Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Broca Area Processing of Information

 

 

Science 16 October 2009: Vol. 326. no. 5951, pp. 445 - 449

Sequential Processing of Lexical, Grammatical, and Phonological Information Within Broca’s Area

Ned T. Sahin,^1,2 Steven Pinker,² Sydney S. Cash,³ Donald Schomer,⁴ Eric Halgren¹

¹ Department of Radiology, University of California–San Diego, La Jolla, CA 92037, USA.
² Department of Psychology, Harvard University, Cambridge, MA 02138, USA.
³ Department of Neurology, Massachusetts General Hospital, Boston, MA 02114, USA.
⁴ Department of Neurology, Beth Israel Deaconess Medical Center, Boston, MA 02215, USA.

[paraphrase]

Words, grammar, and phonology are linguistically distinct, yet their neural substrates are difficult to distinguish in macroscopic brain regions. We investigated whether they can be separated in time and space at the circuit level using intracranial electrophysiology (ICE), namely by recording local field potentials from populations of neurons using electrodes implanted in language-related brain regions while people read words verbatim or grammatically inflected them (present/past or singular/plural). Neighboring probes within Broca’s area revealed distinct neuronal activity for lexical (~200 milliseconds), grammatical (~320 milliseconds), and phonological (~450 milliseconds) processing, identically for nouns and verbs, in a region activated in the same patients and task in functional magnetic resonance imaging. This suggests that a linguistic processing sequence predicted on computational grounds is implemented in the brain in fine-grained spatiotemporally patterned activity.

Within cognitive neuroscience, language is understood far less well than sensation, memory, or motor control, because language has no animal homologs, and methods appropriate to humans [functional magnetic resonance imaging (fMRI), studies of brain-damaged patients, and scalp-recorded potentials] are far coarser in space or time than the underlying causal events in neural circuitry. Moreover, language involves several kinds of abstract information (lexical, grammatical, and phonological) that are difficult to manipulate independently. This has left a gap in understanding between the computational structure of language suggested by linguistics and the neural circuitry that implements language processing. We narrow this gap using a technique with high spatial, temporal, and physiological resolution and a task that distinguishes three components of linguistic computation. According to linguistic analyses, the ability to identify words, combine them grammatically, and articulate their sounds involves several kinds of representations, with logical dependencies among them.

Although the language processing stream as a whole surely exhibits parallelism, feedback, and interactivity, the current results support parsimony-based models in which one portion of this stream consists of spatiotemporally distinct processes corresponding to levels of linguistic computation. Among the processes identified by these higher-resolution data is grammatical computation. The results are consistent with proposals that Broca’s area is not dedicated to a single kind of linguistic representation but is differentiated into adjacent but distinct circuits that process phonological, grammatical, and lexical information.

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