With the rise of non-invasive brain imaging such as functional magnetic resonance imaging (
fMRI), researchers have been granted unprecedented access to the inner workings of the brain. It is now relatively straightforward to put your experimental subjects in an fMRI machine and measure activity 'blobs' in the brain. This approach has undoubtedly revolutionised cognitive neuroscience, and looms very large in people's idea of contemporary brain science. But fMRI has it's limitations. As every student in the business should know, fMRI has poor temporal resolution. fMRI is like a very long-exposure photograph: the activity snapshot actually reflects an average over many seconds. Yet the mind operates at the millisecond scale. This is obviously a problem. Neural dynamics are simply blurred with fMRI. However, probably more important is the theoretical limit.
Electricity is the language of the brain, but fMRI only measures changes in blood flow that are coupled to these electrical signals. This coupling is complex, therefore fMRI can only provide a relatively indirect measure of neural activity. Electroencephalography (
EEG) is a classic method for measuring actual
electrical activity. It has been around for more than 100 years, but again, as every student should know: EEG has poor spatial resolution. It is difficult to know exactly where the activity is coming from. Magnetoencephalography (
MEG) is a close cousin of EEG. Developed more recently, MEG is better at localising the source of brain activity. But the fundamental laws of physics mean that any measure of electromagnetic activity from outside the head will always be spatially ambiguous (
the inverse problem). The best solution is to record directly from the surface of the brain. Here we discuss the unique opportunities in that arise in the clinic to measure electrical activity directly from the human brain using electrocorticography (
ECoG).
Epilepsy can be a seriously debilitating neurological
condition. Although the symptoms can often be managed with medication, some
patients continue to have major seizures despite a cocktail of anti-epileptic
drugs. So-called intractable epilepsy affects every aspect of life, and can
even be life-threatening. Sometimes the only option is
neurosurgery: careful removal of the specific brain area responsible for seizures can
dramatically improve quality of life.
Psychology students should be familiar with the case of
Henry Molaison (aka HM). Probably the most famous neuropsychology patient in history,
HM suffered intractable epilepsy until the neurosurgeon William Scoville removed
two large areas of tissue in the medial temporal lobe, including left and right
hippocampus. This pioneering surgery successfully treated his epilepsy, but
this is not why the case became so famous in neuropsychology. Unfortunately, the
treatment also left HM profoundly amnesic. It turns out that removing both
sides of the medial temporal lobe effectively removes the brain circuitry for
forming new memories. This lesson in functional neuroanatomy is what made the
case of HM so important, but there was also a important lesson for neurosurgery – be
careful which parts of the brain you remove!
The best way to plan a neurosurgical resection of epileptic
tissue is to identify exactly where the seizure is comping from. The best way to map out the affected region is to record activity directly from the surface of the
brain. This typically
involves neurosurgical implantation of
recording electrodes directly in the
brain to be absolutely sure of the exact location of the seizure focus. Activity can then be monitored over a number of days, or even weeks, for
seizure related abnormalities. This invasive procedure allows neurosurgeons to
monitor activity in specific areas that could be the source of epileptic
seizures, but also provides a unique opportunity for neuroscientific research.
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| From Pasley et al., 2012 PLoS Biol. Listen to audio here |
During the clinical observation period, patients are
typically stuck on the hospital ward with electrodes implanted in their brain literally waiting for a seizure to happen
so that the epileptic brain activity can be ‘caught on camera’. This
observation period provides a unique opportunity to also explore healthy brain
function. If patients are interested, they can perform some simple experiments
using computer based tasks to determine how different parts of the brain
perform different functions. Previous studies from some of the
great pioneers in neuroscience mapped out the motor cortex by stimulating different brain areas during neurosurgery. Current experiments are
continuing in this tradition to explore less well charted brain areas involved
in high-level thought. For example, in a
recent
study
from Berkeley, researchers used novel brain decoding algorithms to convert
brain activity associated with internal speech into actual words. This research
helps us understand the fundamental neural code for the internal dialogue that
underlies much of conscious thought, but could also help develop novel tools
for providing communication to those otherwise unable to general natural speech.
In Stanford, researchers were recently able to identify a brain area that
codes for numbers and quantity estimation (read study
here).
Critically, they were even able to show that this area is involved in everyday
use for numerical cognition, rather than just under their specific experimental conditions. See video below.
The great generosity of these patients vitally contributes
to the broader understanding of brain function. They have dedicated their
valuable time in otherwise adverse circumstances to help neuroscientists explore
the very frontiers of the brain. These patients are true pioneers.
Key References
Dastjerdi, M.,
Ozker, M., Foster, B. L., Rangarajan, V., & Parvizi, J. (2013). Numerical
processing in the human parietal cortex during experimental and natural
conditions. Nat Commun, 4, 2528.
Pasley, B. N., David, S. V., Mesgarani, N., Flinker, A., Shamma, S.
A., Crone, N. E., Knight, R. T., & Chang, E. F. (2012). Reconstructing
speech from human auditory cortex. PLoS
Biol, 10, e1001251.
Video showing the use of a number processing brain area in everyday use:
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