2Department of Neuroscience, Brown University, Providence, Rhode Island; 3Institute for Brain Science, Brown University, Providence, Rhode Island; 5Center for Neurorestoration and Neurotechnology, Department of Veterans Affairs Medical Center, Providence, Rhode Island; and
Polymer optical fiber microelectrode array (POF-MEA). A: POF-MEA viewed from the electrode side (top left), from the pad side (bottom left), and attached to two pedestals (Cereport, Blackrock Microsystems) for optical and electrical connections, respectively (right). B: schematics of the POF-MEA; an optical fiber (green) was integrated around the center of the 10 × 10 MEA. C: neural recording and optical stimulation setup. NSP, neural signal processor; DAQ, data acquisition system. D: Monte Carlo simulations of light distribution in the brain under 6-mW laser power shows isointensity contours at 1, 5, and 20 mW/mm2, respectively. (For illustration purpose, microelectrode separation, but not length, is on scale.) For details on the simulation parameters see Wang et al. (2012) and Ozden et al. (2013).
Gamma oscillations induced by constant (square) pulse light stimulation. A: low-pass (<300 Hz) filtered local field potentials (LFPs) from 2 representative recording sites in subjects T (top) and P (bottom). Green bar indicates the duration of 1-s square-pulse light stimulation. Insets: zoomed-in LFP signals. B: trial-averaged (n = 54; 1 session) LFP spectrograms for the corresponding LFP channels shown in A for subjects T and P. The spectrograms show clear optogenetically induced narrowband gamma oscillations. In these 2 sessions, induced gamma had a frequency near 50 Hz in subject T, slowed from 80 Hz to 70 Hz in subject P. C: trial-averaged LFP power spectral density (PSD) based on the time interval [0.1, 0.9] s during stimulation (green curve). (The first and last 100 ms were removed to prevent the contributions of any potential artifacts.) Black curve shows PSD computed for a 0.8-s time window preceding the stimulation onset. Shaded areas: 95% confidence intervals (CIs). D: gamma power during stimulation vs. baseline period in each trial. Each data point in the scatterplot shows the gamma band power in 1 trial for the 2 electrodes as in A–C. x (y)-Axis corresponds to power during baseline (stimulation) period. Diagonal line indicates equality. (n = 54, 54, 34 trials in subject T; n = 49, 99, 50 trials in subject P). E: spatial distribution of optically induced gamma activity over the arrays in subjects T and P, all 6 sessions. Red squares indicate channels in which gamma-band LFP power (40–80 Hz) increased during the stimulation period [P < 0.01; random permutation test with false discovery rate (FDR) correction for multiple testing]. Green circle indicates estimated border for direct optical stimulation effects (threshold of 1 mW/mm2; see also Fig. 1D). The 5 electrode sites on left to the stimulation site (including it) were nonrecording sites because of the optical fiber integration. F: gamma power during stimulation vs. baseline period in each electrode. Each data point shows trial-averaged gamma-band power for 1 electrode that has optically induced gamma. Diagonal line indicates equality. Data from 3 sessions of each subject were included.
Gamma oscillations induced by ramp light stimulation. A: no. of recording sites that showed modulation in multiunit spiking activity (increasing and decreasing) to optical stimulation according to time elapsed since viral injection. B: LFP (<300 Hz, top; 40–80 Hz, bottom) from 2 representative microelectrodes during a 4-s-long light ramp. Insets show position of electrode. C: trial-averaged (n = 36) spectrograms showed elevated LFP power in the gamma band (40–80 Hz) during the late stage of the light ramp. D: evolution of gamma power during light ramps (blue solid line: mean across trials, shaded area: 95% CI). Onset of optogenetically induced gamma oscillations, detected as the lower CI bound crossing the 99% percentile (red line) of the baseline gamma power, occurred once a critical light stimulation level was reached (2.67 mW and 3.06 mW). Time differences in the onset of gamma oscillations in the 2 recording sites in this ramp stimulation condition could result from the progressive recruitment of more distant cortical regions, as the cortical volume affected by light increases with increasing light power.
Spatiotemporal patterns of optogenetically induced gamma waves. A: during stimulation, gamma-band LFPs (40–60 Hz in subject T, 60–80 Hz in subject P) showed time delays between different electrodes. Black dots indicate the peaks of a gamma cycle in the period between the 2 dashed lines across 5 recording sites on the microelectrode array. B: contour map of time delay at each electrode (subject T) with respect to the electrode with the shortest latency (methods), averaged across all gamma cycles from 54 trials. Five nonrecording sites on left of and including the light stimulation site show interpolated values. C: snapshots of LFP amplitude maps (40–60 Hz, subject T; z-scored, interpolated) during a single gamma cycle of 20 ms. Top: gamma waves propagate in an expanding concentric pattern. Bottom: spiral pattern. The shift of the wave “center” to the right (with respect to the fiber optic location) could have resulted from the actual orientation of the tip of the optical fiber or from differences in opsin expression, for example. (The implantation goal was to have the fiber perpendicular to the cortical layer, but the actual orientation might have been different.) D: different patterns of wave propagation illustrated by the averaged (across frames in a 20-ms cycle) wave vector field evaluated via the Horn-Schunck method (Horn and Schunck 1981) corresponding to the same sets of data shown in C. E: contour map of transient time delay corresponding to the 2 gamma cycles shown in C and D.
Modulation of neuronal spiking rates during optogenetically induced gamma LFP oscillations. A: action potential waveforms, interspike time intervals (ISIs), and recording sites for 3 representative neurons recorded from the 2 subjects are shown. Waveform: mean waveform with dashed line envelope showing the corresponding 95% CI for observed waveform values. The modes of ISI of the 3 units are 23, 26, and 29 ms, respectively. B: corresponding raster plots for each neuron (n = 54 trials). C: corresponding peristimulus time histograms (PSTHs) estimated via Bayesian adaptive regression splines (Kass et al. 2003) showing either substantial increase or decrease in spike rates during optical stimulation. Dashed curves indicate the 95% CIs. D: average waveforms of all recorded neurons on the array (subject T, 1 session). Units on the same electrode are shown in different colors. E: proportion of neuronal recordings that showed an increase (yellow), a decrease (orange), or no change (white) in spiking rates during optical stimulation. (subject T: n = 59, 52, 47 neuronal recordings in 3 sessions; subject P: n = 56, 55, 46 neuronal recordings in 3 sessions.)
Spike-LFP pairwise phase consistency (PPC) analysis and predictive power of optogenetically induced gamma oscillations on neuronal spiking. A: waveforms and ISIs of 3 examples of recorded neuronal spiking. Top 2 neurons were recorded in subject T and bottom neuron in subject P. B: corresponding PPC analysis quantifying the phase-locking of neuronal spiking to LFP oscillations during the 1 s before light stimulation (blue) and during the 1-s optical stimulation period (green). Dashed line shows 95% CI for the PPC chance level (methods). In these 3 examples, all neurons showed some level of significant PPC, although weak (PPC = 1 indicates perfect phase-locking) in the corresponding gamma band during optical stimulation. Insets show position of electrode. C: receiver operating characteristic (ROC) curves assessing prediction of spiking for the corresponding 3 neurons using a point process model where the instantaneous rate (intensity) was modeled as a function of the amplitude envelope and phase of the optogenetically induced gamma oscillations (methods). Diagonal dashed line indicates chance-level prediction. TP (FP) denotes true (false) positive rates. Blue and green ROC curves correspond to the baseline and stimulation periods, respectively. D: summary of gamma oscillation PPC over subjects, sessions, and neuronal recordings (N = 45, 30, and 22 neuronal recordings in 3 sessions for subject T, respectively and N = 13, 5, and 5 for subject P, respectively. Only neurons where the same recording site showed induced gamma oscillations were included.) E: gamma predictive power analysis over the same set of data as in D.
Optogenetically induced gamma oscillations during movement preparation are suppressed upon voluntary movement execution. A: cartoon of the task: the subject (subject T) rests his hand on a stationary stick in preparation for a reach-and-grasp movement until an opaque door blocking the food tray is lifted (go cue). The reach-and-grasp movement (toward a food pellet) is then performed. The movement preparation period lasted at least 1 s, and the movement execution period (terminating at the pellet grasping time) ranged from 1 to 1.5 s. Optical stimulation (constant square pulse; green line on plots) was turned ON during the preparation period and lasted 2.5 s, overlapping with the movement execution period. B: positions of the 4 recording electrodes in C and D. C: across-trial (n = 40 trials, 1 session) average LFP spectrogram and PSD for 4 microelectrode recording sites. (Of those 4 sites, 3 showed substantial attenuation of induced gamma on movement execution phase.) PSD was computed on a time window covering 0.5 s before the go cue (black) and 0.5 s starting 150 ms past (blue) go cue onset. Shaded areas indicate 95% CIs. D: same as in C but in control trial (n = 40) where no optical stimulation was presented while the subject performed the same task.
Movement execution attenuates optogenetically induced gamma oscillations: summary over trials and recording sites. A: power of optically induced gamma during movement preparation (horizontal) and execution (vertical) for recording site 95. Each dot is the power of 1 trial (n = 40 trials each session, subject T) in 5 experimental sessions. PSD was computed as described in Fig. 7. B: same scatterplot in control condition (light OFF). C: the level of attenuation in optically induced gamma oscillations during movement execution was anticorrelated with the time delay (linear regression, P = 10−10, r2 = 0.4), indicating that sites away from the directly stimulated area tended to show larger attenuation effects. D, top: recording sites over the array where induced gamma was attenuated during the movement execution phase (red) and where gamma was not affected (light red). Bottom: summary over recording sites on the microelectrode array: the heat map denotes the ratio between the gamma power during movement execution and during preparation. Values < 1 indicate attenuation of the optically induced gamma oscillation. E: average motor evoked potentials in 1 session for all of the recording sites. Motor evoked potentials were detected in all of the recording sites and did not seem to play a role in the suppression or not of optically induced gamma oscillations.
Cover: Electrophysiological and morphological measurements were obtained simultaneously from a single corticospinal neuron. These data served as constraints on evolutionary optimization, generating a family of corticospinal models. A three-dimensional reconstruction serves as the backbone for a pseudo-color visualization of synaptic efficiency as a function of dendritic location, simulated in a single biophysical model selected from the family of optimal individuals. Excitatory synapses at yellow dendritic locations resulted in the largest depolarizations at the soma, while the same synaptic activation at purple locations generated only weak somatic depolarizations. This visualization is surrounded by scatter plots representing the evolutionary optimization: biophysical models optimized across different fitness functions demonstrate tradeoffs between full high-dimensional error (y-axis) and individual error scores (individual x-axes; clockwise order from top
left: subthreshold error, instantaneous firing rate error, spike-shape error, average firing rate error). Color based on 5 error percentiles in increasing instantaneous firing-rate error (purple, red, dark orange, light orange, yellow). From Neymotin SA, Suter BA, Dura-Bernal S, Shepherd GMG, Migliore M, Lytton WW. Optimizing computer models of corticospinal neurons to replicate in vitro dynamics. J Neurophysiol; doi:10.1152/jn.00570.2016.