When ultrashort laser pulses are focused inside transparent materials, extremely high field intensities can easily be achieved in the focal volume leading to nonlinear interaction with the material. In corneal tissue this nonlinear interaction results in an optical breakdown that may serve as a cutting mechanism in ophthalmology. As a side effect of optical breakdown in corneal tissue, streak-like structures have been observed as discoloration in histological sections under a light microscope. To investigate the streak formation, a numerical model including nonlinear pulse propagation due to self-focusing, group velocity dispersion, and plasma defocusing due to generated free electrons is presented. The model consists of a (3+1)-dimensional nonlinear Schroedinger equation, describing the pulse propagation coupled to an evolution equation covering the generation of free electrons. The rate equation contains multi photon ionization as well as avalanche ionization. The model is applicable to any transparent Kerr-medium.