Reaction Time and the Task Evoked Pupil Response as Measures of Non-Volitional and Volitional Cognitive Effort

This study aimed to identify the utility of reaction time (RT) and the task evoked pupil response (TEPR) as measures of non-volitional and volitional cognitive effort. Stimulus probability was manipulated to measure non-volitional changes in cognitive effort toward rare stimuli compared with frequent stimuli. Congruency was manipulated to measure volitional changes in cognitive effort toward incongruent compared with congruent stimulus-response pairings. Participants were 81 typical young adults who completed two blocks of 100 trials in which they responded with key presses using the left or right hand to tones presented monaurally. A 1500Hz tone indicated congruent responding: if the tone was presented in the right ear, the correct response was the right key. A 500Hz tone indicated incongruent responding: if the tone was presented in the right ear, the correct response was the left key. Stimulus probability was manipulated such that each block contained 80 frequent trials (40 congruent, 40 incongruent) and 20 rare trials (10 congruent, 10 incongruent). Hypotheses were as follows: the effect of congruency would be significant for both measures, and larger for RT than for TEPR; the effect of stimulus probability would be significant for both measures, and larger for TEPR than for RT. Results revealed a significant effect of congruency for both RT (d = 0.51) and TEPR (d = 0.55), and a significant effect of stimulus probability for RT only (d = 0.26). Significant block effects occurred such that the effect of stimulus probability for TEPR was significant in the positive direction (d = 0.41) in Block 1 only; the effect was reversed in Block 2. The effect of stimulus probability for RT trended toward significance in Block 2 only. The results of the congruency manipulation of this study indicate that RT and TEPR are sensitive indicators of volitional cognitive effort. Results of the stimulus probability manipulation indicate that TEPR is a sensitive indicator of non-volitional effort toward a rare stimulus, and subsequent decrease in effort when the stimulus has become familiar. Furthermore, results of this study reflect the complex role of the locus coeruleus for non-volitional and volitional cognitive effort.