Early-latency categorical speech sound representations in the left inferior frontal gyrus

• Published in: NeuroImage (Orlando, Fla.) Vol. 129; pp. 214 - 223
• Main Authors: Alho, Jussi, Green, Brannon M., May, Patrick J.C., Sams, Mikko, Tiitinen, Hannu, Rauschecker, Josef P., Jääskeläinen, Iiro P.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.04.2016; Elsevier Limited
• Subjects: Acoustic Stimulation; Adult; Categorical perception; Female; Humans; Inferior frontal gyrus; Listening; Magnetoencephalography; Male; NMR; Nuclear magnetic resonance; Phonetics; Prefrontal Cortex - physiology; Sensory perception; Signal Processing, Computer-Assisted; Sound; Speech; Speech perception; Speech Perception - physiology; Stimulus-specific adaptation; Studies; Young Adult; Stimulus-specific adaptation; Magnetoencephalography; Inferior frontal gyrus; Categorical perception; Speech perception
• ISSN: 1053-8119; 1095-9572
• DOI: 10.1016/j.neuroimage.2016.01.016

Efficient speech perception requires the mapping of highly variable acoustic signals to distinct phonetic categories. How the brain overcomes this many-to-one mapping problem has remained unresolved. To infer the cortical location, latency, and dependency on attention of categorical speech sound representations in the human brain, we measured stimulus-specific adaptation of neuromagnetic responses to sounds from a phonetic continuum. The participants attended to the sounds while performing a non-phonetic listening task and, in a separate recording condition, ignored the sounds while watching a silent film. Neural adaptation indicative of phoneme category selectivity was found only during the attentive condition in the pars opercularis (POp) of the left inferior frontal gyrus, where the degree of selectivity correlated with the ability of the participants to categorize the phonetic stimuli. Importantly, these category-specific representations were activated at an early latency of 115–140ms, which is compatible with the speed of perceptual phonetic categorization. Further, concurrent functional connectivity was observed between POp and posterior auditory cortical areas. These novel findings suggest that when humans attend to speech, the left POp mediates phonetic categorization through integration of auditory and motor information via the dorsal auditory stream. •Phoneme category selectivity was found in the left inferior frontal gyrus (LIFG).•The latency was 115–140ms, compatible with the speed of phonetic perception.•The degree of selectivity correlated with behavioral phonetic categorization.•LIFG constrains the interpretation of speech sounds into discrete categories.