Empirical evaluation of visualizations has so far been typically carried out by measuring the performance of participants that have been shown the visualization in relation to control group by grading programming tasks. Such studies tell little about what effects take place during the visualizations and how these effects build up into the learning effects. To address these issues, we are carrying out a series of experiments using visualization tools whose long-term effects are known.This paper presents a model of the cognitive phenomena that take place during visualization viewing sessions and describes an experiment where two visualization tools that differed in the amount of animation were used. The results show that even when participants were provided with rich visual information in the form of animations, they resorted heavily to the textual cues present on the screen. As the animation proceeded, participants started to follow visual cues more closely. However, if the names of the variables involved in the current operation were not in the visually cued area, participants tended to search for them in the program code. In the absence of animation, participants were more eager to browse the code in general.