On The Prefrontal Cortex Function In Working Memory With Near-infrared Spectroscopy

Working memory (WM) refers to a mental system that provides temporary storage and manipulation of the information necessary for such high-level cognitive tasks as learning, reasoning, problem-solving, and language comprehension. The system can provide the short-term maintenance of an active representation of information so that it is available for further processing. There are abundant evidences coming from positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) studies accumulated to support a key role of prefrontal cortex (PFC) in different WM processes. Near-infrared spectroscopy (NIRS) is a noninvasive optical approach under fast development in recent years. Biological tissues characters low absorption and high scattering in near-infrared range from 700 to 900nm, which allows near-infrared light to probe the human cortex through the intact scalp and skull and carry out the functional information. NIRS has shown great potential in examining functional brain activity during cognitive tasks by enabling an independent measurement of changes in the concentration of the oxygenated hemoglobin (HbO2) and the deoxygenated hemoglobin (Hb) in cortex tissue based on differential absorption at multiple wavelengths. In this thesis, the basic principle of NIRS and the characteristic of brain are introduced. A multi-channel and multi-wavelength NIRS monitoring system is designed and built. As the main application of the system, the PFC functions during two working memory tasks are discussed in detail. 1)Based on the law of the photo-tissue interaction and the characteristic of brain,a 24-channel and three-wavelength continuous-wave-type NIRS monitoring system is designed and built. In the system, the temporal resolution is adjustable and determined by the sampling frequency. Here, we take 1 second as the temporal resolution. The imager possesses a detection area of 6 cm ×12cmcovering most part of PFC. In practical application, the imager is placed on the center of the forehead right above the eyebrow line to the upper and lateral sides. Thus, the detection area corresponds loosely to anterior PFC, dorsolateral PFC (DLPFC) and partial ventrolateral PFC (VLPFC). The main functions can be achieved as follows: sampling the data of light's intensity; calculating and displaying the relative concentration changes of HbO2, Hb and HbT. The capability of the system was validated by the blood model test in vitro. The results indicate that the concentration changes in HbO₂, Hb and HbT can be measured by the NIRS system. 2)PFC activations during verbal and spatial working memory tasks based on item-recognition paradigm were monitored by the NIRS system. On the base of the measurements, both the temporal characteristics of hemodynamic changes in PFC activation and the distributing diversity of the activated cortex area under two different tasks were investigated. The results revealed that there exists a typical activation pattern in which the concentration increases in HbO2 and HbT were frequently accompanied by concentration decrease in Hb. The activations in PFC present a lateral trend. The verbal task mainly activated the left VLPFC and the spatial material activated the right VLPFC. 3)During the verbal n-back task, the PFC activations were also monitored and the behavioral data were recorded by the system above. The effects of the working memory load on subjects'behavioral performances and PFC activation were investigated respectively. The relationships among the response time, accuracy and the PFC activation at higher memory load level were discussed. The results indicated as follow: i) Both the behavioral performances and PFC activation presented significant effects of working memory load. As WM task became more difficult, the activation in activated brain area and the response time increased and the accuracy decreased. ii) The activation in left PFC was obviously larger than that in the right, especially in VLPFC. iii) At higher memory load level, there existed a well-regulated relation among the response time, accuracy and the PFC activation: shorter response time being frequently companied by higher accuracy and smaller activation, in other words, longer response time being frequently companied by lower accuracy and larger activation. Overall, NIRS was firstly applied to the study on working memory in this thesis and thus a new method was provided for the project. In virtue of the characteristics of the high temporal resolution and multi-index measures in NIRS monitoring, we found that there exists a typical activation pattern in the activated cerebral area during the working memory task. The relationship between the PFC activation and the behavior parameters was also firstly investigated in the thesis and the fact was found that there exists a well-regulated relation among the response time, accuracy and the PFC activation at higher memory load level. The findings above provide new evidences for PFC function in working memory and are important for exploring the brain mechanism of the working memory.