Arousal is thought to follow an inverted-U relationship with cognitive performance, with optimal functioning at intermediate arousal levels. However, it remains unclear whether spontaneous trial-by-trial fluctuations in tonic arousal differentially predict response execution and response inhibition within the same task. This study examined whether pre-stimulus pupil size, used as a non-invasive index of tonic arousal, predicts mean go trial reaction time and commission error rate on no-go trials in an auditory go/no-go task. Forty-two participants completed seven blocks of 320 trials. Pre-stimulus pupil size was recorded during the fixation period and divided into five within-block arousal bins per participant. 

For each outcome, a linear and quadratic mixed model were fitted, and a likelihood-ratio test determined which provided the better fit. Bin number significantly predicted mean go RT, with a linear decrease in reaction time as bin number increased. No significant relationship was found between bin number and false alarm rate. An exploratory analysis revealed a significant quadratic relationship between bin number and Inverse Efficiency Score, though this finding should be treated as preliminary. These results provide partial evidence that pre-stimulus pupil-linked arousal is more closely related to response execution than to response inhibition, highlighting the need to distinguish between different components of cognitive control when studying arousal-performance relationships.