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We examine the spatiotemporal dynamics of filamentation and subsequent material changes in two transparent media, fused silica and poly(methyl methacrylate) (PMMA), using inline holographic microscopy. We are able to dynamically observe the nonlinear propagation of femtosecond laser pulses and the consequent evolution of the electronic excitatio n and trapping inside the bulk of both materials. In the case of fused silica we reveal the physical conditions for the formation of nanogratings, measuring excited electron densities well below the critical density while for PMMA we show that excited electrons with densities exceeding 10 cm, exhibit complex trapping dynamics in a 200 fs time scale. The clear demonstration of ultrafast sub-ps photochemical processes that take place during the irradiation of PMMA with femtosecond pulses will have a strong impact on the laser microprocessing of polymers and nanosurgery applications of bio-related materials. [ABSTRACT FROM AUTHOR]