A biophysical cortico-colicular model of saccade initiation based on competitive integration of planned and reactive cortical saccade decision signals in the intermediate layer of the superior colliculus is introduced. The variable slopes of the climbing activities of the input cortical decision signals are produced from variability in the ionic and synaptic conductances of cortical neurons. The model reproduces the unimodal distributions of saccade reaction times for correct antisaccades and erroneous prosaccades as well as the variability of saccade reaction times and the overall error probabilities in a large sample of 2006 young men performing an antisaccade task.