An electroencephalogram (EEG) is a test used to detect any abnormalities that may be present within a test subject's brain waves, which is a term used to describe the electrical activity present within the brain. Typically, an EEG will involve using small metal electrodes directly attached to the test subject's scalp. Since the neurons, which are the cells that make up the brain, are constantly communicating with each other through electrical impulses, this electrical activity will be recorded by the electrodes to provide an EEG reading.
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Animal Models for Electrophysiology
Different electrophysiological signals can be measured and evaluated for animal research studies focused on assessing brain functions and medical drug toxicity. These signals include EEG, electrocardiograms (ECG), electromyography (EMG), and electrocorticogram (ECoG). Within the field of neuropsychopharmacology, animal models are often used to study neural activity and drug response.
Various neurological disorders have also been studied, including schizophrenia, depression, anxiety, all of which can be used to advance the development of adequate treatments further.
One of the best animal models for studying electrophysiology is the dolphin. These mammals have a unique auditory system and well-developed brain that closely resembles both the size and functionality of human brains. In fact, recent studies have found that dolphins also possess a highly differentiated central nervous system responsible for the high level of intelligence often associated with these animals. Despite their utility, dolphins often exhibit unpredictable and uncontrollable behaviors that may interfere with effectively obtaining electrophysiological signals. Furthermore, the speed at which dolphins can swim, often within the range of 7.2 to 9.3 meters per second, can also impair signal acquisition efforts.
Invasive Electrophysiological Devices in Animal Research
To obtain an accurate electrophysiological recording of dolphins, researchers often prefer an invasive recording device. One of the earliest applications of this type of experiment was conducted in 1977, where the researchers implanted intracortical electrodes into the bone aperture in dolphins while under anesthesia.
These EMG electrodes, which had a harpoon shape, were then fixed into the neck and extraocular muscles to allow for invasive signal recording. Although this type of invasive method allowed the researchers to obtain pure electrophysiological signals that were highly accurate, due to their proximity to the brain, surgical techniques and animal anesthesia were administered. Additionally, as with any surgical procedure, the invasive placement of an electrode also increases the risk that the animal will experience an infection or brain damage.
Non-Invasive Electrophysiological Devices in Animal Research
As compared to the invasive methods that can be used, several non-invasive approaches are also available. Typically, these approaches will involve the non-surgical placement of electrodes onto the scalp surface of the test subject. In general, a non-invasive recording system will be comprised of multiple EEG electrodes embedded into suction cups and a signal amplifier, A/D converter, and ground station.
Various non-invasive electrophysiological systems have been tested and evaluated for their utility in animal research. For example, an EEG recording system used to study the neurological activity of zebrafish includes several active electrodes, as well as a single reference electrode placed on a flexible printed circuit board (PCB) to allow for better contact with the head of the zebrafish during signal acquisition.
A group of researchers reported a novel long-term multichannel EEG recording device that could be used on small aquatic animals, like zebrafish, to study epilepsy. This research was published in the journal Scientific Reports on June 8th 2017.In their approach, a four-channel electrode array was printed on a flexible PCB based on a polyimide film, which is biocompatible, flexible, and chemical resistant. The array contained four gold electrodes that could successfully acquire brain signals from the telencephalon and midbrain of each hemisphere of the zebrafish.
In Tokyo, Japan, an MT-11 telemetry system was developed, consisting of several vinyl chloride suction cups that are placed near the eyeballs, blowhole, and dorsal fin of the dolphins. Experiments conducted using this system revealed that the power ratios of the frequency bands were associated with the action of the dolphin.
Future of Technological Devices in Animal Research
There is a need to develop fixed non-invasive electrophysiological devices that will not restrict the animals' movement during behavior-related brain studies. Methods will have to overcome the large onshore device cables connected to the electrodes on the animals' scalp – the main restrictors of movement.
A study published in Sensors recently discusses the development of a portable, wearable, and waterproof EEG acquisition device tested on dolphins. Here, a signal collective device was comprised of a silicon belt, electrodes, and a device box that consisted of all electrical parts. Silicon suction cups with a diameter of 8 centimeters (cm) were used to stick gold-plated EEG electrodes to the skin of the dolphin.
The researchers separated their device into a collecting box and ground station to eliminate the need for a large onshore system, connected through Bluetooth communication. If there is a disruption in this communication system, all acquired signals are written into a memory drive onboard.
When evaluating the efficacy of their novel system, the researchers of the current study found that their device could successfully record and send the dolphin's signals while the dolphin was in different motion states. Although additional device tests must still be performed, the current study found that their device supports long-term portability and, as a result, the ability to collect dolphin EEG signals while in their natural environment.
References and Further Reading
Yu, Y., Li, N., Li, Y., & Liu, W. (2021) A Portable Waterproof EEG Acquisition Device for Dolphins. Sensors 21(10); 3336. Available at: https://doi.org/10.3390/s21103336
Cho, S., Byun, D., Nam, T., et al. (2017) Zebrafish as an animal model in epilepsy studies with multichannel EEG recordings. Scientific Reports 7(3099). Available at: https://doi.org/10.1038/s41598-017-03482-6
Mayoclinic.org. (2020) EEG (electroencephalogram) - Mayo Clinic. [online] Available at: https://www.mayoclinic.org/tests-procedures/eeg/about/pac-20393875
S. Aston-Jones, G. and R. Siggins, G., (2000) Electrophysiology. [online] Acnp.org. Available at: https://acnp.org/g4/GN401000005/Default.htm