This paper reports experimental results on the index of cognitive activity (ICA), a recent micro-level measure in pupillometry, which relates processing load to the frequency of rapid small dilations of the pupil. We collected pupil size during a tracking task which was cast in a simulated driving context, as well as for a dual task of simultaneous tracking and language processing. The present results are the first to evaluate the ICA measure on these tasks. We find that the ICA is sensitive both to the simulated driving and the language task, and that it is more responsive to our driving task than overall pupil dilation. Overall, the use of the ICA as opposed to traditional pupillometry seems promising, as our data provide initial evidence that the ICA may be more responsive, and a more fine-grained measure of cognitive load than traditional macro-scale pupil dilation measures.