Changes in Oscillatory Neural Activity during Wada Test in Stereoelectroencephalography Inform Neural Mechanisms of Language and Memory

Introduction: Intracarotid injection of amobarbital or propofol, the Wada test, is used in neurosurgical resection cases to test for the hemisphere with language dominance. For stereoelectroencephalography (sEEG) patients, these tests present the rare opportunity to measure brain activity invasively in humans while one hemisphere of the brain is anesthetized.

Methods: We recorded from 7 sEEG patients implanted with medically refractory epilepsy at Oregon Health & Science University (6 both hemispheres, 1 only left hemisphere). Patients were given intracarotid injections with either amobarbital or propofol and the time of injection, the time that their hand dropped from anesthesia and the success of memory recall events were recorded. Band-limited power was computed for each sEEG recording contact using a narrowband fourth order Butterworth filter followed by a Hilbert transform for several canonical bands; delta (1 - 4 Hz), theta (4 - 8 Hz), alpha (8 - 13 Hz) and beta (13 - 30 Hz). Changes in power were analyzed relative to the time of hand drop.

Results: After injection of an anesthetic, there were large increases in oscillatory power across bands that peaked within about 3 - 5 seconds which was also the time of hand drop. The recovery to baseline oscillatory activity was complex and depended on the brain area of the recording.

Conclusion : Disruptions in memory consolidation and use of language may be due to changes in hippocampal activity or may be due to hippocampal deafferentation. These data provide evidence for the order that different brain areas return to fully-conscious activity and elucidate a complex picture of how memory consolidation is disrupted during anesthesia.