[acb-hsp] Touching and Hearing

[peter altschul]

How our sense of touch is a lot like the way we he
  Posted by Raymond still December 11, 2012 by Matt Wood
  When you walk into a darkened room, your first instinct is to 
feel around for a light switch.  You slide your hand along the 
wall, feeling the transition from the doorframe to the painted 
drywall, and then up and down until you find the metal or plastic 
plate of the switch.  During the process you use your sense of 
touch to develop an image in your mind of the wall's surface and 
make a better guess for where the switch is.
  Sliman Bensmaia, PhD, assistant professor of organismal biology 
and anatomy at the University of Chicago, studies the neural 
basis of tactile perception, or how our hands convey this 
information to the brain.  In a new study published in the 
Journal of Neuroscience, he and his colleagues found that the 
timing and frequency of vibrations produced in the skin when you 
run your hands along a surface, like searching a wall for a light 
switch, plays an important role in how we use our sense of touch 
to gather information about the objects and surfaces around us.
  The sense of touch has traditionally been thought of in spatial 
terms, i.e.  receptors in the skin are spread out across a grid 
of sorts, and when you touch something this grid of receptors 
transmits information about the surface to your brain.  In their 
new study, Bensmaia, two former undergraduates, and a 
postdoctoral scholar in his lab-Matthew Best, Emily Mackevicius 
and Hannes Saal-found that the skin is also highly sensitive to 
vibrations, and that these vibrations produce corresponding 
oscillations in the afferents, or nerves, that carry information 
from the receptors to the brain.
  The precise timing and frequency of these neural responses 
convey specific messages about texture to the brain, much like 
the frequency of vibrations on the eardrum conveys information 
about sound.  Neurons communicate through electrical bits, 
similar to the digital ones and zeros used by computers.  But, 
Bensmaia said, "One of the big questions in neuroscience is 
whether it's just the number of bits that matters, or if the 
specific sequence of bits in time also plays a role.  What we 
show in this paper is that the sequence of bits in time does 
matter, and in fact for some of the skin receptors, the timing 
matters with millisecond precision."
  Researchers have known for years that these afferents respond 
to skin vibrations, but they studied their responses using 
so-called sinusoidal waves, which are smooth, repetitive 
patterns.  These perfectly uniform vibrations can be produced in 
a lab, but the kinds of vibrations produced in the skin by 
touching surfaces in the real world are messy and erratic.



Responses of an afferent to different stimuli.
  For this study, Bensmaia and his team used a vibratory motor 
that can produce any complex vibration they want.  In the first 
experiment, they recorded afferent responses to a variety of 
frequencies in rhesus macaques, whose tactile nervous system 
closely resembles humans.  In the second part, a group of human 
subjects reported how similar or different two particular 
frequencies felt when a probe attached to the motor touched their 
skin.  When the team analyzed the data recorded from the rhesus 
macaques, they found that not only did the nerve oscillate at the 
frequency of the vibrations, but they could also predict how the 
human subjects would perceive vibrations based on the neuronal 
responses to the same frequencies in the macaques.
  "In this paper, we showed that the timing of spikes evoked by 
naturalistic vibrations matters, not just for artificial stimuli 
in the lab," Bensmaia said.  "It's actually true for the kinds of 
stimuli that you would experience in everyday life." What this 
means is that given a certain texture, we know the frequency of 
vibrations it will produce in the skin, and subsequently in the 
nerve.  In other words, if you knew the frequency of silk as your 
finger passes over it, you could reproduce the feeling by 
stimulating the nerves with that same frequency without ever 
touching the fabric.
  But this study is just part of ongoing research for Bensmaia's 
team on how humans incorporate our sense of touch into more 
sophisticated concepts like texture, shape, and motion.  
Researchers could someday use this model of timing and frequency 
of afferent responses to simulate the sensation of texture for an 
amputee by "replaying" the vibrations produced in an artificial 
limb as it explores a textured surface by electrically 
stimulating the nerve at the corresponding frequencies.
  It could also be used for haptic rendering, or producing the 
tactile feel of a virtual object on a touchscreen (think turning 
your iPad into a device for reading Braille, or controlling 
robotic surgery).  "We're trying to build a theory of what makes 
things feel the way they feel," Bensmaia said.  "This is the 
beginning of a story that's really going to change the way people 
think about the somatosensory system."