  Novel behavior in electroconvection (EC) has been detected in nematic liquid
crystals (NLCs) under the condition of comparable timescales of the director
relaxation and the period of the driving ac voltage. The studied NLCs exhibit
standard EC (s-EC) at the onset of the instability, except one compound in
which nonstandard EC (ns-EC) has been detected. In the relevant frequency
region, the threshold voltage for conductive s-EC bends down considerably,
while for dielectric s-EC it bends up strongly with the decrease of the driving
frequency. We show that inclusion of the flexoelectric effect into the
theoretical description of conductive s-EC leads to quantitative agreement,
while for dielectric s-EC a qualitative agreement is achieved. The frequency
dependence of the threshold voltage for ns-EC strongly resembles that of the
dielectric s-EC.
