asynchronous uncorrelated changes (noise).

18. Such manipulations change the value of the parameter f in themodel. The steady-state amplitude is proportion-al to 1/(1 2 f)r for large r (7). The factor f can be consid-ered to be the fraction of releasable transmitter remain-ing after a spike, so 1 2 f is the fraction of transmitter released. The steady-state synaptic strength is gA with g proportional to the amount of transmitter released, and thus to 1 2 f. As a result, the effective synaptic strength for sustained high rates is proportional to (1 2 f)/(1 2 f)r 5 1/r and is independent of f. 19. See for example R. C. deCharms and M. M. Mer-zenich, Nature 381, 610 (1996). 20. Care and use of animals were in accordance with the guidelines of the Brandeis University animal care com-mittee. We thank P. Dayan, E. Marder, K. Miller, and G. Turrigiano for helpful comments and advice. Sup-ported by the Sloan Center for Theoretical Neurobiol