In [1] a rigorous model for Rx ISI effects caused by optical and electrical filtering in the receiver was presented aiming at 10-40 Gbit/s WDM system design applications. The model is applicable for general types of optical and electrical filtering. The model assumes Gaussian distributed noise contributions in the optical domain for in-phase and quadrature noise components in orthogonal polarization states and that these noise contributions are un-correlated. The optical filter is specified via the correlation function and the electrical filter via the impulse response. A limitation of the model was that only positive impulse responses could be accounted for when the noise properties of the receiver is specified. We present here an extension of the model in [1] to account in rigorous form also for negative impulse responses when determining the noise properties of the receiver. As a major theoretical result the formulation is extended to account for correlation between quadrature noise contributions - as it may e.g. be caused by four wave mixing effects in the transmission fiber - and noise contributions in orthogonal polarizations states - that may be due to saturation effects in cascaded optical amplifiers in the transmission path. The formulation leads to a Hermitian form of the covariance matrix. It is however to be noted that the covariance matrix in the situation with positive impulse response is positive definite (with positive eigenvalues) whereas in the general situation it is not positive definite and may have positive as well as negative eigenvalues. However, the Hermitian form allows in all cases a straightforward analytical formulation of the moment generating function (MGF) which in turn specifies the BER. The model is included in the simulation tool VPI-transmissionMaker™WDM and numerical examples for a 10 Gbit/s pre-amplified Rx shows that the influence of a negative impulse response may be (up to) several orders of magnitude in the BER.