Prasanna Karunanayaka1, Xin Zhang2, Michael Tobia1, Jianli Wang1, Bin Zhang2, Bin Zhu 2, and Qing Yang1
1Radiology, Penn State University, Hershey, PA, United States, 2The affiliated Drum Tower hospital of Nanjing university medical school, Nanjing, China, People's Republic of
Synopsis
Behavioral studies show that human odor perception is
highly dynamic, incorporates both spatial and temporal codes, and is easily
influenced by information from other sensory systems such as vision. However,
the neural representation of odor perception and its dynamic processing by the
brain is poorly understood. In this research, using olfactory task fMRI,
we attempt to unravel how olfactory-related neural networks interact in both
space and time in order to explore how the olfactory and the visual systems integrate information at the central or perceptual levels in the
human brain.Introduction/Purpose
Odor stimulation strongly influences
human memory formation. The role of odors in learning and memory, and the
mechanisms by which visual information influence odor-memory formation,
however, remains poorly understood [1, 2]. In this
study we used neutral visual cues (i.e., symbols “# “and “*”) that were
either paired or unpaired with an odor to determine the dynamic pattern of
brain activity and connectivity of odor-visual association in the human brain.
Using olfactory fMRI and Independent Component Analysis (ICA), we provide new
information about the functional circuitry that may be responsible for forming
rapid olfactory-visual associations in the human brain [2].
Methods
20
healthy subjects (mean age 25 ± 10 yrs.) took part in the fMRI study at the
Gulou Hospital in China with IRB approval. Using the OLFACT™ Computerized Test
Battery, the smell identification ability and odor threshold levels of study
participants were tested and deemed normal. All participants completed the odor-visual olfactory
fMRI paradigm shown in Figure 1. This
paradigm was specifically designed to investigate whether an odor-visual
association could be established after pairing an odor with a neutral visual
cue, i.e., the “#” symbol. An unpaired visual cue, the symbol “*”, was
presented randomly during the paradigm. These two symbols were selected because
they are not likely to be associated with any semantic or affective attributes
of human olfactory function.
An
MR compatible olfactometer with a flow rate of 8 L/min and synchronized with image
acquisition and visual cues was used for stimulus presentation. MR images of
the entire brain were acquired using EPI on a Philips 3T scanner with the
following parameters: TR / TE / FA= 2000 ms / 30 ms / 90°; FOV = 220 mm x 220
mm; acquisition matrix= 80 x 80, 30 slices; slice thickness= 4 mm, and the
number of repetitions= 310.
The
group ICA analysis was based on FastICA algorithm and performed according to
the methods outlined elsewhere [3, 4]. ICA is a data driven method that can
generate scale- and statistically-independent spatial patterns of odor-related
fMRI activation with unique temporal behavior. Using individual IC time
courses, the associated hemodynamic response function (HRF) and the
single-trial response of each IC network was evaluated using methods described
in Eichele et al. [5].
Results
Figure
2 [(a) and (b)] shows
two task-related group IC maps that subserve odor-visual association fMRI paradigm.
These IC maps encompass: (a) primary olfactory cortex (POC), amygdala and
hippocampus and (b) lingual and fusiform gyri.
We quantified the respective IC network
responses during odor-visual association in terms of average β estimates as
shown in Figure 2c. Both primary
olfactory and visual networks showed similar pattern of response decrease
during this paradigm (i.e., roughly exponentially decaying). In IC1 and IC2, the differences in β values for encoding and retrieval 1
conditions did not reach statistical significance after correcting for multiple
comparisons. However, in both networks, the β values for the encoding conditions
were significantly different from retrieval 2 and during conditions where the unpaired
visual-cue ‘*’ was presented. Additionally, for IC 1, the β values for encoding were significantly correlated with
the β values of the retrieval 1 conditions (r=-0.491517,
p< 0.02).
Discussion/Conclusion
Our results demonstrated that a neutral
visual-cue, which was previously paired with an odor, can, in fact, evoke BOLD
activation in primary olfactory cortex in addition to the primary and
associative visual cortex. This process seems to be highly dynamic and rapid in
terms of the time-course signal in respective olfactory and visual systems [2].
Our results provide preliminary evidence for odor-visual association because of
the negative correlation between encoding β values and retrieval 1 β values. Taken
together, these results suggest both a rapid and significant interaction
between olfactory by visual processing in the human brain. In turn, this will
help ask new questions about multi-sensory integration that were not feasible
before.
Acknowledgements
The study was supported by the Department of Radiology at Penn State, George M.Leader Foundation and a grant from the U.S.National Institute of Aging, R01-AG027771References
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