Abstract
The posterior part of the superior temporal gyrus (STG) has long been known to be a crucial hub for auditory and language processing, at the crossroad of the functionally defined ventral and dorsal pathways. Anatomical studies have shown that this “auditory cortex” is composed of several cytoarchitectonic areas whose limits do not consistently match macro-anatomic landmarks like gyral and sulcal borders. The only method to record and accurately distinguish neuronal activity from the different auditory sub-fields of primary auditory cortex, located in the tip of Heschl and deeply buried in the Sylvian fissure, is to use stereotaxically implanted depth electrodes (Stereo-EEG) for pre-surgical evaluation of patients with epilepsy. In this prospective, we focused on how anatomo-functional delineation in Heschl’s gyrus (HG), Planum Temporale (PT), the posterior part of the STG anterior to HG, the posterior superior temporal sulcus (STS), and the region at the parietal-temporal boundary commonly labeled “SPT” can be achieved using data from electrical cortical stimulation combined with electrophysiological recordings during listening to pure tones and syllables. We show the differences in functional roles between the primary and non-primary auditory areas, in the left and the right hemispheres. We discuss how these findings help understanding the auditory semiology of certain epileptic seizures and, more generally, the neural substrate of hemispheric specialization for language.
Highlights
Mapping cortical auditory functions in humans has provided valuable insights about inter-areal anatomo-physiological distinctions, or about left-right functional asymmetries
The electrophysiological mapping of sensory and associative areas involved in cognitive networks should be conducted alongside, to answer the fundamental question of whether there is a spatio-temporal overlap between the epileptogenic and the functional networks
The identification of cortical structures that are essential to cognitive or perceptual functions is challenging because the human brain is a complex system in which a vast range of function arises from coordinated neural activity across diverse spatial and temporal scales (Sporns and Betzel, 2016; Bassett and Bullmore, 2017)
Summary
Mapping cortical auditory functions in humans has provided valuable insights about inter-areal anatomo-physiological distinctions, or about left-right functional asymmetries. Stimulation data consistently reveal two types of positive responses, with a clear-cut difference in the subjective auditory symptoms between the stimulation of BA 41 (primary cortex) eliciting mostly hallucinations and BA 42’s (secondary cortex) provoking illusions This is in line with the functional differences in electrophysiological responses (early vs mid latency evoked components, respectively) recorded from these areas. Liégeois-Chauvel et al (1999) showed lateralized processing of acoustic elements of the French voiced stops (e.g., /ba/) by time locking neural signals in the left dominant auditory cortex to the consonant onset or the release burst These findings have been replicated and used as an electrophysiological marker of the hemispheric dominance for language (Trébuchon et al, 2005). On the healthy auditory cortex, stimulations excite all neurons and elicit hallucinations (as noted above, only in rare cases does it result in temporary deafness)
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