Bistability and Asynchrony in a Boolean Model of the L-arabinose Operon in Escherichia coli.

The lactose operon in Escherichia coli was the first known gene regulatory network, and it is frequently used as a prototype for new modeling paradigms. Historically, many of these modeling frameworks use differential equations. More recently, Stigler and Veliz-Cuba proposed a Boolean model that captures the bistability of the system and all of the biological steady states. In this paper, we model the well-known arabinose operon in E. coli with a Boolean network. This has several complex features not found in the lac operon, such as a protein that is both an activator and repressor, a DNA looping mechanism for gene repression, and the lack of inducer exclusion by glucose. For 11 out of 12 choices of initial conditions, we use computational algebra and Sage to verify that the state space contains a single fixed point that correctly matches the biology. The final initial condition, medium levels of arabinose and no glucose, successfully predicts the system's bistability. Finally, we compare the state space under synchronous and asynchronous update and see that the former has several artificial cycles that go away under a general asynchronous update.

Images of the self in social anxiety: effects on the retrieval of autobiographical memories.

Cognitive models of social phobia propose that negative self-images play an important role in maintaining anxiety. This study examines the effect of holding a positive or a negative image in mind during a speech on the retrieval of autobiographical memories. Twenty high socially anxious participants performed a standard autobiographical memory task (AMT), which used positive, negative and neutral cue words. Participants performed the AMT twice: once after giving a speech holding a positive image and once while holding a negative image. Participants were more anxious and rated their performance worse in the negative image condition. Negative memories were retrieved faster in the negative image condition and positive memories were retrieved faster in the positive image condition. In the negative image condition, positive memories were retrieved more slowly than either negative or neutral memories. Inhibition and facilitation are proposed as two processes that could explain the effects of differently valenced imagery on autobiographical memory. The clear evidence for an inhibitory effect on positive autobiographical memories in the negative imagery condition is considered in relation to Brewin's [(2006). Understanding cognitive behaviour therapy: A retrieval competition account. Behaviour Research and Therapy, 44, 765-784] retrieval competition hypothesis. The paper concludes with a brief discussion of the potential role of inhibition in imagery rescripting.