Neural activity tracks the envelope of a speech signal
at latencies from 50 ms to 300 ms. Modulating this neural tracking
through transcranial alternating current stimulation influences
speech comprehension. Two important variables that can affect
this modulation are the latency and the phase of the stimulation
with respect to the sound. While previous studies have found an
influence of both variables on speech comprehension, the
interaction between both has not yet been measured. We presented
17 subjects with speech in noise coupled with simultaneous
transcranial alternating current stimulation. The currents were
based on the envelope of the target speech but shifted by different
phases, as well as by two temporal delays of 100 ms and 250 ms.
We also employed various control stimulations, and assessed the
signal-to-noise ratio at which the subject understood half of the
speech. We found that, at both latencies, speech comprehension is
modulated by the phase of the current stimulation. However, the
form of the modulation differed between the two latencies. Phase
and latency of neurostimulation have accordingly distinct
influences on speech comprehension. The different effects at the
latencies of 100 ms and 250 ms hint at distinct neural processes for
speech processing.