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.

Figure 1

Velocity discrimination thresholds of schizophrenic patients, their first-degree relatives, and normal controls. The abscissa represents the range of base velocities tested in this experiment. The ordinate, expressed as the Weber fraction (ΔV/V, or the just-noticeable differences between the velocities of the targets to be compared), represents the velocity discrimination threshold values of the three groups. The error bars correspond to ±1 standard error of the mean threshold.

Figure 2

Comparison of velocity discrimination of schizophrenic and normal control groups. (a) Group ratios (schizophrenia/normal control) of Weber thresholds plotted as a function of base velocity. A ratio of unity, shown in the dotted horizontal line, indicates equivalent performance by the two groups. The larger the ratio is, the higher the velocity discrimination threshold of the patients relative to the normal controls. The asterisk and cross sign represent the group ratios after exposure time for the 3.8°/s target (asterisk) and amount of contrast for the 26.2°/s target (cross sign) were randomized. (b) Histograms in the three panels (from left to right) represent distributions of individual patients’ thresholds at the slowest (3.8°/s), middle (10°/s), and fastest (26.2°/s) base velocities. The vertical line in each panel indicates the median threshold of the normal control group.

Figure 3

Comparison of velocity discrimination between first-degree relatives of schizophrenia patients and normal controls. (a) Group ratio (as in Fig. 2, but here for relatives/normal controls) of Weber fraction thresholds plotted as a function of base velocity. The asterisk and cross sign represent group ratios after exposure time and amount of contrast of the two velocity comparison targets were randomized. (b) Histograms in the three panels represent, from left to right, the distributions of individual relatives’ thresholds at the slowest, middle, and fastest velocities. Other details are similar to those in Fig. 2.

Figure 4

The effect of minimizing positional displacement and contrast cues on velocity discrimination. (a) Threshold values (mean and SE) of schizophrenic patients and normal controls obtained during the standard condition (constant exposure time) and after the exposure times of velocity-comparison targets were randomized. (b) Values for the schizophrenic patients and normal controls during the standard condition and after amount of contrast of the targets has been randomized.

Figure 5

Representative responses of eye tracking to the step-ramp target by a normal control (seven trials) and a schizophrenic subject (seven trials). The thick bar denotes the time windows for computation of open-loop acceleration.