Representations and Transformations of Tactile 
Signals in Somatic and Frontal Motor Cortices 
Ranulfo Romo, M.D., Ph.D. — International Research Scholar 
Dr. Romo is Professor of Neurosctence at the Institute of Cellular Physiology, National Autonomous 
University of Mexico, Mexico City. He received his M.D. degree from the National University of Mexico and 
a Ph.D. degree in neuroscience from the University of Paris, France. His postdoctoral work was done with 
Jacques Glowinski at the College of France in Paris, Wolfram Schultz at the University of Pribourg, 
Switzerland, and Vernon B. Mountcastle at the Johns Hopkins University in Baltimore. Dr. Romo has 
received the Demuth Prize of the Swiss Medical Research Foundation and a Guggenheim Memorial 
Foundation fellowship. 
PRIMATES have access to events occurring in 
the external world through specialized sen- 
sory systems. The events are first transduced by 
the sensory receptors and encoded and transmit- 
ted to the central nervous system by the primary 
afferent fibers. These messages are orderly distrib- 
uted and processed in brain centers, where the 
external events are represented and, under some 
conditions, lead to sensation, perception, mem- 
ory, and purposeful motor acts. These phenom- 
ena can only be studied in highly evolved brains. 
Our laboratory is investigating the representation 
of sensory signals in the brain and the mecha- 
nisms by which the motor centers process them 
in order to guide behavior. 
The somatic sensory system of subhuman pri- 
mates appears to be an appropriate model for ap- 
proaching the question of how tactile signals are 
represented in the cerebral cortex, since the 
hands and relevant brain structures are much like 
those of the human. Similar sensory performance 
in somatesthetic tasks has been observed in both. 
Moreover, the exploratory hand movements of 
both primates have similar characteristics, re- 
flecting the fact that their somatic and motor sys- 
tems are similarly linked. These parallels set the 
base for studying the dynamic neural operations 
of the sensory-motor interface — in other words, 
the way sensory representations guide motor 
behavior. 
We have selected an experimental paradigm in 
which monkeys discriminate among sensory stim- 
uli delivered to the skin of their hands as we re- 
cord the associated cortical activity. This allows 
us 1) to define the relevant stimuli among which 
monkeys can discriminate, 2) to follow the trans- 
formation of the initial cortical display in the dis- 
tributed cortical system separating it from those 
cortical areas that drive the difi'erential motor re- 
sponses required for successful execution of the 
task, and 3) to study the details of intracolumnar 
operations in the sensory-association areas of the 
parietal lobe. I will refer to the first point. 
Experiments are in progress to define how the 
direction and speed of a probe moving across gla- 
brous skin of behaving monkeys is represented in 
the activity of somatosensory cortex neurons 
(areas 3b and 1). For this, we have designed and 
constructed a Cartesian robot that allows me- 
chanical stimuli to be presented to the skin of an 
awake primate's hand at specified traverse dis- 
tance, speeds, and directions. We have quantita- 
tively studied many neurons in areas 3b and 1 of 
two alert monkeys performing a behavioral task 
unrelated to the tactile stimuli. The receptive 
fields were scanned with a probe moving at dif- 
ferent speeds in eight directions at preselected 
levels of force exerted by the probe in the skin. 
The first objective in the analysis of the re- 
sponding neurons was to reconstruct the initial 
representations of the peripheral events in areas 
3b and 1 and to identify the possible transforma- 
tions occurring in these stages of the somatic pro- 
cessing system. The preliminary analysis indi- 
cated that it was possible to quantify the 
representation of the physical aspects of the stim- 
uli in the discharges evoked: velocity and posi- 
tion (kinematics) and force (dynamics) . The ac- 
tivity of a large percentage of the recorded cells 
in areas 3b and 1 varied with the speed of the 
stimulus and displayed directional preference. 
This finding suggests that certain sets of neurons 
of the primary somatic areas respond and make a 
neural replica of the mechanical stimuli and that 
there are already some transformations (prefer- 
ence for direction) at this level. Moreover, the 
effects of these two variables (speed and direc- 
tion) were modulated by the force exerted by the 
stimulus on the skin. 
What is interesting in our data is the fact that 
directionality can already be detected at the very 
beginning of the cortical somatic processing sys- 
tem. This finding may explain the presence of 
some nonisomorphic images of complex tactile 
signals in cortical areas 3b and 1 . 
We are presently studying the representation of 
these signals in areas 2, 5, and 7b, applying the 
same experimental protocol. We are also imple- 
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