The Callosal Relay Model of Interhemispheric Communication: New Evidence from Effective Connectivity Analysis

Abstract

Interhemispheric auditory connectivity via the corpus callosum has been demonstrated to be important for normal speech processing. According to the callosal relay model, directed information flow from the right to the left auditory cortex has been suggested, but this has not yet been proven. For this purpose, 33 healthy participants were investigated with 64-channel EEG while performing the dichotic listening task in which two different consonant–vowel syllables were presented simultaneously to the left (LE) and right ear (RE). eLORETA source estimation was used to investigate the functional (lagged phase synchronization/LPS) and effective (isolated effective coherence/ICoh) connectivity between right and left primary (PAC) and secondary auditory cortices (SAC) in the gamma-band (30–100 Hz) during right and left ear reports. The major finding was a significantly increased effective connectivity in the gamma-band from the right to the left SAC during conscious perception of LE stimuli. In addition, effective and functional connectivity was significantly enhanced during LE as compared to RE reports. These findings give novel insight into transcallosal information transfer during auditory perception by showing that LE performance requires causal interhemispheric inputs from the right to the left auditory cortices, and that this interaction is mediated by synchronized gamma-band oscillations.

The role of functional and structural interhemispheric auditory connectivity for language lateralization - A combined EEG and DTI study

“…This finding indicates that an increased gamma-band synchrony goes along with more LE reports and also leads to a reduction of the typical left-lateralization of language. This is in accordance with two prior EEG studies, showing gamma-band oscillations and their synchronization are crucially involved in interhemispheric information transfer during conscious perception of LE syllables 1,21 . It is suggested that synchronized gamma-band activity constitute a fundamental brain mechanism that subserves the cortical computation underlying several cognitive functions 8 , such as auditory perception 58 .…”

“…The dichotic listening task used was the same as in our previous studies 1,15,21 . In brief, six different consonant-vocal syllables (/ba/, /da/, /ka/, /ga/, /pa/, /ta/) were paired and presented simultaneously: one syllable to each ear, resulting in 12 different combinations as only syllables with the same voice-onset time were combined 28 .…”

“…Similar to our previous studies 1,21,32 , all further analyses were executed with the LORETA KEY software package 33 investigating functional connectivity computed as “lagged phase synchronization” (LPS) 34 . LPS was assessed within two a priori defined regions-of-interests (ROIs): one candidate area for the processing of any type of sound is the right and left Heschl’s gyrus (BA41), and another one is pSTG (BA42) which has been related to the processing of complex sounds and speech sounds 35,36 .…”

“…In contrast, the LE syllable initially enters the right auditory cortex and requires further transport through IAPs to be processed in the speech-dominant hemisphere 20 . Using functional and effective EEG-based connectivity analysis, it has been shown that this causal interhemispheric transfer of auditory information during LE performance runs from the right to the left posterior parts of the superior temporal gyrus (pSTG) 21 , and further, is mediated by synchronized gamma-band oscillations 1 . In line with that, a previous DTI tractography study has shown that a larger midsagittal size of the posterior part of the corpus callosum – which is considered as the callosal area (i.e.…”

Comparing the Features of the Diffuse Alpha Pattern with the Normal Alpha Pattern using Wavelet-crosscorrelation Analysis

“…Auditory connectivity via the corpus callosum has been reported to be responsible for the interplay of right and left speech-relevant brain regions [25]. It appears that there is a significantly increased effective connectivity in the gamma band from the right to the left secondary auditory cortex during conscious perception of left ear stimuli [25]. However, it remains unknown in which direction the interhemispheric communication is realized.…”