  Background: Direct electrical stimulation of the brain through intracranial
electrodes is currently used to probe the epileptic brain as part of
pre-surgical evaluation, and it is also being considered for therapeutic
treatments through neuromodulation. It is still unknown, however, how
consistent intracranial direct electrical stimulation responses are across
sessions, to allow effective neuromodulation design.
  Objective: To investigate the cross-session consistency of the
electrophysiological effect of electrical stimulation delivered through
intracranial EEG.
  Methods: We analysed data from 79 epilepsy patients implanted with
intracranial EEG who underwent brain stimulation as part of a memory
experiment. We quantified the effect of stimulation in terms of band power
modulation and compared this effect from session to session. As a reference, we
applied the same measures during baseline periods.
  Results: In most sessions, the effect of stimulation on band power could not
be distinguished from baseline fluctuations of band power. Stimulation effect
was also not consistent across sessions; only a third of the session pairs had
a higher consistency than the baseline standards. Cross-session consistency is
mainly associated with the strength of positive stimulation effects, and it
also tends to be higher when the baseline conditions are more similar between
sessions.
  Conclusion: These findings can inform our practices for designing
neuromodulation with greater efficacy when using direct electrical brain
stimulation as a therapeutic treatment.
