SCIENTIFICAMERICAN.COM | 29cognitive function and, in the more distant future, to
enhance sensory, cognitive or memory capacities. For
example, a recent brain-machine interface provides
completely blind people with limited abilities to per-
ceive light. These tools, however, also reveal the difficul-
ties of fully restoring sight or hearing. They underline
even more the snags that stand in the way of sci-fi-like
enhancements that would enable access to the brain as
if it were a computer storage drive.ANIMAL ELECTRICITY
Nervous systems operate on the flow of electric cur-
rents through ultradense and hyperconnected networks
of switching elements. Countless physicians and scien-
tists have worked on this problem over the past two and
a half centuries, beginning with Italian physician Luigi
Galvani, who in the late 18th century connected a freshly
killed frog to a long metal wire. By pointing the wire
toward the sky during a thunderstorm, he made the frog’s
leg jump and twitch with each flash of lightning. Gal-
vani’s investigations re veal ed that nerve fibers transmit-
ted “animal electricity,” which is no different in kind from
the “atmospheric electricity” that Benjamin Franklin
demonstrated with his kite experiments in Philadelphia
in 1752. In 1802 Galvani’s nephew Giovanni Aldini elec-
trically stimulated the exposed brain of a decapitated
prisoner during a public event. A jaw quivered. An eye
opened. The spectacle may have helped to inspire Mary
Shelley to write the classic gothic novel Frankenstein.
Subsequent animal studies demonstrated that excit-
ing particular brain regions triggered movements in spe-
cific muscles and limbs. These investigations led to the
discovery of the motor cortex in the 1870s. In 1874 Amer-
ican physician Robert Bartholow performed the first
direct brain stimulation of a conscious patient—a pio-
neering act clouded in ethical controversy because it
caused the patient pain and probably hastened her death.
Intracranial electrical stimulation (iES) was re fined over
the following decades. It became part of the neurosur-
geon’s toolbox thanks to the ground-breaking work of
Wilder Penfield of the Montreal Neurological Institute,
who between the 1930s and the 1950s used iES to map
cortical areas that process motor or sensory functions.
In some people with epilepsy, drugs fail to adequately
control the number or severity of seizures. Neurosurgery
becomes an option if those seizures originate in a delim-
ited neighborhood in the cortex—the outermost layer of
the brain involved in perception, motor control, speech,
reasoning, and so on—or in closely related structures,
such as the hippo campus. Uncontrolled hy per ex cit a bil-
ity starts because of local faulty wiring. It can grow and
eventually engulf the rest of the brain. How much tissue
to remove is a dilemma: cut too little, and seizures may
continue; cut too much, and the patient may lose the abil-
ity to speak, see or walk. Surgeons must avoid areas of
the cortex that are crucial for everyday behavior, such as
the primary auditory, visual, somatosensory and motor
cortices and the regions controlling understanding and
producing speech, areas known as the eloquent cortex.SomatomotorVisualDorsal attentionSalienceFrontoparietalLimbicDefault“My left foot
shifted to the right
and the sensation
went all the way
up my calf.” “I feel
tingling in
my leg.”“I see a star
in the top right
center. It was a blue
and silver star.”“I just had the
urge to squeeze
my fingers.
They just closed
by themselves.”“Pulsating feeling,
mostly in the left
chest but also in
the left arm.”
“Just really couldn’t
move (my fingers) too
much; lost the motion. The
hand felt a little tight, but
the thumb was out
of commission.”“I felt like my
arms were moving
but they weren’t. I felt
side-to-side movements,
like floating
in the air.”“Feels like
I’m going in a
circle. Everything is
still and I’m moving.
Makes me
feel sick.”“You just
turned into
somebody else. Your
face metamorphosed.
Your nose got saggy;
went to the left.
Not pretty.”“For being in
this temperature,
just a little more
sensitive. Almost as if
I was in a colder
temperature.”“It smells
funny.
Negative, like
nail polish.”Patient
describes a
feeling of nervous
anticipation.Patient cannot
repeat
“If she comes,
I will go.”Speech
arrest.“Aroused,
calm ...
sexually excited.”Patient
says she
felt dizzy.Patient
describes
a feeling
of fear.Patient
describes a negative
emotional feeling,
seemingly localized
in the chest.
“That
felt good; quite
erotic. I can’t even,
um, I felt good.
I can’t explain it.
Um, yeah, you’re
embarrassing me!”Patient laughs
and says they
“felt well
internally.”Where Experiences
Live in the Brain
An atlas published in 2020 compiled the verbal reports of people with
epilepsy whose cortical areas were stimulated with electrodes during
surgery. What they felt and perceived varied depending on which
brain region was stimulated. All of the 1,537 locations in these 67
patients where current was applied were mapped onto a digital brain
model, a simplified version of which is depicted here. When stimulated
at these sites, patients talked about their experiences.Colors of Cognition
Stimulation was applied to varied
brain networks and regions.Points of Stimulation
Researchers placed dozens of
electrodes onto the surface
of the cerebral cortex accessed
through an opening
in the skull.Illustration by AXS Biomedical Animation Studio