Eye movement and visual motion perception in schizophrenia II: global coherent motion as a function of target velocity and stimulus density

Coherent global motion is a compelling illusion of visual motion that is seen as the result of spatially and successively presented stimuli that are, in fact, stationary. In the present study the threshold perception of global coherent motion was measured using random-dot kinematograms in a group of normal observers and a group with mixed symptoms in schizophrenia who also participated in a companion study on smooth pursuit eye movement (Slaghuis et al. in Exp Brain Res, 2007). The velocity of coherent motion target stimuli was produced by varying the spatial step-size (Äs) between dots to create three target velocities (6.0, 12.0 and 24.0 deg/s) which were measured at three target stimulus densities (100, 200, and 400 dots/deg2). A staircase procedure was used to determine the threshold for the number of target dots that was needed to move in the same direction to detect the direction of motion and which were plotted amongst a field of randomly moving visual noise dots. The findings demonstrate that in comparison with normal observers, the threshold for the perception of coherent motion in the group with schizophrenia was significantly higher at the lowest target velocity of 6.0 deg/s but not at target velocities of 12.0 and 24.0 deg/s. Stimulus density was found to have a significant effect on the perception of coherent motion, but it had no differential effect on performance in the groups. An examination of relationships between coherent motion and smooth pursuit eye movement in the companion study (Slaghuis et al. in Exp Brain Res, 2007) revealed significant, negative, correlations between coherent motion and apparent motion smooth pursuit eye velocity at target velocities of 6.0, 12.0 and 24.0 deg/s in the group with schizophrenia, but no such relationship was found in normal observers. It was concluded that the significant reduction in sensitivity for the perception of coherent motion at the lowest target velocity of 6.0 deg/s in the group with schizophrenia is consistent with an impairment in the detection of visual motion at a local level and in parallel for all parts of the image at striate and extrastiate levels of visual processing.