Human fixation and pursuit in normal and open‐loop conditions: effects of central and peripheral retinal targets.

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Abstract
Eye movements were recorded precisely with a scleral‐coil method under three experimental conditions: fixation of a central, stationary target; pursuit of a central, moving target; pursuit of eccentric, moving targets. Subjects were instructed to attend to and fixate the target and to pursue it when it moved. The target was presented either in darkness (no visible background), on a diffusely lighted background, or on a large, structured background. Target and/or background could be moved independently with single sinusoids, pseudo‐random mixtures of sinusoids or triangular waves. The target was usually presented under normal viewing conditions, but in some measurements (interleaved with normal ones) retinal target motion was uncoupled from eye motion by electronical addition of the eye position to the target position (open‐loop conditions). The gain and phase relations of eye movements induced by motion of the target and/or background were calculated for the total, composite (smooth and saccadic) eye movement and for the reconstructed cumulative smooth component separately. Horizontal motion of a large, structured background induced correlated smooth eye movements while subjects fixated a stationary point target. The induced horizontal movements were very small (gain about 0.05) when the target was seen normally, and larger (gain about 0.20) when the target was horizontally stabilized on the retina. The phase lag of the induced eye movements relative to the background movements was usually smaller than 90 deg. When the target moved vertically and the background horizontally, vertical pursuit was similar to that with a stationary background, but in addition horizontal smooth eye movements, correlated with the background movements, were elicited with a gain of about 0.1 and a phase lag which was usually smaller than 90 deg. Imposed pseudo‐random retinal motion of a central target under open‐loop conditions (retinal image motion uncoupled from eye movements) elicited highly idiosyncratic responses which varied too much among subjects to allow any general conclusions, other than that open‐loop stimulation seems unsuitable as a tool for analysing the response characteristics of the smooth pursuit system. In the absence of a background, an eccentric target configuration (two vertically aligned arrows with the points localized 5 deg above and 5 deg below the fovea) in horizontal motion was pursued equally well as a central target.(ABSTRACT TRUNCATED AT 400 WORDS)