Cracking the
Neural Code with
Phantom Smells
Scientists used light to evoke
an odor directly in a mouse brain—
no nose involved
In the opening of The Matrix, columns
of strange keyboard characters
stream down an old monochrome
computer screen. They represent the
peeled-back digital curtain of experi-
ence, reminding us that every taste,
smell and color that we experience
is, in a way, a deception—a story
computed bit by literal bit in a brain
working in the quiet darkness of the
skull. We don’t need special hard-
ware to enter the Matrix. We just
need to understand the special
hardware we’ve been given: our brain.
The reason we can’t bend experi-
ence to our liking, Matrix-style, is that
we don’t really understand the neural
code. There’s no Alan Turing for the
brain who can study an arbitrary
pattern of brain activity and say,
“Right now an image of a beige cat is
being experienced.” Neuroscientists
know that the specific contents of a
sensory experience have to do with
the timing and/or spatial patterning
of brain activity. But when put to
the test, with even the most basic
mechanistic questions, our ignorance
quickly shows itself. If a couple of
brain cells had fired a half-second
earlier, would you still see the beige
cat? What if three additional cells
had fired in quick succession? In
response, all neuroscientists could do
was shrug and make some genericclaims about codes, patterns and
the likely importance of timing. But
Dmitry Rinberg of New York Univer-
sity and his research group may have
just uncovered a partial answer.
In a fascinating recent paper, the
researchers used precisely controlled
pinpoints of light to directly insert
a phantom smell into a mouse’s
olfactory brain centers, bypassing the
nose altogether. They were also able
to systematically adjust that patternand test how the animal’s experience
changed. The study is one of the
most audacious and systematic
efforts at “experience hacking” yet.
Implanting a specific, reproducible,
easily adjustable and completely
synthetic percept is no small feat.
To do so, Rinberg and his colleagues
used genetically modified mice with
a light-sensitive channelrhodopsin
protein smuggled into their olfactory
neurons. When light shines on one
of these modified neurons, it evokes
neural activity—the brief electrical
“spikes” that are the basic language
of the nervous system—with timing
that can be exquisitely controlled.
Because the part of the brain that
processes sensory information from
the nose is conveniently located
near the surface of the skull, the
researchers were able to skip the
nose and write in an artificial odor
of their own design. By stimulating
the olfactory brain directly, the team
essentially had complete control
over which cells were active, what
their arrangement was and when
they were activated. The scientists
had created odors made to order
with the flip of a simple light switch.
Most natural smells will evoke
widespread and temporally complex GETTY IMAGESNEWS