Yasuji Sawada

Improving the realism of the cellular Potts model in simulations of biological cells

Because the extended or cellular large-Q Potts model (CPM) captures effectively the global features of tissue rearrangement experiments, including cell sorting and tissue engulfment, it has become a common technique for cell level simulation of tissues. However, it omits three key elements of real cells, their fixed membrane area, their attractive binding and the dissipation of making and breaking membrane contacts. In this paper, we modify the Hamiltonian to use negative surface energies, constrained surface area and a spin flip energy threshold to improve the correspondence to reality. We find that the new model correctly predicts several dynamical behaviors of cells which the original CPM does not, including the hierarchy of diffusion constants.

Cell Movements and Mechanical Force Distribution During the Migration of Dictyostelium Slugs

Migration of Dictyostelium discoideum slugs results from coordinated movement of their constituent cells. It is generally assumed that each cell contributes to the total motive force of the slug. However, the basic mechanisms by which mechanical forces (traction and resistive forces) are transmitted to the substrate, their magnitude and their location, are largely unknown. In this work, we performed detailed observations of cell movements by fluorescence microscopy using two-dimensional (2D) slugs. We show that 2D slugs share most of the properties of 3D ones. In particular, waves of movement propagate in long 2D slugs, and slug speed correlates with slug length as found in 3D slugs. We also present the first measurements of the distribution of forces exerted by 2D and 3D slugs using the elastic substrate method. Traction forces are mainly exerted in the central region of the slug. The large perpendicular forces around slug boundary and the existence of parallel resistive forces in the tip and/or the tail suggest an important role of the sheath in the transmission of forces to the substrate.

Subjectivity and awareness

The concept of awareness was discussed from a scientific viewpoint, based on the related experimental results of visual hand tracking and temporal order judgement of successive stimuli, in order to contribute to obtaining a possible design principle of the aware machines. It was proposed that a clock which drives the internal simulator by a faster time than the development of the external world is necessary for an aware machine to make an useful decision for future like higher biological organisms.

The aspects, the origin, and the merit of aware computing

In this paper we tried to understand scientifically the awareness, a daily word. Some aspects of awareness, such as qualitative or quantitative, the targets of awareness, either the external world or the internal world, were discussed. Suggestion on the human awareness was described fromthe experimental results of visual hand tracking. The origin and the merit of awareness in the process of evolution of animals were discussed. Finally some characters of possible aware computers and aware robots were studied.

Roll of the rhythmic component in the proactive control of a human hand

In terms of evolution, the strategy of catching prey would have been an important part of survival in a constantly changing environment. A prediction mechanism would have developed to compensate for any delay in the sensory-motor system. In a previous study, “proactive control” was found, in which the motion of the hands preceded the virtual moving target. These results implied that the positive phase shift of the hand motion represents the proactive nature of the visual-motor control system, which attempts to minimize the brief error in the hand motion when the target changes position unexpectedly. In our study, a visual target moves in circle (13 cm diameter) on a computer screen, and each subject is asked to keep track of the target’s motion by the motion of a cursor. As the frequency of the target increases, a rhythmic component was found in the velocity of the cursor in spite of the fact that the velocity of the target was constant. The generation of a rhythmic component cannot be explained simply as a feedback mechanism for the phase shifts of the target and cursor in a sensory-motor system. Therefore, it implies that the rhythmic component was generated to predict the velocity of the target, which is a feed-forward mechanism in the sensory-motor system. Here, we discuss the generation of the rhythmic component and its roll in the feed-forward mechanism.