velocities (100 mm/s), or broadly tuned (20-100 
mm/s). No differences were encountered between 
areas and submodalities, suggesting that both SAand 
RA neurons in the somatic sensory cortex encode 
the speed of the tactile stimulus. Psychophysical ex- 
periments in humans and monkeys are in progress to 
determine the capacity to detect and discriminate 
velocities, and whether the responses of somatic 
neurons encountered in the study account for this 
property. 
Responses of Single Neurons of Areas 3b 
and 1 to the Direction of Movement 
Across the Primate's Hand 
An analysis of variance was performed to identify 
the neurons whose discharge rate varied signifi- 
cantly with the direction of a stimulus across the 
receptive field, in the skin of the primate's hand 
(ANOVA, F test,p < 0.05). It was found that -74% 
of the 178 tested neurons responded significantly to 
a certain direction of the movement across the re- 
ceptive field. The distribution of the number of neu- 
rons that responded to the direction of movement is 
similar in areas 3b and 1 . A multiple regression 
model was used to determine whether the discharge 
rate varied with the direction of movement. Sixty- 
seven (32 in area 1; 35 in area 3b) neurons showed 
good fits to the model (p < 0.05; r > 0.7). This 
directional tuning develops as a function of speed. 
Higher directional tuning is found at intermediate 
speeds (20-50 mm/s), and lower below and above 
this range. 
These observations point out that neurons (30%) 
in areas 3b and 1 of the somatosensory cortex of 
awake monkeys possess directional tuning func- 
tions. In other words, the discharge rate is a function 
of the difference between the preferent angle and 
the direction of the stimulus. There is a relationship 
between the directional properties and the speed of 
the stimulus, since a large percentage of neurons 
show directional tuning to speeds in the range of 
20-50 mm/s. It is predicted from these results that 
the signaling of the direction of the stimulus is 
carried out by a neuronal population distributed in 
the somatosensory cortex in the form of a neuronal 
population vector. Experiments are in progress to 
determine the discriminative properties of neurons 
of the somatic sensory cortex as the animal discrimi- 
nates the angle of the direction of movement. 
Dr. Romo is Professor of Neuroscience at the 
Institute of Cellular Physiology, National Autono- 
mous University of Mexico, Mexico City. 
THE GENETICS AND EMBRYOLOGY OF DEVELOPMENT 
Janet Rossant, Ph.D., International Research Scholar 
The aim of research in Dr. Rossant's laboratory is 
to understand the development of early cell lineages 
in the mouse embryo, using embryological and ge- 
netic manipulation. Development of the trophecto- 
derm lineage of the blastocyst, and mechanisms of 
anterior-posterior patterning of the early neural ec- 
toderm, are current areas of interest. 
The Role of Mesoderm-Ectoderm Interactions 
in Neural Patterning 
In other vertebrate embryos, such as Xenopus and 
chick, it is known that signals pass from the underly- 
ing mesoderm to initiate neural induction in the ec- 
toderm of the gastrula stage. Regionalization along 
the body axis also involves regionalized inducing 
signals from the mesoderm. There is little evidence 
on the nature of such interactions in the mouse em- 
bryo. An in vitro explant-recombination system has 
been established for the postimplantation mouse 
embryo, in which pieces of germ layers can be 
grown in isolation or in different combinations. 
With this system, it has been shown that markers of 
mid-hindbrain development, the Engrailed- 1 and -2 
genes, are induced by underlying mesoderm at the 
late-primitive-streak/early-head-fold stage. Only an- 
terior and not posterior mesoderm possesses induc- 
ing ability, but posterior and anterior ectoderm are 
capable of responding. Preliminary data suggest that 
the events leading to the expression of other region- 
alized markers in the brain also occur around the 
head-fold stage of development. 
The Role of Retinoic Acid 
in Anterior-Posterior Patterning 
One factor that may be involved in anterior- 
posterior patterning at the gastrulation and neural- 
plate stages of development is retinoic acid (RA) (or 
other retinoids) . There is considerable circumstan- 
tial evidence for this, based largely on the terato- 
genic effects of excess RA and the presence of RA 
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