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 variations in pupil diameter. We collected pupil size data during three language processing tasks (German subject vs. object relative clauses, a grammatical gender violation, and semantic anomalies). We report the first results on the Index of Cognitive Activity for language processing and show that the ICA is responsive to all of our manipulations. We also compare the ICA results to overall pupil dilations and self-paced reading as a measure of processing difficulty. Overall, the use of the ICA as opposed to traditional pupillometry seems promising, as our data provide initial evidence that the ICA may be a faster and more fine-grained measure of cognitive load than overall pupil dilation.