Biomedical subjects

Shahar Hod

Publications and source records attributed to Shahar Hod.

8 recordsLinked to original sources

PubMed2005

Survival probabilities of history-dependent random walks.

We analyze the dynamics of random walks with long-term memory (binary chains with long-range correlations) in the presence of an absorbing boundary. An analytically solvable model is presented, in which a dynamical phase transition occurs when the correlation strength parameter mu reaches a critical value mu(c). For strong positive correlations, mu > mu(c), the survival probability is asymptotically finite, whereas for mu < mu(c) it decays as a power law in time (chain length).

Journal Article↗

PubMed2004

Phase transition in random walks with long-range correlations.

Motivated by recent results in the theory of correlated sequences, we analyze the dynamics of random walks with long-term memory (binary chains with long-range correlations). In our model, the probability for a unit bit in a binary string depends on the fraction of unities preceding it. We show that the system undergoes a dynamical phase transition from normal diffusion, in which the variance D(L) scales as the string's length L, into a superdiffusion phase ( D(L) approximately Lalpha,alpha>1), when the correlation strength exceeds a critical value. We demonstrate the generality of our results with respect to alternative models, and discuss their applicability to various data, such as coarse-grained DNA sequences, written texts, and financial data.

Journal Article↗

PubMed2004

Survival probabilities in time-dependent random walks.

We analyze the dynamics of random walks in which the jumping probabilities are periodic time-dependent functions. In particular, we determine the survival probability of biased walkers who are drifted towards an absorbing boundary. The typical lifetime of the walkers is found to decrease with an increment in the oscillation amplitude of the jumping probabilities. We discuss the applicability of the results in the context of complex adaptive systems.

Journal Article↗

PubMed2004

Evolutionary minority game: the roles of response time and mutation threshold.

In the evolutionary minority game, agents are allowed to evolve their strategies ("mutate") based on past experience. We explore the dependence of the system's global behavior on the response time and the mutation threshold of the agents. We find that the precise values of these parameters determine if the strategy distribution of the population has a U shape, inverse U shape, or W shape. It is shown that in a free society (market), highly adaptive agents (with short response times) perform best. In addition, "patient" agents (with high mutation thresholds) outperform "nervous" ones.

Biological Evolution↗

PubMed2003

Strategy updating rules and strategy distributions in dynamical multiagent systems.

In the evolutionary version of the minority game, agents update their strategies (gene value p) in order to improve their performance. Motivated by the recent intriguing results obtained for prize-to-fine ratios, which are smaller than unity, we explore the system's dynamics with a strategy updating rule of the form p-->p+/-delta(p) (0<or=p<or=1). We find that the strategy distribution depends strongly on the values of the prize-to-fine ratio R, the length scale delta(p), and the type of boundary condition used. We show that these parameters determine the amplitude and the frequency of the temporal oscillations observed in the gene space. These regular oscillations are shown to be the main factors which determine the strategy distribution of the population. In addition, we find that the agents characterized by p=1/2 (a coin-tossing strategy) have the best chances of survival at asymptotically long times, regardless of the value of delta(p) and the boundary conditions used.

Journal Article↗

PubMed2003

Time-dependent random walks and the theory of complex adaptive systems.

Motivated by novel results in the theory of complex adaptive systems, we analyze the dynamics of random walks in which the jumping probabilities are time dependent. We determine the survival probability in the presence of an absorbing boundary. For an unbiased walk, the survival probability is maximized in the case of large temporal oscillations in the jumping probabilities. On the other hand, a random walker who is drifted towards the absorbing boundary performs best with a constant jumping probability. We use the results to reveal the underlying dynamics responsible for the phenomenon of self-segregation and clustering observed in the evolutionary minority game.

Adaptation, Biological↗

PubMed2003

Temporal oscillations and phase transitions in the evolutionary minority game.

The study of societies of adaptive agents seeking minority status is an active area of research. Recently, it has been demonstrated that such systems display an intriguing phase transition: agents tend to self-segregate or to cluster according to the value of the prize-to-fine ratio R. We show that such systems do not establish a true stationary distribution. The winning probabilities of the agents display temporal oscillations. The amplitude and frequency of the oscillations depend on the value of R. The temporal oscillations that characterize the system explain the transition in the global behavior from self-segregation to clustering in the R<1 case.

Journal Article↗

PubMed2002

Self-segregation versus clustering in the evolutionary minority game.

Complex adaptive systems have been the subject of much recent attention. It is by now well established that members ("agents") tend to self-segregate into opposing groups characterized by extreme behavior. However, the study of such adaptive systems has mostly been restricted to simple situations in which the prize-to-fine ratio R equals unity. In this Letter we explore the dynamics of evolving populations with various different values of the ratio R, and demonstrate that extreme behavior is in fact not a generic feature of adaptive systems. In particular, we show that "confusion" and "indecisiveness" take over in times of depression, in which case cautious agents perform better than extreme ones.

Adaptation, Psychological↗