[DSRP Evidence](https://dsrpevidence.org/)

# Perceptual warping exposes categorical representations for speech in human brainstem responses

Carter & Bidelman, 2023, NeuroImage — Neuroscience

Patterns: [Distinctions](https://dsrpevidence.org/pattern/distinctions)

## In short

The nervous system converts a continuous acoustic gradient into discrete phonetic distinctions.

## What they found (results)

Speech-evoked frequency-following responses showed categorical (not linear) encoding, and responses to physically identical tokens shifted trial-by-trial in line with listeners' perceptual boundaries.

## What they set out to do (purpose)

To test whether the drawing of phonetic category boundaries emerges subcortically in the brainstem rather than only in cortex.

## In more detail

The nervous system converts a continuous acoustic gradient into discrete phonetic categories as early as the brainstem — distinction-making below conscious cortex. Purpose: To test whether the drawing of phonetic category boundaries emerges subcortically in the brainstem rather than only in cortex. Finding: Speech-evoked frequency-following responses showed categorical (not linear) encoding, and responses to physically identical tokens shifted trial-by-trial in line with listeners' perceptual boundaries. [Added 2026-07-30 as new evidence beyond the original 109; DSRP structural basis: D axiom (Mutual Negation): identical input is warped toward one category identity vs its 'other,' showing boundary-drawing is built into early sensory encoding.]

## Abstract

The brain transforms continuous acoustic events into discrete category representations to downsample the speech signal for our perceptual-cognitive systems. Such phonetic categories are highly malleable, and their percepts can change depending on surrounding stimulus context. Previous work suggests these acoustic-phonetic mapping and perceptual warping of speech emerge in the brain no earlier than auditory cortex. Here, we examined whether these auditory-category phenomena inherent to speech perception occur even earlier in the human brain, at the level of auditory brainstem. We recorded speech-evoked frequency following responses (FFRs) during a task designed to induce more/less warping of listeners' perceptual categories depending on stimulus presentation order of a speech continuum (random, forward, backward directions). We used a novel clustered stimulus paradigm to rapidly record the high trial counts needed for FFRs concurrent with active behavioral tasks. We found serial stimulus order caused perceptual shifts (hysteresis) near listeners' category boundary confirming identical speech tokens are perceived differentially depending on stimulus context. Critically, we further show neural FFRs during active (but not passive) listening are enhanced for prototypical vs. category-ambiguous tokens and are biased in the direction of listeners' phonetic label even for acoustically-identical speech stimuli. These findings were not observed in the stimulus acoustics nor model FFR responses generated via a computational model of cochlear and auditory nerve transduction, confirming a central origin to the effects. Our data reveal FFRs carry category-level information and suggest top-down processing actively shapes the neural encoding and categorization of speech at subcortical levels. These findings suggest the acoustic-phonetic mapping and perceptual warping in speech perception occur surprisingly early along the auditory neuroaxis, which might aid understanding by reducing ambiguity inherent to the speech signal.

These researchers were not testing DSRP. The finding is theirs; the correspondence to DSRP is drawn by this site.

[Source](https://doi.org/10.1016/j.neuroimage.2023.119899)