Alex T. L. Leong1,2, Xunda Wang1,2, Russell W. Chan1,2, Karim El Hallaoui1,2, and Ed X. Wu1,2
1Laboratory of Biomedical Imaging and Signal Processing, The University of Hong Kong, Hong Kong, China, 2Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, China
Synopsis
The sensory system is topographically
organized by highly interconnected excitatory thalamo-cortical and
interhemispheric cortical-cortical projections. However, little is known at present regarding
the existence of spatiotemporal neural activity pattern(s) that may dictate the
propagation of activity between cortices at both hemispheres. Here, we employed
a novel paired-pulse optogenetic fMRI stimulation paradigm to reveal a
temporally-specific neural activity pattern initiated from the somatosensory
thalamus that could drive activity propagation from the ipsilateral to
contralateral cortex. We found
that propagation from the ipsilateral to contralateral somatosensory cortex was
facilitated when paired optogenetic stimulation pulses were spaced at 100ms,
but not 50ms.
Purpose
One hallmark of long-range excitatory projections
in the brain is the presence of the corpus callosum, a massive white-matter
structure that interconnects the two cortical hemispheres1. Interhemispheric
callosal/cortico-cortical projections predominantly interconnect sensory
cortices2. Recently,
it has been shown that neural activity inputs from the thalamus are critical for
the development of inter- and intracortical laminar organization3. These
studies suggest the importance of structural and functional integrity in the
thalamo-cortical and interhemispheric cortico-cortical projections as
prerequisites for normal sensory processing. However, little is known at present regarding
the spatiotemporal neural activity pattern(s) initiated from the thalamus that may
dictate the propagation of activity between the sensory cortices at both
hemispheres. In this study, we employed a novel paired-pulse optogenetic fMRI
stimulation paradigm to activate ventral posteromedial thalamus (VPM)
excitatory thalamocortical neurons. We aim to reveal the temporally-specific neural activity pattern that could drive
activity propagation from the ipsilateral to contralateral hemisphere using
the well-defined topographically-organized somatosensory thalamo-cortical network as our model.Methods
Animal preparation and optogenetic stimulation:
3μl of AAV5-CaMKIIα::ChR2(H134R)-mCherry
was injected to VPM of adult rats (200-250g, male, SD strain, n=6; Figure 1a). Four weeks after injection,
an opaque optical fiber
cannula (d=450μm) was implanted at the injection site (Figure 1b). Blue (473nm) light was presented to animals expressing
ChR2 with a paired-pulse paradigm (10ms pulse width, interstimulus interval, ISI=100ms
or 50ms, 40mW/mm2; Figure 1c).
fMRI acquisition and analysis:
fMRI data was acquired at 7T using GE-EPI (FOV=32×32mm2,
matrix=64×64, α=56°, TE/TR=20/1000ms, 16 contiguous slices with 1mm thickness).
Data were preprocessed before coherence analysis was applied to identify
significant BOLD responses (p<0.001). BOLD signal profiles were extracted
from anatomically defined ROI.
Electrophysiological
recordings and analysis: Recordings
at bilateral S1 were performed using linear microelectrode arrays (n=3; 16 electrode
channels equally spaced at 100μm, 1.5MΩ; sampled at 30kHz). Recorded data was downsampled
to 2kHz, band-pass filtered (1-300Hz) and notch filtered for 50Hz before
further local field potential (LFP) and current source density (CSD) analyses were
performed.
Results
Pulsed optogenetic stimulation at the VPM
thalamocortical excitatory neurons activated the ipsilateral primary
somatosensory cortex (S1; Figure 2). However, the BOLD response was stronger
(4% vs. 2%), reached its peak later (8s vs. 7s) and prolonged (25s vs. 14s) when
the ISI of the paired-pulse was 100ms in comparison to 50ms. This finding
indicates differences in the underlying evoked neural activity in ipsilateral
S1. Interestingly, we detected robust BOLD activation at the contralateral S1
when the ISI was 100ms, but not 50ms.
Furthermore, we examined the spatiotemporal characteristics of neural
activity propagation between the bilateral S1, initiated at the VPM. We
performed multi-channel depth electrophysiological recordings at the
ipsilateral and contralateral S1 (Figure 3a). We measured a latency of
~13ms between the onset of the first optogenetic pulse and the first evoked LFP
in L4; indicating direct propagation through thalamo-cortical projections (Figure
3b). An additional latency of ~6ms was measured before the first evoked LFP
was recorded at L2/3 and L5 of the contralateral S1. These results corroborate
the conduction delays across interhemispheric cortico-cortical projections4,5. We then generated CSD maps from the LFP recordings (Figure 3b, c).
It was observed that long-lasting current source and sink patterns (~50ms) were
successfully evoked across the layers in the bilateral S1 by the paired optogenetic
pulses with ISI=100ms (Figure 3b). However, only the first optogenetic
pulse evoked such a source and sink pattern with ISI=50ms (Figure 3c).
Discussion and Conclusion
In this study, we demonstrate that by
probing the somatosensory thalamo-cortical network with a paired-pulse stimulation
paradigm, a temporally-specific interhemispheric neural activity propagation
pattern was revealed. We found that robust positive BOLD responses were weaker
and shorter in ipsilateral S1, and absent in the contralateral S1 when the ISI
was decreased from 100ms to 50ms. These findings indicate that the underlying neural
activity that occurs at long time-scales may be absent. Multi-channel depth
recordings revealed that when ISI=50ms, ipsilateral S1 did not respond to the second
optogenetic pulse and bilateral S1 lacked the characteristic late neural
activity (>100ms after onset of stimulation; Figure 3b, c). Interestingly, we observed
long-lasting neural activity (~50ms) evoked by the first optogenetic pulse across all S1
layers. This suggests the presence of recurrent cortical activity, which has
been reported to require long-time windows for integration across layers6. Such activity in S1 may impede any subsequent response
to the second optogenetic pulse at ISI=50ms and disrupt propagation to the contralateral
S1. In conclusion, our work offers a novel optogenetic fMRI approach to
investigate neural activity propagation dynamics in the brain. Acknowledgements
This work was supported by the Hong Kong Research Grant Council (Grants C7048-16G and HKU17103015 to E.X.W.).References
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