As people study more facts about a concept, it takes longer to retrieve a particular fact about that concept. This fan effect (Anderson, 1974) has been attributed to competition among associations to a concept. Alternatively, the mental-model theory (Radvansky & Zacks, 1991) suggests that the fan effect disappears when the related concepts are organized into a single mental model. In the present study, attentional focus was manipulated to affect the mental model to be constructed. One group of participants focused on the person dimension of person-location pairs, whereas the other group focused on the location dimension. The result showed that the fan effect with the focused dimension was greater than the fan effect with the nonfocused dimension, which is contrary to the mental-model theory. The number of associations with a concept is indeed crucial during retrieval, and the importance of the information seems to be accentuated with attentional focus.
The fan effect refers to an increase in response time and/or error rates on a memory test with an increase in the number of competing associations to that memory probe. The associations to a concept were assumed to "fan" out of the concept node, hence the name. Since its first demonstration by Anderson (1974), the fan effect has been replicated in many different experimental paradigms with different types of stimuli (Lewis & Anderson, 1976; Reder & Ross, 1983;Zbrodoff, 1995). The assumptions underlying the account of the fan effect specify how and why retrieval processes interact with memory representations. Specifically, multiple facts linked to a concept in the probe will interfere with each other during retrieval because of limited cognitive resources allocated to the probe. As more associations are attached to the probe, the amount of activation that spreads down any path from the probe is reduced, requiring more time for a particular fact to be retrieved. Alternatively, however, there has emerged a competing view that emphasizes a representational account based on situation models (e.g., Radvansky & Zacks, 1991 ). In the present study, we seek to incorporate the different views of the fan effect and to test the predictions of these accounts.
In the fan paradigm, participants learn arbitrary associations between concepts (e.g., "Hippie-Park"). In the present study, participants memorized a set of 28 facts about people in locations. Figure 1 shows a basic network representation of some facts and their associated concepts. These facts are constructed so that one, two, or three facts are studied about each person and location. After committing these facts to memory, participants are tested on their ability to recognize person-location pairs previously studied (targets), and to reject novel combinations of the same people and locations (foils). The fan of a probe is the number of facts associated with the person and the location, and the reaction latency increases with the fan. The fan effect holds for both targets and foils, although sometimes the effect size varies (Anderson, 1976).
The present study is concerned with better understanding why the size of the fan effect for different dimensions of the stimuli (e.g., person vs. location) sometimes varies and what influences that variation. In some studies, the size of the fan effect is comparable for both dimensions (Anderson, 1974). However, some types of material have produced different size fan effects for different dimensions (Radvansky & Zacks, 1991 ), the phenomenon known as the differential fan effect. First, we will describe the mentalmodel theory that was initially proposed to explain the differential fan effect. second, we will describe the ACT-R theory, and how it differs from the mental-model theory.
According to mental-model theory, facts are organized into mental models when the material is studied. For example, when object-location pairs are studied (e.g., "The potted plant is in the hotel"), these associations should be organized into location-based mental models because a location can have many items in it, but an object can be in only one place at a time. …