Pauses In Motion Research Articles

Exact solution of an anisotropic 2D random walk model with strong memory correlations

Over the last decade, there has been progress in understanding one-dimensional non-Markovian processes via analytic, sometimes exact, solutions. The extension of these ideas and methods to two and higher dimensions is challenging. We report the first exactly solvable two-dimensional (2D) non-Markovian random walk model belonging to the family of the elephant random walk model. In contrast to Lévy walks or fractional Brownian motion, such models incorporate memory effects by keeping an explicit history of the random walk trajectory. We study a memory driven 2D random walk with correlated memory and stops, i.e. pauses in motion. The model has an inherent anisotropy with consequences for its diffusive properties, thereby mixing the dominant regime along one dimension with a subdiffusive walk along a perpendicular dimension. The anomalous diffusion regimes are fully characterized by an exact determination of the Hurst exponent. We discuss the remarkably rich phase diagram, as well as several possible combinations of the independent walks in both directions. The relationship between the exponents of the first and second moments is also unveiled.

Differential Labeling of Myosin V Heads with Quantum Dots Allows Direct Visualization of Hand-Over-Hand Processivity

The double-headed myosin V molecular motor carries intracellular cargo processively along actin tracks in a hand-over-hand manner. To test this hypothesis at the molecular level, we observed single myosin V molecules that were differentially labeled with quantum dots having different emission spectra so that the position of each head could be identified with ∼6-nm resolution in a total internal reflectance microscope. With this approach, the individual heads of a single myosin V molecule were observed taking 72-nm steps as they alternated positions on the actin filament during processive movement. In addition, the heads were separated by 36 nm during pauses in motion, suggesting attachment to actin along its helical repeat. The 36-nm interhead spacing, the 72-nm step size, and the observation that heads alternate between leading and trailing positions on actin are obvious predictions of the hand-over-hand model, thus confirming myosin V’s mode of walking along an actin filament.

Streaming and bouncing: observations on motion defined objects.

When two identical objects move in opposite directions on the same path and at the same speed, they can appear, after crossing over to continue n their original directions (streaming), or to reverse direction (bouncing). In order to be able to man pulate visibility by adding no se, we used objects defined by contrast, flicker, or motion, and thereby extended previous findings on luminance-defined objects. Two identical rectangles (1.1 x 1.4 degrees) composed of random dot patterns moved toward each other at a speed of 3.5 degrees/s. In experiment I we used backgrounds of a grey field, static random dots, or dynamic noise, and examined the effect of introducing a pause in motion and a visual distractor. In experiment 2 we introduced visual noise at four levels. For all three types of motion display, we found an increase in the proportion of the bouncing percept when either a pause in motion or an attentional distractor was introduced. Experiment 2 showed that neither of these effects depends on the visibility of the moving objects. An increase in the bouncing percept, due to a pause in motion or the distraction of attention, can be observed for all types of object definition, and is not affected by decreas ng the visibility of the motion-defined objects.This finding suggests that the role of attention in determining the perception of bouncing does not lie in the modulation of object visibility.

Palaeomagnetic evidence from DSDP cores of northward drift of India

EVIDENCE of latitudinal changes associated with the northward movement of India during the Cainozoic have been described1 from remanence inclination measurements made on sediments and basalts recovered at DSDP sites in the Arabian Sea (Fig. 1). When combined with palaeomagnetic data from continental India2–4 the results showed an initial northward movement at a mean rate of 26 cm yr−1 from the Cretaceous to at least Middle Eocene times and a subsequent slower mean rate of 16 cm yr−1 being resumed in the Miocene (Fig. 2a). The original analysis implied a possible pause in motion between the two phases during the Oligocene, for which period no data were available.