MEG source imaging detects optogenetically-induced activity in cortical and subcortical networks

Abstract

Magnetoencephalography measures neuromagnetic activity with high temporal, and theoretically, high spatial resolution. We developed an experimental platform combining MEG-compatible optogenetic techniques in nonhuman primates for use as a functional brain-mapping platform. Here we show localization of optogenetically evoked signals to known sources in the superficial arcuate sulcus of cortex and in CA3 of hippocampus at a resolution of 750 µm³. We detect activation in subcortical, thalamic, and extended temporal structures, conforming to known anatomical and functional brain networks associated with the respective sites of stimulation. This demonstrates that high-resolution localization of experimentally produced deep sources is possible within an intact brain. This approach is suitable for exploring causal relationships between discrete brain regions through precise optogenetic control and simultaneous whole brain MEG recording with high-resolution magnetic source imaging (MSI).

Fig. 1. Location of optogenetic protein expression and light evoked responses.

a Cortical transduction and implantation. i ChR2-eYFP labeled neurons confined to gray matter of the posterior wall of right arcuate sulcus corresponding to area 8a. Coronal (ii) and sagittal (iii) MRI demonstrating location of the optrode in terminal MRI (arrow). b Hippocampal transduction and implantation. i ChR2-eYFP labeled neurons in the anterior dorsolateral aspect of left hippocampus corresponding to area CA3 indicate transgene expression; inset demonstrating labeled neuron. Coronal (ii) and sagittal (iii) MRI demonstrating location of the optrode in terminal MRI (arrow). c Example of averaged response to stimulation at five intensity levels (gray to black) in hippocampus showing absence of response in week 2 (upper trace) compared to the large, biphasic LFP in week 7 (lower trace). Blue bar indicates stimulus onset and duration. d Average intensity-response curves for each of the three-time points in representative animal M1.Bands = 95% CI. e Maximum intensity stimulation (9.53 mW) of cortex and hippocampus at 7 weeks post-injection elicited a response as measured by the LFP electrode that was greater than at 2 weeks post-injection in representative animal M1. (n = 20 trials, except n = 10 trials for cortex at 5 weeks). Center line = mean, error bars = 95% CI.

Fig. 2. SAM source localization of optogenetically evoked MEG responses in the right arcuate sulcus in two representative animals and stimulation types.

a An example of optogenetic stimulation localized by dual-state SAM to the posterior bank of right arcuate sulcus for NHP M1 in the coronal (i), sagittal (ii), and axial planes (iii), for a stimulus of 50 ms square light pulses. The whole brain, un-thresholded, dual-state SAM SPM (voxel size of 750 µm³, iv) reveals synchronized activity (red voxels) arising from the stimulation site and surrounded by desynchronization (blue voxels). Green dot and number indicate a peak in the SPM plus the associated pseudo-t value. b Right SAM arcuate peaks for four additional and different optical stimuli in two different NHPs. (bi–ii) present arcuate activations for M1 and (iii-iv) shows arcuate stimulation for M2. (bi–ii) is the same subject (M1) as presented in (ai–iv); (i) depicts the arcuate peak for 8 Hz sine waves, and (ii) shows the arcuate peak for 40 Hz sine waves. Arcuate stimulations in M2 are presented in (iii–iv); (iii) shows the arcuate peak for 10 ms square pulses and (iv) shows the arcuate peak for 20 Hz square wave pulse trains. A single trial (v) of an LFP recorded by the optrode and a simultaneous SAM virtual electrode for an arcuate peak (as seen in biii). Blue vertical line indicates stimulus onset (10 ms pulses). Both the LFP (white) and virtual electrode (red) exhibit a rapid peak following stimulation and have similar features and time courses. One gray square in the graph = 100 ms on the abscissa and for the ordinate, 35 μV for the LFP or 15 nA m for the virtual electrode. All maps follow radiological conventions.

Fig. 3. SAM source localization of optogenetically evoked MEG responses in the left hippocampus.

a SAM left hippocampal peak in coronal (i), sagittal (ii), and axial planes (iii) for a 20 Hz sine wave input in NHP M3. The whole brain, un-thresholded, SAM SPM shows synchronized activity arising from the stimulation site and surrounded by desynchronization (iv). b Left SAM hippocampal peaks for four additional and different stimuli in two different NHPs (M1 and M3). (bi–ii) is the same subject (M3) as presented in (a); (i) and (ii) show hippocampal peaks for single 150 ms ramp trials or 10 ms square pulses, respectively. (biii–iv) show hippocampal peaks for 8 Hz or 40 Hz sine waves, respectively, in the same subject (M1) as in Fig. 2a. A single trial (v) of a simultaneous LFP and SAM virtual electrode (both from 1 to 35 Hz) for hippocampus (as seen in bi). Blue vertical line indicates stimulus onset (ramp inputs). Both the LFP (white) and virtual electrode (red) peak rapidly following stimulation and have similar features and time courses. One gray square = 50 ms (abscissa) and 5 V for the LFP (amplified) or 500 nA m for the virtual electrode (ordinate).

Fig. 4. Arcuate optogenetic stimulation results in propagation through conserved networks across animals.

a Identification of cortico-striato-pallido-thalamo-cortical (CSPTC) network activity in response to 8 Hz sine wave stimulation of right arcuate sulcus in NHP M1. SAM analysis of optical input to arcuate sulcus reveals a known functional network with peaks in: (i) right inferior bank arcuate sulcus, (ii) right superior bank arcuate sulcus, the site of stimulation, (iii) right corona radiata/dorsolateral caudate n., (iv) right caudate n. (v) right putamen/ external segment of globus pallidus, (vi) and (vii) posterior bank of contralateral (left) arcuate sulcus, coronal and sagittal slices, respectively, (viii) right globus pallidus, (ix) right ventroposterior medial/ventroposterior lateral n. of thalamus, (x) right medial dorsal n. of thalamus, (xi) right ventral posterior thalamus, and (xii) parietal-occipital association area of the intraparietal sulcus. b A second example of right hemispheric CSPTC network activity in a different NHP (M2) in response to 20 Hz square wave pulse trains with SAM peaks as follows: i caudate n. (green) and cortical area 3 (yellow), (ii) thalamus (green) and caudate n. (yellow), (iii) posterior dorsal bank of the arcuate sulcus (green) and two thalamic peaks (yellow), (iv) thalamus, (v) posterior ventral bank of arcuate sulcus (green, site of stimulation), corpus callosum (yellow), and caudate n. (yellow), (vi) caudate n. (green) and mesencephalic nuclei (yellow), (vii) mesencephalic n., (viii) substantia nigra, (ix) tegmental n., (x) pontine or mesencephalic n., and (xi), and (xii) all peaks in primary motor cortex (area 4).

Fig. 5. Hippocampal optogenetic stimulation results in propagation through downstream networks across animals.

a Functional hippocampal and temporal networks in NHP M3 engaged by a 20 Hz optical sine input to left hippocampus (v) with SAM peaks as follows: (i) left insular proisocortex/temporopolar proisocortex, (ii) left para-insular cortex, (iii) left dorsolateral amygdala (lateral n.), (iv) left basolateral/lateral amygdala, (v) left hippocampus, site of stimulation, (vi) left parasubiculum/presubiculum of hippocampus, (vii) contralateral (right) hippocampus, and (viii) left pontine reticular activating formation. b A second example of hippocampal network activity in a different NHP (M1) in response to 8 Hz sine wave input with SAM peaks as follows: (i) left putamen, (ii) PGa/IPa association areas of left temporal cortex (lateral yellow peak), left hippocampus (green peak, site of stimulation), and possible left red n. (yellow, medial peak), (iii) area TEA/TEM of left temporal cortex, and (iv) area TL/TFM of right temporal cortex. c A third example of hippocampal network activity in response to 40 Hz sine wave input (same subject as in b) with SAM peaks as follows: (i) left anterior hippocampus, (ii) left hippocampus, site of stimulation, (iii) left deep mesencephalic n., and (iv) left pulvinar n. of thalamus.