Perceiving and misperceiving speech: lexical and sublexical processing in the superior temporal lobes

Area of Science:

• Neuroscience
• Auditory Perception
• Speech Processing

Background:

• Listeners utilize predictive processing for noisy speech, leveraging prior knowledge to enhance comprehension.
• This predictive mechanism, while beneficial, can also lead to speech misperceptions.
• The specific neural underpinnings of speech perception and misperception remain incompletely understood.

Purpose of the Study:

• To investigate the causal roles of the left and right posterior superior temporal gyri (pSTG) in the perception and misperception of degraded speech.
• To differentiate the functions of the left and right pSTG in processing clear versus distorted auditory information.
• To examine the impact of targeted neural stimulation on predictive speech processing.

Main Methods:

• Employed a prime-probe paradigm with spectrotemporally degraded auditory stimuli.
• Utilized transcranial magnetic stimulation (TMS) to inhibit activity in the left and right pSTG, and a control vertex site.
• Created pseudo-sentences through homophonic nonword transformations to elicit misperceptions.

Main Results:

• Inhibitory TMS over the left pSTG selectively impaired priming for real sentences, but not for pseudo-sentences (misperceptions).
• Inhibitory TMS over the right pSTG enhanced priming for pseudo-sentences (misperceptions) but did not affect real sentence perception.
• These findings reveal distinct, asymmetric causal roles for the left and right pSTG in degraded speech processing.

Conclusions:

• The left pSTG is causally involved in the accurate perception of degraded speech, particularly for linguistically coherent stimuli.
• The right pSTG plays a distinct causal role in the misperception of degraded speech, potentially through sublexical processing.
• Results support bilateral models of speech processing, highlighting functional specialization within the posterior superior temporal gyri.