Fast (100–175 ms) components elicited bilaterally by language production as measured by three-wavelength optical imaging

Optical imaging has been gradually utilized to investigate language functions in the brain. The majority of hemodynamic response (slow signal) measurements have been applied to receptive and productive language processing, while several event-related optical signal (EROS) measurements on neuronal response (fast signal) have focused on receptive language processing. Therefore, an investigation of language production based on fast signal measurement is yet to be realized. Using a continuous-wave near-infrared spectroscopic (CW-NIRS) method with three long wavelengths in close ranges (780, 805, and 830 nm), which are suitable for the detection of fast optical signals, the current work investigated whether absorbance-based EROS components during overt language production might be elicited bilaterally in each wavelength with a 25 ms sampling time. Healthy adult subjects read aloud Japanese noun phrases (NP) presented on a computer screen. Two conditions (short/long-vowel duration) included either initial [s]- or [k]-phoneme types in the first words of the NP. The cognitive subtraction method achieved by deducting short-duration from long-duration conditions showed that in both phoneme types, reliable fast optical components with a peak latency of about 100–175 ms post initial-consonant onset were bilaterally elicited by long vowels. This result suggests that the present CW-NIRS methodology can clearly detect such early optical signals with good temporal resolution and with a good signal-to-noise ratio (SNR) obtained from a small number of stimuli. The fact that optical absorbance values at all three wavelengths had the same positive deflections during the initial-syllable production demonstrates that the elicitation of fast optical components may directly represent neuronal activity.