Volunteers will receive a weight-based opioid (remifentanil) infusion for 10 minutes. In the first run, serial pupillary measurements (pupillary unrest, pupil diameter) will be taken at baseline, and at 2.5-minute intervals during the infusion and a 25-minute recovery period afterwards. After a washout period, the experiment will be repeated in each subject (second run). The two runs differ only by presence versus absence of verbal interaction.

Healthy volunteers aged 20-55 will receive a weight-based opioid (remifentanil) infusion for 10 minutes. Vital signs including SpO2, transcutaneous CO2, and respiratory rate will be continuously measured.

In the first run, serial pupillary measurements (pupillary unrest, pupil diameter) will be taken at baseline, and at 2.5-minute intervals during the infusion and a 25-minute recovery period afterwards. After a washout period, the experiment will be repeated in each subject (second run). The two runs differ only by presence of ongoing verbal interaction versus complete avoidance of verbal interaction.

Incidence of respiratory depression will be compared in the two protocols with or without verbal interaction. Correlation between pupil diameter and pupillary unrest with estimated opioid concentrations will be determined by regression. Difference between pupil measurement versus opioid concentration regressions will be compared by chi2.

Inclusion Criteria:

• Healthy
• Able to provide informed consent

Exclusion Criteria:

• Active substance use disorder
• Prior opioid use disorder
• Opioid use within 30 days.