The purpose of this experiment is to measure the latency to onset of the contraction of the pupil, as a function of the size of positive steps in luminance, starting at various luminance levels to which the eye has been adapted prior to the stimulus steps. In the past, latency of the pupil response has been inaccurately measured, owing to the difficulty of separating the end of the latent period from the slow beginning of con- traction. To overcome this, a digital curve-fitting technique involving a time delay followed by a modified second-order step response was developed. Latency was defined as the time delay giving the most accurate fit. Because the curve-fitting procedure needed a response with less random variation than is normally pre- sent, an average was used. Such averaging was first justified by using the standard deviation to show that there is probably no significant variation of latency for responses of a given subject under identical stimulus conditions. This analysis also showed that 20 responses is an efficient number to average for the pupil- contraction system. The excellent agreement between each average experimental response and the computed fit verified the value of delay used in the computation. Latency, thus defined for each stimulus condition, was found to be primarily a function of luminance during the step and only secondarily of the ratio of the step change of luminance to the adaptation luminance.
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