Pupillary responses and reaction times index different cognitive processes in a combined Go/Switch incidental learning task

In previous studies we have provided evidence that performance in speeded response tasks with infrequent target stimuli reflects both automatic and controlled cognitive processes, based on differences in reaction time (RT) and task-related brain responses (Cheyne et al. 2012, Isabella et al. 2015). Here we test the hypothesis that such shifts in cognitive control may be influenced by changes in cognitive load related to stimulus predictability, and that these changes can be indexed by task-evoked pupillary responses (TEPR). We manipulated stimulus predictability using fixed stimulus sequences that were unknown to the participants in a Go/Switch task (requiring a switch response on 25% of trials) while monitoring TEPR as a measure of cognitive load in 12 healthy adults. Results showed significant improvement in performance (reduced RT, increased efficiency) for repeated sequences compared to occasional deviant sequences (10% probability) indicating that incidental learning of the predictable sequences facilitated performance. All behavioral measures varied between Switch and Go trials (RT, efficiency), however mean TEPR amplitude (mTEPR) and latency to maximum pupil dilation were particularly sensitive to Go/Switch. Results were consistent with the hypothesis that mTEPR indexes cognitive load, whereas TEPR latency indexes time to response selection, independent from response execution. The present study provides evidence that incidental pattern learning during response inhibition tasks may modulate several cognitive processes including cognitive load, effort, response selection and execution, which can in turn have differential effects on measures of performance. In particular, we demonstrate that reaction time may not be indicative of underlying cognitive load.