In science especially, visual images are preferred for displaying multiple relationships and
processes that are difficult to describe. Research studies indicate that the type of visual
representation could determine how powerful the illustration will be as a learning aid (Carney
& Levin, 2002; Mayer, 1993). Mayer (1993) summarized four types of illustrations, modified
from Levin's system of classifying illustrations (see Table 1). Mayer concluded that
explanative illustrations, those illustrations with a verbal explanation that describe how
scientific systems or processes work, elicit the highest level of cognitive processing. Other
types of illustrations, like decorative color pictures, may not even affect cognitive processing.
Research has shown that promoting cognitive interest is more important than promoting
emotional interest. Students perform worse on retention tests when entertaining text and/or
illustrations have been added (Harp & Mayer, 1997). Therefore, explanative illustrations are
more effective than illustrations designed for affective purposes; illustrations designed to
promote interest in and motivation toward content materials have not been found to improve
student learning (Park & Lim, 2007)