The Brain Senses
Touch beyond
the Body
You detect a tool’s contact
with an object as if you placed
your own finger on it
Luke Miller, a cognitive neuroscien-
tist, was toying with a curtain rod in
his apartment when he was struck by
a strange realization. When he hit an
object with the rod, even without
looking, he could tell where it was
making contact like it was a sensory
extension of his body. “That’s kind
of weird,” Miller recalls thinking to
himself. “So I went [to the lab], and
we played around with it in the lab.”
Sensing touch through tools is not
a new concept, although it has not
been extensively investigated. In the
17th century, philosopher René
Descartes discussed the ability of
blind people to sense their surround-
ings through their walking cane. While
scientists have researched tool use
extensively, they typically focused on
how people move the tools. “They, for
the most part, neglected the sensory
aspect of tool use,” Miller says.
In a 2018 Nature study, Miller and
his colleagues at Claude Bernard
Lyon 1 University in France reported
that humans are actually quite good
at pinpointing where an object
comes into contact with a handheld
tool using touch alone, as if the
object were touching their own skin.
A tool is not innervated like our skin,
so how does our brain know when
and where it is touched? Results
in a follow-up study, published last
December in Current Biology, reveal
that the brain regions involved with
sensing touch on the body similarly
process it on the tool. “The tool is
being treated like a sensory exten-
sion of your body,” Miller says.
In the initial experiment, the
researchers asked 16 right-handed
subjects to determine where they
felt touches on a one-meter-long
wooden rod. In a total of 400 trials,
each subject compared the locations
of two touches made on the rod: If
they were felt in different locations,
participants did not respond. If they
were in the same location, the people
in the study tapped a foot pedal to
indicate whether the touches were
close or far from their hand. Even
without any experience with the rod
or feedback on their performance,the participants were, on average,
96 percent accurate.
During the experiment, researchers
recorded subjects’ cortical brain
activity using scalp electrodes and
found that the cortex rapidly pro-
cessed where the tool was touched.
In trials in which the rod was touchedin the same location twice in a row,
there was a marked suppression
of neural responses in brain areas
previously shown to identify touch
on the body, including the primary
somatosensory (touch) cortex and
the posterior parietal cortex.
There is evidence that when the LOTTIE CLARKSomatosensory system, running from brain to hand, extends imaginatively outward and
into the stick the woman holds.NEWS