| Evoked potentials (EPs) are the weak electrical signals reflecting the neural activities in response to the external stimulus. According to stimulus type, they may be classified as somatosensory evoked potentials (SEPs), visual evoked potentials (VEPs) and auditory evoked potentials (AEPs). Following electroencephalogram (EEG) and electromyogram (EMG), EP has been developed into the third neural electrophysiological measurement with scientific and clinical applications. Currently, it is widely used for the study of signal processing mechanisms along the neural pathway and an objective assessment of neural system, offering an effective approach to the monitoring of the brain status and contributing to the diagnosis of neural dysfunction and the identification of loci of suspicious neural damage.An EP acquisition system consists of two main components:stimulus generator and data acquisition subsystem. The stimulus generator (SG) produces stimulus of specific parameters, which is passed onto the subjects through a proper transducer (e.g., an earphone for auditory stimuli, or CRT monitor for visual stimuli). The SG also produces a trigger to signal the occurrence of stimulus onset, in order to synchronize the data acquisition (DAS) for data acquisition. The DAS is responsible for signal amplification, filtering, analog-digital-conversion, averaging and a series of post-processing for effective extraction of EP signal.With the wide acceptance of clinical EP by domestic hospital due to its indispensable superiority, the demand of EP recording is constantly increasing.Different application purposes request rather different specification requirements, such as portability, flexibility, and low power consumption. When applied in scientific research, a specific study may also demand particular functions. For example, research into steady-state visual evoked potentials require the SG flexibly implement the generation of visual stimuli of different stimulation rate, and precisely produce the synchronization trigger. However, most of the EP systems in hospitals and labs are imported, which are usually expensive, more importantly, functionally constrained by the closed design. It cannot offer the flexibility, and portability required for many clinical and scientific applications.In order to meet the new requirements, we developed a high-rate visual stimulus generator based on FPGA, and an embedded-flexible auditory stimulus generator based on chip WM8731, and a high-performance EEG data acquisition based on chip ADS 1299. Besides, we also implemented a portable system for AEP extraction based on a single STM32 microprocessor architecture.With regard to the visual stimulus generation, there are two traditional approaches. The first makes use of single-chip microprocessor (SCM) in generating both the visual stimulus and the trigger for synchronization. Although this design offers reliable functions, it is difficult to flexibly adapt it for different specific applications because of the limited internal resources of SCM. The second is a soft approach relying on programming on general purpose PC. While this design allows flexible modification in stimulus parameters, the time accuracy of trigger signal depends on both the hardware and software factors. In terms of hardware, the performance of CPU and graphics card influences the timing of trigger; in software, multitasking operating system such as Windows may cause significant timing uncertainty. Therefore, we develop a FPGA-based visual stimulus generator. It can generate visual stimulus according to the stimulation mode command from its host computer (PC). When the actual stimulus is sent for displaying on a computer screen, the stimulus generator sends a trigger signal to external data acquisition device at the same time. The experiment result showed that this FPGA-based SG can flexibly generate different modes of visual stimulus, and that it also can ensure precise timing of the trigger signal. The synchronization accuracy can be up to μs level.The AEP extraction system consists of the auditory stimulus generator (ASG) and the EEG acquisition and processing unit. Traditionally, the ASG was based on the specialized sound equipment. This approach can achieve good stimulation effect, but it is functionally limited by the closed design, in addition to the disadvantage of high cost. Therefore, we designed an ASG based on an audio processing chip WM8731 which was featured of low power consumption, low cost, flexibility and portability. This ASG allows the user selects different stimulation modes through the LCD screen and keyboards, then its STM32 microcontroller reads the selected audio data of sound stimulus stored in an SD card. At the same time when WM8731 chip plays the sound stimulus, a high-speed I/O port outputs a trigger to signal the EEG data acquisition unit. The data acquisition unit was designed around an analog front-end integrated circuit chip ADS 1299. The above-described embedded auditory stimulus generator and the data acquisition unit are then combined with a single microprocessor STM32. Using two direct memory access (DMA) controllers, the system achieves an alternate storing mechanism of two caches. The trigger synchronizes the onset of sound stimulus and the acquisition of EEG data. Experiment results showed that our system can effectively extract AEP signal. |
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