86 Olfaction and Autism
The human olfactory system is extensively represented in the cortical areas
of the brain. This intricate system allows people to perceive odor and to
remember it. When they inhale, or exhale, communication between cortical
areas facilitates this perception and categorization of odors. The olfactory
areas in the so called “gray matter” of the cerebral cortex are known to be sensi-
tive to disease. Perception can be disrupted by many disorders, including
autism, Parkinson’s disease, depression, and Alzheimer’s disease. Exposure to
toxins early in life can also disrupt odor perception. Most of the cortices
responsible for so called primary sensing are, in fact, multisensory. In olfaction
in humans and other mammals, the sensory cortex is located near the brain’s
periphery. The brain senses and remembers smell through coordinated assis-
tance of olfactory sensory neurons (OSNs), the olfactory bulbs, and the olfac-
tory cortex. Located between the eyes, the olfactory bulbs transmit signals that
allow the brain to register the smell of odors that enter the nose (Figure 3.1).
This intermediary role between nose and brain, as remarkable as it is, is just
one reason why the olfactory bulb (the singular term bulb is often used to rep-
resent the plural reality of bulbs) is helpful to humans, and to researchers.
It also plays a barometer‐like role in diagnosis of diseases. When an olfactory
bulb does not function well, the chances are that the human involved has other
serious problems besides an inadequate sense of smell. Given the obvious and
visible importance of the senses of sight and hearing, it may come as no sur-
prise that the sense of smell has been seriously understudied, but that has been
changing in the recent past. As we will describe in detail, in a typical adult there
are only two known areas in the brain that have the capacity to regenerate:
olfactory cells; and dentate gyrus (see below). Our environment contains mil-
lions of odors, which are combinations of chemical odorants. Since there are so
many faculties of the brain that are intimately involved in olfaction, there must
be multiple mechanisms through which early sensory dysregulation in ASD
could lead to social deficits throughout brain development, before and after
birth. Future research is needed to clarify these mechanisms, and specific focus
should be given to distinguish between deficits in primary sensory processing
and altered top‐down attentional and cognitive processes.
The olfactory system can detect and identify many thousands – and perhaps
millions – of odorant molecules. No one knows exactly how many. Just imagine
that each human possesses over 400 different odor recognition neurons. It is
like there being over 400 different letters in an alphabet. How many words
could be made from these 400+ letters? Odorants are small molecules that
easily evaporate and become airborne. When one inhales the air, odorant
molecules are drawn into the nose to enter the nasal cavity, entering a complex
system of nasal passages (Figures 3.2).
Lining a portion of the nasal cavity is the olfactory epithelium, a thin sheet of
mucus‐coated sensory neurons that contains the olfactory receptor cells, along
with supporting cells and basal (stem) cells.