Lateralized Activation in the Inferior Frontal Cortex During Syntactic Processing: Event-related Optical Topography Study

Functional imaging with near-infrared light has the potential to provide novel information that cannot be obtained with other imaging techniques. An event-related paradigm has not been fully established for studying human cognitive functions with near-infrared optical imaging. We conducted language experiments to develop an event-related paradigm with optical topography (OT). We directly compared cortical activation during syntactic and semantic decision tasks, both of which involved error detection in a sentence stimulus that consisted of a noun phrase and a verb. In the syntactic decision task, subjects judged whether the presented sentence is syntactically correct, where the syntactic knowledge about the distinction between transitive and intransitive verbs was required. In the semantic decision task, subjects judged whether the presented sentence is semantically correct, where the lexico-semantic knowledge about selectional restrictions was indispensable. We found local increases in oxyhemoglobin concentration, which were selectively associated with the syntactic decision task. Activation in the left inferior frontal gyrus was detected when syntactically anomalous sentences were presented, whereas there was no significant activation in this region when semantically anomalous sentences were presented. Moreover, identical stimuli of normal sentences elicited activation in the left inferior frontal gyrus, only when the employment of syntactic knowledge was required. This task-selective activation was not observed in any other measured regions, including the right homologous region. These results demonstrate that OT techniques, when coupled with the event-related paradigm, are useful for studying the higher cognitive functions of the human cerebral cortex.