摘要：目的 用近红外光学功能成像技术研究语义与知觉编码过程中的左前额叶活动。方法 实时记录22名青年被试者（男女各半）在语义和知觉加工过程中760 nm和850 nm两个波长近红外连续光透过额骨和左前额皮层后的漫射光强变化，并以此表征该皮层区血容（CBV）和血氧的变化。语义加工要求被试者用两个非相关的汉字词对组成一个有意义的句子；知觉加工则要求被试者判断两个词是否具有相同的结构组合以及两个词对中左右结构的差别。 结果 结果显示，与知觉加工相比，语义加工时左下前额叶活动增多。该区域大致对应布德曼45、47区的一部分。 结论 成功地利用近红外功能成像技术观测了语义与知觉编码过程中的左前额叶活动，获得了与fMRI和断层成象技术(PET)研究相近的结果，进一步证实了左前额叶在语义加工中的作用。

中图分类号：R319；R338.2文献标识码：A文章编号：1002-0837(2000)02-0079-05

Left Prefrontal Cortex Activation during Semantic Encoding Accessed with Functional Near Infrared Imaging

LI Peng-cheng,GONG Hui,YANG Jiong-jiong,ZENG Shao-qun

(Institute of Biomedical Photonics, Huazhong University of Science and Technology, Wuhan 430074)

abstract: Objective To investigate the left prefrontal lobe activation during semantic and non-semantic encoding tasks with functional near-infrared imaging (fNIRI) technique. Method 22 healthy subjects were assigned semantic encoding and non-semantic encoding tasks. During semantic encoding tasks, subjects were asked to make a meaningful sentence including two unrelated Chinese word pairs, while during non-semantic encoding task they were asked to judge whether the two Chinese word pairs had the same morphological structure or not. Light intensity of two wavelengths (760 nm and 850 nm) diffused through skull and left prefrontal lobe were real-time recorded and used to reconstruct the brain activation image during the experiment. Result With the fNIRI, significant activations were observed in the left inferior prefrontal cortex (Brodmann' areas 45 and 47) during the two tasks, but the evoked activations were more significant for semantic than non-semantic task. These observations were consistent with the results reported by others with functional megnetic resonance imaging(fMRI) and positron-emission tomographyPET.Conclusion The results suggest that fNIRI provides an important, non-invasive way to map the prefrontal activation during cognitive tasks.

Key word:near-infrared spectroscopy;semantic encoding;non-semanticencoding;left prefrontal activation

Modern functional neuroimaging methods, such as positron-emission tomography (PET), functional MRI (fMRI) and optical imaging with NIRS, utilize activity-dependent hemodynamic changes to obtain indirect maps of the evoked electrical activity during brain activation. These imaging techniques rely on various types of activity-dependent hemodynamic changes: Regional changes in cerebral blood flow (CBF) were used for functional mapping of the brain by PET,single photon emission computed tomography (SPECT) and flow-sensitive MRI. The optical imaging technique relies on the optical properties of cortex change due to regional changes of either cerebral blood oxygenation or cerebral blood volume (CBV) changes when the cortex is functionally active. Since near-infrared light can penetrate deeply into tissues, optical method employing near-infrared radiation could detect the absorption or scattering changes of the cortex through skin and skull.

Since Petersen et al^［1］ provided the first functional neuroimaging evidence accessed with PET implicating left prefrontal regions in the semantic analysis of words, subsequent PET and fMRI studies have found similar left prefrontal activations indicating LIPC involving the semantic encoding^［2］.In these studies, the left inferior prefrontal cortex (LIPC) area, anterior to Broca's area, showed greater activation during semantic than non-semanticencoding^［2,3］.As a unique non-invasive technique for brain function imaging, near-infrared spectroscopy has been used to study brain activities, including motor^［4］, visual^［5］ and cognitive^［6］ activities. However, this technique has not been used in studying the brain activation during semantic encoding tasks. In this paper, left prefrontal activation during encoding unrelated word pairs were measured by means of functional near-infrared imaging (fNIRI) and difference between activations during semantic encoding tasks and non-semanticencoding tasks were demonstrated.

Methods

SubjectsThe subjects of the experiment were 22 right-handed healthy students of Huazhong University of Science and Technology (1/2 were female, aged 18 to 22 years). All subjects are native speakers of Chinese and had no history of neurological or psychiatric disorders.

MaterialsThe stimuli were taken from 50 unrelated Chinese word pairs with no obvious semantic relation between the two words in each word pair. The difficulty and structure of these word pairs are carefully selected and matched in difficulty and frequency. Of the 50 word pairs, 40 pairs were used as material for experiment. The remaining 10 pairs were used for exercising and filling. The 40 experimental word pairs were divided into 2 groups. Each group contained 20 word pairs used for semantic or non-semanticencoding session. Word pairs in each group were presented to the subjects in a random order by a computer.

ProcedureThe procedure of the experiment included:(a) Rest condition (silent, baseline): The subjects were instructed to stare at a “+" character at the center of the screen and keep as calm as possiblei;(b) Semantic encoding task: The subjects were asked to make a meaningful sentence including the presented word pair on the screen and speak it out;(c) Non-semantic encoding task: The subjects were asked to judge whether the two words in the presented word-pairs had the same morphological structure or not. If not, figure out the difference in LEFT-RIGHT structure( LEFT-RIGHT structure is one of the 5 morphological way to construct a Chinese character).

The experiment sequence for each subject is, baseline (30 s), task (150 s), rest (60 s), the other task (150 s), and rest (60 s). The order of altering encoding tasks was counterbalanced across subjects. Each word pair was presented centrally on the computer screen for 5 s, followed by the character “+" presented in a second interval before the appearance of the next word pairs. Word pairs orders were kept randomly across subjects.

Principle of functional Near-Infrared ImagingThe fNIRI used in this study is designed basing on the dual wavelength (760 nm and 850 nm) near-infrared spectrophotometry^［7,8］.It is capable of picking up small difference in absorption characteristics between the oxygenated and deoxygenated states of the hemoglobin at 760 nm and 850 nm. Utilizing these two wavelengths, it is possible to determine the relative concentrations of oxygenated (HbO₂), deoxygenated (Hb) hemoglobin and blood volume between the two states^［9］. Since the neural activity is associated with hemodynamic changes, it can be accessed by fNIRI. The detail description of fNIRI was given in reference ［8］.

Experiment setup The diagram of functional near-infrared imager used in this study is shown as figure 1. Nine Tungsten lamps and four dual wavelength detectors make 16 lamp-detector pairs covering an area of 9 cm by 4 cm. In each detection section, the distance between lamp and detector pair is 2.5 cm. Each dual wavelength detector pair ha