Language
Do front and back parts of Broca's area handle grammar differently?
Open access · cc by · source: Europe PMC
Briefly disrupting the front of the left inferior frontal gyrus slowed people most when starting a phrase with a grammatical word, while stimulating the back part shortened how long they took to say that grammatical word.
Study at a glance
- Design
- Human experiment — Within-subject TMS study: effective vs sham 10 Hz bursts to left anterior or posterior IFG while participants produced Danish determiner-adjective-noun phrases in a grammatical or lexical condition
- N
- Healthy adult volunteers; the participant count is not given in the available text (the participants section appears to be missing)
- Population
- Healthy Danish-speaking adult volunteers
- Outcome
- Response time to speech onset (primary), duration of the determiner and duration of the adjective-noun phrase
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Key findings
Effective stimulation generally delayed the start of speech compared with sham, and the delay was larger for the anterior site. At the anterior site this delay was stronger for the grammatical article than for the numeral, which the authors had not predicted. Grammatical articles were spoken more briefly than numerals overall, and effective stimulation of the posterior site selectively shortened them further, which the authors interpret as a facilitation of phonological planning. Filler trials with the same colour and number demands did not show the anterior grammar-specific delay, suggesting it was not simply due to retrieving colours.
Methodology
Healthy volunteers heard a spoken prompt and then described a coloured picture with a short Danish phrase that began either with the article 'a' (grammatical condition, contrasting colour) or the numeral 'one' (lexical condition, contrasting number). The two responses sounded almost the same and differed mainly in stress. On each trial, a robot-held coil delivered a short burst of three magnetic pulses at 10 Hz to either the anterior or posterior left inferior frontal gyrus, at either an effective intensity or a low sham intensity. Speech recordings were used to measure response time and the durations of the determiner and of the rest of the phrase.
Limitations
The findings do not cleanly support the classic division of labour in which the posterior region handles grammar and the anterior region handles meaning; the key effects were partly unexpected, and the authors offer post hoc explanations. The grammatical and lexical tasks were not perfectly matched, since the grammatical task required retrieving one of four colours rather than one of two numbers. Even with a small focal coil and neuronavigation, the region between the two targets may have been co-stimulated, so the results argue for an anterior-to-posterior gradient rather than sharply separate areas. The number of participants is not reported in the text available here, which limits judging the study's power.
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