Neuroscience is a broad scientific field that encompasses many different types of research. This research ranges from the microscopic level such as basic protein interactions, genes, and cells, to the marcoscopic level such as brain regions and behavior. The study of neuroscience is not limited to humans. Rather, various animal models, such as flies, rodents and non-human primates, can be used to understand how the brain works.
But how can we study the link between behavior and specific brain areas?
How can we study the link between behavior and specific brain areas?
In 1848, a railroad worker named Phineas Gage suffered an accident at work: A 1.1 m long, 6 mm in diameter and 6 kg heavy pole impaled his head, via his left cheek, and exited out the top of his skull. Could you imagine a pole piercing your brain? Despite the accident, Mr. Gage did not show any significant neurological deficits and was left only with a blind left eye and facial weakness… or so it was thought. Following the accident, his family and friends described a sudden change in in Mr. Gage’s personality: he became fitful, irreverent, and even profane at times, a change that cost him his job (O’Driscoll & Leach, 1998).
Many years later, in the 1950s, Brenda Milner, a then graduate student, was invited to study the case of Henry Molaison, also known as ‘Patient HM’. He suffered from severe epilepsy and had his medial temporal lobes removed in attempt to ease his epileptic seizures. Milner observed that, although patient HM did not show any signs of intellectual disability or perceptual deficits, he was unable to create new memories (Squire, 2009).
These two patients are some of the most famous cases in the study of psychology and neuroscience, as they help us understand how certain brain areas are linked to certain functions. We now understand that the change in Mr.Gage’s personality was due to damage to his prefrontal cortices, which play a crucial role in cognitive control functions. On the other hand, the amnesia in Patient HM occurred because he had his hippocampi removed, which play a key role in memory function.
But how can we study the link between brain and behavior beyond these patient cases?
Alba participating in an EEG study.
Neuroimaging is a subfield of neuroscience that investigates healthy and diseased brain functioning, combining efforts from psychologists, physicists, chemists, computer scientists and neuroscientists to develop technology to investigate brain structure and function. Some common neuroimaging techniques used nowadays are magnetic resonance imaging (MRI), positron emission tomography (PET) and electroencephalography (EEG), which can be used in combination with behavioral or pharmacological tests.
EEG is a neuroimaging technique that dates back to the 1920s, when Hans Berger, a German psychiatrist with a strong interest in telepathic phenomena, first recorded the brain’s rhythmic fluctuations (Biasiucci, Franceschiello, & Murray, 2019). Yes, you read that correctly! Mr. Berger’s interest was in understanding how two brains might communicate with each other, but instead he developed what we nowadays know as EEG. When neurons communicate with each other, they exchange chemical signals that lead to changes in electrical activity. When this signaling happens in a lot of neurons at the same time, particularly when the activity of neurons is aligned, we can measure it with EEG. Therefore, the EEG measures neural oscillations and their characteristics.
The early use of nuclear medicine for imaging techniques developed into PET imaging. Patients and research participants are injected with radioactive tracers which emit particles called positrons. When positrons collide with their antiparticles, electrodes, they emit gamma rays in opposite directions that can be detected by a PET scanner (Turkington, 2001). When this happens many times at once, scientists can use this information to know where in the brain these events took place.
These tracers are made by chemists in a type of particle accelerator called cyclotron, by combining a radioactive atom into specific chemical substances. For example, a common type of PET scan is FDG (fluorodeoxyglucose), which is used to study brain metabolism by looking at the presence of glucose. In its early stages, PET imaging could only be conducted at specific facilities. As the strength of the tracers decays very quickly, they could only be made by chemists in facilities that had a cyclotron on site. Nowadays, these tracers can be made in different centers and sent quickly into hospitals and research centers for imaging purposes.
Later on, in the 70s and 80s, the first magnetic resonance images were taken. MRI is based on the principle that the nuclei of certain atoms act as spinning magnets which align when placed in a strong magnetic field. Scientists then apply a radiofrequency pulse that causes the little spinning magnets to spin out of equilibrium, and, when the pulse is turned off, they re-align. The atoms in different types of tissue realign at different times when the scientists turn on and off these pulses, allowing us to differentiate between different types of tissue in our brains. Structural MRI scans allow us to identify brain structure, and functional MRI scans allow us to investigate functional brain activity. Way back in the 1800s, Angelo Mosso and William James suggested that there is an increase in blood flow to a brain area when it is active (Raichle, 2010). Nowadays, fMRI lays on a similar principle, by measuring what is a called Blood-Oxygen-Level-Dependent (BOLD signal), which is based on the change of oxygenation in our blood and used to infer brain function and activity (Poldrack, Mumford, & Nichols, 2011). Besides structural and functional MRI, we can also use diffusion MRI to investigate the diffusion of water molecules. This imaging technique allows us to investigate parts of the brain where water molecules behave differently, such as in different orientations, integrities, and microarchitecture. This magnetic resonance technique and the modelling approaches to its analyses, allows us to study the brain’s white matter tracts, which act as long-distance cables that connect distant brain regions. And, it produces amazing images! Even the music group Muse used an image of brain’s white matter tracts to make the cover of their album The 2nd Law.
Depending on the questions we are interested in asking about the brain structure and function, neuroscientists use one neuroimaging technique over another, or even combine them to get a fuller picture on how the brain works! Which one is your favorite?
Written by Alba Peris-Yagüe
Illustrated by Himani Arora
Edited by Sarah Wade and Talia Oughourlian
Biasiucci, A., Franceschiello, B., & Murray, M. M. (2019). Electroencephalography. Current Biology, 29(3), R80–R85. https://doi.org/10.1016/j.cub.2018.11.052
O’Driscoll, K., & Leach, J. P. (1998). “No longer Gage”: an iron bar through the head. Early observations of personality change after injury to the prefrontal cortex. BMJ, 317(December), 1997–1998.
Poldrack, R. A., Mumford, J. A., & Nichols, T. E. (2011). Handbook of Functional MRI Data Analysis. Cambridge University Press.
Raichle, M. E. (2010). The origins of functional brain imaging in humans. History of Neurology (3rd ed., Vol. 95). Elsevier B.V. https://doi.org/10.1016/S0072-9752(08)02118-0
Turkington, T. G. (2001). Introduction to PET Instrumentation. Journal of Nuclear Medicine and Technology.