suggests that while MT signals provide a part of the 
foundation for this class of visuomotor behavior, 
other pathways as yet undescribed physiologically 
must also contribute. 
Dr. Movshon is also Professor of Neural Science 
and Psychology at New York University and Ad- 
junct Professor of Physiology and Biophysics at 
New York University School of Medicine. 
Books and Chapters of Books 
Landy, M.S., and Movshon, J.A., editors. 1991. 
Computational Models of Visual Processing. 
Cambridge, MA: MIT Press. 
Articles 
Movshon, J.A., and Newsome, W.T. 1992. Neural 
foundations of visual motion perception. Curr 
Dev Psycholog Sci 1 :36-39. 
Skottun, B.C., De Valois, R.L., Grosof, D.H., Mov- 
shon, J. A., Albrecht, D.G., and Bonds, A.B. 1991. 
Classifying simple and complex cells on the basis 
of response modulation. Vision Res 31:1079- 
1086. 
SIGNAL TRANSDUCTION AND PROCESSING IN THE OLFACTORY SYSTEM 
Randall R. Reed, Ph.D., Associate Investigator 
The mammalian olfactory system is an exquisitely 
sensitive sensory organ responsible for encoding in- 
formation on the intensity and the identity of chemi- 
cal stimuli. The initial events in olfactory signal 
transduction occur in a complex sensory organ, the 
nose. Molecules that comprise the chemical stimuli 
perceived as odors are first solubilized and concen- 
trated by protein components of the aqueous me- 
dium that bathes the tissue. The neuroepithelium 
that lines the nasal cavity contains the sensory neu- 
rons responsible for the conversion of the external 
stimulus into an electrical signal. Each of these sen- 
sory neurons extends a dendritic process to the lu- 
minal surface, where a small number of cilia extend 
into the mucous layer. These cilia, the presumed 
site of odorant recognition, likely contain the ma- 
chinery required for signal transduction. Consider- 
able electrophysiological evidence has accumu- 
lated to suggest that individual receptor neurons 
respond differently to each odor. The axons extend 
from the cell bodies of the sensory neurons located 
in the epithelium and project on particular second- 
order neurons in the glomerular tufts of the olfac- 
tory bulb. 
One of the most remarkable aspects of the olfac- 
tory neurons is their ability to be replaced from a 
population of precursor cells. This replacement of 
olfactory neurons from neuroblast precursors oc- 
curs continually in adult animals. More signifi- 
cantly, acute injury to the olfactory bulb or to the 
receptor neurons leads to the rapid loss of the sen- 
sory cells and their subsequent, synchronous re- 
placement. Complex regulatory mechanisms must 
underlie this neuronal replacement and the forma- 
tion of appropriate connection between the epithe- 
lium and the olfactory bulb. 
The Mechanism of Olfactory Signal 
Transduction: A G Protein-coupled Cascade 
Receptor proteins present in the cilia membranes 
of the sensory neuron are presumed to provide the 
specificity of odorant recognition. These receptor 
proteins might then converge on a common intra- 
cellular pathway. The membrane-bound receptors 
that couple to G proteins in a wide variety of systems 
share considerable structural similarities. Each 
member of the family is glycosylated and crosses the 
membrane seven times. Recently, Drs. Linda Buck 
and Richard Axel (HHMl, Columbia University) 
have identified several new members of a large G 
protein-coupled receptor family expressed exclu- 
sively in olfactory epithelium that likely encode the 
odorant receptor proteins. Currently, Dr. Reed's lab- 
oratory is examining the genomic structure of this 
family to understand the mechanism by which ex- 
pression of individual receptors is directed to partic- 
ular cells. Preliminary results indicate that an indi- 
vidual receptor is expressed in perhaps only 0. 1% of 
the sensory neurons. Expression of these receptor 
proteins in heterologous systems may help to eluci- 
date the relationship between the structure of the 
receptors and the odorant ligands that they bind. 
Dr. Reed has identified several other components 
in the presumptive pathway for olfaction. The labo- 
422 
