SOC in a population model with global control

We study a plant population model introduced recently by J. Wallinga [OIKOS {\bf 74}, 377 (1995)]. It is similar to the contact process (`simple epidemic', `directed percolation'), but instead of using an infection or recovery rate as control parameter, the population size is controlled directly and globally by removing excess plants. We show that the model is very closely related to directed percolation (DP). Anomalous scaling laws appear in the limit of large populations, small densities, and long times. These laws, associated critical exponents, and even some non-universal parameters, can be related to those of DP. As in invasion percolation and in other models where the r\^oles of control and order parameters are interchanged, the critical value $p_c$ of the wetting probability $p$ is obtained in the scaling limit as singular point in the distribution of infection rates. We show that a mean field type approximation leads to a model studied by Y.C. Zhang et al. [J. Stat. Phys. {\bf 58}, 849 (1990)]. Finally, we verify the claim of Wallinga that family extinction in a marginally surviving population is governed by DP scaling laws, and speculate on applications to human mitochondrial DNA.Comment: 19 pages, with 10 ps-figured include

Patterns of cognitive dysfunction in progressive MS

Background: Progressive MS is associated with a high frequency of cognitive impairment. However, it is not clear to what extent this reflects global dysfunction, or independent deficits in specific functions

Temporal judgments in multi–sensory space

To successfully interact with the environment requires a combination of stimulus recognition as well as localization in both space and time, with information moreover coming from multiple senses. Several studies have shown that auditory stimuli last subjectively longer than visual ones of equal duration. Recently, it has also been suggested that stimulus position affects duration perception. The present study investigated how lateral spatial presentation influences sub-second visual and auditory duration judgments. Five experiments were conducted using the duration discrimination paradigm, wherein two stimuli are presented sequentially and participants are asked to judge whether the second stimulus (comparison) is shorter or longer in duration than the first (standard). The number of stimulus positions and the way in which different modality trials were presented (mixed or blocked) varied. Additionally, comparisons were made either within or across modalities. No stable effect of location itself was found. However, in mixed modality experiments there was a clear over-estimation of duration in visual trials when the location of the comparison was different from the standard. This effect was reversed in the same location trials. Auditory judgments were unaffected by location manipulations. Based on these results, we propose the existence of an error-mechanism, according to which a specific duration is added in order to compensate for the loss of duration perception caused by spatial attention shifts between different locations. This mechanism is revealed in spatial and modality-mixed circumstances wherein its over-activation results in a systematic temporal bias