We explore the response of a uniaxial nematic liquid crystal with a negative dielectric anisotropy to the electric field applied normally to the director. The geometry causes two effects: field-induced biaxiality order (FIBO) and differential quenching of uniaxial fluctuations (DQUF). We separate the two effects by tracing the field-induced changes in optical response at the timescales of nanoseconds. The study paves the way to establish the possibility of field-assisted formation of the biaxial nematic phase and to explore the likelihood of the appearance of biaxial order in the field-free conditions.