The use of methodologies such as eye-tracking and fMRI are part of an evolving research agenda, and are well applicable to visual aesthetics, and the cognitive and affective outcomes for users related to their reactions to visual design principles.
In this regard, Soussan Djamasbi has examined online viewing and aesthetic preferences using an eye-tracking device.
These methodologies offer precise insights into why users respond as they do - that serves to develop or elaborate design theory.