Infant Pupil Diameter Changes in Response to Others' Positive and Negative Emotions
Figure 3
The effect of stimulus luminance on pupil dilation.
We explored whether variations in stimulus luminance across video frames have a significant effect on the recorded changes in pupil size. It was predicted that as stimulus luminance increases, an adjustment reaction with decreasing pupil size would be recorded. It was found that the emotional valence of the stimuli explained a big proportion of the variation, partial η2 = .32, while variations in luminance across the movies had an extremely small effect, partial η2 = .04. The relation between the effect of luminance on pupil size differed across emotions (F(2, 7500) = 217.77, p<.001). When controlling for other sources of variations between videos [69], the relation between luminance and pupil size is positive (.42), and this could be due to the overall tendency of the pupil to increase with stimulus presentation. Illuminance measured at the level of the eye during the presentation of each video was within a range not expected to yield significant differences in pupil size (15.0±0.5 Lux) [50]. All this information suggests that the observed differences in luminance and illuminance between stimuli could not produce the differences in pupil size related to the emotional valence.