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In this study in stable periodontal maintenance patients, a modest but statistically significant positive correlation was observed between the levels of IL-1β, a major proinflammatory cytokine in PISF, and WC, a reliable measure of central obesity.

We thank many individuals, teachers, and colleagues who have both inspired us and aided our research and thinking on difficult and complex issues, notably William Timberlake (for RIB). Josep Call provided useful suggestions on an earlier version of this chapter, as did Todd Freeberg, Serge Pellis, and Charles Snowdon.

If acquired associations are to accurately represent real relevance relations, there is motivation for the hypothesis that learning will, in some circumstances, be more appropriately modelled, not as direct dependence, but as conditional independence. In a serial compound conditioning experiment, two groups of rats were presented with a conditioned stimulus (CS1) that imperfectly (50%) predicted food, and was itself imperfectly predicted by a CS2. Groups differed in the proportion of CS2 presentations that were ultimately followed by food (25% versus 75%). Thus, the information presented regarding the relevance of CS2 to food was ambiguous between direct dependence and conditional independence (given CS1). If rats learnt that food was conditionally independent of CS2, given CS1, subjects of both groups should thereafter respond similarly to CS2 alone. Contrary to the conditionality hypothesis, subjects attended to the direct food predictability of CS2, suggesting that rats treat even distal stimuli in a CS sequence as immediately relevant to food, not conditional on an intermediate stimulus. These results urge caution in representing indirect associations as conditional associations, accentuate the theoretical weight of the Markov condition in graphical models, and challenge theories to articulate the conditions under which animals are expected to learn conditional associations, if ever.