Synapses with short-term plasticity are optimal estimators of presynaptic membrane potentials.

The trajectory of the somatic membrane potential of a cortical neuron exactly reflects the computations performed on its afferent inputs. However, the spikes of such a neuron are a very low-dimensional and discrete projection of this continually evolving signal. We explored the possibility that the neuron's efferent synapses perform the critical computational step of estimating the membrane potential trajectory from the spikes. We found that short-term changes in synaptic efficacy can be interpreted as implementing an optimal estimator of this trajectory. Short-term depression arose when presynaptic spiking was sufficiently intense as to reduce the uncertainty associated with the estimate; short-term facilitation reflected structural features of the statistics of the presynaptic neuron such as up and down states. Our analysis provides a unifying account of a powerful, but puzzling, form of plasticity.

System model for studying fibre grating dispersion compensators

A complete optical system model has been developed and used to assess chirped fibre Bragg grating dispersion compensators. Gratings suitable for dispersion compensation in both laser based and modulator based optical communications systems have been modelled. A grating 10 cm in length has been shown to permit virtually dispersion free transmission over 425 km, when used in an externally modulated system. Long haul dispersion compensation using several 2 cm gratings spaced at intervals along the fibre is also modelled, illustrating viable 10Gbit/s transmission over a distance in excess of 168 km.

Extending the transmission distance of a directly modulated laser source using Bragg grating dispersion compensators

Comprehensive computer modelling has been used to investigate the dependence of dispersion penalty on transmission length in an optical communications system employing a directly modulated 2.5Gbit/s DFB laser source and an optimised fibre grating dispersion compensator. Two grating apodization schemes, tanh and Gaussian, have been compared. The 2dB dispersion penalty transmission distance is shown to be approximately 520km along standard monomode fibre after compensation with a 5cm tanh grating. This represents a great improvement over the 150km range expected for a similar uncompensated system.