Sound identification in human auditory cortex: Differential contribution of local field potentials and high gamma power as revealed by direct intracranial recordings

Abstract

High gamma power has become the principal means of assessing auditory cortical activation in human intracranial studies, albeit at the expense of low frequency local field potentials (LFPs). It is unclear whether limiting analyses to high gamma impedes ability of clarifying auditory cortical organization. We compared the two measures obtained from posterolateral superior temporal gyrus (PLST) and evaluated their relative utility in sound categorization. Subjects were neurosurgical patients undergoing invasive monitoring for medically refractory epilepsy. Stimuli (consonant-vowel syllables varying in voicing and place of articulation and control tones) elicited robust evoked potentials and high gamma activity on PLST. LFPs had greater across-subject variability, yet yielded higher classification accuracy, relative to high gamma power. Classification was enhanced by including temporal detail of LFPs and combining LFP and high gamma. We conclude that future studies should consider utilizing both LFP and high gamma when investigating the functional organization of human auditory cortex.

The neural oscillations of speech processing and language comprehension: state of the art and emerging mechanisms

“…Top‐down coupling has been described between the phase of theta‐band oscillations and the amplitude of gamma‐band oscillations within the left hemisphere, in the vicinity of the auditory cortex (Canolty et al ., ; Morillon et al ., ). When assuming that theta–gamma phase–amplitude coupling subserves the binding of phonemic representations inside syllabic representations (Giraud & Poeppel, ), such results conflict with a body of studies that describe theta‐band phase synchronisation to syllables relatively stronger in the right as compared to the left hemisphere (Luo & Poeppel, ; Abrams et al ., ; Hämäläinen et al ., ; Gross et al ., ; but see Howard & Poeppel, ; Peelle et al ., ), and conversely, gamma‐band amplitude synchronisation to subsyllabic aspects of speech relatively stronger in the left as compared to the right hemisphere (Lehongre et al ., ; Morillon et al ., ; Gross et al ., ; but see Nourski et al ., ). It is conceivable that syllabification of subsyllabic aspects of speech operates indirectly by relaying right‐hemispheric theta via transcallosal phase–phase coupling to left‐hemispheric theta, in turn phase–amplitude coupling locally with left‐auditory‐cortical lower‐gamma‐band oscillations.…”

“…In addition, electrocorticography work suggests that phonemic‐categorical information is only reflected in the amplitude synchronisation of neural oscillations in the high gamma band (i.e. > 70 Hz; Nourski et al ., ), likely reflecting spiking activity of auditory‐cortical neurons sensitive to individual phonemes (Mesgarani et al ., ; Nourski et al ., ). In sum, there are indications for a relationship between neural oscillations at gamma‐band frequency and sound amplitude modulations at subsyllabic frequencies.…”

Can you hear me yet? An intracranial investigation of speech and non-speech audiovisual interactions in human cortex

“…First, we computed the power of cortical activity in the high gamma (70–150 Hz) frequency range, which is thought to reflect relatively localized activity of neural ensembles in human cortex (Crone et al, 2006; Steinschneider, Fishman & Arezzo, 2008). High gamma activity in STG has been shown to carry substantial information about the content of the speech signal (Mesgarani et al, 2014; Steinschneider et al, 2011 but see Nourski, Steinschneider et al, 2015). Moreover, activity in the high gamma band correlates with the BOLD fMRI response (Logothetis, Pauls, Augath, Trinath, & Oeltermann, 2001; Mukamel et al, 2005; Niessing et al, 2005), offering us the ability to compare our results with previous AV speech studies.…”

Re-visiting the electrophysiology of language

“…This title has been chosen to emphasise the re-fuelled interest in the neural dynamics that we measure as fluctuations of electrical neural signalsnon-invasively from the scalp, or invasively directly from neural tissue. It is maybe noteworthy that this issue conjoins both approaches, yet reverses the usual method-to-species assignment: We here present scalp electroencephalography (EEG) data from behaving macaques (Attaheri et al, 2014) and invasive electrocorticography (ECoG) data from human patients undergoing epilepsy treatment (Nourski et al, 2015).…”

“…That is, electrophysiological measures akin to the ones used prominently in current auditory and speech research (e.g., Gross et al, 2013;Henry & Obleser, 2012;Lakatos, Karmos, Mehta, Ulbert, & Schroeder, 2008) can inform us also about infants' abilities to compute the statistical structure of language. Next, Nourski et al (2015) turn to the electrophysiological signal as it can be recorded directly from the cortical surface. The authors utilise the ECoG technique in neurosurgical patients and focus on syllable categorisation in the posterior superior temporal cortex.…”