A low-power digitally programmable neural recording amplifier with selectable multiunit activity (MUA) and local field potential (LFP) recording modes
Introduction
Neural signal recording is used to study the local neural activities in many applications like visual neuroscience and brain research [[1], [2], [3], [4]]. Neural recording techniques are necessary to pinpoint the origin and localizing seizure activities such as Epilepsy, Parkinson's disease, and etc [5]. These techniques are also widely used in cognitive science for human cognition and cortical mapping. The information can then be applied to brain machine interface (BMI) technologies for brain control of external devices [6]. BMIs record brain signals and decode an intended response to control the movement of an external device.
Neural recording is a method of measuring the electrophysiological activities of neurons. Neurons are the basic functional units in the brain and communicate with each other using electrical signals. The electric current flow through the neuron creates a voltage potential on the nerve cell. These electrical currents generate magnetic fields in the brain. Neuronal activation and cerebral blood flow are also coupled, since blood flow increases in the active area of the brain. Therefore, brain activities can be captured in various ways, ranging from imaging techniques by recording magnetic fields in magnetoencephalography (MEG), and detecting changes associated with blood flow in functional magnetic resonance imaging (fMRI) [7,8], to the recording of electrical signals by electrodes in electroencephalography (EEG) and electrocorticography (ECoG). EEG is an electrophysiological monitoring method to record brain activities from outside the skull. It is typically noninvasive, since the electrodes are placed along the scalp. ECoG is another type of electrophysiological monitoring that uses electrodes placed directly on the exposed surface of the brain to record electrical activity from the cerebral cortex. Invasive microelectrodes are used in ECoG, and a craniotomy (a surgical incision into the skull) is required to implant the microelectrodes. Neural recording can be combined with other techniques, e.g., microelectrode recording combined with fMRI technique has been reported in Refs. [7,8].
The recording microelectrodes, placed near the generating cell, isolate and record the neural signals. Highly integrated multichannel recording devices with large channel counts have been developed utilizing multisite neuronal electrodes [9], such as Michigan probe [10] and Utah array [11]. Recently, wireless neural systems have gained popularity for ECoG from the surface of the cerebral cortex, due to the capability of compressing and lowering the transmitted data rate [12]. A multi-channel neural recording system, shown in Fig. 1, consists of several sections. After flowing through the neural amplifiers, the recorded neural signals are collected by an analog multiplexer (MUX), then the signals are sampled and digitized to be transmitted to the digital signal processing (DSP) unit for further processing [13]. The modules placed before DSP may require digital control signals. These digital signals can be provided by the DSP. Every recording channel requires its own neural amplifier. The design and specifications of neural amplifiers have a key role in these systems.
Neural signal, generally known as extracellular signal, includes multiunit activity (MUA) and local field potential (LFP) [[2], [3], [4]]. MUA represents the average spiking activity of small neural populations around the electrode, within a sphere of approximately one-third of a millimeter radius with the microelectrode at its center. It can include dendritic spikes and activity from interneurons [8]. LFPs, predominantly reflect synaptic events, including synchronized afferent or local spiking activity. Spatial summation of LFPs occurs within a couple of millimeters from the electrode tip [8]. LFPs are generated by the electrical activity of a larger population of neurons and can be used to monitor the brain state during sleep, behavioral activation, and etc [1]. Although MUA and LFP research are mostly based on animal experiments, ECoG is often recorded and analyzed in human clinical experiments; real-time estimations of finger movements and arm trajectories can also be produced [14]. Due to their small signal amplitudes, MUAs are easily interfered by internal flicker noise and external noise sources. Therefore, a high-gain and low-noise analog recording front-end amplifier is required to process the weak neural signals acquired by electrode interfaces [15]. In order to achieve implantable biomedical systems for long-term ECoG signal monitoring and processing, both power consumption and chip area of the circuits should be minimized [16]. Accordingly, various types of automatic biopotential detectors are used to locate the targeted MUA waveforms, and process only the relevant contents instead of the entire signals [13,17]. Significant bandwidth reduction can be achieved with such a scheme, since it enables fitting more transmitted channels over a given wireless link. However, these benefits come at the cost of increased complexity.
In the signals recorded by extracellular microelectrodes, MUAs occupy the 0.1–10 kHz frequency band, with amplitudes typically lower than 500 μV. The LFPs occupy the lower frequencies, below 100 Hz, with amplitudes below 5 mV. An effective approach to reduce the required bandwidth and power consumption for implantable and wireless devices is separating the LFP and MUA signal bands at the recording front-end. It also provides separated signals for the next processing stages, like analog to digital converter (ADC) and DSP [8,[17], [18], [19]]. However, the system dynamic range requirement is an issue. Since the MUAs have ten times lower magnitudes than the LFPs, they can be amplified only to one-tenth of the full output swing. To use the full output dynamic range, MUA and LFP signals need to be amplified with different gains. A higher amplification gain is required for MUA compared to LFP signal. A programmable gain amplifier (PGA) can solve the dynamic range issue. Logarithmic PGAs can also be used to compress the high dynamic range of the input signal [[37], [38]].
The main goal of this paper is to develop a low-power digitally programmable neural amplifier for implantable wireless microelectrode recording applications. However, it can be used in other electrophysiologic signal processing applications. The structure is intended to be capable of separating the LFP and MUA signals by selecting between two frequency bands. Moreover, different gains should be applied depending on the selected recording mode. Due to different amplitudes in LFP and MUA recording modes, the structure should be capable of programming the gain and selecting the bandwidth, simultaneously. Low power consumption, low noise, and small area per channel are other requirements for such a structure.
This paper is organized as follows: In section 2, the structure of the proposed neural amplifier is presented. The four stages of the structure are discussed in detail in subsections. The simulation results for the proposed neural amplifier and comparison with recent similar works are presented in section 3, and section 4 concludes the paper.
Section snippets
Neural amplifier design
The proposed neural amplifier in this paper consists of a cascade of 4-stages, including instrumentation amplifier (IA), buffer, high-pass filter (HPF) and programmable gain amplifier (PGA), as shown in Fig. 2. The proposed structure operates in two different modes, i.e., MUA and LFP recording modes. In the state of φ = 0 for the switches S1−5, MUA signals are recorded; and for φ = 1, LFP signals are recorded. In LFP recording mode, the proposed neural amplifier operates with a constant gain,
Simulation results
The proposed programmable neural amplifier in this paper is designed and simulated using a 130 nm CMOS technology. According to subsection 2.4, PGA is digitally programmed into eight gain states by changing the states of the switches S6−8. PGA gain is programmed in the range of 9.4 dB–30.5 dB. The total gain of the structure during MUA recording is programmed in the range of 58.4 dB–79.5 dB. Table 1 shows the PGA and total gains of the proposed structure for eight states of the switches.
The
Conclusion
The paper presents the structure, operation, and simulation results for a low-power programmable neural recording amplifier. It has two selectable recording modes, i.e., MUA and LFP recording modes. The low and high cutoff frequencies of the proposed neural amplifier in LFP and MUA recording modes are 0.36 Hz–480 Hz, and 330 Hz - 8.7 kHz, respectively. By biasing the transistors in the subthreshold region, power dissipation is decreased to 1.9 μW. The proposed neural recording amplifier is
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