Event

Title:  Phonetic categories in speech emerge subcortically: Converging evidence from the frequency-following response (FFR)

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. In multiple studies, we are examining whether such auditory-category phenomena inherent to speech perception occur even earlier in the human brain, at the level of auditory brainstem. In experiment #1, 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. In experiment #2, we examined FFRs to dichotically presented, “duplex” speech stimuli that are only perceived categorically only after sound information is integrated between the ears. We find FFRs are stronger for categorically perceived speech relative to category-ambiguous tokens but also differentiated phonetic categories for both diotically and dichotically presented speech sounds. Collectively, our data expose the presence of binaurally integrated, category-level information in FFRs and suggest top-down processing actively shapes the neural encoding and categorization of speech at subcortical levels. These findings suggest human brainstem processing reflects a surprisingly abstract level of the speech code typically circumscribed to much later cortical processing.