Feedforward excitation and inhibition evoke dual modes of firing in the cat's visual thalamus during naturalistic viewing

Abstract

Thalamic relay cells transmit information from retina to cortex by firing either rapid bursts or tonic trains of spikes. Bursts occur when the membrane voltage is low, as during sleep, because they depend on channels that cannot respond to excitatory input unless they are primed by strong hyperpolarization. Cells fire tonically when depolarized, as during waking. Thus, mode of firing is usually associated with behavioral state. Growing evidence, however, suggests that sensory processing involves both burst and tonic spikes. To ask if visually evoked synaptic responses induce each type of firing, we recorded intracellular responses to natural movies from relay cells and developed methods to map the receptive fields of the excitation and inhibition that the images evoked. In addition to tonic spikes, the movies routinely elicited lasting inhibition from the center of the receptive field that permitted bursts to fire. Therefore, naturally evoked patterns of synaptic input engage dual modes of firing.

Figure 1. Spatially Opponent Excitation, or Push, and Inhibition, or Pull, in the Relay Cell’s Receptive Field

(A) Anatomical reconstruction of an On center Y cell in layer A1; scale bar, 100 μm. (B) Averaged responses to ten trials of bright (red) and dark (blue) disks and annuli in the receptive field center and surround; disk size 1°, annulus size 2°, 20°; horizontal bars under the traces mark stimulus duration; scale bars are 10mV and 200 ms in this and the following panel. (C) Voltage dependence of the hyperpolarization evoked by suppressive stimuli; response recorded at different holding currents: -0.2 nA (dark gray line), 0 nA (black line), -0.2 nA, -0.4 nA (light gray line). (D) Wiring diagram for feedforward, push-pull responses mediated by local interneurons (left) and for feedback inhibition from the perigeniculate nucleus (right). Cells are represented as their receptive fields; blue indicates Off subregions; red, On subregions; and minus signs, interneurons; solid and dashed lines indicate excitatory and inhibitory connections, respectively. Drawings of overlapping On and Off receptive fields in the retina and the lateral geniculate nucleus are offset in the figure for the purpose of illustration.

Figure 2. Intracellular Responses of a Relay Cell to Repeated Presentations of Natural Movies

(A) Anatomical reconstruction of an Off center X cell in layer A; scale bar, 100 μm. (B) Insets show examples of movie frames that introduced marked changes in luminance in the receptive field center (blue ellipse); arrows point to the time at which each frame appeared. (C) Receptive field of the retinal input reconstructed from responses to repeated presentations of the movie; white ellipse is the 1.5σ contour from a 2D Gaussian fit. (D) Clips of responses to three presentations of the movie recorded in current-clamp mode; scale bars, 200 ms and 10mV. (E) Responses to the same segment of the movie recorded in voltage-clamp mode, which damped intrinsic conductances. (F) Separation of components of the raw data, as illustrated for the lowest trace in (E). The black trace shows spikes, the red trace shows templates fit to each EPSC, and the blue trace shows the residual (spike and EPSC subtracted) currents, which were mainly hyperpolarizing but also contained occasional putative T-currents (asterisk); scale bars, 200 ms and 100 pA.

Figure 3. Receptive Fields of Synaptic Excitation, or Push; Inhibition, or Pull; and Spikes Reconstructed with Natural Movies

(A) Anatomical reconstruction of six relay cells. Anatomical class and center polarities are as labeled; scale bar, 100 μm. (B) Receptive fields of the push (top), the pull (middle), and spikes (bottom); white ellipses are 1.5σ contours of 2D Gaussian fits of the centers. (C) Time course of the responses. (D) Scatter plot of the overlap index versus the ratio of sizes of the push to the pull. Histograms of the distributions of the overlap index (top) and the ratio of sizes (right) are shown next to graphical depictions of the two measures; crosses outline the mean ± standard deviation. (E) Elliptic eccentricities of the receptive field centers of push, pull, and spikes; horizontal bars indicate the mean ± SEM. Values for X cells are in red; Y cells, in blue; W cells, in green; and remaining unlabeled cells, in black.

Figure 4. Pull from the Center of the Receptive Field Precedes Bursts, but Not Tonic Spikes, Evoked by Natural Movies

(A and B) Burst (bSTA) and tonic (tSTA) spike-triggered averages of membrane current (I_m) and voltage (V_m) of two relay cells (black traces). Sigmoid fit of bSTA and single exponential fit of tSTA (gray bands) was used. Scale bars are 100 ms and 2mV in (A), and 100 ms and 25 pA in (B). (C) Time courses of the hyperpolarization, or outward currents, and depolarization, or inward currents, preceding burst versus tonic spikes. Values for membrane currents are shown in white and membrane voltage in black for different cell types as indicated. Error bars show mean ± SEM.

Figure 5. Model Prediction of the Occurrence of Burst and Tonic Spikes Based on Feedforward Synaptic Inhibition

(A) Receptive field of synaptic inhibition reconstructed for the cell in Figure 4B. (B) Predicted response of the pull by the receptive field (red) overlaid with the actual pull response (black); scale bar is 1 s. (C) bSTA (red) and tSTA (black) of the pull for the cell in Figure 4B; gray shades a 500 ms time window before the occurrence of spikes. Scale bars are 100 ms and 10 pA. (D) Scatter plot of the second versus first principle components of the pull waveform in a 500 ms window before tonic (black dots) and burst (red dots) spikes. Green arrow represents the vector linking the tSTA to the bSTA. (E) Distribution of tonic (black) and burst (red) events along the tSTA-bSTA axis; black ticks mark the different threshold levels used to generate receiver operating characteristic (ROC) curves in (F), and the blue line shows the threshold level used to predict event identity in (G). (F) ROC curves of the model’s performance on another movie (black) and for bootstrap resampling (gray). Dots represent different threshold levels. Gray lines mark the 95% bootstrap confidence interval, and the filled blue circle marks the threshold level used in (G). (G) Prediction of spike type (tonic or burst) based on the inhibitory waveforms taken from responses to another movie; thick red lines represent bursts. Scale bar is 1 s. Note that the model produces serial bursts because it does not include a refractory period.

Figure 6. Retinogeniculate Synaptic Inputs Trigger Putative T-Currents that Evoke Bursts

(A) Cross-correlogram of putative T-currents with EPSCs (EPSC at t = 0). The shift predictor has been subtracted from the raw correlogram; bin width, 1.0 ms; n = 41. (B) Cross-correlogram of the first spikes in bursts with the putative T-currents (T-current at t = 0); conventions and sample size as in (A). (C) Conditional probability densities of the interval between a burst and most recent retinal EPSC (EPSC at t = 0); time bin, 0.5 ms. Within-trial (solid line) and cross-trial (dashed line) probability densities intercept at about 8 ms (arrow); the contribution value, i.e. the shaded area, is 0.80 in this case; n = 41. (D) Contribution values calculated for retinal inputs to putative T-currents and putative Tcurrents to bursts for intervals of 0 to 12 ms and -5 to 10 ms, respectively. (E) Examples of retinal inputs that trigger T-currents that initiate bursts in turn; scale bars, 50 pA and 20 ms.

Figure 7. Extraction of Neural Events and Components from Voltage-Damped Recordings

(A) Raw recording of membrane current. (B) The same signal smoothed and differentiated; all local minima were used as candidate neural events. Events ultimately classified as EPSCs are labeled with red, and those classified as spikes are labeled with green in this and remaining panels. (C) Scatter plot of the candidate events plotted as peak value (slope in the raw signal) against area under the peak (magnitude in the raw signal). (D) Trains of spike times including bursts (magenta). (E) EPSC times. (F) Illustration of spikes (action currents) isolated from the raw signal. (G) Isolated action currents. (H) Fitting the template to model EPSCs. Parameters are as they appear in Equation 1. (I) Modeled EPSCs as the EPSC train convolved with the template. (J) Residual currents remaining after subtraction of the action currents and modeled EPSCs; blue line indicates one-tenth spike height. (K) Times of putative T-currents, defined as events that crossed and remained below the blue line for longer than 10 ms as in (H). Scale bar, 50 pA and 200 ms for panels on the left.

Figure 8. Explanation and Controls for Receptive Field Reconstruction

(A) Fraction of explained variance by the fitted response (blue) and the predicted response (red) as a function of λ, the Lagrange multiplier for regularization (Equation 4); prediction is made with a different movie. (B) The kernel that best predicted the receptive field was chosen as the reconstructed receptive field (indicated by the gray arrow in A). (C) Predicted response (red) by the optimized kernel overlaid with the actual response (black); scale bar, 500 ms. (D and E) Side-by-side comparisons of receptive fields reconstructed from responses to movies (left) and receptive fields mapped with sparse noise (right). White ellipses are 1.5σ contours of 2D Gaussian fits of the centers; yellow ellipses replicate the white ones in the panels to the left.