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.

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