The Neuroanatomy of Speech Processing: A Large-scale Lesion Study

Area of Science:

• Neuroscience
• Psycholinguistics
• Cognitive Neurology

Background:

• The neural basis of spoken word recognition remains incompletely understood.
• Existing functional-anatomical models propose diverse critical brain regions for receptive speech.
• Discrepancies arise from studies focusing on different aspects of the sound-to-meaning pathway and employing varied tasks.

Purpose of the Study:

• To investigate the neural substrates underlying syllable discrimination and word comprehension.
• To compare the brain networks involved in these two distinct auditory processing tasks.
• To test competing functional-anatomical models of receptive speech.

Main Methods:

• Assessed syllable discrimination (words/nonwords) and word comprehension (clear/noisy speech) in 158 individuals with focal brain damage and 26 controls.
• Employed lesion-symptom mapping analyses to correlate brain lesions with task performance.
• Utilized behavioral and neurological dissociations between discrimination and comprehension tasks.

Main Results:

• Syllable discrimination and word comprehension are behaviorally and neurologically doubly dissociable.
• Clear speech comprehension was largely preserved in left stroke patients, supporting bilateral involvement.
• Right temporal damage impaired syllable discrimination, while left superior temporal gyrus was implicated in both tasks.
• Word comprehension additionally engaged the left posterior middle temporal gyrus, whereas discrimination involved dorsal sensorimotor regions.

Conclusions:

• The findings support a bilateral model for clear speech comprehension and highlight distinct neural networks for discrimination and comprehension.
• Left superior temporal gyrus is crucial for both syllable discrimination and noisy word comprehension.
• Specific regions within the temporal lobe and sensorimotor cortex differentially contribute to auditory processing.