Bipolar and schizophrenic patients differ in patterns of visual motion discrimination

Abstract

Background: Since Kraepelin's early distinction between bipolar disorder and schizophrenia, it has been assumed that these disorders represent two different pathophysiological processes, although they share many clinical symptoms. Previous studies showed that velocity discrimination, a sensitive psychophysiological measure of the visual motion system, is deficient in schizophrenia. Here we examined whether the motion processing impairment found in schizophrenia also occurs in bipolar disorder.

Methods: We compared 16 bipolar patients, 25 schizophrenic patients, and 25 normal controls on a velocity discrimination task. We measured the psychophysical threshold for velocity discrimination and contrast detection (as a control task) in all subjects.

Results: Bipolar patients showed normal velocity discrimination thresholds at intermediate velocities, the range in which velocity cues dominate velocity discrimination, and at low velocities. Schizophrenic patients, however, showed elevated velocity discrimination thresholds at intermediate and low velocities. At higher velocities, both bipolar and schizophrenic patients showed elevated thresholds. All subjects showed normal contrast detection thresholds.

Conclusions: Normal velocity discrimination in the intermediate range of velocity indicates unimpaired motion processing in bipolar disorder. The abnormal velocity discrimination of both schizophrenic and bipolar patients at higher velocities may reflect impaired temporal processing rather than impaired motion processing per se. These results suggest that the pathophysiological processes of bipolar disorder and schizophrenia diverge at the stage of visual motion processing, a sensory component mediated primarily in the extrastriate cortex.

Fig. 1

Schematic illustration of velocity discrimination and contrast detection tasks.

Fig. 2

Mean velocity discrimination thresholds of bipolar patients (BP, gray circles), schizophrenic patients (SZ, black circles) and normal controls (NC, open circles). The abscissa presents the range of base velocities. The ordinate, a logarithmic scale, is expressed as the Weber fraction (ΔV/V, or the just-noticeable difference between the velocities of the targets to be compared) and represents the mean velocity discrimination threshold values of the groups. The error bars correspond to ±1 standard error of the mean threshold. Sample sizes per target velocity are as follows: BP: n=16 for 3.8, 10.0, and 26.6°/s; n=14 for 6.2 and 16.2°/s; SZ: n=21 for 3.8°/s; n=17 for 6.2°/s, n=25 for 10.0°/s; n=16 for 16.2°/s, and n=20 for 26.6°/s; NC: n=24 for 3.8, 10, and 26.6°/s; n=21 for 6.2 and 16.2°/s.

Fig. 3

Scatter plot of individual subjects' thresholds at the slowest (3.8°/s), the middle (10.0°/s) and the fastest (26.6°/s) velocities. Data from bipolar patients, normal controls and schizophrenic patients are represented by gray, white and black symbols, respectively.