Pupil dilation predicts modulation of direct gaze on action value calculations.

Perceiving direct gaze facilitates social cognition and behaviour. We hypothesized that direct gaze modulates decision-making, particularly calculations of action values. To test our hypothesis, we used the reinforcement learning paradigm in situations with or without direct gaze. Forty adults were recruited and participated in pupil size measurements and a two-armed bandit task. The task was conducted with 70% and 30% reward probabilities for each option. During the task, a female showing the Direct Gaze (DG) or Closed Eyes (CE) condition was presented from the start of each trial. The results showed that behavioural bias to choices with 70% reward probability increased more in the DG condition than in the CE condition and the expected reward value. This bias to choices with 70% reward in the DG condition was predicted by pupil dilation to DG. These results suggest that participants over-evaluated the expected reward value in the DG condition, and this DG effect may be related to subjective expectations of rewarding events indexed by pupil dilations. It is considered that perceiving direct gaze is a driver of reward expectations that modulate action value calculations and then cognitive processing and behaviours are facilitated.