A study of the eye-movement control system shows the dependence of many of the system parameters on target luminance and contrast. Saccadic reaction time was found to decrease from a high value toward a fixed minimum as target luminance was increased, whether with a zero background (high contrast) or a fixed low contrast with respect to the background. The magnitude of the visual dead zone created when target luminance went below foveal threshold was also measured as a function of target luminance. The closed-loop gain of the eye-movement control system for ±2° sinusoidal target motion was measured as a function of luminance for high- and low-contrast targets. The results showed two changes of system gain as target luminance was decreased: (a) There was a decrease of the high-frequency response associated with target energies (luminance-by-time products) falling below a critical value required to produce visual sensation, resulting in a cutoff frequency; (b) for high-contrast targets only, there was an over-all decrease of system gain as target luminance was decreased, for luminances well above foveal threshold and for frequencies well below cutoff. This latter, unexplained effect cannot be interpreted as resulting from an increase of retinal latency, the effect of a visual dead zone, or the lack of sufficient target energy for visibility. A similar tracking experiment was performed for “unpredictable” target motion. Several changes were observed in the response of the eye-movement control system, and these were related to the effects of luminance upon system parameters and target predictability.
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