newsome et al 1985). Our psychophysical experiments confirmed that schizophrenic patients and a portion of their relatives do have difficulty in accurately detecting the speed of moving objects, although their other visual capacities, such as detection of color, contrast, or position, are normal ( Chen et al 1999b( Chen et al , 1999c. Indeed, our studies have shown that they tend to substitute position and contrast cues for velocity cues in order to adapt to the moving environment around them (Chen et al 1999a).…”