88 Olfaction and Autism
human, there are two types of neurons that constantly regenerate, olfactory
odor neurons and the dentate gyrus neurons, which are involved in fortifying
our memory [23].
The orbitofrontal cortex plays a particularly important role in the brain’s
processing of olfactory input. This highly multisensory part of the brain is
critical in the perception of flavor, an olfactory experience about which humans
are intensely conscious. The perception of flavor is a particularly multisensory
activity, with the taste (gustatory) component but one of several sensory
contributors. If any of the odorant recognition systems becomes nonfunctional
(sometimes due to an accident that results in a brain injury), food will no longer
taste the same. Therefore, how an item of food feels in the mouth (somatosen-
sory), smells (retronasal olfactory), and looks (visual) also contribute to an
overall unitary perception of flavor. The orbitofrontal cortex, which receives
input from the thalamus via the mediodorsal nucleus is crucial in information
gathering and decision making; it is a highly developed multisensory and
chemosensory area that helps humans perceive flavor and smell.
Olfactory cortical areas, comprised of both six‐layer neocortical and three‐
layer archicortical parts, contribute to the processing of normal memory and
odor perception. They help effect the seemingly fantastic transformation of
a physicochemical substance into a mental perception. When the molecules of a
substance are sufficiently volatile to travel into the nasal passages, the original
substance typically loses an imperceptively small amount of mass. Our olfac-
tory epithelium sensory neurons at the back of the nose, where millions of such
neurons are located, start a bodily process that will initially detect that an odor
is present, then discriminate, further recognize, and finally identify the odor.
In addition, the brain will then remember what has been learned. It is not
inconsequential that 5% of human coding DNA is tied to olfactory ability, and
that humans have roughly 450 different kinds of receptors that, like locks that
have met their odoriferous keys, assist in this complicated process. Dogs, noted
for their olfactory capacity, have about double the number of receptors. They
outperform their human owners at tracking because of these receptors, which
bind the volatile odor molecules [1,2]. Odors are able to induce hedonic
responses, which are based on a person’s perception of how pleasant or
unpleasant an odor is. Two people can smell cinnamon and recognize it as
cinnamon, yet their hedonic responses would vary if one finds the smell of
cinnamon more pleasant than the other.
During gestation, if a synthetic chemical found in fragrances and other
substances enters the developing fetal brains they can be recognized by the
primordial odorant neurons, just like they are recognized in the adult orthona-
sal olfaction process, or by the retronasal olfaction process. Once they bind any
of these particular type(s) of odorant neurons they kill them (just as in the
adult human where a particular neuron degenerates just after stimulation of an
electrical signal). However, at early stages of gestation (in the developing fetal