title: Electrocorticographic delineation of human auditory cortical fields based on effects of propofol anesthesia.
abstract: The functional organization of human auditory cortex remains incompletely characterized. While the posteromedial two thirds of Heschl's gyrus (HG) is generally considered to be part of core auditory cortex, additional subdivisions of HG remain speculative. To further delineate the hierarchical organization of human auditory cortex, we investigated regional heterogeneity in the modulation of auditory cortical responses under varying depths of anesthesia induced by propofol. Non-invasive studies have shown that propofol differentially affects auditory cortical activity, with a greater impact on non-core areas. Subjects were neurosurgical patients undergoing removal of intracranial electrodes placed to identify epileptic foci. Stimuli were 50Hz click trains, presented continuously during an awake baseline period, and subsequently, while propofol infusion was incrementally titrated to induce general anesthesia. Electrocorticographic recordings were made with depth electrodes implanted in HG and subdural grid electrodes implanted over superior temporal gyrus (STG). Depth of anesthesia was monitored using spectral entropy. Averaged evoked potentials (AEPs), frequency-following responses (FFRs) and high gamma (70-150Hz) event-related band power were used to characterize auditory cortical activity. Based on the changes in AEPs and FFRs during the induction of anesthesia, posteromedial HG could be divided into two subdivisions. In the most posteromedial aspect of the gyrus, the earliest AEP deflections were preserved and FFRs increased during induction. In contrast, the remainder of the posteromedial HG exhibited attenuation of both the AEP and the FFR. The anterolateral HG exhibited weaker activation characterized by broad, low-voltage AEPs and the absence of FFRs. Lateral STG exhibited limited activation by click trains, and FFRs there diminished during induction. Sustained high gamma activity was attenuated in the most posteromedial portion of HG, and was absent in all other regions. These differential patterns of auditory cortical activity during the induction of anesthesia may serve as useful physiological markers for field delineation. In this study, the posteromedial HG could be parcellated into at least two subdivisions. Preservation of the earliest AEP deflections and FFRs in the posteromedial HG likely reflects the persistence of feedforward synaptic activity generated by inputs from subcortical auditory pathways, including the medial geniculate nucleus.
authors.affiliation: Department of Neurosurgery, The University of Iowa, Iowa City, IA, USA. Electronic address: kirill-nourski@uiowa.edu.
authors.forename: Kirill V
authors.initials: KV
authors.lastname: Nourski
authors.affiliation: Department of Anesthesiology, University of Wisconsin - Madison, Madison, WI, USA.
authors.forename: Matthew I
authors.initials: MI
authors.lastname: Banks
authors.affiliation: Departments of Neurology and Neuroscience, Albert Einstein College of Medicine, Bronx, NY, USA.
authors.forename: Mitchell
authors.initials: M
authors.lastname: Steinschneider
authors.affiliation: Department of Neurosurgery, The University of Iowa, Iowa City, IA, USA.
authors.forename: Ariane E
authors.initials: AE
authors.lastname: Rhone
authors.affiliation: Department of Neurosurgery, The University of Iowa, Iowa City, IA, USA.
authors.forename: Hiroto
authors.initials: H
authors.lastname: Kawasaki
authors.affiliation: Department of Anesthesia, The University of Iowa, Iowa City, IA, USA.
authors.forename: Rashmi N
authors.initials: RN
authors.lastname: Mueller
authors.affiliation: Department of Anesthesia, The University of Iowa, Iowa City, IA, USA; Department of Anesthesiology, University of Minnesota, Minneapolis, MN, USA.
authors.forename: Michael M
authors.initials: MM
authors.lastname: Todd
authors.affiliation: Department of Neurosurgery, The University of Iowa, Iowa City, IA, USA; Pappajohn Biomedical Institute, The University of Iowa, Iowa City, IA, USA; Iowa Neuroscience Institute, The University of Iowa, Iowa City, IA, USA.
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Howard
mesh_terms: D000161:Acoustic Stimulation; D000328:Adult; D018686:Anesthetics, Intravenous / Q000008:administration & dosage; D001303:Auditory Cortex / Q000187:drug effects* / Q000502:physiology*; D001307:Auditory Perception / Q000187:drug effects / Q000502:physiology*; D000069280:Electrocorticography; D005072:Evoked Potentials, Auditory / Q000187:drug effects*; D005260:Female; D065818:Gamma Rhythm; D006801:Humans; D008297:Male; D008875:Middle Aged; D015742:Propofol / Q000008:administration & dosage*
chemical_list: D018686:Anesthetics, Intravenous; D015742:Propofol
keywords: Frequency-following response, Heschl's gyrus; General anesthesia; High gamma; Intracranial recordings