MIT Encyclopedia of the Cognitive Sciences - Cryptome

Further Readings
Braun, A. R., T. J. Balkin, N. J. Wesensten, R. E. Carson, M.
Varga, P. Baldwin, S. Selbie, G. Belenky, and P. Herscovitch.
(1997). Regional cerebral blood flow throughout the sleepwake
cycle. Brain 120: 1173–1197.
Hennevin, E., B. Hars, C. Maho, and C. Bloch. (1995). Processing
of learned information in paradoxical sleep: Relevance for
memory. Behavioural Brain Research 69: 125–135.
Hobson, J. A., R. W. McCarley, and P. W. Wyzinki. (1975). Sleep
cycle oscillation: Reciprocal discharge by two brainstem neuronal
groups. Science 189: 55–58.
Hobson, J. A., R. Lydic, and H. Baghdoyan. (1986). Evolving concepts
of sleep cycle generation: From brain centers to neuronal
populations. Behav. Brain Sci. 9: 371–448.
Hobson, J. A., and M. Steriade. (1986). The neuronal basis of
behavioral state control. In F. E. Bloom, Ed., Handbook of
Physiology—The Nervous System, vol. 4. Bethesda, MD:
American Physiological Society, pp. 701–823.
Hobson, J. A., and R. Stickgold. (1995). Sleep, the beloved
teacher. Current Biology 5: 35–36.
Maquet, P., J. M. Peters, J. Aerts, G. Delfiore, C. Degueldre, A.
Luxen, and G. Franck. (1996). Functional neuroanatomy of
human rapid-eye-movement sleep and dreaming. Nature 383:
163.
McCarley, R. W., and J. A. Hobson. (1975). Neuronal excitability
modulations over the sleep cycle: A structured and mathematical
model. Science 189: 58–60.
Nofzinger, E. A., M. A. Mintun, M. B. Wiseman, D. Kupfer, and
A. Y. Moore. (1997). Forebrain activation in REM sleep: An
FDG PET study. Brain Res. 770: 192–201.
Smith, C. (1996). Sleep states, memory processes, and synaptic
plasticity. Behavioural Brain Research 78: 49–56.
Steriade, M., and R. W. McCarley. (1990). Brainstem Control of
Wakefulness and Sleep. New York: Plenum Press.
Smell
The olfactory system is a phylogenetically ancient sensory
system capable of detecting and discriminating among a
vast number of different odorants. Olfaction is critical to the
survival of a variety of lower animals ranging from insects
to mammals, while in humans it has been considered less
important than the other senses. Understanding how the
olfactory system encodes and decodes information is not an
easy task, given the lack of a clear physical energy continuum
to characterize and control stimulus presentation, like
wavelength for COLOR VISION (see VISUAL ANATOMY AND
PHYSIOLOGY) or frequency for auditory pitch (see AUDITION
and AUDITORY PHYSIOLOGY). The situation is made even
more complex by the findings that similar chemical substances
can sometimes have quite different odors and some
substances with altogether different chemical formulas can
smell alike.
The olfactory organ of vertebrates is a complex structure
designed to collect odorant molecules and direct them to the
sensory neurons. Although the chemoreceptive endings and
neural projections of the olfactory nerve are primary to the
sense of smell, other cranial nerves are involved, namely,
the trigeminal, glossopharyngeal, and vagus. These accessory
cranial nerves possess at least some chemoreceptive
endings which line the nose, pharynx, and larynx, giving
Smell 775
rise to the pungent or irritating quality often experienced as
part of an odor sensation.
For much of the animal kingdom olfaction is basic to the
maintenance of life, regulating reproductive physiology,
food intake, and social behavior. In fact, the essence of ANI-
MAL COMMUNICATION is chemical, relying on odors produced
by body glands, feces, and urine. For example, the
male silk moth uses olfactory cues to find his mate, as does
the adult salmon to return to the place where it was spawned.
Many mammalian species, ranging from deer to cats and
dogs, mark their territory with urine or other secretions.
These chemical messages provide the animal sampling the
scent mark with information regarding whether it came from
a conspecific, if the depositor was male or female, dominant
or submissive, and even its reproductive status.
There is also a dependency of reproductive and sexual
behavior on olfactory cues. Introducing the odor of a male
mouse or his urine can induce and accelerate the estrus
cycle of a female. Moreover, appropriate odor cues from a
female are important in attracting the male’s interest during
estrus and promoting copulation. In some species sexual
dysfunction and even retarded development of the sex
organs results when olfaction is compromised.
In humans, the sense of smell is considered less critical
to survival than the other special senses, although the detection
of stimuli such as smoke, gas, or decaying food prevents
bodily harm. Instead, civilized society appears to
emphasize the importance of olfaction on the quality of life.
People attempt to modify attractiveness by adding perfumes
to their bodies and incense to their homes. Consider the
plethora of commercial products for use against “bad”
odors. One instance in which smell plays a major role is in
flavor perception and the recognition of tastes. Much of
what people think they TASTE they actually smell and a
large percentage of people coming to chemosensory clinics
complaining of taste problems actually have smell dysfunction
(consider what happens to food appreciation when a
cold strikes). In fact, disorders of the sense of smell can
often be profoundly distressing, as well as harbingers of
more general disease states.
Although not as extensively documented as in animals, a
relationship between olfaction and sex seems likely in
humans. Olfactory acuity in women seems better at ovulation
than during menstruation and there is evidence that
olfactory cues (i.e., human pheromones) among women can
synchronize the menstrual cycle and that odors serve as
attractants to the opposite sex.
In vertebrates, olfactory receptor neurons (ORNs) differ
in the number and profile of odorants to which they respond
(see NEURON). For example, one electrophysiological study
of single ORN responses to twenty different stimuli demonstrated
that individual neurons responded to as few as two of
the odorants within the panel. Furthermore, despite sampling
over fifty neurons, each had a distinct odorant
response profile. Thus, ORN responses define the range of
odorants that can elicit a response in a given cell (termed its
molecular receptive range [MRR], analogous to the spatial
receptive field in the visual system). Emerging evidence further
suggests that a cell’s MRR may reflect interactions with
particular ligand determinants (compounds with similar