A team of engineers, surgeons and medical researchers has published data from both humans
and rats demonstrating that a new array of brain sensors can record electrical signals
directly from the surface of the human brain in record-breaking detail.
The new brain sensors feature densely packed grids of either 1,024 or 2,048 embedded electrocorticography
(ECoG) sensors.
The paper was published by the journal Science Translational Medicine.
These thin, pliable grids of ECoG sensors, if approved for clinical use, would offer
surgeons brain-signal information directly from the surface of the brain's cortex in
100 times higher resolution than what is available today.
Access to this highly detailed perspective on which specific areas of the tissue at the
brain’s surface, or cerebral cortex, are active, and when, could provide better guidance
for planning surgeries to remove brain tumors and surgically treat drug-resistant epilepsy.
Longer term, the team is working on wireless versions of these high resolution ECoG grids
that could be used for up to 30 days of brain monitoring for people with intractable epilepsy.
The technology also holds potential for permanent implantation to improve the quality of life
of people who live with paralysis or other neurodegenerative diseases that can be treated
with electrical stimulation such as in Parkinson’s disease, essential tremor, and the neurological
movement disorder called dystonia.
Recording brain activity from grids of sensors placed directly on the surface of the brain
– electrocorticography (ECoG) – is already in common use as a tool by surgeons performing
procedures to remove brain tumors and treat epilepsy in people who do not respond to drugs
or other treatments.
The new work in Science Translational Medicine provides a wide range of peer-reviewed data
demonstrating that grids with 1,024 or 2,048 sensors can be used to reliably record and
process electrical signals directly from the surface of the brain in both humans and rats.
For comparison, the ECoG grids most commonly used in surgeries today typically have between
16 and 64 sensors, although research grade grids with 256 sensors can be custom made.
Being able to record brain signals at such high resolution could improve surgeons' ability
to remove as much of a brain tumor as possible while minimizing damage to healthy brain tissue.
In the case of epilepsy, higher resolution brain-signal recording capacity could improve
a surgeon's ability to precisely identify the regions of the brain where the epileptic
seizures are originating, so that these regions can be removed without touching nearby brain
regions not involved in seizure initiation.
In this way, these high resolution grids may enhance preservation of normal, functioning
brain tissue.
Demonstrating that ECoG grids with sensors in the thousands function well also opens
new opportunities in neuroscience for uncovering a deeper understanding of how the human brain
functions.
Basic science advances, in turn, could lead to improved treatments grounded in enhanced
understanding of brain function.
Recording brain signals at higher resolution is attributable to the team's ability to place
individual sensors significantly closer to each other without creating problematic interference
between nearby sensors.
For example, the team's three-centimeter-by-three-centimeter grid with 1,024-sensors recorded signals directly
from the brain tissue of 19 people who agreed to participate in this project during the
"downtime" of their already scheduled brain surgeries related to either cancer or epilepsy.
In this grid configuration, the sensors are one millimeter apart from each other.
In contrast, the ECoG grids already approved for clinical use typically have sensors that
are placed one centimeter apart.
This provides the new grids 100 sensors per unit area compared to 1 sensor per unit area
for clinically used grids, offering 100 times better spatial resolution in interpreting
brain signals.
While using platinum-based sensors for recording electrical activity from neurons in the brain
is not new, the research team is using platinum in a novel way: nanoscale platinum rods.
The nano-rod shape offers more sensing surface area than flat platinum sensors, which helps
to make the sensors more sensitive.
The sensing system is based on the fact that the electron count in the platinum nano-rods
changes in response to neurons firing in the brain.
Charged ions move in and out of a neuron when it fires.
This movement of charged ions causes changes in the voltage potential in the cerebrospinal
fluid that the neurons are bathed in.
These changes in voltage potential in the brain tissue and cerebrospinal fluid change
the population count of electrons in the platinum nano-rods by charge screening processes.
In this way, the firing of neurons at or near the surface of the cerebral cortex is recorded
by the platinum nanorods in near real time and with high precision.
The human brain is always moving.
With each heartbeat, for example, the brain moves with the pulsating blood flowing through
it.
The platinum nano-rod based sensor grids are thinner and more flexible than today's clinically
approved ECoG grids.
The thinness and flexibility allows the sensor grids to move with the brain, enabling a closer
connection and better readings.
In addition, the grids are manufactured with small, ring-shaped holes that allow cerebral
spinal fluid to pass through.
In this way, these perfusion holes support a better interface between the sensor grid
and the brain surface by allowing the sensor to easily and safely displace the fluid.
The new platinum nano-rod brain sensor grids are ten micrometers thick, approximately one
tenth the size of a human hair and 100 times thinner than the one millimeter thick and
clinically approved ECoG grids.
The nano-rods are embedded in a transparent, soft and flexible biocompatible material called
parylene which is in direct contact with the surface of the brain.
The electrical signals travel from the brain, through the cerebrospinal fluid, and reach
the exposed surfaces of the platinum nano-rods which are recessed within the parylene.
This design yields a sensor grid that forms a close and stable connection with the surface
of the brain, improving signal quality.
The manufacturing process used also allows for a wide variety of sizes and shapes, which
opens up new possibilities for greater and more customized cortical coverage.
Collecting signals from larger areas of the brain at the same time could unlock more of
the brain's mysteries.
One of the challenges of removing brain tumors is that the presence of the tumor triggers
changes in the brain, including changing what areas of the brain are involved in what functions.
These changes make it critical for the surgical team to make a personalized map of the patient's
brain – "functional maps" – in order to decide where to cut and where not to cut while
removing as much of the tumor as possible.
The authors of the Science Translational Medicine paper demonstrated that these functional maps
can be made extremely precise using their platinum nano-rod ECoG sensors.
In particular, the team developed functional maps in four different people of a boundary
in the brain called the central sulcus.
In these four individuals, the researchers placed the platinum nano-rod grids on the
surfaces of the subjects’ brains and asked them to do a number of activities, including
hand grasping.
With this information, the researchers reconstructed the actual location of this key finger sensation
and hand grasping.
The results from the platinum nano-rod grids aligned with the results from the lower-resolution
ECoG grids already approved for clinical use, but with more precision about exactly where
this critical functional boundary is between the somatomotor cortex and the somatosensory
cortex.
The newly delineated curvilinear functional boundary unique to each patient’s brain
is superior to the often extrapolated and linear boundary that is determined from today's
one-centimeter-spaced clinical grids.
Some of the team's newly published data from studies in rats also demonstrate the utility
of the grids for opening new avenues in fundamental neuroscience research.
The Science Translational Medicine paper, for example, includes what the researchers
believe is the first mapping of a cortical column in a rat from brain-surface recordings.
In the past, the mapping of cortical columns has only been done by placement of an individual
needle at the surface of the brain and sequential electrical stimulation and movement of the
needle across the brain surface.
More generally, the fact that the platinum nano-rod grids provide high resolution data
in both time and space opens up many new possibilities for uncovering new knowledge about how the
brain works.
Another observation enabled by the new grids is uncovering the short and local as well
as long and broad range brain waves associated with brain function all at the same time.
This high- spatial and time-varying (dynamic) picture of the brain activity was documented
in several supplementary movies associated with the Science Translational Medicine paper
and was used to interpret hand motion in new ways utilizing brain wave patterns.