Influence of stimulus size and contrast on the temporal parameters of saccadic eye movements: implications for road traffic.

Saccadic eye movements are required for the recognition of peripheral objects in road traffic. Their latencies largely determine reaction time in emergency situations. The aim of this study was to investigate the influence of stimulus and surround parameters on the temporal characteristics of saccadic eye movements under conditions of object size, contrast and luminance corresponding to nighttime traffic. Square stimuli of 1 degree or 5 degrees size were presented under an eccentricity of 5 degrees and 15 degrees. The luminance of the surround was 4 x 10(-4) cd/m2 and 1 cd/m2. More than approx. 2,000 saccades of 7 normal subjects were registered and evaluated with respect to the latency, maximal velocity, and frequency of secondary saccades. At high stimulus contrast, latency approaches a minimum of approx. 200 ms. Latency increases with decreasing contrast up to maximal values of more than 600 ms. Transforming the contrast values into a relative decibel scale shows that this increase in latency occurs at significantly higher relative contrast values under scotopic as compared with photopic conditions. Our results demonstrate that an overall latency of 200-300 ms is not adequate for the assessment of accidents, especially under the stimulus conditions of nighttime traffic.