RESUMEN
We discuss activated escape from a metastable state of a system driven by a time-periodic force. We show that the escape probabilities can be changed very strongly even by a comparatively weak force. In a broad parameter range, the activation energy of escape depends linearly on the force amplitude. This dependence is described by the logarithmic susceptibility, which is analyzed theoretically and through analog and digital simulations. A closed-form explicit expression for the escape rate of an overdamped Brownian particle is presented and shown to be in quantitative agreement with the simulations. We also describe experiments on a Brownian particle optically trapped in a double-well potential. A suitable periodic modulation of the optical intensity breaks the spatio-temporal symmetry of an otherwise spatially symmetric system. This has allowed us to localize a particle in one of the symmetric wells. (c) 2001 American Institute of Physics.
RESUMEN
Zebra fish and pearl danios were either cross-reared or raised with conspecifics and tested as adults for species preference over 4 days. Subjects were then placed with conspecifics by group for 30 days and retested over 2 days. Initial testing showed less preference (species identification) for conspecifics by cross-reared subjects and less conspecific preference by pearls than zebras. At retesting pearls showed a great increase in preference for conspecifics whereas zebras showed little change.
