Biomedical Engineering Reference
In-Depth Information
With the sole exception of the olfactory system, all peripheral sen-
sory neurons synapse within domain-specific regions of the thala-
mus, before reaching the primary sensory cortical regions. When
air containing odorant molecules is inhaled into the nasal cavities,
a rich collection of receptor neurons in the olfactory epithelium
(and which later comprise the outermost olfactory nerve layer in
the olfactory bulb) converts the information into electrical form.
As a molecule binds to a matching receptor, an impulse is sent to
the olfactory bulb, which, in the rodent, is located just anterior to
the frontal lobe and is separated into two “identical” hemispheres,
each with a multi-layered organization
(66)
. Signals from recep-
tor neurons arrive at the glomerular layer, where they are inte-
grated by a large number of mitral/tufted cells that project their
axons out of the olfactory bulb onto the olfactory cortex, specifi-
cally piriform and entorhinal cortices, amygdala, and the olfactory
nucleus.
The olfactory bulb, more specifically the glomerular layer, has
been shown “chemotopic maps” with exposure to specific odor-
ants (i.e. responses to different odorants are processed in anatom-
ically distinct locations in the glomerular space) using a variety of
techniques
(67-71)
including fMRI
(26,28,54-56,72,73)
.There-
fore, functional mapping at the level of individual glomeruli is
important and this spatial resolution is quite commonly attainable
with optical imaging
(58,68)
. However, this technical feat has also
been achieved in the past with fMRI using voxel sizes less than 10
nL
(74, 75)
. For the current studies, however, we acquired data
with high BOLD sensitivity in slightly larger voxels to cover the
entire bulb at a temporal resolution superior to most other whole
bulb mapping techniques. Furthermore, the minimally invasive
approach of fMRI allowed repeated perturbations on the same
subject to provide comparison across different experimental runs
(from the same subject) without any averaging.
With our stimulation device, we were able to test response
variability to different odorants (
Figs. 10.2 and 10.3
)aswell
as their concentrations (data not shown). We observed diffuse
BOLD responses in the bulb, where a large proportion of the
bulb was active for any given odorant and the activity patterns
were specific to the odorant type (
Fig. 10.2)
. While we corre-
lated LFP signals to a few medial and lateral BOLD foci (
Fig.
10.3
), understanding the complex frequency components within
the LFP responses and their relationship to the BOLD response
remain a future area of research. Given that fMRI can inherently
detect both hemispheres simultaneously, the hypothesized “iden-
tical” function of each bulb could also be tested in the future.
Since we could not vary the incident direction of the odorants
with the current delivery device, plans are underway to include
spatial specific delivery tubes for directional delivery inside the
4.1.1. Non-tactile:
Olfactory Stimulation
