Methods and apparatuses for amplitude modulation of all or a portion of an ensemble waveform to modify a user's cognitive state by transdermal electrical stimulation (TES). An ensemble current waveform may include a sequence of component biphasic current waveforms in which one or more parameters (such as frequency, current amplitude, percent duty cycle, and percent charge imbalance) vary between sequential component waveforms. These ensemble waveforms may be specifically configured to evoke a particular cognitive state when applied by TES. In particular, described herein are methods for applying amplitude modulation to all or a sub-set of the component waveforms in the ensemble waveform which may dramatically enhance the efficiency and effectiveness of the TES. Also described herein are methods and apparatuses for applying amplitude modulation to all or a portion of an ensemble waveform without truncating pulses of the component waveforms forming the ensemble waveform.
