one can think of a car by thinking of the word "car", or by forming a mental image of a car. The verbal and image systems are correlated, as one can think of the mental image of the car and then describe it in words, or read or listen to words and then form a mental image. DCT identifies three types of processing: (1) representational, the direct activation of verbal or non-verbal representations, (2) referential, the activation of the verbal system by the nonverbal system or vice versa, and (3) associative processing, the activation of representations within the same verbal or nonverbal system. A given task may require any or all of the three kinds of processing. Verbal system units are called logogens; these units contain information that underlies our use of the word. Non-Verbal system units are called imagens. Imagens contain information that generates mental images such as natural objects, holistic parts of objects, and natural grouping of objects. Imagens operate synchronously or in parallel; thus all parts of an image are available at once. Logogens operate sequentially; words come one at a time in a syntactically appropriate sequence in a sentence. The two codes may overlap in the processing of information but greater emphasis is on one or the other. The verbal and non-verbal systems are further divided into subsystems that process information from different modalities. Many experiments reported by Paivio and others support the importance of imagery in cognitive operations. In one experiment, participants saw pairs of items that differed in roundness (e.g., tomato, goblet) and were asked to indicate which member of the pair was rounder. The objects were presented as words, pictures, or word-picture pairs. The response times were slowest for word-word pairs, intermediate for the picture-word pairs, and fastest for the picture-picture pairs. Empirical evidence DCT research focused initially on memory and soon expanded to other cognitive phenomena. Memory remains crucial, however, because it is the basis of all knowledge and thought. The memory emphasis is further justified here because learning and memory are at the heart of educational goals. The effects can be explained by two DCT hypotheses. One hypothesis is that nonverbal and verbal codes, being functionally independent, can have additive effects on recall. For example, participants in free recall experiments are likely to name presented objects covertly and thus create a nonverbal (pictorial) and a verbal memory trace. They can also