Introduction:
The brain is the most complex and significant organ of the human body and is the command center that controls and manages cognitive abilities and physiological functions. Neurophysiological research on brain development has always been done, but it was not until recently that techniques were developed to assess fetal neural activities. The first successful fetal magnetoencephalography (fMEG) was done in 1985. The fetal brain is vulnerable to external and internal influences. Therefore, research on the prenatal brain is essential to understand brain development during this period. fMEG is not affected by tissue separating the fetus and is noninvasive, and usually induces auditory and visual effects in the fetus at 34 weeks to 37 weeks.
What Is Fetal Magnetoencephalography (fMEG)?
Fetal Magnetoencephalography (fMEG) is a noninvasive biomagnetic imaging method with a superior temporal resolution. It is used to identify induced brain responses and spontaneous brain activity by the fetus in the uterus. The brain cells generate electric currents during their biological activity, producing magnetic fields. Even with different layers of tissues, the magnetic signals are not distorted. Both these signals (electrical and magnetic) can be detected through the scalp noninvasively.
In basic stimulus-response studies, the auditory response of the fetus and visually evoked fields have been recorded. Studies like mismatch response and habituation, which were applied successfully with fMEG, also show late cognitive development in the prenatal stage. Parameters of the fetal heart rhythm were also recorded.
Magnetoencephalography has undergone many technological advancements since its establishment. A modern multichannel whole-head system with highly sensitive superconducting quantum interference devices (SQUID) is now established in clinical and research settings. The magnetic fields are recorded with 156 highly sensitive sensors. They are shaped to fit the anterior surface of the mother's abdomen, beginning from the perineum to the top surface of the uterus. Neither the mother nor the child is exposed to any radiation. This device also has a cradle attachment that will help track the child's postnatal development. Applications like these have significantly contributed to understanding the fetus's normal cortical processes and abnormalities.
What Are the Studies Done With fMEG?
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Initial Studies: The initial studies were done to investigate a basic stimulus response from the fetus. Among 80 % of the fetus, auditory responses were detected in response to pure tone stimuli. After the onset of the stimulus, the cortical responses occurred at latencies of around 200 milliseconds. But with advanced gestational age, they seem to decline. Shorter latencies show more mature responses from the brain. Other than auditory stimulation, visual fields were also induced. Auditory stimulation was induced successfully using a light flash.
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In studies conducted subsequently, the spontaneous fetal brain activity of the fetus was investigated. Different segments of fetal heartbeat and heart signals were detected through fMEG. Recent studies showed fetal heart rate could be differentiated between low and high-risk conditions when recorded with fMEG.
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Habituation - Habituation is one of the primary phenomena and another paradigm used to identify factors responsible for cognitive development. It assesses the neurological integrity of the fetal central nervous system. Habituation is seen as an indicator of developmental delays and a predictor for cognitive development in the postpartum period. Characteristics of the mother, like smoking, stress, and diabetes, are shown to have a negative impact on prenatal habituation.
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Mismatch Response (MMR) - Mismatch response studies were done to assess the higher-level cortical functions in neonates and fetuses. MMR, or discriminative response, is a study that detects a participant's ability to discriminate auditory information. Here, rare deviant stimuli are intermixed with frequent standard stimuli, and the participant's ability to differentiate the two stimuli is noted. The difference in the wave is acquired by subtracting the response from the standard stimuli from the response from the deviant stimuli. Studies have shown that MMR is more evident in full-term and preterm neonates. In addition, recent studies have shown that MMR was found among 66 % of fetuses, starting from the gestational age of 28 weeks.
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Rapid Temporal Processing (RTP) - RTP is seen to predict later speech performance and has been recorded successfully with fMEG. Rapid temporal processing (RTP) processes rapidly present auditory stimuli to investigate higher-level processes. The RTP creates enough sound perceptions to constitute a predecessor for developing speech perceptions. The study showed that neonates at about three weeks could process rapidly presented tones, whereas fetuses and younger newborns showed only one response to the presented tone pair. The result indicates immature processing during the early developmental stages.
What Are the Current Limitations of Fetal Magnetoencephalography (fMEG)?
The current limitations of fMEG include artifacts, source determination, and monitoring fetal positions during recording.
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Artifacts - Environmental artifacts like environmental noise and terrestrial magnetic fields are much larger than the small magnetic field created by the fetus's brain. So fMEG recordings are usually conducted in a magnetically shielded room to detect cortical activity without any disturbance from the environmental artifacts. Near-field biological signals like maternal and fetal heart, intestinal movements, motion artifacts, and uterine contractions also cause magnetic interference. They have to be extracted to observe brain response in analyzing data.
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Source Determination - Another challenge of the MEG recordings is source determination. MEG has a limited spatial resolution but a high temporal resolution. The spatial resolution is usually low for fetal recordings and is determined by the ratio of the signal to noise. Hence determining the exact source of the recorded signal can take time and effort. The reconstruction of the signal sources can be possible only with an adequate model of the investigated structure. Information on fetal recordings can be obtained with the help of other imaging systems like ultrasound.
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Monitoring the Fetal Positions During Recording - Fetal states are categorized into four states by combining the body movements with the fetal heart rate variability. They are F1 or quiet asleep, F2 or active asleep, F3 quiet awake, and F4 or active awake. Every recording of fMEG contains body movements and fetal heart rate acceleration. These recordings are extracted and inspected visually to determine the changes in fetal posture.
What Is the Significance of an fMEG?
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An fMEG enables a direct investigation of brain activity in utero (while in the uterus).
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In at-risk populations, fMEG can be used to evaluate and implement specific programs for fetal intervention.
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Maturation deficits and delays can be prevented with a detailed understanding of brain development during prenatal life.
Conclusion:
Fetal Magnetoencephalography (fMEG) is the only noninvasive technique used for the prenatal assessment of cognitive development. The goal of fMEG is mainly to understand neural activity and the healthy brain development of a fetus. Despite its limitations, fMEG is very promising in assessing cortical developments in prenatal.
