The oculomotor smooth pursuit system is driven by the slip
of the target image upon the retina arising from errors in
matching eye and target velocities. However, pursuit of an
object moving against a structured background causes most
retinal flow to be in the direction opposite to target movement.
Central mechanisms allow these distracting signals to be
overridden effortlessly. To isolate the anatomical substrate
of this capacity we studied the effect of the presence of a
structured background upon smooth pursuit in 26 patients
with focal cerebral lesions. In normal control subjects, studies
confirmed that a background has little effect upon pursuit.
Eye movements were recorded by the scleral search coil
method or by infra-red oculography. The target was a
bright spot moving horizontally in a triangular waveform
of amplitude ±11.25° visual angle, at either 10, 20, 30 or
36.5°/s. Data were collected in darkness and with a structured
background: 14 patients showed a significant reduction of
gain with a structured background, while the remaining 12
showed little or no effect. Comparison of the location of the
cerebral lesions in these two groups suggested that lesions
in the inferior parietal cortex (area 40) or in white matter
containing parieto-frontal connections result in disruption of
pursuit in the presence of a background.