Perceptual warping exposes categorical representations for speech in human brainstem responses

Area of Science:

• Neuroscience
• Auditory Perception
• Speech Processing

Background:

• The brain categorizes continuous acoustic speech signals for perception.
• Phonetic categories are adaptable, influenced by stimulus context.
• Previous research localized speech-category mapping to the auditory cortex.

Purpose of the Study:

• To investigate if speech categorization occurs earlier than the auditory cortex, specifically in the auditory brainstem.
• To examine the role of the auditory brainstem in acoustic-phonetic mapping and perceptual warping of speech.
• To determine if neural responses in the auditory brainstem reflect phonetic category information.

Main Methods:

• Recorded speech-evoked frequency following responses (FFRs) using a clustered stimulus paradigm.
• Employed a behavioral task with varying stimulus presentation orders (random, forward, backward) to manipulate perceptual warping.
• Compared FFRs during active listening with acoustical stimuli and computational models.

Main Results:

• Serial stimulus order induced perceptual shifts (hysteresis) near category boundaries.
• Neural FFRs during active listening showed enhanced responses for prototypical versus ambiguous speech tokens.
• FFRs were biased towards the listener's phonetic label, even for acoustically identical stimuli, indicating central processing.
• These effects were absent in stimulus acoustics and computational models of peripheral auditory processing.

Conclusions:

• Frequency following responses (FFRs) encode speech category-level information.
• Top-down processing influences neural encoding and categorization of speech at subcortical levels.
• Acoustic-phonetic mapping and perceptual warping occur earlier in the auditory neuroaxis than previously thought, aiding speech comprehension.