The location of fluctuations occurring in a filamentary current flow is detected by spatially resolved photocurrent measurements using optical interband excitation in n-GaAs at low temperatures. Current fluctuations are only observed in the filament boundaries. In the presence of an external magnetic field, characteristics like the frequency and amplitude of the spontaneous current oscillations are different for opposite edges of the filament pattern. The observed phenomena are discussed in terms of electron generation-recombination kinetics.