Task difficulty differentially affects two measures of processing load: the pupil response during sentence processing and delayed cued recall of the sentences.

Purpose: In this study, the authors assessed the influence of masking level (29% or 71% sentence perception) and test modality on the processing load during language perception as reflected by the pupil response. In addition, the authors administered a delayed cued stimulus recall test to examine whether processing load affected the encoding of the stimuli in memory.
Method: Participants performed speech and text reception threshold tests, during which the pupil response was measured. In the cued recall test, the first half of correctly perceived sentences was presented, and participants were asked to complete the sentences. Reading and listening span tests of working memory capacity were presented as well.
Results: Regardless of test modality, the pupil response indicated higher processing load in the 29% condition than in the 71% correct condition. Cued recall was better for the 29% condition.
Conclusions: The consistent effect of masking level on the pupil response during listening and reading support the validity of the pupil response as a measure of processing load during language perception. The absent relation between pupil response and cued recall may suggest that cued recall is not directly related to processing load, as reflected by the pupil response.