The experiment was controlled by a desktop computer running an HTC Vive Virtual Reality device equipped with an SMI eye tracker. The HTC Vive headset has a resolution of 1080 x 1200 pixels, with a 110° by 113° field of view and a refresh rate of 90 Hz. The virtual experimental set-up was built using the Unity platform (Unity Technologies, 2017) [31 ]. The SMI eye tracker has a sampling rate of 250 Hz and was controlled using the SMI-designed Unity plugin. Tracking accuracy was maintained by performing a calibration every 20 trials. Calibration consisted of following a moving white circle with a red dot in the middle to 5 different regions. The SMI Unity plugin reports only success or failure of calibration, not accuracy, so calibration was repeated upon failure. Head movements (movements of the HTC Vive headset) were tracked using inertial measurement units inside the headsets that were calibrated via two infrared base stations (60 Hz) located on opposite corners of the room [32 (link)].
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