Hemispheric asymmetry for auditory processing in the human auditory brain stem, thalamus, and cortex

Area of Science:

• Neuroscience
• Auditory Neuroscience
• Human Brain Imaging

Background:

• The human brain exhibits functional asymmetries for various sensory processes.
• Auditory processing, particularly spatial sound localization, is known to involve hemispheric specialization.

Purpose of the Study:

• To investigate context-dependent functional asymmetries in the human auditory system.
• To determine if neural responses in the auditory midbrain, thalamus, and cortex vary with the presence of binaural sounds.

Main Methods:

• Two functional magnetic resonance imaging (fMRI) experiments were conducted.
• Neural activity was measured in the cochlear nuclei, inferior colliculi (ICs), medial geniculate bodies (MGBs), and auditory cortices (ACs).
• Monaural sounds were presented with and without concurrent binaural stimuli to assess contextual modulation.

Main Results:

• In the absence of binaural stimulation, auditory structures showed stronger responses to contralateral ear stimulation.
• The presence of binaural sounds abolished this contralateral preference in right-hemisphere structures (ICs, MGBs, ACs).
• This modulation occurred over tens of seconds, indicating a slow, top-down mechanism.

Conclusions:

• Functional asymmetries in human auditory processing are not solely stimulus-dependent but are modulated by auditory context.
• The findings suggest a top-down regulatory mechanism influencing auditory processing, characterized by a long time constant.
• The observed asymmetry modulation is comparable to that seen in spatial sound processing.