Pupillary responses to intranasal trigeminal and olfactory stimulation

The aim of the present study was to investigate whether pupillary responses to odorous stimuli reflect their intensity or hedonic tone. A total of 21 healthy subjects participated in the study. Using a computer-controlled olfactometer, subjects received intranasal stimuli including odors of rose (PEA; 2 concentrations), lemon and rotten eggs, plus the trigeminal irritant CO₂ (also at two concentrations). Changes in the pupil diameter were obtained ipsilaterally to the side of stimulus presentation. Both trigeminal and olfactory stimulation produced an increase in pupillary diameter. Latencies for pupillary reaction were fastest for the higher concentration of CO₂ and slowest after the presentation of PEA at the low concentration. Response amplitudes were largest in response to stimulation with CO₂ at the high concentration, while they were smallest in response to odorous stimulation with PEA. Response latencies decreased with increasing stimulus intensity. No such correlation was found for hedonic ratings and pupillary reactions. Thus, the change in the pupillary diameter indicates differences between stimulus modalities and stimulus strength, but not pleasantness or unpleasantness of the odors.