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 50 Hz 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 (70–150 Hz) event-related band power (ERBP) 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.

Anesthesia-induced loss of consciousness disrupts auditory responses beyond primary cortex

“…In contrast to the "thalamic gating" hypothesis whereby the relay to primary sensory regions is disrupted (38), our results support the notion of preserved PAC responses under light anesthesia (29,33,45), as was also found for LFP responses to visual stimulation (21). Two recent studies examining the effects of propofol on human LFP auditory responses reported preserved early evoked potentials in the most posteromedial part of Heschl's gyrus when compared to intermediate and anterolateral regions showing robust attenuation (46), as well as preservation of novelty responses over short (local deviant) time scales in auditory cortex (47). Our study extends these observations along several dimensions, most importantly by recording spike responses to complex word stimuli.…”

Disruption of cortical network activity by the general anaesthetic isoflurane

“…exhibiting a state dependence with useful diagnostic implications. 36 The bursts we observed here are similar to brief UP states or network events observed in vivo under non-rapid eye movement sleep and anaesthesia, 37 38 and are thought to represent fragments of the desynchronized state associated with active engagement with the environment. 14 39 The observation that this type of bistable network activity exists under waking conditions as well 38 40-42 lends further motivation to study this activity in slices.…”

Interactive effects of linguistic abstraction and stimulus statistics in the online modulation of neural speech encoding

“…In light of the recent findings of potential contributions from phase-locked activities of the primary auditory cortex to FFRs (Coffey et al, 2016(Coffey et al, , 2017Bidelman 2018), one may also consider the possibility of attention-related cortical mechanisms known to inhibit phase-locking at the auditory cortex to be a factor in modulating the contextdependent FFRs. 1 Recent electrocorticography evidence has suggested that phase-locked neural responses at human posteromedial Heschl's gyrus (HG) is more robust during anesthesia (Nourski et al, 2017). The interpretation to this result was that without anesthesia, simultaneous nonphase-locked synaptic events initiated from other cortical regions may project to posteromedial HG to inhibit phaselocking synaptic events therein.…”

“…The predictive tuning mechanism is likely to be in a constant push-pull with other neural mechanisms such as SSA local to the IC (Lau et al, 2017) as well as cortical inhibition at the HG (Nourski et al, 2017) in mediating sensory encoding in different auditory contexts, as indexed by the FFR. Indeed, a recent FFR study found that the predictability enhancement could be reversed when FFRs were elicited in an irrelevant visual task with high processing load (Xie et al, 2018).…”